Two holes (or features) used as one datum.
Two holes (or features) used as one datum.
(OP)
I've recently learned that more than two or more holes (or features of size) can be used as one datum. How do you tolerance them relative to each other? I looked for the attached example on this thread http://www.eng-tips.com/viewthread.cfm?qid=335267 but couldn't find it.





RE: Two holes (or features) used as one datum.
I'd use position tolerance with a basic dimension between them, the FCF only having the one datum for perpendicularity.
Remember the rest of the features would then be dimensioned to the 2 datum planes created by the hole pattern.
What is Engineering anyway: FAQ1088-1484: In layman terms, what is "engineering"?
RE: Two holes (or features) used as one datum.
RE: Two holes (or features) used as one datum.
Then, when you want to reference back to that datum (of two or more features of size), you'd probably want to modify the datum reference with "M". See the attached graphic for an example from the Y14.5 standard; notice that the two small holes create datum B, but first they are positioned to each other -- and also to datum A (thus the perpendicularity aspect).
John-Paul Belanger
Certified Sr. GD&T Professional
Geometric Learning Systems