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Easy Guide to GD&T – Concentricity[ Symbol, Tolerance, Measurement ]
GD&T Concentricity Symbol
In GD&T, Concentricity can be used to establish a tolerance zone for the median points of a cylindrical or spherical feature on a part. Median Points are two points that oppose each other on the feature. So, for example, imagine the points on a circle that are exactly one diameter apart. Draw a line between any two of those end points and the midpoint of that line is a Median Point.
In other words, Concentricity is a tolerance on the axis of a feature versus an ideal axis.
It’s difficult and time-consuming to establish median points with measuring equipment, so wherever possible, it is preferably to use a runout or position tolerance instead of concentricity. Only very high precision parts that depend on control of median points for proper operation should use concentricity.
Concentricity Symbol, Example, and Callout on Drawings
The Concentricity callout points the arrow to the feature while the datum is the axis. The feature must be concentric with the axis within 0.020.
Concentricity Tolerance Zone
Continuing with the example of the circle, Concentricity controls how much the distance between any median point and the ideal axis may vary, hence it establishes a tolerance zone that looks like a cylinder concentric with the datum axis.
Looked at another way, imagine that all the median points are plotted relative to the axis of a cylinder. They may not be further from the axis than the Concentricity tolerance.
Gaging and Measurement of Concentricity
Measuring Concentricity is difficult because it relies on the measurement of a derived axis rather than on the measurement of an actual surface on the part. In practice, measuring concentricity requires taking many measurements (as many as is realistic) to establish the theoretical central axis. The surface must be mapped as well as possible and the median points calculated.
That process is so time consuming that typically Concentricity is only measured using a coordinate measuring machine (CMM) or other automated approach, such as a probe. Consequently, most experienced machinists will tell you to avoid using Concentricity where possible.
Concentricity vs Runout
Runout is a combination of Concentricity and Circularity. If a part is perfectly round, then the runout equals the concentricity.
Because Concentricity is so hard to measure, always consider whether Runout is a better tolerance for the part.