{"id":9661,"date":"2021-03-18T16:36:09","date_gmt":"2021-03-18T14:36:09","guid":{"rendered":"https:\/\/fractory.com\/?p=9661"},"modified":"2025-01-06T12:09:15","modified_gmt":"2025-01-06T10:09:15","slug":"straightness-gdt","status":"publish","type":"post","link":"https:\/\/fractory.com\/straightness-gdt\/","title":{"rendered":"Straightness (GD&T) Explained"},"content":{"rendered":"

ASME Y14.5 2009<\/a> GD&T Standards lists out 14 types of geometric tolerances<\/a>. These 14 tolerances are bunched up into five different groups based on the type of control they offer. Among these five groups is form control.<\/p>\n

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.<\/p>\n

What is Straightness (GD&T)?<\/h2>\n

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.<\/p>\n

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.<\/p>\n

Also, the feature control frame is different in each case. Let\u2019s see what we mean by either of these functions.<\/p>\n

Surface straightness<\/h3>\n

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<\/strong> and has 2 types of applications:<\/p>\n

The first type is a flat surface such as a face of a cube.<\/p>\n

The second type is a cylindrical surface in the axial direction.<\/p>\n

\"straightness
2D tolerance zone along a line<\/figcaption><\/figure>\n

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

Axis straightness<\/h3>\n

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<\/strong>.<\/p>\n

\"axial<\/p>\n

The tolerance zone, instead of applying to the surface, applies to the part\u2019s axis in this case. Also, instead of being above and below the axis, the tolerance zone forms a cylindrical area around the centre axis<\/strong>.<\/p>\n

Feature Control Frame (FCF) of Straightness<\/h2>\n

The feature control frame tells us all the necessary information about the tolerance.<\/p>\n

Surface straightness FCF<\/h3>\n

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.<\/p>\n

\"Surface<\/p>\n

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.<\/p>\n

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

Axis straightness FCF<\/h3>\n

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.<\/p>\n

\"axis<\/p>\n

Another difference is that for axis straightness, the leader arrow, instead of marking the surface, points to the part\u2019s diametric size dimension.<\/p>\n

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\u2019s diametric dimension indicates that the callout controls the part\u2019s axis.<\/p>\n

How to Measure Straightness?<\/h2>\n
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