True Position Formula?

DeKamp

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Not using the diametric approach indicates a lack of understanding of one of the prime reasons for using GD&T, which is to assure that mating parts will actually mate as planned. Turning the tolerance zone into a square makes no sense in terms of mating in an assembly.
Hi Jim, so technically me telling my supplier to utilize the diametric approach is the best way to ensure Fit, Form, and Function?

To tell the truth, I have never paid attention whether or not if there was ever a Ø symbol because the Feature Control Frame is stating a True Position call out, and the only way I have ever known to calculate that is the formula utilized.

I cannot find anywhere in plain English where it states measurement method for True position if there is no Ø symbol. And my supplier is in China, so there is the language barrier i have to deal with.
 

Cari Spears

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True Position Formula?
 

Cari Spears

Super Moderator
Leader
Super Moderator
If in accordance with ASME Y14.5, it is 10.4.4 - Bidirectional Positional Tolerancing of Features of Size. See Figure 10-29 for rectangular coordinate method.
 

DeKamp

Involved In Discussions
Hello Cari, Jim,

I appreciate everything you have shown me today, thank you very much. I feel very dumbed down today.

I am still a little confused on how to mathematically check this even with placing on a CMM. Is there a cut and dry formula to use? Can I use deviations for thickness to ensure conformity?

Its very easy for True Position due to taking deviations from center and using a formula. But I am finding myself at a loss here.

You can visually see below the Wall Thickness is different when compared to one side and the other by over 1/2 mm.
True Position Formula?

True Position Formula?


Thank you.
 

Cari Spears

Super Moderator
Leader
Super Moderator
Hi Jim, so technically me telling my supplier to utilize the diametric approach is the best way to ensure Fit, Form, and Function?
If your company provided that drawing to your supplier, they have to inspect in accordance with that drawing. You shouldn't expect them to do otherwise unless you give them a deviation approval or change your drawing.
 
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