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Question on gages and datums
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Posted by: lvw

10/11/2006, 11:57:14

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In designing a gage to check parts using datums (-A-, -B- and -C-). Where -A- is the plane -B- is feature of size (slot) and -C- is also feature of size (hole). The callout is a hole POS Dia .015 A Bm Cm. Would the gage have to restrain the part on all datums so as not to move or would you be able to use mmc calculation built into the size of the gage so that the part could float about within its mmc tolerance without actually touching the datums -A- and -B- of the gage. I have seen it both ways at my company with the above description but also with a gage restraining on all datums and using the mmc for the hole position of the part. Which way is correct?








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Re: Question on gages and datums
Re: Question on gages and datums -- lvw Post Reply Top of thread Forum
Posted by: traingdt

10/11/2006, 14:27:48

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Since datum feature A is a surface, your part must always stay in primary contact with that -- it would be represented by a flat plate forming the base of your gage.
But, the MMC modifiers on the two datums mean that you CAN move the part around on those 2 features. There is no restraint. That's the very purpose of the MMC on a datum -- to tell you that looseness is OK. Without those two M's, you would be required to have a variable gage (such as two expanding pins) that would lock firmly onto the hole and slot.
A minor point -- I suspect in your question that datum feature B should be the hole, and C is the slot. (Since a slot probably only controls one direction, it is usually tertiary.)

John-Paul Belanger








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