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.
Circularity Callout on Drawings
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.
If you search for GD&T circularity examples, you’ll likely find drawings of shapes with tolerances such as ±0.050” or ±0.030”. Here at ByTune, where we specialize in very small, precision parts, 0.050” or 0.030” out of round is huge. Although uniform diameter is not a perfect definition of what circularity is, most of ByTune’s customers find it meets their requirements for controlling roundness and helping to ensure that parts fit properly, move smoothly, and wear evenly.