Datum features for blind holes

[310toumad]

Lets say you have a cylindrical part with two different pilot diameters on each end, and tapped holes on both faces of the part. What is the proper way to locate the holes on each face of the part? I can see a few scenarios:

1.) You have four datums total. The two surfaces which the holes lay on, and each of their respective pilot diameters. So holes on one face would have a positional tolerance relative to the surface they lay on, and the pilot diameter. Ex: Position[Dia.001][A] for one set of holes, and then Position[Dia.001][C][D] for another set.

2.) Or is it redundant to have both surfaces act as a datum, so instead you'd have three datums total, pick one surface to control orientation of the holes, and then the two separate pilot diameters. Then the surface not being designated as a datum has a parallelism tolerance back to the primary.

I suppose in the first example you would also have a parallelism tolerance relating the datums to one another, which is why I'm thinking you can just eliminate one of the surfaces as being called out as a datum and just move the parallelism callout to that surface as in scenario two.

 

[chez311]

The proper way would depend on how your part assembles/functions. There is no single correct way which covers all applications.

 

[drawoh]

310toumad,

Try to design a fixture.

If your datum feature is an accurate, blind hole on the opposite face, it can pick up a dowel pin on your manufacturing/inspection fixture.

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JHG