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True Position with MMC

True Position with MMC

True Position with MMC

Need a little help figuring out MMC on some holes and whether they pass or fail. As an example, Datum A is the bottom, B and C are two sides,so i set up a coordinate system between all three. Hole is basic dimension 19.738 in one direction and the other is basic dimension 30.833, hole diameter is supposed to be .438 +.006 / -.001, True Position with .007 MMC to Datums A,B,C. The hole diameter i measure is .440 and basic dimensions are 19.7409 and 30.837. I get stuck on trying to manually figure out weather it passes or not on the position. I know we get an extra bonus tolerance but on how to calculate it is where i get stuck. Can someone help? Thanks ahead to all those that can.

RE: True Position with MMC

M4C7's table is correct in determining the allowable position tolerance. But that is a diametric tolerance (presuming the given .007 was preceded by a diameter symbol in the callout).
So you need to take your data of 19.7409 and 30.837 and convert the deviation in those two directions into a diagonal, using the Pythagorean Theorem:

19.7409 - 19.738 = .0029 deviation in one direction
30.837 - 30.833 = .004 deviation in the other direction
.0029^2 + .004^2 = .00002441
Then... square root of that = .00494065 which is the radial deviation

M4C7's table shows that your allowable position tolerance is a diameter of .010, which allows for a radial deviation of .005. So in a pure mathematical sense your part is within tolerance. But I suppose we have to account for the number of significant digits and the accuracy of the measurements.
Anyhow, I hope this helps you see how to convert X-Y data into the diametric shape that is usually preferred in GD&T.

John-Paul Belanger
Certified Sr. GD&T Professional
Geometric Learning Systems

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