DRF on fixture

[[To...]]

I want to use a fixture to hold a part. If I do an alignment on the fixture, I can pick up datum features B and C relative to the fixture. When I put the part on the fixture, I can measure datum A. Here's where I get stuck. How do I measure features relative to DRF A-B-C? I want to use a "Loaded Alignment" so all the operator needs to do is drop the part on the fixture and go.

The more I think about this, I am realizing that the pins for B and C cannot truly represent their orientation relationship to A, unless the bottom surface is held perfect parallel to A. Oh well.
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[[Cl...]]

Just a thought, cones instead of dowels. The weight of the part applies
downward force to make the cones self center into -C- & -B-. Then you
can probe the remaining exposed cone surface to establish -C- & -B- ?

[[De...]]

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Yeah this is definitely the case. And actually even if the bottom face is perfectly parallel to A there is no guarantee the holes are perpendicular to A. You also have to account for the clearance you allow between the holes and the pins.

If the check includes MMC then the pins could be made at low limit and give a representation of the function of the part. If it does not then any clearance between the hole and the pin would allow for potential error in your results.

Generally, In a situation like this turning the part on its side to allow for measuring the pin holes and the datum at the same time is the best bet.

All that said, you could make a base alignment to measure the fixture and pins, then put in a programmable stop, where you add the part, then have it continue and measure the A datum.

Another option would be along the same lines - have the base alignment measure the pins, then use current alignment to actually run the part and check datum A so it does not have to re-measure the pins each time.

[[An...]]

Just my two cents.
See attached.

Contribution_23_02_2019.pdf

[[To...]]

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Datum B has a 0.018 diameter tolerance and C has a 0.033 diameter tolerance. I would be checking Profiles and Positions with 0.3mm tolerance, so I was anticipating the position and profile features would be held much closer and absorb the sloppiness of the pins. I've moved on to Plan B which involves probing B and C from underneath with a J probe.

[[Ri...]]

I like to measure all my machined features first, at the end of the program the part is removed and then measure datum targets.
I put a CMM Stop in my last measured machined feature to remove the part from the fixture.