# Why do we double hypotenuse when doing TP calcs?

Six Sigma – iSixSigma Forums Old Forums General Why do we double hypotenuse when doing TP calcs?

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• #46401

Ropp
Participant

I know this is the convention, but someone asked me this today and I can’t honestly think of a good reason why. Digging out my GD&T handbook I can see that in every example we use the pythagorean theorum, then multiply by two to get the true out of tolerance calculation.
I’m sure this has something to do with the difference between diameters and radii, but I’m not sure why. Can someone give me a quick explanation? My CMM can tell me the difference between the calculated center of a hole and the theoretical. Why are we doubling the distance from centerline to centerline?

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#153235

BTDT
Participant

Dave:Tp is a diameter, calculations are in radii.Found it here with a single search using the string “Tp Calcs gd&T”http://www.pcdmisforum.com/showthread.php?t=3659&page=4Cheers, BTDT

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#153237

Jim Shelor
Participant

When using a triangular distribution you need to calculate the upper endpoint, lower endpoint and the mode.
If you assume the mode is in the center, you would simply calculate either the upper or the lower endpoint and multiply by 2.

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