I have just been trying to get my head around a true position geometric tolerance for some holes that references 4 (yes 4) Datum features.

These holes are machined perpendicular to a face (Datum A) and

positioned to centrelines (Datums B and C)

Datum Description

A Face

B Centreline of main bore

C Centreline of main bore @ 90° to B

No problems so far, but the then the geometric tolerance calls up Datum D which is a group of 4 holes machined in the same face

I think the draughtsman has got it wrong and there should be 2 seperate geometric tolerances.

The first one referencing Datums A B C

The second on referencing Datum D

Then I would report the holes twice, first time to Datums A B C, second time to Datum D.

Any thoughts?

Bob Fairbrother

These holes are machined perpendicular to a face (Datum A) and

positioned to centrelines (Datums B and C)

Datum Description

A Face

B Centreline of main bore

C Centreline of main bore @ 90° to B

No problems so far, but the then the geometric tolerance calls up Datum D which is a group of 4 holes machined in the same face

I think the draughtsman has got it wrong and there should be 2 seperate geometric tolerances.

The first one referencing Datums A B C

The second on referencing Datum D

Then I would report the holes twice, first time to Datums A B C, second time to Datum D.

Any thoughts?

Bob Fairbrother

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