Use profile or symmetry to define surface feature

[Astorite]

Hi,

First post here so any advice would be appreciated as I'm primarily a mechanical analyst so rarely venture in the realms of GD&T.

However, I'm getting a test piece made to verify some simulation work and need guidance on the best way to dimension the part. I've attached a sketch showing the proposed part.

In terms of importance, the critical aspect is that the raised features in the centre are located centrally about the aperture. My DO colleague has proposed to use a combination of symmetry and parallelism to define the relationship but I think the part should be defined in term of a profile constraint. Incidentally, the values assigned aren't correct as they are only to illustrate the concept.

Many thanks

 

[KENAT]

What drawing standards are you workig too?

If ASME then I'd be cautious about using symmetry.

In fact, from a quick look I'd probably use position rather than either of your approaches though that would depend a bit on some functional aspects.

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[Astorite]

The drawing would be against ISO BS8888. Could you elaborate on the the use of position in this example?

 

[KENAT]

Hmm, that may change things as I can't recall the differences.

I'll try to throw together a sketch in a bit if I have time.

Posting guidelines faq731-376

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

 

[Belanger]

I think your sketch is A-OK as it is now. Profile is a good choice. The only advantage to using position is that it would allow you to separate the tolerance for location from the shape/size tolerance. Profile, used with appropriate datums, covers everything (size, form, location, orientation) as long as you can live with the same tolerance value for those 4 qualities.

John-Paul Belanger
Certified Sr. GD&T Professional
Geometric Learning Systems

http://www.gdtseminars.com

 

[MechNorth]

Normally I'm the first advocate of Profile controls, however when you say that you want it centered, I automatically jump to position as well. If the tolerance on the profile is adequately tight, you're in decent shape as far as centering is concerned. On the other hand, remember that profile is a per-surface control, and the both opposed faces could both be shifted to the same side (i.e. either left or right) within the individual tolerance zone and be acceptable though not centered.

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

http://www.profileservices.ca

TecEase, Inc.

http://www.tec-ease.com

 

[Astorite]

The application is as test piece for RF circuitry so the part needs to be as close as possible to its specified sizes (otherwise the performance is significantly degraded).
Therefore, if profile is used to control the feature we are probably looking at a tolerance of 0.04mm!!
However, the idea of using position to control the feature is appealing for the reasons mentioned in an earlier post.

 

[MechNorth]

0.04mm is pretty wide compared to my history; 0.0025mm surface profile was my zone of expertise. After form, size is the easiest aspect of a feature (location, orientation, size, form) to control. Location is the hardest. Keep that in mind as you decide which control to use.

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

http://www.profileservices.ca

TecEase, Inc.

http://www.tec-ease.com

 

[Astorite]

Thanks for the advice, Jim but I don't think I'll quote you on that 0.04 is a pretty wide tolerance (as my RF colleagues would easily settle on 0.0025mm) but it's the usual trade off of performance versus cost.

 

[pmarc]

Astorite,
I do think profile is quite a good choice, though I still have some comments.

Per ISO GD&T use of symmetry is also absolutely legal - symmetry is only a special case of position used for features shown coplanar on the prints, unlike to ASME, where its interpretation is totally different to position.

Is the position of width (basic 10) of the top-center block 20x10 critical to your design? Current print is not controlling it anyhow - all around profile is not doing it.

Remember also that ISO's position/symmetry indirectly controls form of the feature, again ulike to ASME's.

 

[Belanger]

pmarc, are you sure about that last statement?

Position (and ISO's symmetry) really only feel the actual mating envelope, so it doesn't control form in the strict sense.

John-Paul Belanger
Certified Sr. GD&T Professional
Geometric Learning Systems

http://www.gdtseminars.com

 

[pmarc]

J-P,
ISO's position (and symmetry) has nothing to do with actual mating envelope. If you have a possibility, take a look to appendix A.3 of ISO 17450:2011. It nicely shows step-by-step procedure of how hole's positional tolerance assessment should be done. Basically it verifies whether imperfect median line, which is a collection of cross-sectional center points, fits into positional tolerance zone.

Maybe I should be a little bit more specific when I said that position controlled form. In OP's case it would indirectly control flatness of median plane. In case of simple cylidrical hole it would indirectly control straightness of median line. But I would not like to be understood that it would control circularity of the hole too.

 

[fsincox]

J-P,
I believe pmarc is correct at least according to Henzold. His book has a section where he compares the ASME control of an AME and ISO control of the actual feature.
pmarc,
I was thinking the same thing as Henzold explains it coaxiality and symmetry are really just special cases of position like parallel and perpendicular are special cases of angularity. It seems they are the same as our old concept of position.
Another thing I see is I have been misusing their concentricity, it is 2D like ours, they use coaxiality a common axis control which has the same symbol so it is rally a matter of teminology.
Frank

 

[pmarc]

Frank,
Yes, you are right regarding concentricity/coaxiality naming. The way to distinguish 2D concentricty from 3D coaxiality on ISO print - assuming one want's to specify 2D concetricity - is to place ACS (any cross-section) modifier close to the tolerance frame.

 

[fsincox]

pmarc,
They actually use symmetry for a coplainar condition and not just to describe centered non-round features? What is the datum, itself?
Frank

 

[pmarc]

Frank, I didn't mean coplanar like two interrupted surfaces. I meant two centerplanes coplanarity. Apologies for my wording not being precise enough.

 

[Belanger]

OK, I agree that position (and symmetry in ISO) controls the centerplane form. Thanks for the clarification.

John-Paul Belanger
Certified Sr. GD&T Professional
Geometric Learning Systems

http://www.gdtseminars.com

 

[fsincox]

Thanks, I get it, now. Sorry for my confusion once I got my head out of the ASME coplanarity world it makes good sense, actually centerplanes in a common zone.
Frank

 

[MechNorth]

One comment on the use of Henzold's material as reference. Several years ago, I reviewed some of his first publication on ASME and ISO in the same text; I found a number of fundamental flaws in his understanding of ASME GD&T, and a couple of his interpretations of ISO didn't mesh with the actual ISO document. When I inquired about it with EU colleagues, they reviewed the original and the English and I was told that the original text in German was different from the English version which was translated by a British fellow who didn't have a background in GD&T (or engineering as I recall). So, if reading an English translation of Henzold, then caveat emptor.

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

http://www.profileservices.ca

TecEase, Inc.

http://www.tec-ease.com

 

[pmarc]

Jim,
I was not referring to Henzold. I was referring to official ISO standard which is the indication for me in this case.

Henzold's publication indeed should be read carefully. Apart of being oddly translated to English, it contains a lot of his personal interpretations not covered by any ISO standard. However in this particular situation his interpretation is in line with the meaning of the standard.