Posted by John-Paul Belanger (184.108.40.206) on January 09, 2003 at 11:06:32:
In Reply to: Concentricity Vs Positional tolerance (and their relation to TIR) posted by Venkataraghavan on January 03, 2003 at 09:49:09:
I'll also add my take on this question...
Concentricity, runout, and position can all be used to control the general quality of "coaxiality". When you speak of TIR, that is checking runout, because you are not only measuring the alignment, but the circularity of the diameter. So this is not what your drawing specified.
Position and concentricity are similar in that they tolerance the axis-to-axis alignment, but how the axis is derived is different. Concentricity asks you to derive the center point of each cross-section along the piece, and each of those center points must be within an imaginary cylinder of 0.15. This is difficult to measure, and the CMM is probably the best way.
Finally, position is usually the most common choice, and it asks for the axis to be derived from the "mating envelope". This means a perfect cylinder formed by the outermost points of the part -- so the circularity is not a factor here. The axis of this outer envelope must now fall within a dia. 0.15 around the datum axis. A CMM may be used again, but the axis to measure is taken from the outermost points, not a best fit.
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