Two holes (or features) used as one datum. 2

[Bflo]

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.

 

[KENAT]

Typically...

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.

Posting guidelines faq731-376

http://eng-tips.com/market.cfm?

(probably not aimed specifically at you)
What is Engineering anyway: faq1088-1484​

 

[Bflo]

So by saying "...the FCF only having the one datum for perpendicularity." , the perpendicularity would be the median perpendicularity between the two hole axis, and where would that plane lie in relation to those features (i.e. at the end of one hole vs. the other, half way down the holes, etc?)

 

[Belanger]

You'd tolerance the holes with position to only one datum (this is what Kenat was saying) and also use the MMC modifier in there. It doesn't control the median perpendicularity or anything like that. It would just control each hole to be perpendicular to datum A, but at the same time control their location from each other.

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