{"id":10390,"date":"2021-06-03T16:55:30","date_gmt":"2021-06-03T13:55:30","guid":{"rendered":"https:\/\/fractory.com\/?p=10390"},"modified":"2024-01-26T14:24:04","modified_gmt":"2024-01-26T12:24:04","slug":"cylindricity-gdt-explained","status":"publish","type":"post","link":"https:\/\/fractory.com\/cylindricity-gdt-explained\/","title":{"rendered":"Cylindricity (GD&T) Explained"},"content":{"rendered":"

The GD&T standards in ASME Y14.5<\/a>-2009 define fourteen types of geometric tolerances. These fourteen geometric tolerances are divided into 5 main types of geometric control. These are form, location, orientation, profile and runout.<\/p>\n

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).<\/p>\n

As the name suggests, designers and manufacturers use cylindricity to create accurate cylindrical parts.<\/p>\n

What Is Cylindricity?<\/h2>\n

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

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.<\/p>\n

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.<\/p>\n

The cylindricity callout specifies how close the cylindrical dimensions of an actual part need to be to an ideal cylinder.<\/p>\n

\"illustrative
The size and alignment of each disc is taken into account<\/figcaption><\/figure>\n

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

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.<\/p>\n

Cylindricity Tolerance Zone<\/h2>\n
\"cilindricity
Green represents the tolerance zone. Purple is the surface of the cylinder and has to fit into the zone.<\/figcaption><\/figure>\n

The cylindricity tolerance zone is represented by two concentric cylinders<\/strong>. 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\u2019s entire surface.<\/p>\n

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.<\/p>\n

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<\/strong> between these two concentric cylinders for approval.<\/p>\n

Cylindricity vs Other Callouts<\/h2>\n

Each callout in GD&T<\/a> 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.<\/p>\n

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.<\/p>\n

Two callouts that function somewhat similar to cylindricity are circularity and total runout. Let\u2019s compare them with cylindricity.<\/p>\n

Cylindricity vs circularity<\/h3>\n

Cylindricity is to circularity<\/a> what flatness is to straightness. In both cases, an additional dimension is introduced.<\/p>\n

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

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.<\/p>\n

Cylindricity can also be understood as a combination of circularity and straightness<\/a> callout. Consider the previous example of a disc stack.<\/p>\n

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.<\/p>\n

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

Cylindricity vs total runout<\/h3>\n

Cylindricity control and total runout<\/a> control are pretty much the same feature characteristics with some minor differences.<\/p>\n

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.<\/p>\n

A key difference between the two is that total runout, when controlling a cylindrical feature, is concerned with where the centre<\/strong> of each cross-section lies with respect to its ideal position. Cylindricity, on the other hand, forms direct boundaries around the entire surface<\/strong> of the cylinder without any concern for the position of each cross-section\u2019s centre.<\/p>\n

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.<\/p>\n

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<\/p>\n

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<\/strong>. Even using a tighter cylindricity tolerance limit will not control the size.<\/p>\n

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