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True Position Question

bikerjeff11

Plastic
Joined
Apr 27, 2021
Hi All,
Im curious if anyone has come across a true position callout that is to two datums, both datums being holes?
Its my understanding that a hole used as a datum is like a coordinate system. How do you dimension something to 2 coordinate systems?

Thanks in advance!datums.jpg
 

JonFinn

Aluminum
Joined
Apr 25, 2013
Location
CA, USA
Datum B is used to to orient the part about Datum A. If there was only the A Datum, you could put the hole anywhere around A at a given radius.
 

jccaclimber

Hot Rolled
Joined
Nov 22, 2015
Location
San Francisco
Datum B is used to to orient the part about Datum A. If there was only the A Datum, you could put the hole anywhere around A at a given radius.

^This is the correct answer.

If you really want to go letter of the law and be pedantic you could assume the part has some thickness is which case:
Hole A sets the X, Y, and 2 axis of rotation. IE, if hole A isn't perpendicular to the face, this other hole ideally would be off by the same amount.
Hole B sets the 3rd axis of rotation, and Z direction position is neither set nor relevant because the part is the same at all cross sections.
 








 
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