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GD&T - keeping features on center line of part

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badger2011

badger2011

Bioengineer
Jul 1, 2011
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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.
 
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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.
 
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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 (probably not aimed specifically at you)
What is Engineering anyway: faq1088-1484​
 
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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
 
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dgallup said:
However, symmetry seems to be out of political favor and generally frowned upon for reasons that escape me.
Click to expand...

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