【转帖】Runout conflict

cad13992

Runout conflict
When measuring runout of 2 concentric circles I get 2 different numbers depending on which circle I put in first. When I enter the inside circle to the outside circle I get .0019. When I enter the outside circle to the inside circle I get .0033. I would think these numbers should be the same or nearly so. Am I missing something?

cad13992

It has to do with the LENGTH of the feature and which one is datum. You should be getting the lowest results when using the LONGEST feature as the datum. Think of it this way....

This example will NOT match exactly what you have, but will demonstrate the problem.

Take a paperplate. Put a 1/2" diameter pin in the middle of the plate. Now, if you call the plate the master and spin it centered on the pin, the pin will hardly wobble since the 'datum' is a big feature. Now, chuck on the pin and spin, the plate will flop all over the place because your 'master' is now the little pin.

cad13992

If you are using runout as your dimension, then what you are seeing is the form error (roundness) factored in with the amount of position error of the first feature selected (no form error is figured on the second feature).

When you select them in the opposite order, you then see the form error included with the position error of the other diameter. That is why it is important to select in the correct order based on the drawing requirement,

FYI - concentricity does not work this way because it evaluates the axis' of the feature.

cad13992

haven't we concluded that using the CMM to check runout isn't your best option?  

You are correct......both runout and concentricity are dynamic (rotational) checks. The CMM checks parts statically (at rest). The only relative comparison on a CMM is position.