×
INTELLIGENT WORK FORUMS
FOR ENGINEERING PROFESSIONALS

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

How not to promote bad practice

How not to promote bad practice

RE: How not to promote bad practice

I agree, misuse of terms like this can create confusion and misinterpretation by those who are not aware of the subtle differences between MMC/LMC bonus tolerance and the concepts of MMB/LMB and datum feature shift. The safest bet is to never conflate the two concepts, and leave out the word bonus whenever discussing MMB/LMB shift.

To your question I believe the use here can be correct if we interpret "the bonus is quite complicated" to refer to analysis of the 0.3 MMC position tolerance applied to the 2x 8mm holes. If we instead consider it, and all other uses, to instead apply to analysis of datum feature shift relative to the |A|B(M)| DRF then I would say it is incorrect. Under certain circumstances it may look like a bonus tolerance and for worst case stack up purposes may be convenient to calculate it as such, but I think utilization of the term bonus in this sense is incorrect.

RE: How not to promote bad practice

greenimi,

I don't see much of a problem here. I would can design an inspection fixture with two pins 7.7mm in diameter. At the datum B feature's MMC and maximum error, the part will be fixtured precisely. At the holes open up and are located more accurately, the part can be wiggled around to make the other tolerances work.

I don't understand the concept of sharing a bonus tolerance.

--
JHG

RE: How not to promote bad practice

Drawoh,

I'm not familiar with the specifics but i believe why its more challenging or complex is because i think in terms of CMM programming and software that MMB and MMC are relatively difficult to analyze. Doubly so when those features are instead of a single feature but part of a pattern. Its relatively easy to shift a part so it fits on a physical gauge. I believe its more difficult for a computer to do the same with a fitting routine.

As far as "sharing" the tolerance, i think it just means that the allowable shift is affected by the combined effects of variation of all features in a pattern, instead of a single feature which would presumably be easier to analyze.

RE: How not to promote bad practice

Datum shift is not bonus, CMM doesn't like datums that move, company is trying to sell the product.

What else is new?

"For every expert there is an equal and opposite expert"
Arthur C. Clarke Profiles of the future

RE: How not to promote bad practice

Datums don't move, but datum features can, relative to datum feature simulators.

The unanswered question is about providing a unique, single, numeric value for MMB/MMC datum feature/feature reference for the purpose of QA/QC reporting. I think that's the huge irritant for the "show me the number" crowd.

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! Already a Member? Login


Resources


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