True Position Question

1. Plastic
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## True Position Question

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?

2. Looks like it's calling the holes as references.

3. Aluminum
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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.

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Originally Posted by JonFinn
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.

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.