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GD&T - keeping features on center line of part

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.

RE: GD&T - keeping features on center line of part

For this I like to use symmetry. However, symmetry seems to be out of political favor and generally frowned upon for reasons that escape me.

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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

What drafting standards, if ASME I'd suggest not using symmetry as such.

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

Posting guidelines FAQ731-376: Eng-Tips.com Forum Policies http://eng-tips.com/market.cfm? (probably not aimed specifically at you)
What is Engineering anyway: FAQ1088-1484: In layman terms, what is "engineering"?

RE: GD&T - keeping features on center line of part

badger2011,

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.

--
JHG

RE: GD&T - keeping features on center line of part

Quote (dgallup)

However, symmetry seems to be out of political favor and generally frowned upon for reasons that escape me.

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

Pretty much what J-P said. I tell my students, "You're not in high school any longer, so time to accept the new definition of symmetry / concentricity for this subject." It's a mindset.

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

Adding a single center line from left to right will imply that the features share a center plane, which in a lot of cases would be enough... but to solidify it, as stated, apply a datum to the center plane and use a position tolerance.

RE: GD&T - keeping features on center line of part

It implies that the features are to share a center line, but it doesn't quantify a tolerance for how accurate the alignment must be. So a GD&T callout or a note is actually required.

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

Sorry EMorel, but per ASME Y14.5M-1994 section 2.7.3 just putting in the center-line doesn't imply anything from a standards/dimensioning point of view. Like Belanger says, it doesn't give any information on how well aligned they need to be.

Posting guidelines FAQ731-376: Eng-Tips.com Forum Policies http://eng-tips.com/market.cfm? (probably not aimed specifically at you)
What is Engineering anyway: FAQ1088-1484: In layman terms, what is "engineering"?

RE: GD&T - keeping features on center line of part

I stand corrected :P

RE: GD&T - keeping features on center line of part

dgallup,
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

Obviously, position is the way to go under the current ASME system. I am personally particularly interested in being able to express the definition of a functional axis to axis relation when it is required in the design. I know many do not seem to feel these actually exist, but the ball and roller bearing manufactures’ seem to disagree.

RE: GD&T - keeping features on center line of part

The datum feature A symbol should be an extension of .783 dimension line if center plane is going to be a datum.
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

(OP)
Thanks for your help resolving this issue!

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

First things first, do you have a copy of the standard?

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.

Posting guidelines FAQ731-376: Eng-Tips.com Forum Policies http://eng-tips.com/market.cfm? (probably not aimed specifically at you)
What is Engineering anyway: FAQ1088-1484: In layman terms, what is "engineering"?

RE: GD&T - keeping features on center line of part

Agreed, If you are only going to have one, the real standard is the best.

RE: GD&T - keeping features on center line of part

Quote (Belanger)


However, symmetry seems to be out of political favor and generally frowned upon for reasons that escape me.

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.

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

The definition of concentricity (ASME) says that the median points of each diametrically opposed element shall be within the specified tolerance zone. Sounds great on paper, but imagine how tedious it is to find each median point -- something that can't be touched, but rather is purely based on mathematical derivation. Then there's the issue of how many of these "median points" to check; that's a subjective thing that's usually left up to the inspector.

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

I understand, it may be looked at that the retention of symmetry and concentricity was just as much of a political of a decision, but since they do not accept the same definitions it seems a failed attempt at best.
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

I like the ISO datum symbol FSincox because it's easier for me to pick out, but then that's the only symbol that I've worked with too. The "unification" of the standards is a long and ongoing story of good intentions and bad delivery.

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

Looking to the newest editions of some important ISO GPS standards (8015:2011, 1101:2012, 5459:2011 or 14405-1:2010) I am afraid we can forget about ISO/ASME harmonization once and for all.

RE: GD&T - keeping features on center line of part

There is always an easy way.

RE: GD&T - keeping features on center line of part

I have suspected as much, pmarc. When they swung onto the tangency of a metrology-based standard, there was little hope of reconciling into a single cohesive standard. There have been genuine efforts and concessions by ASME, but the response from ISO seems to be to push for more rather than migrate to common ground.

Jim Sykes, P.Eng, GDTP-S
Profile Services www.profileservices.ca
TecEase, Inc. www.tec-ease.com

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