When runout is checked on a surface plate the part is rotated under an indicator and the deviation between the highest point and the lowest point is the called runout. Typically this is related to a circular part with a shaft or a cylinder as the reference datum. As the part is rotated any deviation in the datum is taken into effect.
Runout on a CMM is static. The part is not turned but is measured by points or scanned and these points make up the best-fit circle that will be use to check to the datum. The datum is also made up of measured features that are fitted into an element.
This brings up a dilemma. Which tolerance strategy is better, runout or position? This is something you will need to clear up with your customer so that both parties clearly understand what the data off the coordinate measuring machine (CMM) is really giving you. In most cases, position is the best strategy to use. Using a positional tolerance will give you more accurate numbers when using a CMM. Using the runout tolerance is not incorrect but when factoring in the best-fitting functions of CMM calculations then the positional tolerance is best approach.
Mark Boucher, CMM Quarterly http://www.cmmquarterly.com/
