GD&T Circularity and Cylindricity

What is Circularity form control in GD&T and how to interpret it ? and how is it inspected?

Circularity is the degree of roundness of a circular feature like a hole or a pin or a shaft .

Any manufactured part from even the most precise is not perfectly circular. There are always variations in its shape depending on the precision of the manufacturing process.

Here is an example of deviation from roundness

A Manufactured part which turns out to be a polygon (with many sides) deviating from the roundness or circularity.

Here is a drawing of a cylindrical part with a circularity GD&T callout

The hashed boundaries indicate the tolerance zone. The actual shape of the part should lie within the tolerance zone for acceptance.

The tolerance zone is in 2D that is a disc formed by two concentric circles .

Circularity when applied to a cylinder like a shaft has to be verified at each and every cross section of the part .

Circularity controls only form which means that it is not with respect to any reference datum. The circularity or roundness is controlled with respect to itself.

Here is one way to inspect to circularity.

The part is installed on a turntable which rotates and a measuring probe or gauge records the variations. The total variation should be within the value in feature control frame callout.

Cylindricity is a more stringent control of form for cylindrical parts as compared to circularity

The tolerance zone for cylindricity is 3D in shape which is space between two concentric cylinders spaced at the value mentioned in feature control frame (geometric tolerance)

Here is an example of a pin.

Definition of the tolerance zone :

Inspection of cylindricity is similar to circularity , the only exception is that the complete surface is inspected as a whole for every instance whereas in circularity each cross section is verified individually.

Manufactured parts which are off from the true cylindrical shape :

Circularity and cylindricity is very difficult to measure on features which cannot be easily fixtured
This is a reason why its substituted for runout which gives information about location too and its cheaper and practical.
Circularity and cylindricity are used only when the form of a feature is absolutely critical to function ,

Importance of circularity in bearings:

A good example of usage of circularity is the inner and outer race of bearings.