Hole position inspection without CMM, primary datum simulation 1

[CTengIS]

Suppose that blind holes for location of dowel pins have a tolerance of position specified. The primary datum feature A is the functional mating surface the holes are perpendicular to (the one in which the holes were drilled).

For manual inspection without CMM, how is the primary datum being simulated, typically?

For example, if someone uses a granite slab and 2 angle blocks to simulate the datums A,B,C once the part is put on the granite slab the holes become inaccessible for measurement. How is this handled?

 

[geesaman.d]

I've seen it done by placing the part with the machined/dowelled face vertical to the table and then set dead perpendicular to the table using a precision square. Then the height gauge will give direct readings in that coordinate direction.

Then to get the other coordinate you repeat the process on the table turned 90 degrees. It's a very slow process, getting the part set up dead square to the table each time.

I'm not a huge fan of arm CMMs but they do well probing all in the same direction from one setup, so if it's dowel pins and bores being measured from that one side they can give fairly good data.

David