Log In

Come Join Us!

Are you an
Engineering professional?
Join Eng-Tips Forums!
  • Talk With Other Members
  • Be Notified Of Responses
    To Your Posts
  • Keyword Search
  • One-Click Access To Your
    Favorite Forums
  • Automated Signatures
    On Your Posts
  • Best Of All, It's Free!
  • Students Click Here

*Eng-Tips's functionality depends on members receiving e-mail. By joining you are opting in to receive e-mail.

Posting Guidelines

Promoting, selling, recruiting, coursework and thesis posting is forbidden.

Students Click Here


Two holes (or features) used as one datum.

Two holes (or features) used as one datum.

Two holes (or features) used as one datum.

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.

Posting guidelines FAQ731-376: Eng-Tips.com Forum Policies http://eng-tips.com/market.cfm? (probably not aimed specifically at you)
What is Engineering anyway: FAQ1088-1484: In layman terms, what is "engineering"?

RE: Two holes (or features) used as one datum.

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?)

RE: Two holes (or features) used as one datum.

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

Red Flag This Post

Please let us know here why this post is inappropriate. Reasons such as off-topic, duplicates, flames, illegal, vulgar, or students posting their homework.

Red Flag Submitted

Thank you for helping keep Eng-Tips Forums free from inappropriate posts.
The Eng-Tips staff will check this out and take appropriate action.

Reply To This Thread

Posting in the Eng-Tips forums is a member-only feature.

Click Here to join Eng-Tips and talk with other members!


Close Box

Join Eng-Tips® Today!

Join your peers on the Internet's largest technical engineering professional community.
It's easy to join and it's free.

Here's Why Members Love Eng-Tips Forums:

Register now while it's still free!

Already a member? Close this window and log in.

Join Us             Close