GD&T - keeping features on center line of part
GD&T - keeping features on center line of part
(OP)
I'm having some issues keeping a feature on center. Can you please look at the attached image?
I need a good GD&T call-out to keep these features on center. First move is to add a center line to this print and I’m assuming adding a datum. I would like the .563" thickness to stay on center with the .783" thickness to within .005". I'm used to working with round parts, so a run-out symbol came to mind, but those can only be used with round parts.
Thanks for any help you can offer here. I couldn't find anything well explained in my "Fundamentals of GD&T" book.
I need a good GD&T call-out to keep these features on center. First move is to add a center line to this print and I’m assuming adding a datum. I would like the .563" thickness to stay on center with the .783" thickness to within .005". I'm used to working with round parts, so a run-out symbol came to mind, but those can only be used with round parts.
Thanks for any help you can offer here. I couldn't find anything well explained in my "Fundamentals of GD&T" book.





RE: GD&T - keeping features on center line of part
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The Help for this program was created in Windows Help format, which depends on a feature that isn't included in this version of Windows.
RE: GD&T - keeping features on center line of part
Instead perhaps make the .783 width a datum feature (so the center-plane of it becomes the effective datum) then center the .563 dimension on it using position tolerance.
Your current drawing doesn't appear to have any control over how well centered the .563 dimension is. See ASME Y14.5M-1994 section 2.7.3
What is Engineering anyway: FAQ1088-1484: In layman terms, what is "engineering"?
RE: GD&T - keeping features on center line of part
The position tolerance should work for you. You should consider (the implications of) calling up the .783 datum at MMB.
I have no objections to the symmetry tolerance. I just don't see why I would ever use it.
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JHG
RE: GD&T - keeping features on center line of part
I wouldn't really say it's "political." Symmetry -- in the GD&T language, not Webster's dictionary -- is based on very precise definitions. It's just that ASME and ISO have slightly different definitions and the ASME one makes it much more tedious to inspect.
Both definitions are trying to align two or more centers, but the difference is how those centers are derived. So like everything else, it depends on function, and most functional scenarios don't require the tedious ASME version of symmetry, but usually position.
John-Paul Belanger
Certified Sr. GD&T Professional
Geometric Learning Systems
http://www.gdtseminars.com
RE: GD&T - keeping features on center line of part
Jim Sykes, P.Eng, GDTP-S
Profile Services www.profileservices.ca
TecEase, Inc. www.tec-ease.com
RE: GD&T - keeping features on center line of part
RE: GD&T - keeping features on center line of part
John-Paul Belanger
Certified Sr. GD&T Professional
Geometric Learning Systems
http://www.gdtseminars.com
RE: GD&T - keeping features on center line of part
What is Engineering anyway: FAQ1088-1484: In layman terms, what is "engineering"?
RE: GD&T - keeping features on center line of part
Thanks again!
RE: GD&T - keeping features on center line of part
RE: GD&T - keeping features on center line of part
I agree it seems to be a political issue, because I know the history behind it. The ASME "powers that be" wanted it gone in 1982; normal English language usage supports its existence. The "powers that be" in the ISO seem to support it. The insistence of the precise ASME definition is itself an attempt to make it irrelevant. No one I know actually insists a check be preformed on every single point of a flat surface.
Frank
RE: GD&T - keeping features on center line of part
What is Engineering anyway: FAQ1088-1484: In layman terms, what is "engineering"?
RE: GD&T - keeping features on center line of part
RE: GD&T - keeping features on center line of part
RE: GD&T - keeping features on center line of part
The way as it is now tells that only upper face of the feature serves as datum feature A. So there is a difference.
RE: GD&T - keeping features on center line of part
What is Engineering anyway: FAQ1088-1484: In layman terms, what is "engineering"?
RE: GD&T - keeping features on center line of part
One last questions: any recommendations for GD&T reference/self-teach guide?
I searched this site and found a few ideas: YouTube videos, some older "textbooks", etc., but you all seem very knowledgeable so I thought I would ask!
Thanks again!
RE: GD&T - keeping features on center line of part
Yes it can be tough going at times and many areas benefit from more explanation and examples etc. however, if I only had the choice of one 'book' on the subject that's the one I'd pick.
What is Engineering anyway: FAQ1088-1484: In layman terms, what is "engineering"?
RE: GD&T - keeping features on center line of part
RE: GD&T - keeping features on center line of part
I have not checked the ASME definition of symmetry carefully. I think I would use symmetry in conjunction with sloppy profile tolerances. I don't care what the outlines look like, as long as they are symmetric. I do not know why I would do this. If I want something balanced, I will explicitly specify the balancing test, eg. 4000rpm, 1oz centrifugal force maximum.
--
JHG
RE: GD&T - keeping features on center line of part
So I guess your comments are about right: a sloppy profile tolerance could allow an egg-shaped part, while still being concentric in the ASME world. But if balance is what you're after, I guess there are other direct ways to say that.
John-Paul Belanger
Certified Sr. GD&T Professional
Geometric Learning Systems
http://www.gdtseminars.com
RE: GD&T - keeping features on center line of part
See, I remember the talk, after 1982, when they were all going to harmonize for the next version. They did end up adopting the ISO datum look, which I have mentioned before was a silly idea if they didn’t go for the rest. Typical for today’s society though, “make it look like” but “not actually be like”.
RE: GD&T - keeping features on center line of part
Jim Sykes, P.Eng, GDTP-S
Profile Services www.profileservices.ca
TecEase, Inc. www.tec-ease.com
RE: GD&T - keeping features on center line of part
RE: GD&T - keeping features on center line of part
RE: GD&T - keeping features on center line of part
Jim Sykes, P.Eng, GDTP-S
Profile Services www.profileservices.ca
TecEase, Inc. www.tec-ease.com