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Tuesday, August 26, 2014

Total positional tolerance at material condition

Total positional tolerance at material condition

Suppose the Ø 1.005 / 1.010 hole is inspected and there are six parts with different ID dimensions. Their actual sizes checked with run out methods give that their actual axis is to be .006” over and up from the true position even though they have different actual ID’s. We want to know which part is within true position tolerance at MMC. Parts to be acceptable require some calculation when is used the run out method.

By applying the Pythagorean Theorem we find out the radius. To get the diameter of the actual cylindrical zone of the actual hole location we need only to multiply the radius times two.

The actual cylindrical zone DIA is .016” and the True Position Tolerance is only .003” but there is an MMC modifier. The following formulas are used to calculate the bonus tolerance and total positional tolerance:

Bonus = Actual feature of size-MMC
Total positional tolerance= Bonus + Geometric tolerance

 We have 6 parts in which a hole is produced at:

1. Part#1 Ø 1.005
2. Part#2 Ø 1.006
3. Part#3 Ø 1.007
4. Part#4 Ø 1.008
5. Part#5 Ø 1.009
6. Part#6 Ø 1.010

Calculating the bonus and the total positional tolerance for all samples:

As you can see, even the parts have bonus tolerances they are out of the specification because the calculated Total Position Tolerances have a max of .008”, which is < .016.

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