Use profile or symmetry to define surface feature
Use profile or symmetry to define surface feature
(OP)
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
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





RE: Use profile or symmetry to define surface feature
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.
What is Engineering anyway: FAQ1088-1484: In layman terms, what is "engineering"?
RE: Use profile or symmetry to define surface feature
RE: Use profile or symmetry to define surface feature
I'll try to throw together a sketch in a bit if I have time.
What is Engineering anyway: FAQ1088-1484: In layman terms, what is "engineering"?
RE: Use profile or symmetry to define surface feature
John-Paul Belanger
Certified Sr. GD&T Professional
Geometric Learning Systems
http://www.gdtseminars.com
RE: Use profile or symmetry to define surface feature
Jim Sykes, P.Eng, GDTP-S
Profile Services www.profileservices.ca
TecEase, Inc. www.tec-ease.com
RE: Use profile or symmetry to define surface feature
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.
RE: Use profile or symmetry to define surface feature
Jim Sykes, P.Eng, GDTP-S
Profile Services www.profileservices.ca
TecEase, Inc. www.tec-ease.com
RE: Use profile or symmetry to define surface feature
RE: Use profile or symmetry to define surface feature
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.
RE: Use profile or symmetry to define surface feature
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
RE: Use profile or symmetry to define surface feature
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.
RE: Use profile or symmetry to define surface feature
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
RE: Use profile or symmetry to define surface feature
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.
RE: Use profile or symmetry to define surface feature
They actually use symmetry for a coplainar condition and not just to describe centered non-round features? What is the datum, itself?
Frank
RE: Use profile or symmetry to define surface feature
RE: Use profile or symmetry to define surface feature
John-Paul Belanger
Certified Sr. GD&T Professional
Geometric Learning Systems
http://www.gdtseminars.com
RE: Use profile or symmetry to define surface feature
Frank
RE: Use profile or symmetry to define surface feature
Jim Sykes, P.Eng, GDTP-S
Profile Services www.profileservices.ca
TecEase, Inc. www.tec-ease.com
RE: Use profile or symmetry to define surface feature
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.
RE: Use profile or symmetry to define surface feature
Jim Sykes, P.Eng, GDTP-S
Profile Services www.profileservices.ca
TecEase, Inc. www.tec-ease.com
RE: Use profile or symmetry to define surface feature
Frank
RE: Use profile or symmetry to define surface feature
Jim Sykes, P.Eng, GDTP-S
Profile Services www.profileservices.ca
TecEase, Inc. www.tec-ease.com