For ease of understanding, these 14 controls are divided into five main groups. These are form, profile, orientation, location and runout.

The tolerances under each of the five categories primarily control the feature of the category they belong to. For instance, the tolerances under the runout category (circular and total runout) control the part runout. But under certain circumstances, they may control other features as well.

Of the five groups, orientation tolerances control the tilt associated with part features. Angularity is a type of orientation control besides parallelism and perpendicularity.

In this article, we shall take a look at the angularity tolerance, its various aspects, feature control frame and measurement methods. Let us start by defining what angularity is.

What Is Angularity?

Angularity is a 3-D GD&T callout that helps maintain a specified angle between a feature (line or surface) and a reference feature. It can be used to reference a line with respect to another line but more commonly it is used to maintain a surface at an angle to a datum or reference plane. This is especially important for parts with angled surfaces that mate with other parts in an assembly.

At times, angularity finds use in aligning a feature of size, such as pins or holes in a surface, to the desired angle. In those cases, angularity controls the center axis of the hole to fix its orientation.

Angularity is also used to control the orientation of non-circular features such as tabs and slots. The angularity callout creates a tolerance zone around the mid-plane to control these features.

Angularity Tolerance Zone

The biggest misconception regarding angularity is that it creates an angular tolerance zone such as 30 degrees +/- 10′. This would mean that the feature is supposed to be at a 30 degrees angle in relation to a datum and is allowed an angular variation of 10 minutes on either side. But this is not the case with angularity.

The shape of the tolerance zone changes with the feature being controlled. In the case of surfaces, the tolerance zone is expressed by two parallel planes that are aligned with the exact angle required by design. In other words, the zone is oriented to the datum at a basic (or theoretically exact) angle and the surface variation is allowed to float between the two planes.

If the zone width increases, the degree to which an erroneous angle is accepted also increases. Thus, it is the width of the zone that is indirectly restricting the permissible variation of the angle. For such a part to be in spec, all the points on the surface must lie within the envelope.

In the case of a circular feature of size, such as a cylindrical pin or a hole, the zone is expressed as a cylindrical tolerance zone around the axis of the said feature. This zone is aligned to the basic angle with respect to the datum and the diameter of the zone is specified in the feature control frame. Same as before, the zone’s diameter determines the permissible angular variation. As long as the center axis of the feature lies within the tolerance zone, the part is approved.

When used to control non-circular features of size, such as tabs and slots, angularity creates a planar tolerance zone with one plane on either side of the central plane. The center of the feature at each point must lie between the two planes of the tolerance zone.

GD&T Angularity Tolerance Zone Explained

Angularity vs Other Callouts

Angularity vs flatness

Angularity can be understood as oriented flatness. It uses the same concept and tolerance zone as flatness but is oriented at a specific angle to the datum. All the points on the part surface must lie between the two planes for both callouts.

The difference is that flatness is measured relative to itself and hence, works without a datum whereas the angularity tolerance references a datum. Thus, angularity controls the angle as well as the flatness of a surface.

If we only want to control the angle instead and not the flatness, we can use the tangent plane modifier (T in a circle) which controls the tangent plane to the surface instead of the entire surface thickness. This is especially important when working with natural materials as they have a certain thickness (plywood, wood, metal sheets) and defects such as wood knots.

Angularity vs perpendicularity and parallelism

Angularity bears similarity to perpendicularity and parallelism too. In fact, many books refer to perpendicularity and parallelism as special cases of angularity. They all require datum elements and create total wide tolerance zones to define the expected accuracy of a part. In this way, angularity is more similar to perpendicularity and parallelism than it is to flatness.

The only difference is that in the case of perpendicularity and parallelism, the angles are 90 degrees and 0 degrees respectively. Whereas for angularity, it can be any value from 0 to 90 degrees including both.

Thus, it is perfectly alright to apply angularity anywhere you would use perpendicularity or parallelism without any issues.

Angularity Feature Control Frame

Angularity uses a simple and easy-to-understand feature control frame (FCF). The FCF marks the feature under control using a leader arrow that points to the feature or its extension line. The feature control frame can be divided into three distinct parts. Going from left to right, these three parts are:

• Geometric tolerance block

• Feature tolerance block

• Datum block

Geometric tolerance block

This block in the FCF gives information about the type of geometric tolerance applied to the feature.

Each tolerance in GD&T has been assigned a specific symbol. The symbol denoting angularity is an acute angle opening to the right (⦟).

Feature tolerance block

This is the second block in the FCF and it explains the shape and magnitude of the tolerance zone for the callout. The first symbol in this block is for the shape of the zone.

As discussed earlier, there are two main types of tolerance zones in angularity. When applied to pins and holes, angularity creates a cylindrical tolerance zone which requires a diameter symbol to be placed in this block.

If the tolerance is a total wide tolerance zone as in the case of lines, surfaces and non-circular features, no extra symbols are needed as this is the default tolerance zone.

This symbol is followed by a numerical value known as the tolerance value. ASME defines it as the total amount a specific dimension is permitted to vary. It is the difference between the maximum and minimum limits.

For angularity, this is the width between the parallel planes of the planar tolerance zone or the zone diameter in the case of a cylindrical tolerance zone.

The next symbol is for material condition modifiers. When it comes to angularity, it is often called out with MMC in the case of pins and holes to ensure that little to no interference occurs between mating parts during assembly. This is represented by the letter M in a circle.

Datum block

This is the third and final block in the FCF. It gives information about the datum elements used as references for the angularity callout. The datum axis or the surface is represented by a letter on the drawing which is subsequently placed in the FCF.

Pay attention that the angularity FCF doesn’t talk about the angle it needs to maintain to the datum. Then where do we get this information?

The angle that the surface under control must maintain with the datum is shown on the drawing as a basic angle. In the case of parallel and perpendicular angularity measurements, the zero and 90-degree angles are never called out, but they are assumed. All other angle values are marked on the drawing.

How to Measure Angularity

Inspecting Angularity Using a Dial Indicator and a Sine Bar

Angularity maintains the flatness of a surface at a specific angle. Hence, the methods used for measuring flatness can also be applied to angularity, albeit at an angle. If all the points on the surface in either direction (length and breadth) lie between the two planes, that part is within the specified angularity tolerance and is said to be in spec.

For the cylindrical zone, we derive the axis of the feature of size and ensure that it lies entirely within the zone.

Metrology offers a variety of options for measuring angularity for both cases. From the more advanced methods, optical comparators or CMMs are often used. For high-volume production lines, companies might opt for custom go/no-go gauges.

The traditional method of measuring angularity uses a sine plate. The feature under inspection is tilted to the basic angle in the drawing so that it is horizontal, and therefore, parallel with the granite slab. The machinist then checks whether the flatness of the surface is within permissible limits using a dial gauge.

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In this article, we shall take a look at the profile of a line tolerance that belongs to the profile group along with profile of a surface. Line profile tolerance is quite common and it helps us to manufacture parts with complex shapes. But first, let’s start by defining what it is.

What Is Profile of a Line in GD&T?

The profile of a line tolerance is a 2D GD&T callout that can define and control a linear/curved feature or a surface cross-section within specified limits. We can use it to control the form, location, orientation and size of a feature. This callout can only be used on surfaces and cannot be used to locate a central axis or plane.

The line profile callout is typically used in the case of complex curves. These could be simple or advanced algebraic curves. The callout simply mimics the curve irrespective of the complexity.

Profile of a Line Tolerance Zone

The line profile tolerance creates a true profile where the curve should ideally be located. The actual curve is evaluated with respect to this true contour. The actual curve must conform as much as possible to this true profile.

The tolerance zone is made of two parallel lines that follow the true profile in either direction. Unless stated otherwise, these parallel lines are assumed to be equidistant from the true profile.

The distance between the two parallel curves is the tolerance limit. Every point on the actual curve must lie between these two lines of the tolerance zone for approval.

Line Profile vs Other Callouts

GD&T Profile of a Line

Profile of a line vs profile of a surface

The profile of a surface tolerance, is most closely associated with profile of a line. In fact, it is the 3D equivalent of the line profile tolerance. Profile of a line controls individual line elements on a single cross-section or any other linear feature, whereas surface profile controls the entire curved surface by taking the variance of adjacent cross-sections into account.

Profile of a line vs straightness/circularity tolerance

Straightness and circularity are great for controlling straight lines and circles respectively. They are easier to measure and control and this can be done with simple measurement instruments. In addition to both straight lines and circles, profile of a line can effectively control highly complex curves. This is not the case with straightness and circularity.

Besides, straightness and circularity tolerances can only control the form, whereas profile of a line can control the form, location, orientation and the size of a feature.

Line Profile Feature Control Frame

The feature control frame (FCF) for any GD&T callout contains all the information required to correctly define how it applies to a feature or surface. It connects to the feature using an arrow known as the leader arrow. A typical FCF consists of three main blocks. These are:

• Geometric tolerance block

• Feature tolerance block

• Datum block

Geometric tolerance block

This block gives information about the type of geometric tolerance applied to a feature. The tolerance is represented by its symbol in the first block. The symbol for profile of a line is an inverted semicircle.

Feature tolerance block

This block gives information about how the geometric tolerance applies to a feature. It contains information about the shape of the tolerance zone, the tolerance limit and material modifiers, if any.

In the case of profile of a line, the tolerance zone is a 2- dimensional total wide tolerance zone. This zone is distributed bilaterally in relation to the true profile. This is the default zone and requires no special notation.

This is followed by the tolerance limit which specifies the width of the tolerance zone. The third symbol is for material modifiers but these do not apply when using profile of a line. This callout is only controlled by basic dimensions.

Datum block

This block represents datum elements that are used as references to locate/orient a part feature. Profile of a line tolerance may or may not be called out with a datum. In the absence of a datum, the line profile tolerance acts as a form control (such as straightness, circularity, flatness, etc) and is a refinement of a size tolerance.

With one or two datums, profile of a line controls the orientation besides the form. Location requires two to three datums. Whenever we use a datum, it is important to notify the distance using basic dimensions. Using basic dimensions, a profile control can also control the feature size.

How to Measure Profile of a Line

The means to measure the line profile will depend on the type of curve. The profile tolerance can very well be used as a replacement for different form controls in which case a simple setup consisting of a dial gauge and a surface plate would be sufficient.

When it comes to complex curves, more advanced and reliable instruments such as a CMM, VMM (vision measuring machine) or a 3D scanner are required. In each case, the actual surface must lie within the tolerance zone to prevent rejection. For complex curves, we trace the contour of a part using a probe, record multiple measurements and compare it to a virtual boundary through a CMM.

Uses for Profile of a Line Tolerance

A large number of parts in modern machines feature advanced curves and cross-sections that have to be manufactured accurately for functional reasons such as aerodynamics, ergonomics, etc. The profile of a line tolerance helps us achieve that. It can accurately define and communicate geometry requirements for parts with curved shapes. Some examples of parts where we use profile of a line are:

1. Turbine blades

2. Bearing housings

3. Car hoods

4. Airplane wings

The post Profile of a Line (GD&T) Explained appeared first on Fractory.

Many of these 14 tolerances have features similar to each other in the way they control a part’s shape and dimensions, therefore it is important to understand what is the intended use of the part beforehand.

For ease of use, these 14 tolerances are divided into five main groups.

1. Form

2. Orientation

3. Location

4. Profile

5. Runout

In this article, we shall study the profile of a surface (or surface profile) callout that is part of the profile group mentioned above. These two callouts can help us manufacture geometries that have complex outer shapes with impressive details.

What Is the Profile of a Surface in GD&T?

The profile of a surface is an extremely powerful and versatile GD&T callout. It can be used on nearly all kinds of complex outer shapes where other tolerances are not easily applicable.

Some examples of surface profile use:

1. Aerospace: airplane wing, air intake for engine, turbine blades

2. Automotive sector: BiW, A-pillar, complex outer shape

3. Product design: design of complex outer shapes for consumer appliances such as coffee makers, smartphones and displays.

GD&T Profile of a Surface

Profile of a surface controls a part’s surface profile in accordance with the CAD model or the drawing. Many engineering parts such as turbine blades, car BiWs and airplane wings have highly complex surfaces. These surfaces need to be manufactured to great detail for functional reasons. The profile of a surface callout can help us manufacture these parts by controlling the surface profile during manufacturing and measuring results after production.

The profile of a surface generates a virtual surface that acts as the baseline to measure the actual surface. Thus, it is possible to create highly intricate surface profiles as long as they can be created as a CAD model. The required surface’s specification must also be within the capabilities of the fabrication process to be used.

Surface Profile Tolerance Zone

The surface profile tolerance zone consists of two parallel planes, bilaterally disposed on each side of the ideal curved surface (aka the true profile). Both planes follow the shape of the ideal surface and the distance between them is the tolerance limit for the callout. A smaller tolerance limit gives tighter control but may be difficult to manufacture. The points on the part’s entire surface must lie within the specified tolerance zone for it to be accepted.

Surface Profile Relation to Other GD&T Symbols

As mentioned above, the profile of a surface can effectively replace almost all of the GD&T Callouts whether they are curved or flat. Let’s see a few examples:

Profile of a surface vs form controls

When used without a datum, the profile of a surface can replace all the form controls.

The four form controls are

1. Flatness

2. Cylindricity

3. Circularity

4. Straightness

These form controls constrain the form of a part by creating a tolerance zone between two parallel surfaces. These planes can be either flat or cylindrical.

Profile of a surface vs orientation controls

When used with a datum callout, the profile of a surface can replace all the orientation controls.

The orientation controls are:

1. Parallelism

2. Perpendicularity

3. Angularity

In order to orient a part feature in GD&T, we need a reference point such as a datum plane, line, or axis. By specifying a datum in the profile of a surface’s feature control frame, we can replicate the function of all orientation controls to maintain a surface in a particular position.

Profile of a surface vs line profile tolerance

The profile of a surface tolerance is the 3D equivalent of the line profile tolerance. While the profile of a line controls a specific cross-section on the part, the profile of a surface controls every cross-section across the entire length of the surface.

There are times when both profile controls may be applied together. In such a case the line profile’s tolerance is tighter than the profile of a surface. This is done to achieve greater control at critical cross-sections while the profile of a surface maintains a looser overall control over the full surface.

Profile of a Surface Feature Control Frame

A leader arrow connects the feature control frame (further referred to as FCF) of the profile of a surface tolerance to the part feature. Like any other FCFs in GD&T, the profile of a surface FCF can be divided into three distinct blocks. These are:

• Geometrical tolerance block

• Feature tolerance block

• Datum block

Geometrical tolerance block

This block defines the geometrical tolerance applied to a feature by housing its symbol. The profile of a surface symbol is an inverted semicircle with a horizontal diameter connecting its two ends.

Feature tolerance block

This block contains specific information about how a tolerance applies to a feature. Since for the profile of a surface, the tolerance zone is a total wide tolerance zone which is the default zone, there is no special symbol for it. This is followed by the tolerance limit. This limit, also known as the tolerance value, is the distance between the two planes of the tolerance zone and is represented by its numerical value in the FCF.

This number is typically followed by material condition modifiers (MMC, LMC, etc.), but these do not apply to the profile of surface control.

Datum block

This is the third block in GD&T that houses the reference planes for the tolerances. In the case of a profile of surface control, the use of a datum is optional. When a datum is not present, the callout only controls the feature’s form. But if we want to control additional aspects such as the orientation, location, and size, we must define one or more datums as needed.

When to Use Profile of a Surface Tolerance?

As mentioned in the above examples, the profile of a surface can effectively replace almost all of the form and location tolerances. So the question then becomes:

Why isn’t it used everywhere?

The answer boils down to pricing. In order to measure the surface profile, the part has to be measured via a CMM machine. CMM machines can measure either with a probe or with a laser. Regardless both of these options are expensive because:

1. It takes time to program the machine and then measure. This is not feasible for parts in running production.

2. They require skilled operators: The operator should not just be trained in measurement, but he/she should also know how to interpret the drawings. Such operators don’t come cheap.

Hence it is strongly advised to engineers to make sure that the surface profile tolerance is necessary and no other GD&T can fulfil the requirement.

Important Points to Remember

Here are some best practices and important points to consider when using the profile of surface tolerance.

1. As with all GD&T, make sure that basic dimensions are used, when adding numerical dimensions for features controlled with surface profile control.

2. The numerical tolerance range for the surface profile depends on the following factor:

   1. Is the intended surface exposed or assembled?

   2. What kind of environmental factors will the part be subject to? Putting tight profile control on a bumper panel for example does not make sense if the part will expand and contract depending on the temperature

Therefore it is extremely important that tolerance chain analysis is conducted beforehand to find the numerical values that are put in the drawing.

Conclusion

The surface profile is an extremely powerful GD&T symbol that can be used for complex surface profiles for all kinds of parts. However, it should be used keeping in mind the associated costs for measuring and verification of parts.

The post Profile of a Surface (GD&T) Explained appeared first on Fractory.

Under orientation control, we have parallelism, perpendicularity and angularity tolerances. These tolerances control the orientation of a feature (such as a line, axis or surface) in respect to another reference feature (datum element).

In this article, we will explain the concept of parallelism GD&T tolerance. We will also go over its tolerance zone, feature control frame and measurement methods.

What Is Parallelism in GD&T?

Parallelism is a 3D GD&T orientation tolerance which maintains that two part features are parallel to each other. You can use it to control centerlines, center planes, cylindrical and planar surfaces parallel to the datum elements.

There are two types of parallelism in GD&T. It may either refer to surface parallelism or axis parallelism depending on whether you use it to control a surface or an axis. The use of surface parallelism is more common than axis parallelism.

With both types of GD&T parallelism, the goal is to maintain parallelism (0° alignment) with the datum element (axis or plane) according to the limits specified in the feature control frame.

Parallelism Tolerance Zone

In the case of surface parallelism, the parallelism tolerance zone is made of two theoretically exact parallel planes. The distance between the two planes is the tolerance limit for the callout. All the points on the planar surface or center plane must lie within the two parallel planes for a part to be approved. 

It is evident from the shape of the zone that the parallelism tolerance doesn’t create an angular tolerance zone to control the 0° alignment between the controlled surface and the datum plane.

Instead, it fixes the tolerance zone at a basic (or exact) 0° angle and the permissible variation is controlled by widening or tightening the two surfaces of the zone. The greater the distance between the zone’s two planes, the more error it can accommodate.

Axis parallelism creates a cylindrical tolerance zone. It is used to maintain the axis of a feature of size such as cylindrical pins or holes parallel to a datum. All the points of the feature’s center axis must lie in this cylindrical zone for a part to be in spec. In this type of zone, the permissible angular deviation can be controlled by reducing or increasing the diameter of the cylindrical zone.

It is worth mentioning here that the parallelism callout cannot control the location of the tolerance zone. It is only concerned with orientation. The tolerance zone exists at the location of the surface.

Parallelism vs Other Callouts

Due to many similarities between parallelism and some other tolerances, it is often confused with flatness and other orientation controls. It is advisable to understand the difference between them for their correct application in GD&T.

Parallelism and flatness

Parallelism and flatness resemble each other on many levels. They both have similar tolerance zones. They essentially control the flatness of the surface they are applied to. Their measuring methods are identical.

There are some key differences though, that help us distinguish between the two. For instance, parallelism, as with all other orientation controls, cannot function without a datum. The surface’s tilt is compared with a datum plane to create the tolerance zone.

On the other hand, when flatness tolerance is used, the surface is measured against itself. The tilt does not matter as long as the flatness is within limits. With the parallelism callout, the zone may translate or shift but it cannot tilt in respect to the datum.

Parallelism and angularity

Parallelism and angularity are both orientation controls. They are similar in almost every way except that angularity can maintain part features at any angle between 0 and 90°, whereas parallelism can only maintain a 0° angle.

Perpendicularity, on the other hand, is used for perpendicular orientations. Many books refer to parallelism and perpendicularity as refined forms of angularity. Angularity can replace both parallelism and perpendicularity in all cases but the opposite is not true.

Parallelism Feature Control Frame

The parallelism control is applied to part features through feature control frame (FCF). A leader arrow points the FCF to a feature or its extension line. A generic FCF is divided into three main blocks for ease of understanding. These blocks are:

• Geometric tolerance block

• Tolerance block

• Datum block

Geometric tolerance block

This block contains information about what geometrical tolerance is being applied to the feature. In our case, it will contain the parallelism symbol. The parallelism symbol consists of two oblique parallel lines (//).  

Tolerance block

This block shows how a GD&T callout applies to a feature. The first symbol in this block is for the shape of the tolerance zone.

In the case of axis parallelism, the zone’s shape is cylindrical which is represented by a diameter symbol in this block. For surface parallelism, the zone is made of two parallel planes. This is the default zone in GD&T and does not require a symbol.

The next component is the tolerance value. For axis parallelism, the value is the diameter of the cylindrical zone. For surface parallelism, this value is the distance between the zone planes.

The next symbol is for the surface modifier. When parallelism tolerance is in reference to a surface plane, material modifiers cannot be used. But in the case of axis parallelism, both LMC (least material condition) and MMC (maximum material condition) are allowed. The bonus tolerance (difference between LMC/MMC and the actual condition) comes into the picture when we apply these modifiers.

Axis parallelism is not commonly used but check out our article on perpendicularity for a more detailed overview of how an axis is controlled in GD&T.

Datum block

Many GD&T callouts are called in reference to a datum feature. The third block in the FCF houses the datum symbol. As with other orientation controls, parallelism cannot be called without a datum element.

How to Measure Parallelism

Parallelism and flatness have similar measurement methods, except that in parallelism the part must be held against a flat surface that acts as the datum element.

A CMM is the most accurate tool to measure parallelism. However, in the absence of one, an inspector may use a surface plate and height gauge to measure parallelism with reasonable accuracy.

Parallelism GD&T

To measure parallelism using a surface plate and a height gauge, the inspector follows the following steps:

1. Place the datum surface from the FCF against the surface plate. The surface plate now acts as a datum simulator creating a theoretical reference plane. This plane is known as a ‘simulated datum’ in ASME Y14.5 M.

2. Set the dial indicator at a fixed height with its probe touching the surface under control.

3. Reset the dial indicator to zero.

4. Sweep it across the entire surface and record the highest and lowest values obtained. Subtract these values from each other to obtain the full indicator movement (FIM) for the surface.

5. Compare the FIM value to the tolerance limit. For a part to be in spec, this value must be smaller than the tolerance limit.

Important Points to Remember

• Parallelism can never be called without a datum feature.

• Due to the envelope principle (GD&T rule #1), the parallelism tolerance cannot be greater than the size tolerance.

• Temperature becomes an important factor when measuring parallelism with extremely small tolerance limits as some materials expand/contract more than others with temperature.

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The 14 geometric tolerances are classified into 5 main groups – form, location, profile, orientation and layout. Symmetry is one of the three tolerances under location control (the other two being true position and concentricity).

As the name suggests, it controls the symmetry of part features such as tapers, holes, chamfers, curves, etc. This may not be required in general applications. However, in special applications where balance and equidistant loading is of great concern (high-speed applications), symmetry becomes increasingly important.

What is Symmetry?

GD&T symmetry is a 3D tolerance that ensures that part features are symmetrical about a datum plane. The callout defines a central plane and creates a tolerance zone around it.

The GD&T symmetry callout ensures symmetry control by checking the distance between any two corresponding points on either side of the datum plane and calculating their median points. These median points must lie near the datum plane and be within the symmetry tolerance zone specified in the feature control frame.

Theoretically, the inspector must check all the median points and find them within the tolerance zone. However, for practical purposes, fewer points at different cross-sections are inspected. The remaining median positions are interpolated to achieve the median plane.

Symmetry Tolerance Zone

The symmetry tolerance consists of two parallel planes, one on each side of the datum center plane. The distance between the two parallel surfaces is the tolerance limit for the callout. For instance, if the tolerance limit is set at 0.03 mm, the two planes will be at a distance of 0.015 mm on either side of the datum plane. This type of zone is the default tolerance zone type in GD&T. It is also sometimes known as the total wide tolerance zone.

All the points on the median plane must lie in the volume between the two planes of the tolerance zone for approval.

Symmetry vs Other Callouts

The GD&T symmetry callout is a type of location control. It ensures that two features are at their proper locations when checked against the datum plane. Other location controls can also perform the same job, although using a different method and tolerance zone type. The symmetry tolerance is comparable to concentricity and true position in terms of what they can achieve.

Symmetry and Concentricity

The concentricity callout controls the concentricity of cylindrical surfaces whereas symmetry controls are typically applied to any non-cylindrical surface. Many refer to concentricity as the circular version of symmetry. ASME Y14.5M-1994, 5.14 states that: “…symmetry and concentricity controls are the same concept, except as applied to different part configurations.”

GD&T symmetry controls the location of two features by establishing a datum plane. The concentricity symbol, on the other hand, checks the concentricity by establishing a central datum axis. It then measures the spread of actual centers of cylindrical cross-sections; and if they are within the cylindrical tolerance zone around the ideal datum axis. Concentricity derives an actual central axis instead of a median plane.

Both symmetry and concentricity are incredibly difficult to measure. For accurate measurements, a coordinate measuring machine (CMM) is a must.

Symmetry and True Position

Symmetry and true position can both be used to define the ideal location for a part feature. They may even be used interchangeably in some situations. However, true position is far more versatile compared to symmetry. It can do everything symmetry can do but the opposite is not true.

The true position callout can establish a total wide tolerance zone as well as a circular zone. This increases the range of features that can be controlled by it. True position allows for bonus tolerances, whereas symmetry does not. Symmetry also does not allow datum feature shift and projected tolerance zone, both of which are possible with true position.

Another difference is that true position can be called Relative to Feature Size (RFS), or with Least/Maximum Material Condition (LMC/MMC). Symmetry is always applied RFS.

Symmetry Feature Control Frame

The feature control frame (FCF) for symmetry is one of the easiest to understand and use. On the drawing, the FCF is connected to the feature using a leader arrow. It points to the feature’s surface or its extension line.

The FCF gives all the required information about a callout using a set standard. A general FCF can be divided into three main blocks:

• Geometric tolerance block
• Feature tolerance block
• Datum block

Geometric tolerance block

This block gives information about the geometric tolerance applied to the feature. It houses the symbol for the callout. The GD&T symmetry symbol consists of three horizontal lines on top of each other, with the middle line being slightly longer than the other two. The middle line represents the datum plane while the other two represent a feature subjected to the symmetry requirement.

Feature tolerance block

This block gives information about the type of tolerance zone, the tolerance limit and material condition modifiers, if any. The tolerance zone for symmetry is a total wide tolerance zone. No symbol is required as it is the default zone type.

The tolerance limit represents the distance between the two parallel planes. The lower the number, the tighter the tolerance.

The symmetry callout is always applied RFS, neither MMC nor LMC applies here. RFS is the default condition and does not require a symbol.

Datum block

The datum block houses the datum axis, points or planes that act as references for callouts. The symmetry callout requires a datum that will act as the reference plane for the measurement. The tolerance zone is placed evenly on either side of this plane. Measurements are taken across this plane during an inspection. This datum plane’s name is placed in the datum block.

How to Measure Symmetry

Among all the GD&T callouts, symmetry is one of the more difficult callouts to measure. The median points that must lie within the tolerance zone are a derived feature and there are no actual surfaces readily available for measurement. The symmetry symbol requires the calculation of these median points along with the feature under symmetry control. Such calculations require a lot of time and a skilled operator.

There are two main ways in which symmetry tolerance can be measured.

• Using a caliper or a micrometer
• Using a coordinate measuring machine

Using a caliper or a micrometer

It is possible to measure symmetry with an analog caliper or micrometer in some simpler cases. However, the operator’s skill and the instrument error can affect the accuracy of such measurements and thus it’s not generally recommended.

Different instrument designs are available for different form and location measurements. They can measure the size effectively but may not be as accurate when verifying the form. Another disadvantage is that this method requires manual recording of measurements.

Using a coordinate measuring machine

This is the most common method of measuring symmetry. A coordinate measuring machine (CMM) can plot all the median points by just bringing the stylus into contact with the opposing points. This method provides comparatively greater accuracy when compared to a caliper or a micrometer.

Initially, the CMM is set up to establish the theoretical center plane. Then, both symmetrical sides are measured using the CMM stylus to calculate where the median points fall. The positions of all the median points along the feature’s length are compared with the datum plane. The inspectors approve the part as long as no median point exceeds the tolerance limits around the datum plane.

The CMM records the measurements. Although this method requires less from the operators it is still relatively complex to achieve accurate results.

Uses of Symmetry

Symmetry finds use in very specific applications where there is a need for even load or form distribution. Symmetry is preferred for:

• High-speed applications where static and dynamic balancing is of great concern
• Machine elements under heavy load to prevent uneven wear
• Fluctuating loads to prevent fatigue failure due to disproportionate loading

Wherever possible, manufacturers avoid the use of symmetry tolerance as it is a difficult and expensive callout to measure.

Important Points to Remember

• It is worth noting that the symmetry callout has been removed from the 2018 edition of ASME Y14.5-2018, as it can be replaced easily by true position. Symmetry was in the 1994 and 2009 editions which are still predominantly used in the industry today.
• Straightness and parallelism can also replace symmetry in some instances.

The post Symmetry (GD&T) Explained appeared first on Fractory.

Location controls maintain tight control on a feature’s position with respect to a datum. Concentricity, symmetry and true position are the controls under the location category. In this article, we shall take a look at concentricity, its various aspects, uses and measurement methods. Let us start by defining concentricity.

What is Concentricity?

Many mechanical parts require a highly accurate concentric design for a satisfactory operation. Parts such as tubes that endure high pressures require a design with uniform wall thickness to prevent any weak structural points. Concentricity is a 3D GD&T callout that ensures that one or more part features are concentric about a datum axis.

However, in GD&T, concentricity has a slightly different meaning than the literal definition that most engineers are aware of. The function of concentricity callout is to ensure that the midpoint of two diametrically opposite points lies within a specified tolerance zone. The circular feature may have notches, dips or other surface variations but the mass distribution about the central axis should be uniform.

This balanced mass distribution is important in applications where the part undergoes high-speed rotation and there is a risk of oscillation or uneven wear. But concentricity is a difficult characteristic to achieve and measure during manufacturing.

In most applications, simpler callouts such as circular runout, total runout, position or profile can do the job equally well. Wherever possible, they must be used to avoid concentricity.

Concentricity Tolerance Zone

The zone for GD&T concentricity is a cylindrical tolerance zone. The feature control frame specifies a datum axis that is used as a reference point to develop this zone. The diameter of this cylindrical zone is the permissible tolerance value for the callout.

In order to ensure concentricity, the actual median axis for the part must be derived by calculating midpoints of diametrically opposed points. When all such median points are connected, we obtain the median axis. All the points on the median axis must lie within the cylindrical tolerance zone for the approval of the part.

Concentricity vs Other Callouts

Concentricity is a necessary callout in many specialized applications where a uniform mass distribution is of the utmost importance. But due to the difficult and costly process involved in its application, it is important to be aware of other callouts that can replace concentricity without compromising the required specifications.

Circular runout and true position (also sometimes known as ‘position’) are the two most closely related callouts that can replace concentricity in many applications.

Concentricity vs Circular Runout

The difficulty in measuring concentricity arises from the need to find the derived median axis of the part. There is no method in which such a calculation can be carried out reliably without the use of a computer.

On the other hand, the runout of a part can be measured easily from the surface since it is a tangible feature. Moreover, simple instruments such as a V-block and a dial indicator can give reliable runout measurements.

Runout is popularly defined as the sum of circularity and concentricity. If a part is perfectly round, runout tolerance becomes equal to concentricity tolerance.

Specially designed gauges can also measure runout in a quick (<10 seconds), effective and relatively inexpensive manner. Thus, from all angles, runout is a better alternative to concentricity and must be used wherever possible.

Concentricity vs True Position

True position in GD&T is a fairly simple callout that can fix the position and size of different features. In many cases, true position callout can effectively replace concentricity. Standard hole sizes and positional tolerances are better than concentricity when there is no need for a precise mass distribution.

Concentricity Feature Control Frame

We use feature control frames (here on out referred to as ‘FCF’) to explain the manufacturing conditions, controls and tolerances placed on a part feature. A single part may have multiple features toleranced by GD&T. Each feature’s tolerance is represented by its own FCF. The FCF connects to the feature under control or its extension line using a leader arrow.

The feature control frame for GD&T concentricity is pretty straightforward. It follows the general layout of an FCF which consists of three distinct blocks. The blocks for concentricity can be understood as “relative to datum A, all median points of opposing elements on this cylindrical surface must lie within a cylindrical tolerance zone of 0.03”.

Each block gives information about a different aspect of the GD&T tolerance. These three blocks are:

• Geometric characteristic block
• Feature tolerance block
• Datum block

Geometric characteristic block

This is the first block in the concentricity FCF. It contains the symbol of the geometric tolerance applied to the feature. The concentricity symbol consisting of two concentric circles is placed in this block to specify this tolerance.

Feature tolerance block

This is the second block in the FCF which provides information about the type and size of the applied tolerance zone.

In the case of concentricity, the zone’s shape is cylindrical. This zone is also known as a diametral tolerance zone and is specified using a diameter symbol in this block. The diameter of this cylindrical zone is the tolerance value or the maximum allowable deviation for the derived median points of the part.

Material modifiers cannot be used with concentricity as the bonus tolerance would come into the picture. This extra tolerance enlarges the tolerance zone which leads to a step function of varying diameters, causing sharp changes in the surface diameter.

Datum block

This block contains information about the datum element. It may be a center point, a center line or a datum plane depending on the requirement. In the case of concentricity, the datum element is a datum axis derived from a datum feature. Sometimes, concentricity FCF may contain multiple datums. That is the case with shafts with multiple diameters.

How to Measure Concentricity

The procedure for measuring concentricity is the prime reason why most designers and machinists choose to avoid it. Taking necessary measurements is difficult, time-consuming and costly.

The inspector needs to build the actual central axis of the part by joining the center points of successive circular cross-sections. This is why a reliable concentricity measurement requires the use of a coordinate measuring machine (CMM) or some other computer-aided measuring method such as an optical shaft measuring system or a laser micrometer with a concentricity extension.

In many cases, engineers record the difference between the highest and lowest point on the surface using a dial gauge. They are under the impression that they are measuring concentricity when they are actually measuring runout. As we saw earlier that for runout to be equal to concentricity, the section under observation must be a perfect circle, which is rarely the case. Passing runout measurements as concentricity allow circularity errors to creep into the concentricity tolerance.

Now, let’s take a brief look at the step-by-step process to measure concentricity.

Measuring concentricity using a CMM

Measuring Concentricity with CMM

The concentricity tolerance can be measured using a CMM in four distinct steps:

Step 1: Securing the part and fixing the datum axis (theoretical axis)

The first step is to lock all degrees of freedom by constraining the part in a suitable position. The position must allow access to the entire cylindrical surface for measurement. This ensures that there is no need for repositioning during the whole course of taking measurements.

Then we need to establish the datum axis. We recommend choosing the bearing end of the shaft for the datum axis as the rotation of the assembly will be directed from that end.

Step 2: Finding the center point for one cross-section

The second step is to plot the control surface using a CMM stylus. We need multiple pairs of diametrically opposed points at each cross-section of the cylindrical part. A minimum of three such pairs is recommended at every cross-section. For these three pairs, we might receive three distinct median points (unless some coincide). The average of those three median points is selected to determine the center point for the cross-section.

Step 3: Repeating for multiple cross-sections across the cylindrical part length

The center point for multiple cross-sections must be calculated. On joining these points, we obtain the measured axis or the actual central axis of the part. This axis is also known as the derived axis.

Step 4: Check whether the measured axis lies within the tolerance zone

After obtaining the measured axis of the cylindrical (or spherical part), we check its position in reference to the datum axis. Each point on this axis must lie within the cylindrical tolerance zone specified in the FCF.

Uses of Concentricity

Most professional engineers understand that the concentricity tolerance must not be used unless absolutely necessary. But there are still many applications that require it. Some of these are:

• Precision ball bearings
• Transmission gears
• Medical-grade tubing
• High pressure piping

Precision ball bearings

These are high precision parts used in a variety of industries to reduce energy losses. Their manufacturing must achieve tight tolerances for a satisfactory operation. Concentricity tolerance is used between various elements of a ball bearing to ensure it meets specifications.

Transmission gears

The manufacturing of transmission gears requires concentricity to line up the axes perfectly. This prevents sideways movement and minimises wear rate. However, in some cases, runout may provide enough accuracy.

Medical-grade tubing

Concentricity is also used to control the tubing wall thickness in medical devices. These parts can be tiny and require high precision for an acceptable product.

High-pressure piping

Quite often, high-pressure piping may be manufactured using concentricity tolerance. These parts require a minimum wall thickness to prevent any thin points along the length where the tube may rupture due to the high pressures.

FAQ

Has concentricity been removed from ASME Y14.5-2018?

Yes, the 2018 ASME standard does not include concentricity tolerance. However, this isn’t very shocking as all previous versions also recommended using position and runout wherever possible. Besides, the majority of companies still use the 1994 standard while the remaining use the 2009 version. Thus, concentricity controls can still be used, unless the previous versions are restricted.

What is eccentricity?

Eccentricity is the measure of the offset with respect to the geometric center of the tube profile. It is a vector quantity with magnitude as well as direction. Quite often, the value of eccentricity is found to be half that of concentricity.

The magnitude of eccentricity is calculated as:

Eccentricity = (maximum wall thickness – minimum wall thickness)/2

The post Concentricity (GD&T) Explained appeared first on Fractory.

Perpendicularity is a type of orientation control. Orientation controls define the orientation of a feature with reference to a datum plane or axis. Angularity and parallelism are other callouts in orientation control besides perpendicularity.

This article will explain the various aspects of perpendicularity and its two types in detail.

What Is Perpendicularity?

Exact perpendicularity between features is very difficult to achieve. The perpendicularity callout establishes limits within which a feature must lie to be accepted as reasonably perpendicular.

Similar to GD&T straightness, there are two ways in which perpendicularity may be applied. It can control the perpendicularity of a surface or an axis. Let us explore each type.

• Surface perpendicularity
• Axis perpendicularity

Surface Perpendicularity

Surface perpendicularity is a 2-dimensional GD&T callout that controls the perpendicularity between two surfaces. The surfaces must be perpendicular within the tolerance limits specified in the feature control frame. Surface perpendicularity does not directly control the angle between the surfaces. Instead, it ensures perpendicularity by defining the location where the surface must lie for approval.

Axis perpendicularity

Axis perpendicularity ensures that the axis of a feature is within the perpendicularity limits in the feature control frame (FCF). The feature may be positive such as a pin, or negative such as a hole.

Axis perpendicularity is a 3D tolerance that specifies a cylindrical boundary where the axis of the referenced feature must lie.

Perpendicularity Tolerance Zone

As with all other GD&T callouts, the perpendicularity callout sets up a tolerance zone at the ideal location of the feature. The zone particulars, however, are different for surface and axis perpendicularity.

Let us see how these two zones work and the differences between them.

Surface perpendicularity tolerance zone

The tolerance zone for surface perpendicularity is made of two parallel planes. The surface under inspection must lie in between the two planes for approval. The feature control frame controls the spacing between the two planes—the smaller the spacing, the tighter the zone.

As it is apparent from the shape, the zone does not directly control the angle between the two surfaces. Instead, it creates a zone perpendicular to the datum surface and maintains the flatness of the perpendicular surface.

Axis perpendicularity tolerance zone

For axis perpendicularity, the tolerance zone is cylindrical. The zone is created around a theoretical axis that is perfectly perpendicular to the datum feature. All the points on the feature’s actual axis must lie within the zone for approval.

Since the zone is cylindrical, the feature control frame contains a diameter symbol to denote it.

Perpendicularity vs Other Callouts

The perpendicularity callout symbol bears some similarities to other commonly used GD&T callouts. This section covers some of those callouts.

Perpendicularity vs Flatness

Similar to flatness, surface perpendicularity measures surface variation between two parallel planes. But unlike perpendicularity, flatness (like all other form controls) applies to a surface without a datum.

Another difference is that perpendicularity controls the angle (90° with respect to the datum feature) while flatness does not. Flatness is only concerned with the smoothness of a surface and the angular variation from the desired range does not make any difference.

Perpendicularity vs Angularity

All orientation controls are similar to each other in a way. Perpendicularity is a specialized form of angularity, as is parallelism. While angularity can maintain orientation at any specific angle, parallelism and perpendicularity are set at 0°/180° and 90° respectively.

Perpendicularity Feature Control Frame

The feature control frame (FCF) for perpendicularity is pretty straightforward. The leader arrow of the FCF points to the feature under control or its extension line. Compared to an axis, there are some minor differences when perpendicularity is applied to a surface. This section will explain the FCF for perpendicularity in both cases.

To understand the FCF better, we shall divide it into its constituent blocks as follows.

• Geometric characteristic block
• Feature tolerance block
• Datum block

Geometric characteristic block

The geometric characteristic block houses the GD&T symbol for the geometrical tolerance. Perpendicularity symbol is ⊥ and it is used for both surface and axis perpendicularity.

Feature tolerance block

The feature tolerance block contains information about the shape and size of the tolerance zone and any material condition modifiers in that order.

If no symbols are present for the zone’s shape, total wide tolerance zone is considered as the default. This is the case for surface perpendicularity.

In the case of axis perpendicularity, a cylindrical tolerance zone must be indicated. To denote this, the feature tolerance block contains a diameter symbol in front of the tolerance value.

This symbol is followed by the tolerance value or limit that defines the width of the tolerance zone.

When it comes to material modifiers, perpendicularity is often called with either MMC (Maximum Material Condition) or LMC (Least Material Condition). If LMC/MMC symbols are not present, the reader understands that the default condition of Regardless of Feature Size (RFS) is in use.

Datum block

The datum block contains information about datums used as a reference for the feature under control. Orientation controls cannot function without a datum. For perpendicularity, we often associate a surface as a datum. This surface’s name is placed in the datum block.

At times, multiple datum surfaces may be used. These datums are placed one after the other and are referred to as primary datum, secondary datum and so on.

For example, let’s say we need to design a cube-shaped product and the base and the first wall have tolerances already applied. We can use the perpendicularity callout for the adjacent wall.

To ensure that the second wall is perpendicular to the base and the first wall, we use both of them as datums in the perpendicularity callout.

How To Measure Perpendicularity

PET Prefrom Perpendicularity Gauge

Both surface and axis perpendicularity are relatively easy to measure when comparing them to other GD&T callouts.

Let us see how this is done.

Surface perpendicularity measurement

Measuring surface perpendicularity requires the use of a height gauge. The datum surface is kept in contact with a surface plate during the measuring process.

The degree of freedom of the gauge (or the part) is restricted by only allowing relative motion in the perpendicular direction. The deviation on the height gauge’s dial gives the perpendicularity of the feature.

Axis perpendicularity measurement

Axis perpendicularity is usually measured with custom gauges. These gauges are built with keeping the Virtual Condition of the feature in mind.

Axis perpendicularity is often called out at MMC to ensure a good fit in all cases. Bonus tolerance also plays a role in defining the range of sizes that will pass the tolerance check.

Uses of Perpendicularity

Perpendicularity is a very common callout in GD&T due to its simplicity and usefulness. Both surface and axis perpendicularity are often used in engineering drawings to guarantee the desired perpendicularity.

In its surface form, perpendicularity can control planar as well as curved surfaces. For instance, it can ensure the perpendicularity of a cylinder’s curved surface with respect to the cylinder’s bottom. It may also be used for centre plane tolerancing.

In its axis form, perpendicularity can control the centre axis of various positive and negative features. It is predominantly used to ensure the perpendicularity of holes and pins to make sure they mate perfectly in an assembly.

Bonus Tolerance in Perpendicularity

Bonus tolerance refers to the increase in allowed tolerance limits as the size of a positive feature shifts from the MMC condition towards the LMC condition. Let us explain this with a simple example.

Imagine that two pipes need to be connected by flanges. The faces must be perpendicular to the pipe length. The pipes are usually manufactured at their MMC to fix the Virtual Condition (VC) size. This size represents the maximum allowed size for the pipe, inclusive of the perpendicularity tolerance.

But as we depart from the MMC size of the pipe, the outer diameter reduces and it does not have to be as perpendicular to the flange face as before to fit into the flange bore.

Thus, the reduction in pipe diameter translates into increased flexibility (or tolerance) in perpendicularity. This increased allowable tolerance is the bonus tolerance. In other words, it is the extra tolerance on top of the perpendicularity tolerance limit mentioned in the FCF.

The post Perpendicularity (GD&T) Explained appeared first on Fractory.

In engineering, runout refers to an error in the rotation about a central axis of rotating mechanical systems. Any kind of wobble or eccentric rotation can cause problems in the functioning of various machines and must be minimised as much as possible. The runout control helps us achieve that.

The runout group houses circular runout and total runout.

Like profile control, runout controls are combination controls as they affect multiple physical characteristics of a part such as its location, size and form.

But more on that later. Let us start with defining what total runout represents.

What is Total Runout?

Total runout is a composite tolerance that controls the location, orientation and cylindricity of the entire surface simultaneously. It does so by specifying a datum axis and rotating the part by 360 degrees. 

Any peaks and valleys on the surface are observed with respect to the applied total runout tolerance zone. All points on the surface must lie within the tolerance zone, and the difference between the highest and the lowest point on the entire surface must be less than the applied tolerance limit.

In the case of cylindrical parts, besides controlling surface irregularities, total runout controls any axial variations in a part. Bends, if any, along the part length should not cause the part to breach the runout tolerance zone.

Let us understand how total runout works with the example of a coin stack. 

Total runout ensures that all the coins in the stack are perfectly round. It also ensures that they are stacked perfectly straight and that none of them is jutting out along the length of the stack, and also that the stack is located at its defined position in the correct orientation.

This kind of tight control isn’t needed in all applications, but many parts could not function satisfactorily without such accuracy, especially in high-speed applications.

A second way to apply total runout is to measure the surface variations on a flat surface. Think of a solid cylindrical part with flat faces at each end. Total runout can control the flatness of the front face and ensure that it is perpendicular to the datum axis.

Total Runout Tolerance Zone

The tolerance zone is of two types, depending on what type of surface total runout controls.

For a cylindrical part, the tolerance zone is a 3-dimensional cylindrical sleeve around the referenced surface. The inner and outer limit is marked by two coaxial cylinders whose central axes coincide with the specified datum axis.

When total runout is applied to a flat surface perpendicular to the central axis, the tolerance zone is made of two flat planes located on either side of the surface referenced in the feature control frame. All the surface elements must lie in the space between these planes for approval.

Total Runout vs Other Callouts

Total runout places many restrictions on the surface. These restrictions actually enable total runout to control multiple characteristics of a part. It may thus be used to replace individual callouts that control one attribute at a time.

This is highly beneficial as it eliminates the need to inspect each attribute with a different inspection method and replaces it with one standard method that measures the total runout of the part. Total runout controls the following attributes of a part at a time.

• Circularity
• Coaxiality
• Concentricity
• Straightness
• Cylindricity
• Surface taper
• Parallelism
• Angularity
• Perpendicularity
• Profile
• Flatness (when applied to a planar surface)

Thus, as a callout, total runout bears some similarity to many other GD&T callouts. Let’s explore these similarities as well as any differences between total runout and other callouts.

Total Runout vs Circular Runout

Circular runout and total runout bear the most resemblance to each other.

Circular runout, or as it is more popularly known, simply “runout”, is also a combinational control like total runout, controlling the location and orientation in addition to a part’s form. But there are some key differences between the two.

While circular runout controls a single cross-section at a time, total runout inspects the entire length of the cylindrical part simultaneously. It is the 3D version of circularity. 

The runout tolerance can control a variety of surfaces such as cones, cylinders, and spheres, whereas total runout controls only cylindrical surfaces.

As compared to circular runout, a surface with a total runout control is more expensive and tougher to produce and inspect. Designers should, therefore, prefer circular runout if the application can function satisfactorily without cylindricity or flatness control.

Total Runout vs Cylindricity

Cylindricity combines circularity and straightness to measure how closely a part feature resembles a perfect cylinder. Any deviation in the form is expressed as increased cylindricity.

Cylindricity is applied to cylindrical parts only. The use of total runout for parts that are not cylindrical is highly unusual but possible. It may be used to measure flatness, as we already saw in the initial description.

The key difference lies in the need for a datum. Cylindricity does not need a datum axis/surface as a reference. It doesn’t verify the location of the part and is only concerned with the shape of the part feature.

On the other hand, total runout measures circular runout along the length of the part. With the help of a datum, total runout ensures that the location, orientation, and size is accurate in reference to other part elements, besides controlling any form variation.

A second difference between the two is that total runout is concerned with ensuring the axis of the cylindrical surface remains under control, whereas cylindricity focuses on the entire surface without worrying about the centres of different cross-sections.

This difference is apparent even in the way the two callouts are measured. When measuring cylindricity, the part is fixed on the turntable and rotated to measure it with the help of a dial indicator.

For total runout measurement, the cylindrical part is held by fixing the centres of the opposite faces measuring along the length with a dial indicator.

Total Runout Feature Control Frame

The feature control frame (FCF) of total runout describes how it applies to the specified feature. It uses a standard layout and symbols to convey the tolerance type, tolerance limit, specific conditions and reference points to give complete information about the applied total runout callout.

The leader arrow of the FCF points to the surface under control or its extension line.

The FCF for total runout is a fairly straightforward one. As with all other GD&T callouts, it consists of three blocks.

• Geometric characteristic block
• Feature tolerance block
• Datum block 

Geometric characteristic block

This block gives information about what callout is applied by housing the total runout symbol. You may already know that the symbol for (circular) runout is an arrow pointing northeast.

Since total runout measures the runout across the entire length, the runout symbol is made of two arrows pointing northeast with their tails connected by a horizontal line.

The arrows signify that total runout measures circular runout from one end of the specified part surface to the other, with the horizontal line representing the surface under control.

Feature tolerance block

This block gives information about how the callout applies to the surface. It gives information about the type of tolerance zone, tolerance limit, and material condition modifiers, if any.

The tolerance zone is not diametral, hence there is no diameter symbol in this block. The block contains the tolerance limit for the surface under control.

For a cylindrical surface, this stated limit represents the radial separation between the concentric cylinders that make up the tolerance zone. For a flat surface, the limit represents the difference between the two virtual planes of the total wide tolerance zone.

In all cases, total runout controls a surface without a material condition modifier. It is always applied RFS (Regardless of Feature Size) which is the default mode for all geometric tolerances.

Datum block

Total runout requires a datum in the FCF to derive the runout tolerance zone for the callout. It may use a datum axis or a datum surface, depending on the type of control needed.

Choosing the right datum is important as it will dictate how well it rotates in service. For most applications, it will be the axis of the shaft with the bearing surface that will be used as the datum.

Another fairly common feature seen with total (as well as circular) runout is that of a multiple datum or a compound datum feature. A callout can reference multiple datums to define part requirements better and each of them can be used for as many FCFs as needed.

The datums are placed one after the other, each in a separate box, and are known as primary datum, secondary datum and so on. Multiple datums usually find use in shafts with multiple diameters. 

Compound datums are when more than one datum is placed in the same box, separated by a dash. They are two datums but they work as one. When measuring in such a case, the part is held along both axes, but together, they form a single axis.

How to Measure Total Runout

Metrology offers several ways of measuring total runout. Inspectors may use a CMM or manual method.

Using a CMM offers greater accuracy but requires a skilled operative. The manual method is easier and cheaper to implement.

Let us see the step-by-step process to measure total runout using the manual method.

Setting up the apparatus

The apparatus to measure total runout includes two large precision V-blocks, a small V-block, a straight edge (a flat, straight piece of metal), a dial or height gauge, and the part under observation.

The precision V-blocks are connected securely to the surface plate or any other smooth surface (usually a highly ground granite block) for stability. The inspector then places the cylindrical part’s (rotor, shaft, etc.) surface with the datum axis on the V-blocks.

The next step is to align the dial gauge to obtain a linear, smooth and continuous motion along the entire part surface. We start on this with a straightedge for accuracy.

The straightedge is held flush against the precision V-blocks. Inspectors sometimes use petroleum jelly to ensure smooth relative motion between the straightedge and the V-blocks. The small V-block is inverted, and the inspector connects the dial gauge to this V-block.

The dial gauge with a V-block is now held against the straightedge and adjusted in a manner such that the tip of the dial gauge connects with the part surface.

The idea is that precision V-blocks line up against the datum axis of the part, and the dial gauge lines up with the datum axis through the straightedge.

Measurement

The inspector now butts up the dial gauge along the cylindrical part at one of its ends. It is important to ensure that there is no gap between the straightedge and the V-blocks. One must also make sure that there is a small pressure on the dial gauge tip to measure variation in both directions.

Calibrate the dial gauge to zero, spin the part on the V-block and make a note of the maximum reading. Now start moving the dial gauge in a straight line along the part surface without spinning it.

Whenever a movement is observed on the dial gauge, wait and spin the part and record the maximum value. Continue this motion until the dial gauge reaches the other end of the part.

Final results

The inspector now compares the variations on the dial gauge at the different positions along the part length. The highest variation obtained is the Total Runout tolerance for the part. If this variation is within the specified tolerance limit in the FCF, the inspector approves the part.

Uses of Total Runout

The use of total runout is not as common as other circular callouts, as it places very tight restrictions on part geometry. Total runout mainly finds application in high-speed rotating parts with high surface contact area. A low total runout effectively prevents vibration, oscillation, and noise in the entire part when it rotates at such speeds.

Some parts where total runout is used are as follows.

• Large pump shafts
• Motor rotors
• Complex gears
• Drills
• Transmission shafts
• Axles
• Conveyor rollers
• Bearing journals

Important points to remember

1. Total runout applies to the entire surface simultaneously. It puts tight restrictions on a part and is therefore used sparingly.
2. The leader arrow points to the surface or its extension line.
3. It cannot be called without a datum.
4. The total runout tolerance zone is the spacing between two concentric cylinders or between two flat planes, depending on the feature under total runout control.
5. Total runout is always applied RFS and never with MMC/LMC (no modifiers).
6. Due attention must be given to any other geometric tolerances that may be indirectly controlling the size, shape, orientation or location of the feature under total runout control.

The post Total Runout (GD&T) Explained appeared first on Fractory.

Form controls determine the form of individual part features. In this article, we shall learn about cylindricity tolerance which is one of the four types of controls in the form category (the other three being straightness, flatness and circularity).

As the name suggests, designers and manufacturers use cylindricity to create accurate cylindrical parts.

What Is Cylindricity?

The cylindricity control is a GD&T tolerance from the form control group and guarantees a part’s cylindric shape by determining the two key aspects of roundness and axis straightness.

Many cylindrical parts that fit into a tight assembly must be “cylindrical enough” for a good fit. This is especially true for parts that fit into long, tight bores where the circularity, straightness, and taper of the cylindrical part must be within tight specifications.

Take the example of a pin that needs to pass through a hole with tight diametral tolerance. Even if the pin is perfectly round (good circularity), a small deviation from desired straightness (bend along the length) will prevent it from passing through the hole.

The cylindricity callout specifies how close the cylindrical dimensions of an actual part need to be to an ideal cylinder.

We can explain the working of the cylindricity callout by means of a disc stack. The cylindricity control, besides checking the circularity of each disc, also checks that the discs are stacked straight.

If even one of the discs deviates too much in size or roundness compared to the others or it shifts to one side more than allowed, the whole stack would fail to adhere to the tolerance limits.

Cylindricity Tolerance Zone

The cylindricity tolerance zone is represented by two concentric cylinders. These cylinders run along the entire length of the curved surface, one on the inside and the other on the outside, creating a perfect cylindrical boundary around the part’s entire surface.

The cylindricity tolerance zone is the volume enclosed by the radial separation between these two concentric cylinders. The difference in their sizes is the applied cylindrical tolerance limits. Thus, the zone is such that the entire surface of the part is constrained.

The common axis of the concentric cylinders in the tolerance zone coincides with the axis of the cylindrical part. All points of the surface under control must lie within the zone between these two concentric cylinders for approval.

Cylindricity vs Other Callouts

Each callout in GD&T has specific applications where it will work just perfectly. Before applying them, a designer considers factors such as the desired degree of accuracy, tolerance limit and ease of measurement.

Cylindricity bears certain similarities to other callouts. This can be a source of confusion for many engineers. It is necessary to have a good understanding of the similarities and differences between their characteristics and how we apply them.

Two callouts that function somewhat similar to cylindricity are circularity and total runout. Let’s compare them with cylindricity.

Cylindricity vs circularity

Cylindricity is to circularity what flatness is to straightness. In both cases, an additional dimension is introduced.

While circularity applies to one cross-section at a time as it has a flat (2D) circular tolerance zone, the cylindricity tolerance zone covers all the cross-sections at once (3D). Thus, cylindricity controls the entire surface as opposed to a single cross-section in circularity.

It is as though the circularity tolerance zone is stretched in the third dimension along the full length of the cylindrical part. Hence, cylindricity is also sometimes appropriately referred to as the 3D version of circularity.

Cylindricity can also be understood as a combination of circularity and straightness callout. Consider the previous example of a disc stack.

Circularity would only be concerned with each disc being perfectly round or having good circularity, whereas cylindricity control would also consider the straightness of the whole stack. So when the taper of a cylindrical part in an application does not bear much importance, it is better (easier to check and cheaper to ensure) to use circularity.

However, when a perfect cylinder with good circularity and taper control (near-perfectly straight) is needed, cylindricity is the way forward.

Cylindricity vs total runout

Cylindricity control and total runout control are pretty much the same feature characteristics with some minor differences.

Similar to cylindricity control, total runout is also a 3D callout that controls the entire surface of the part. Total runout is most commonly applied to cylinders but may be used for other features on rare occasions.

A key difference between the two is that total runout, when controlling a cylindrical feature, is concerned with where the centre of each cross-section lies with respect to its ideal position. Cylindricity, on the other hand, forms direct boundaries around the entire surface of the cylinder without any concern for the position of each cross-section’s centre.

The most obvious difference between the two, however, is the need for a datum feature. Total runout cannot be defined without a datum feature but cylindricity can.

This means that total runout can control orientation, location, and form (only if the total runout tolerance is tighter than size tolerance) whereas cylindricity only controls the form

Just like all other form controls, cylindricity does not use a datum feature and controls only the shape while needing other controls to control size. Even using a tighter cylindricity tolerance limit will not control the size.

Cylindricity Feature Control Frame

The feature control frame is a means to describe how a geometrical control applies to a feature. It uses standard layout and symbols to convey the tolerance type, tolerance value, specific conditions and reference points to give complete information about the applied callout.

The feature control frame for cylindricity, compared to other GD&T callouts, is quite simple and easy to apply. It points to a feature on paper by means of an arrow known as the leader arrow/line. This arrow indicates the feature under control. If it points to an axis, the axis is under control. If it points to a surface, the callout controls the surface.

As cylindricity is always applied to an individual surface, the leader arrow/line always points to a surface. It may point towards the surface or its extension line in the rectangular view (when viewing the cylinder from the front) or the circular view (when viewing the cylinder from the side).

A general feature control frame consists of three blocks known as:

• Geometric characteristic block
• Feature tolerance block
• Datum block

Geometric characteristic block

The geometric characteristic block houses the symbol for the type of geometric tolerance in a feature control frame. The cylindricity symbol is a circle enclosed by parallel lines on each side. When writing, it is often written as /o/.

Feature tolerance block

The feature tolerance block gives information about the type of tolerance zone, the tolerance value and material condition modifiers if any.

In the case of cylindricity, the zone is not a diameter tolerance zone. Hence, there is no diameter symbol in this block. The cylindricity tolerance zone is similar to flatness – if the flatness tolerance zone was wrapped around the surface of the cylinder.

So this block states the tolerance value for the feature. This value is the radial separation between the two coaxial cylinders of the tolerance zone.

As for the material condition modifiers, they do not apply to cylindricity (both MMC and LMC) as cylindricity is a form control. It does not control size. Therefore, Regardless of Feature Size or RFS applies to cylindricity at all times.

Datum block

The datum block holds any datum references that act as reference points for relationship geometrical controls. Cylindricity is a form control applied to the entire surface independent of any other feature.

Thus, it does not need a datum feature and so the feature control frame for cylindricity is complete without a datum block.

How to Measure Cylindricity

The cylindricity control finds less use compared to other form controls due to it being restricted to cylindrical parts. Depending on the tolerance value, it can also be difficult to measure.

From wobbling and vibration in high-speed applications to bearing and bushing failure, wrong measurements can cause a variety of problems in service. Thus, it is important to ensure that cylindricity is within specifications.

The cylindricity control also detects surface flaws such as pits or bumps that may otherwise go undetected.

Machinists use a variety of methods to measure/inspect cylindricity. Let’s take a look at some popular methods for measuring cylindricity.

Using a roundness measuring instrument

Roundness measuring instruments are used to measure cylindricity and circularity. To verify the accuracy of a cylindrical feature, we check for radial as well as longitudinal distortion in the cylinder.

In order to measure cylindricity error using a roundness measuring instrument, follow the steps below.

• Firmly secure the cylindrical part on the turntable. Ensure that the part rotates along its central axis.
• Put the measuring instrument’s probe or stylus in touch with the circular element. The range of the measuring instrument should be greater than the tolerance limit on the part.
• Turn the part using the turntable. Record the different values along the cross-section on a polar graph or analyze using a computer algorithm. In the case of a large part, the inspector can move the stylus instead of the part.
• Repeat a similar measurement at multiple points along the full length of the cylinder.
• For approval, the stylus’ movement range must be less than the specified tolerance value.

A roundness measuring instrument is great for measuring cylindricity but is limited by the fact that it can only measure circularity and cylindricity. When using it, great care must be taken to ensure that the turntable axis matches the feature axis for an accurate measurement.

Using a Coordinate Measuring Machine (CMM)

Spiral cylindricity measurement on CMM

Many times, accurate cylindricity measurements will not be possible without a CMM. A CMM is far more capable of GD&T measurements compared to manual alternatives. An advantage of CMM is that besides cylindricity, it can measure a variety of other geometric tolerances.

To measure cylindricity error using a CMM follow the steps below.

• Select a cross-section and mark four or more points for point measurement.
• Use the CMM manually or by computer control and take measurements at the marked points of the circular element. The CMM stylus is extremely flexible and can take measurements from various angles for different points. These may also be plotted on a polar graph manually or using a computer algorithm.
• Repeat measurements at various points along the entire length of the cylindrical part for a complete cylindricity measurement.
• For approval, the total variation on CMM must be less than the tolerance amount.

CMM is a highly versatile measuring instrument. The use of CMM for cylindricity measurements provides high accuracy, reduced setup time and increased productivity.

Uses of Cylindricity

If precise circularity, taper, and straightness are important, cylindricity is the most suitable GD&T callout. The callout finds abundant use in mechanical and automotive applications.

The different parts that frequently use the cylindricity control are as follows:

● Shafts

● Pins

● Cylinder liners

● Bearings and bushings

● Hydraulic and pneumatic cylinders

● Regular and telescopic pipes

● Camshafts

All these parts often have shape constraints for a good fit. The cylindricity control can ensure that the shape of these parts corresponds to a near-ideal cylinder.

Important Points to Remember

● The cylindricity callout does not need a datum.

● The tolerance value for cylindricity cannot be greater than the tolerance value for size (diameter).

● The envelope boundary remains unaffected by the cylindricity tolerance.

● There is no symbol for diameter in the tolerance block of the FCF as the cylindricity callout does not control the axis but the entire surface profile.

● We cannot use the MMC or LMC condition modifiers with the cylindricity callout. Hence, there is no bonus tolerance associated with it.

● Cylindricity must be considered in a statistical tolerance stack as it controls the form of the entire surface.

● Normal tolerance stack does not need cylindricity to be included as per rule #1 of GD&T which states that the maximum envelope (boundary) for a FOS is its MMC. This means that the radial boundary in cylindricity cannot be greater than the maximum diametral tolerance.

The post Cylindricity (GD&T) Explained appeared first on Fractory.

Circularity belongs to the form control group. It controls the geometry of circular features such as cones, cylinders and spheres.

In this article, we shall learn about the circularity callout and how we can use it to ensure the final part’s maximum closeness to its intended design.

What is Circularity?

The geometrical tolerance of circularity is one of the four types of form control, the others being straightness, flatness and cylindricity. Also known as roundness, it control’s a feature’s circular nature such as the diameter of a cylindrical pin or a hole.

The aim is to set a limit to the desired accuracy of the circular feature in relation to a perfect circle.

Circularity Tolerance Zone

The circularity callout defines a two-dimensional tolerance zone for the actual part surface. The tolerance zone consists of two concentric circles that lie on a plane that is perpendicular to the central axis of the part feature.

The difference between the radii of these two circles defines the permissible tolerance limit for the feature.

To understand it better, one may imagine an infinite number of tolerance zones in contact with one another to cover the entire surface (like discs in a stack). All the zones may not be of the same size (as in the case of a cone).

For part approval, all the points on a circular feature’s cross-section must lie in their respective tolerance zone, i.e. between the two circles. Thus, a sequence of circular tolerances can be used to determine the conformity to requirements of various cross-sections.

Circularity vs Other Callouts

Circularity may sometimes be confused with other callouts. Each callout has a specific function and a method of measurement.

A designer chooses the right callout for an application after considering various factors such as the degree of accuracy, tolerance limit, and ease of measurement. The following information will help us understand the difference between the different radial callouts in geometric dimensioning and tolerancing, and in making wiser choices regarding the same.

Circularity vs cylindricity

Cylindricity is the 3D counterpart of circularity. While the latter concerns itself with only the roundness of the feature, the former also controls the straightness of the circular feature’s central axis.

Cylindricity tries to bring the feature’s form as close as possible to a perfect cylinder.

Designers use cylindricity when, in addition to the diametral tolerance, the feature’s straightness plays an important role in the part assembly. For instance, a pin may have the diametral variation well within limits, but if it isn’t straight enough, it will not fit the hole.

Cylindricity is also different from circularity in that it is meant for features with a constant diameter, thus not suitable for conical shapes, for example.

Circularity vs coaxiality

The coaxiality callout keeps the difference between the central axes of a part’s multiple circular features within limits.

A part may consist of multiple circular features, like in the image above, situated at various positions along its length. Even if all the features are perfect circles, there will be wobble should the axes not align sufficiently.

In order to prevent wobbling, the central axis of all features must coincide. This is especially important for high-speed applications. Coaxiality keeps this variation under control.

Circularity is applied to a single feature, whereas coaxiality needs multiple features. Another key difference is that circularity does not need a datum whereas coaxiality cannot function without a datum axis.

Circularity vs concentricity

Concentricity is a special case of coaxiality where multiple features exist on the same plane.

When a plane perpendicular to the part axis contains diameters of multiple features (for example, inner and outer diameter of a hollow pipe), the concentricity callout ensures their centres are close enough to prevent wobbling.

Circularity vs runout

Runout (or circular runout) combines circularity and concentricity to control the complete form of the feature. The tolerance zone for runout is similar to a circularity zone, hence it is also a 2D measurement.

Runout captures the errors in circularity and concentricity into a single measurement. It is the sum total of circularity and concentricity errors.

In case a part is perfectly concentric, runout measurements give the circularity error. Similarly, when a part has perfect roundness, runout represents the error in concentricity.

Unlike circularity, runout also needs a datum axis.

Circularity Feature Control Frame

We use a feature control frame to apply the circularity control to a surface. It specifies all the important information for the callout in a standard manner for easy understanding by everyone who interacts with the concerned part’s drawing.

The feature control frame for circularity is quite easy to implement. As with other GD&T callouts, we will explain circularity’s feature control frame using the three general compartments as follows:

• Geometric characteristic block
• Feature tolerance block
• Datum block

Geometric characteristic block

This block contains the symbol for the circularity callout. Circularity is represented by a circle in this block.

Feature tolerance block

This block specifies the tolerance limit applied to the surface for the callout. Material condition modifiers are not applicable to circularity, hence they are not present in this block. Thus, it only contains the numerical value of the tolerance limit.

Datum block

The circularity callout does not need a datum because we apply it to individual features. The callout controls only the form of the surface and has nothing to do with the cross-section’s position on the part.

Measuring Circularity

There are many methods to measure circularity. All these methods require some skill and can be difficult to perform in the beginning. The means for measuring circularity are as follows:

• Using a height gauge
• Using a CMM
• Using a micrometre

Using a height gauge

Circularity can be measured using a turntable and a height gauge. To measure circularity, we carry out the following steps:

• Fix the part in a turntable (or a vee block) and constrain it so that it rotates along the central axis.
• Select a cross-section and set a height gauge probe at this cross-section. When selecting the height gauge, the inspector must ensure that the range of the height gauge (or dial gauge) is greater than the tolerance limit for the part.
• Ensure the height gauge is touching the part and calibrate it to zero.
• Rotate the part and log down the readings for a complete rotation.
• Plot the recorded values onto a polar graph or feed them into a computer program to create graphs that easily convey the part’s form. Check whether the part tolerance is within limits by ensuring that the total variation on the gauge is less than the specified tolerance limit.
• Repeat the same procedure at other cross-sections to obtain a complete picture of the part’s circularity.

Using height gauge to measure circularity

This type of setup may sometimes be referred to as a roundness measuring instrument. It contains a turntable and an adjustable stylus that can measure many other characteristics as well.

Besides circularity, it can measure straightness, perpendicularity, coaxiality, cylindricity, circular runout, total runout and parallelism.

Using a CMM

An alternate method of inspecting circularity includes the use of a coordinate measuring machine (CMM). The machine’s stylus takes measurements at four (or more) points of a particular cross-section. The variation is calculated by using the least-squares method.

The machine repeats the procedure at multiple cross-sections to ensure the entire part meets the roundness specification.

Roundness measurements recorded through this method are the most accurate.

Using a micrometre

A micrometre can also measure the circularity of a part, especially if it is the outer form (as in a pin). It is measured as a two-point measurement at various points of the same cross-section.

After recording these measurements, the minimum value obtained is subtracted from the maximum value and halved to get the roundness measurement.

The accuracy of the measurement improves with the increase in the number of measurements at a cross-section. An amazing feature of this method is that the only equipment needed is a micrometre, hence it is easy to perform with simple tools available at hand.

Note on measurement

Circularity inspection in the case of spheres is difficult to measure as any cross-section passing through the centre of the sphere is subject to tolerance limits. Thus, unlike cylinders and cones, measurements need to be taken in multiple planes to satisfactorily inspect a part.

Most machined parts do not have an oval shape and are usually made of a number of lobes. Circularity inspections can give the wrong measurements when the part is made of an odd number of lobes.

When we use a two-point measurement method (e.g. a micrometre) on a part that has an odd number of lobes that are evenly distributed, the results will show that the part is perfectly round when it’s not.

This error can lead to the approval of parts that need further machining. This is how circularity measurements can be tricky and, therefore, need skilled inspectors.

Uses of Circularity

Circularity is an extremely common callout that is used quite frequently in the manufacture of many different products. It controls the circular shape in circular, cylindrical, conical and spherical features.

It is used in parts such as bearings, shafts, pins, spools, pipes, etc. Circularity ensures that these parts function without any wobble and wear out evenly. This is especially important in high-speed applications.

Thus, circularity is often a part of many engineering drawings.

Points to Remember

1. The tolerance zone for circularity is a radial tolerance zone, not a diametral one.
2. The control works only when applied to a round feature.
3. At each cross-section, the callout applies independently of other cross-sections.
4. Circularity measurements can give the wrong measurements in some scenarios.
5. No material condition modifiers (LMC /MMC) are part of the feature control frame.
6. The circularity tolerance limit must be less than that of any other callout that also controls the circularity of that feature.

The post Circularity (GD&T) Explained appeared first on Fractory.

Form controls determine the shape of individual features in a part. They consist of the following four types of geometric tolerances – straightness, flatness, circularity and cylindricity.

In this post, we shall learn about the flatness callout and how to use it in the right place for maximum efficiency.

What is Flatness?

Many applications need parts with a flat surface. No surface is perfectly flat but using GD&T, we can develop parts with a surface that is flat enough for our application.

The flatness callout controls the uniformity of a surface or a median plane as needed. It defines two parallel planes on either side of the flat surface as the tolerance zone for the surface. All the points on the specified surface must lie between these two planes for part approval.

As flatness refines a surface, we can also use it in a tolerance stack without interfering with other requirements.

Flatness vs Other Characteristics

It may seem like flatness is very similar to other geometric as well as regular tolerances in terms of the final result. So let’s do some 1:1 comparisons in order to make sure the difference is clear to everybody reading this article.

Flatness vs straightness

Flatness is the 3D equivalent of the surface straightness control. While straightness has parallel lines representing its tolerance zone, the flatness tolerance zone is formed by two parallel planes.

Thus, while straightness only makes sure that a single line on a surface has to be within the limits, GD&T flatness does the same for a collection of lines – a surface.

Flatness vs parallelism

These two are often confused. Parallelism is not a standalone callout. It needs another feature such as an axis or a surface to relate to. It cannot function without a datum.

On the other hand, flatness does not need a datum. We can use flatness on a surface that is not parallel to any other surface, so there is no reference point to compare the result with other than the closed system itself.

Flatness vs surface finish

This probably causes the most confusion out of these comparisons.

While both control surface variations, the surface finish does it on a much finer scale. The measurement for the surface finish is shown as an average while for flatness, the difference between the maximum height and depth is shown as the worst case.

Flatness vs regular tolerancing

The image above has a tolerance of +/- 0.1 mm for thickness. All in all, this gives exactly the same result in terms of flatness – it guarantees it as does the one below.

But this one has both the flatness callout AND a +/- tolerance for part thickness. As you can see, the flatness is still inside the same limits – 0.2 mm in total. But now the thickness of the part can vary up to 0.4 mm in both ways or 0.8 mm in total.

Thus, flatness can be achieved without restricting any other dimensions, making it easier to obtain and lowering the total cost.

How to Show Flatness on a Drawing?

We show flatness tolerance on a drawing through a feature control frame. The feature control frame of flatness is quite straightforward.

The first block contains the geometric characteristic symbol for flatness. It is represented by a parallelogram.

Since the tolerance zone for flatness is a total wide zone, there is no need for a symbol in the second block for the tolerance type as this is the default zone. The second block, therefore, contains only the tolerance value and the material modifiers as needed.

Similar to other form controls, the flatness callout does not need a datum for reference. The leader arrow points to the surface that is under control.

At times, the leader arrow may point towards a size dimension. This indicates that the derived median plane is under flatness control.

What is Flatness at Maximum Material Condition?

Flatness with an MMC modifier can be a bit confusing since flatness is a form control. Form controls do not work with material condition modifiers. So is this callout even valid?

The validity of this callout depends on the type of feature it is applied to. If we call it for single planar surfaces, then this will not be a valid callout.

Flatness with an MMC modifier is a valid callout only when we apply it to a Feature of Size. When the callout is applied for a FOS such as width, instead of controlling the flatness of the surface, it controls the derived median plane. This callout is found in ASME Y14.5-2009, para 5.4.2.1.

Designers use this callout when a certain local size (width, for instance) needs a control tighter than the overall form.

As Rule no. 1 states, a size tolerance controls form as well. The tolerance zone of the size tolerance restricts the controlled feature within the stated measurements. This requirement is no longer in effect though, when we use the flatness callout with MMC as the geometric tolerance adds to the size tolerance (this condition overrides rule no. 1).

In other words, the flatness callout now controls the form and the size tolerance controls only the local width.

Measuring the Tolerance

There are different ways to check if the final measurements conform to the tolerance set by flatness. The method depends on the surface, so we are going to discuss each instance separately.

Single planar surfaces

Flatness measurements require a surface plate and a height gauge, probe, or a surface of some type. We cannot measure it by simply placing the part on a surface plate or a slab and using a height gauge as this would mean we are measuring parallelism with reference to the bottom surface.

Using a height gauge

How to Accurately Inspect a Flat Surface

To measure flatness with a height gauge, we need to hold the reference feature parallel. Advanced CMMs (Coordinate measuring machines) can inspect flatness very well. They create virtual planes that mimic the surface to be inspected. This gives accurate measurements.

The height gauge is then run over the entire surface in such a way that it covers every area. We add the maximum positive and negative measurements on the height gauge to calculate the total variance. This variance must be less than the flatness tolerance value to approve the part.

Using a surface plate

Machinists sometimes use a surface plate to inspect flatness. The part is held face down on the surface plate and a height gauge is brought in contact with the specified surface through a hole in the surface plate.

Then, the height gauge and the parts are moved in a manner that covers the full length and width of the surface and the flatness variance of the actual surface is calculated.

Feature of size (Flatness at MMC)

When we are measuring flatness at MMC, we are in fact measuring the flatness of the derived median plane. To inspect flatness when applied to a feature of size, we have two methods:

Using a functional gauge

In this method, we hold two height gauges at opposite ends of the feature of size. Consider a flat plate and the FOS under control through the flatness callout is the width.

We hold a height gauge on the top and the bottom surface in line with each other. The height gauges measure the local thickness. We move them all over the surface to ensure that the entire surface is within the size tolerance.

The second method is to use a gauge that has a cavity that can fit the plate at the virtual condition boundary. Virtual condition boundary is the total available tolerance limit when we add the geometric tolerance and MMC. For approval, the plate must fit in this gauge.

Using a CMM

The measurement of flatness, perpendicularity, parallelism

A CMM is capable of making many different types of measurements. But measuring this callout requires some additional preparations.

Consider the same plate as before with the same FOS under control. The plate would have to be positioned in a manner such that the probe can reach both surfaces. Then we mark points on the surface and measure the local thickness at these points. If these thicknesses are within the size limits, we start calculating the midpoint of these opposing points and join them together. The derived median plane then starts to take shape.

We arrive at the flatness tolerance by subtracting the maximum local thickness of the plate. Now, if the derived median plane’s flatness variance is less than the allowable flatness tolerance, the part is within specification.

Benefits of Using Flatness Tolerance

Engineering tolerances come in many different shapes and forms (literally), each with its own nuances. Hence, they have different applications and benefits. Flatness is no exception.

Flatness controls the waviness or variation in the surface without putting tighter constraints on the surface.

We use flatness in parts where good mating of two surfaces is crucial but the orientation isn’t that important. Sometimes, designers use the flatness callout to give the entire surface an equal amount of wear. This prevents any wobble when the parts mate.

One of the most common use cases is applying flatness to sheet metal parts. Laser cutting is one of the most widely used thermal cutting processes. Depending on various factors, the sheet may get deformed during the cutting operation.

Specifying the flatness levels with GD&T will help to ensure that the sheet conforms to your application’s requirements, whether it ends up as a tabletop or a side sheet for a conveyor.

All in all, it is a simple-to-use tolerance that can help in a lot of cases to avoid any setbacks in the latter stages (assembly phase) of a project.

The post Flatness (GD&T) Explained appeared first on Fractory.

Form control limits the final shape’s deviation from its ideal form. And GD&T straightness is one of the tolerances to assure a feature’s closeness to the ideal.

What is Straightness (GD&T)?

Straightness tolerance is a 2-dimensional GD&T callout that controls the straightness of part features. No axis can be perfectly straight. The goal is to ensure it is straight enough for the application. This callout sets a standard on how straight a feature must be along its length.

Straightness can control two very different types of functions. It is the only callout that can control either lines on a surface or a FOS (feature of size). It may be used to control the straightness of a surface or an axis.

Also, the feature control frame is different in each case. Let’s see what we mean by either of these functions.

Surface straightness

When we apply this callout to specify surface straightness, the tolerance zone forms a total wide zone above and below the ideal surface position and controls any deviations. Surface straightness controls the form of a line anywhere on the surface and has 2 types of applications:

The first type is a flat surface such as a face of a cube.

The second type is a cylindrical surface in the axial direction.

In both cases, the tolerance zone forms a 2D plane. It is shown as two parallel lines (also parallel to the surface), one above and the other below the surface.

Axis straightness

The second function that this callout can control is the straightness of an axis. The amount of linear deviation in the axis is an important feature that must be controlled for a seamless assembly. The straightness callout can be used to keep this deviation of the derived median line within permissible limits.

The tolerance zone, instead of applying to the surface, applies to the part’s axis in this case. Also, instead of being above and below the axis, the tolerance zone forms a cylindrical area around the centre axis.

Feature Control Frame (FCF) of Straightness

The feature control frame tells us all the necessary information about the tolerance.

Surface straightness FCF

When controlling the surface straightness GD&T, the geometric characteristic block contains the symbol for straightness. The symbol for straightness is a short horizontal line, much like a hyphen.

The second block contains the type of tolerance zone, the tolerance value, and material modifiers (e.g. maximum material condition) if any. Since the tolerance zone type is a total wide zone, no symbols are needed as this is the default zone.

The straightness callout (as all other form controls) does not need a datum. The leader arrow only marks the surface to be controlled.

Axis straightness FCF

When it comes to axis straightness, the feature control frame remains similar for the most part, except for an added symbol for the type of tolerance zone. Since this zone is a cylinder as mentioned before, the second block contains the diameter symbol to denote the same.

Another difference is that for axis straightness, the leader arrow, instead of marking the surface, points to the part’s diametric size dimension.

When the arrow marks a particular size dimension, the FCF is understood to be controlling the centre plane or the axis of the feature. Thus, pointing towards the part’s diametric dimension indicates that the callout controls the part’s axis.

How to Measure Straightness?

Measuring Straightness

The method for measuring surface straightness and axis straightness is different. We shall look at how to inspect each type of function.

Surface straightness

Measuring surface straightness is pretty straightforward. A height gauge is secured on the specified location on the surface and moved in a straight line in the direction called out in the feature control frame.

As the gauge slides along the surface, any variations in the straightness or flatness of the surface are observed on the gauge and recorded.

Axis straightness

There are two methods available for measuring axis straightness:

Dial gauge

The first method is used when we need accurate deviations of the part axis and the number of parts is limited. This method uses dial gauges. The steps in the first method are as follows:

1. Secure the part at both ends in such a way that it rotates along the axis under straightness control.
2. Place two dial gauges at opposite positions of the cylinder’s curved surface. Two dial gauges work in tandem to locate the median point at that cross-section.
3. At the same cross-section, the part is rotated to record measurement readings at various angles.
4. Averaging multiple sets of measurements at various angles, we calculate the median point. This median point is a point on the part’s axis.
5. The machinists then repeat the dimensioning and calculation procedure at multiple cross-sections and plot the part’s complete axis in 3D.
6. If this axis is within the specified tolerance zone, the part is approved.

Cylinder gauge

The second method is more suitable for mass production scenarios where a large number of parts must be examined in a short time. This method also works for parts that do not need a high degree of accuracy.

It uses a cylinder gauge that is slightly larger than the cylindrical part. If the part fits into this gauge, it is approved without taking a single measurement. The testing equipment is called a go/no-go gauge.

To further limit the variation in part diameter and ensure better assembly, we call straightness tolerance at maximum material condition. This ensures that the part will always fit in a hole of a specific size.

Why Use Straightness Tolerance?

Straightness comes in handy when two parts in an assembly need to have a line contact. Using straightness, we mark the mating surface so that special attention is given to it during machining.

Straightness control is used in the design and manufacture of hydraulic sleeves, tubes, and covers that need perfect contact to hold high pressures.

Axis straightness is applied in the design of pins and cylindrical parts that need to fit into bores or holes to function. Straightness callout ensures that the parts mate even at the maximum material condition.

FAQ

What is the derived median line?

The derived median line is a line formed by joining the median points at each cross-section of a part. The meridian line should match the criteria set by GD&T straightness control. If the part matches the tolerancing requirements, it passes for quality.

What is bonus tolerance?

The bonus tolerance is an additional tolerance that comes into play when the straightness callout comes with a maximum material condition (MMC) modifier. In simple terms, as a part’s actual size departs from the MMC size, the difference between the actual size and the MMC size is added to the straightness tolerance value. This is known as bonus tolerance.

For example, imagine a pin that needs to fit into a hole of a given size at MMC. Now, at MMC, the pin needs to be extremely straight or have the perfect form (as per rule no. 1 of GD&T) to ensure proper assembly. As the pin diameter becomes smaller (deviates from MMC), the restrictions on the part’s straightness are relaxed.

In other words, the part needs to be less and less straight as the pin size reduces to fit the hole. This means that pins with a barrel, waist, or concave/convex shape will still clear the go/no-go gauge designed for them as they depart from MMC.

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