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What’s the “CMM runout error”?



What’s the “CMM runout error”?

When you measure the runout on CMM, the CMM induces a lot of error into the measurements. If you have 2 axes in your CMM program for the runout, for example, the axis of a feature, a circle, and the axis of the datum -A-, a cylinder, each axis of movement during the measurement is going to add an element of error. The axis of the features and datum -A- always are off to the axes of CMM. That “off” is a very little angle, A1/A2 angles, but that “little angle” end up magnifying error substantially when , for example, it is about a .0005 tolerance.

You can simply see it if you examine the difference in the A1/A2 angles between the features. You cannot change the A1/A2 angles on CMM. On a Roundness tester you make A1/A2= zero before measuring the runout. A roundness tester allow us to correct the A1/A2 angles, to center the part axis on the spindle so that the runout will be in the part in which the axis feature-datum have 0° angles to the axes of the machine.


The A1/A2 angles are the root cause of the CMM runout error, and of the deviation between 2 different fixturing on 2 different CMM’s, between 2 different CMM’s, between the CMM and the Roundness tester, and between the CMM and the Dial indicator.

As a conclusion for the CMM runout error: No problems when the part is perfect .When the parts are not perfect the CMM may reject good parts, parts in which the runout is on the high side of tolerance.

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