True Position Help needed....
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Posted by: CapstoneIII ® 10/22/2009, 12:10:52 Author Profile eMail author Edit

I have a callout on a print (This a true position for a hole to hole calculation): Dia. 8.0 +.08/-0.00 Positional callout- Dia. 0.1max, Datum A(max), Datum B(max), Datum C(max) My question is what amount of bonus tolerance do I apply? Do I add all three up? Any help would be appreciated. Thx CapIII

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: True Position Help needed....
: True Position Help needed.... -- CapstoneIII	Post Reply	Top of thread	Engineering Forum

Posted by: traingdt ® 10/22/2009, 12:44:12 Author Profile eMail author Edit

You probably know this already, but the first M symbol provides bonus based purely on the hole size. Therefore, if the hole is made a 8, the position tolerance is 0.1, and if the hole is made at 8.08, the position tolerance becomes 0.18. The other M symbols contribute what is often called "shift tolerance." (This term helps us to distinguish in from "bonus tolerance.") If all three datum features are at the worst case virtual condition, they contribute nothing extra to the position tolerance. But if one or two or all three datums vary from that worst case, they contribute a "shift" to where the hole might be. The problem is that you can't add the effects of these other 3 M symbols together and get one grand total for your position tolerance. That's because the datums act in different directions. Think about it -- if there's any looseness around datum A and also around datum B, they won't allow the part to shift in the same way. A CMM can calculate the different combinations if you really need it. Or of course a physical functional gage will check the part without spitting out any actual numbers. Hope that helps! John-Paul Belanger

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: : True Position Help needed....
: : True Position Help needed.... -- traingdt	Post Reply	Top of thread	Engineering Forum

Posted by: CapstoneIII ® 10/22/2009, 13:28:15 Author Profile eMail author Edit

Thanks for your explanation and help.

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