Pattern of Multiple Features with Composite Positioning Tolerance 4

Phnx34,

That's quite a question for a first post. You're obviously not new to GD&T.

For this application, it would be better to use multiple single-segment tolerancing instead of composite.

The lower segments of composite FCF's are not subject to the rule of Simultaneous Requirements, and so the features would not be treated as one composite pattern. Your FRTZF's only reference datum feature A anyway, so you don't need the composite FCF's property of waiving location to the datum reference frame.

"Regular" multiple single-segment FCF's are subject to Simultaneous Requirements, which would satisfy the design intent.

Evan Janeshewski

Axymetrix Quality Engineering Inc.

Phnx34,

I agree with Evan,

it would be better to use multiple single-segment tolerancing instead of composite

Click to expand...

to accomplish your functional objective of keeping the hole and rectangle relationship more refined than its relationship to the plate's outer profile... but relying on the more indirect method of linking those feature tolerances via simultaneous requirements is often more risky than directly specifying them.

I would try other method to directly specify that functional relationship. I took a stab at it... see attached

Paul

Paul,
I agree with Evan and you, you must work at the kind of places that I do. It is an easy solution that would be rejected by my shop as too vague. It is one of the things, like the envelope principle, that makes the standard almost irrelevant to most of the people who are supposed to use it.
Frank

Thanks for the help, I really appreciate all of your comments!

I definitely need to brush up on my Composite Position vs. Multiple Single Segment Control basics!

Evan's suggestion reminded me of a Tec-Ease tip I came across regarding simultaneous requirements:


Is it possible to use this concept in conjunction with Paul's suggestion to reference the hole locations directly from the rectangular cutout? (see attachment)

Thanks again!

P.S. Although I used tolerance of position in the new drawing, I thought that profile tolerancing was a very clever way of accomplishing the goal. I find that, with GD&T, there are so many different ways to do things that you can go crazy looking for the "most-elegant" solution. ;-)

Looks like you have got it. I would bring the centerlines out farther, just to include the outer rectangle, but that is more my personal preference.
Frank

Don't forget to add diameter symbols to both positional callouts for holes.

Phnx,

I would still caution against the reliance on indirect association of the feature controls... many people don't understand the sim req't.

Although Don's example mirrors what you are proposing the tolerance 0(M) controlling orientation and simultaneous position of the center hole draws attention to itself for use as the defacto secondary datum feature simply because of the zero. When an attribute gage is not built to measure this... I would suspect that most inspectors would indeed set up on the center diameter and as the secondary and be left with resolving the rotational orientation in their reasoned approach to inspect the remainder.

Your example doesn't have that same attractive solution... If I were you I would dismiss the elegant and embrace the obvious.

Paul

Frank, I guess you are familiar with ISO. So what does ISO say about the simultaneous requirements?

Simultaneous Requirements is the default in ASME, but as mentioned, most people don't understand it or recognize it.

An alternative may be to use a composite profile tolerance to control the feature geometries and locations (per 2009). Depends on whether you can live with the slight difference in tolerance zones for the holes or not. That way, it's elegant, technically correct, and more visibly related.

Jim Sykes, P.Eng, GDTP-S
Profile Services TecEase, Inc.

bxbzq,
I am trying to learn. pmarc has posted their position above and I believe it is safe to say it is a little more restrictive. They have it for features shown on the same centerline, as indicated above, and can invoke it by adding CZ (common zone) as required to the FCF. This is according to Georg Henzold's book, anyway.
Frank

Thanks for all of the great responses! I have to agree that the simultaneous requirements of the 1994 standard can be a bit confusing, albeit technically correct.

As Paul and Jim mentioned, perhaps profile tolerancing provides a more intuitive solution. I've been delaying the inevitable, so I guess now is as good a time as any to check out the 2009 standard. Thanks again for all of the help!