Circularity Tolerance Definition Essay

In GD&T, circularity tolerance is used to control the roundness of circular parts or features.  Some examples of circular features include cylinders, spheres, and cones.  Sometimes circular surfaces are used for moving parts such as ball bearings or spools, in which case circularity helps ensure these parts move smoothly and wear evenly.  Because circularity is applied to an individual surface, this tolerance does not need to be related to a datum.

The first example of circularity tolerance is shown below. Although a cylinder is shown. this example applies for conical shapes as well.  The simple pin shown in the left figure has a circularity tolerance applied to it.  The boxed symbols can be read “each circular cross section of this cylinder must lie between two concentric circles spaced 0.25 apart”.  The right figure shows a sample cross section A-A and its possible roundness relative to the tolerance zone.  Note that circularity applies to the entire length of the cylinder, and would therefore require multiple checks to verify the pin meets the specified tolerance over its entire length.

The second example of circularity is shown below and is applied to a sphere.  In the case of a sphere, the circularity tolerance now applies in all directions since any cross section through the center of the sphere should be a circle.  The verification of the sphere is therefore more involved than for a cylinder or cone, which would require measurement in only one plane.

Easy GD&T: Circularity / Roundness [ Symbol, Tolerance, Measurement ]

GD&T Circularity / Roundness Symbol

Definition of Circularity (Roundness)

In GD&T, Circularity describes how close an object should be to a true circle. It is also sometimes called Roundness. Examples of features that might have Circularity controls would be cylinders, spheres, and cones. For moving parts, circularity is often used to help ensure the parts move smoothly and wear evenly. For example, it might common be used to control the roundness of a shaft.

Since we apply circularity to a particular surface, it does not need to be related to a datum. Circularity is the 2D version of Cylindricity.

Circularity Callout on Drawings

 

Circularity callout…

The Circularity callout points the arrow to the surface and gives the tolerance. The Circularity Feature Control Block might also be applied to the Diameter dimension of a circular feature.

Circularity Tolerance Zone

The Tolerance Zone for Circularity is two concentric circles, one inside the other. The circles lie on a plane that is perpendicular to the axis of the circular feature. All the points on the circular surface being controlled must fall between the inner and outer circles that make up the Circularity Tolerance Zone.

In the drawing callout above, every point on the cylinder must be within the two concentric circles and they differ in diameter by 0.050″.

Gaging and Measurement of Circularity

Gaging Circularity is straightforward. Constrain the part so it can be rotated around the central axis and measure the deviation of the surface with a height gage or other measuring device.

GD&T Table of ContentsGD&T Symbols

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