Please review and provide feedback

Hi guys,
I've put together a list of fundamental GD&T concepts for your review. This list is based on frequent errors I see when checking prints. I thought I would put them all in one place and make the list available to the designers and engineers here. I want to submit it for peer review first though. The list is per the 94 standard since that's what we're still on here.
Thanks,

Powerhound, GDTP T-0419
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[CheckerHater]

As "tolerances cannot be implied", what is your take on General Tolerances?

 

[drawoh]

Symmetry and Concentricity are useless. Never use them.

I do not know why I would use them. That does not meant that they cannot solve someone else's problem.

Positioning a bolt hole circle at LMC does not work.

Is somebody really doing this? LMC makes no functional sense for a clearance hole. You have a general requirement for tolerances to make functional sense. Someone could interpret this as an insult.

13. When a hole pattern is clearance for fasteners, modify the positional tolerance with MMC. Use the fixed or floating fastener formula. (Appendix B)
14. When holes are for press fits, leave the positional tolerance at RFS. MMC will serve no functional purpose.

I would be concerned about someone preparing drawings and having to follow rules like this. I would want people to understand the geometry needed to make holes and fasteners line up. For most practical purposes, tapped holes are equivalent to press-fit dowel pins.

You also need to know when a clearance hole is too accurate to be positioned, and you need to slot things. This is not just a problem with dowel pins. I have done this while designing sheet metal.

Otherwise, your document looks okay.

JHG

 

[KENAT]

Powerhound, I believe I get where you're coming from, no it's not ideal that you even need to provide a document like this but perhaps it has merit.

I'll have a look in my files when I get a chance, I got pressured into doing things like this and may have something. I think mine may have been a 'peer review checklist' but it's been a while.

Checker Hater, if by "General Tolerances" you mean block tols or iso 2768 or similar than I don't think they fall under 'implied' so long as they are referenced properly on the drawing with the usual "Unless otherwise stated..." type wording.

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[CheckerHater]

No, I was genuinely curious which technique OP prefers. I just have seen spectrum of opinions ranging as far as "direct tolerances ALL dimensions" sometimes, so I was wandering where powerhound stands.

For example, to see opinion on how to combine "no trailing zero for metric" requirement with "X.XXX" general notes; what exactly is happening "unless otherwise specified", etc.

I believe an attempt to create list like that is great idea, I just was never able to accomplish it, because it starts behaving like rising dough rather quickly

KENAT, you have a thing for 2768, have you?

 

[Belanger]

If you want to get nit-picky, item #6 shouldn't say "always" because of the AVG idea on flexible parts (see paragraph 6.8 in 1994; about two-thirds into that paragraph they say that the form tolerance could exceed the size).

I'd also add something saying that a position tolerance can't be used if it's not controlling location. (It's been discussed on the forum: some folks try to use position when it's really only doing perpendicularity.)

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

http://www.gdtseminars.com

 

[KENAT]

Well yes I have a thing about) 2768 but that's not my intent in bringing it up. It was the 1st 'general tolerance' that sprang to mind other than block tols.

I agree with you on the dough aspect, how far to you get before you've rewritten Y14.5! Maybe picking a arbitrary number like '15 most common offences' or similar is the way to limit it.

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[powerhound]

Thanks for the feedback guys. I appreciate you taking the time.

CH,
I'm perfectly fine with general tolerances but they should not apply to coaxial or coplanar tolerances for a couple of reasons. Also, paragraph 2.7.3 says that those things must be controlled to avoid an incomplete drawing. For other individual features, I think they work fine.

drawoh,
Yes, somebody was really doing this (hole pattern at LMC) and the bad part is that he was encouraging its use based on misunderstanding of the concept. Also, I did not make the list as a set of rules to follow or really even as a teaching tool. It's more of a list of things to be aware of on a high level. I do get your point though and your input is appreciated.

J-P,
That's a good point. Considering the case of the example in the standard, should I leave it as it is and include the caveat or should I remove the word "always" and specify the one time that it doesn't have to be?

Ken,
Thanks. Any other suggestions you may have for the list are welcomed.

Thanks again for taking the time guys,

Powerhound, GDTP T-0419
Engineering Technician
Inventor 2010
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[fsincox]

Sorry,
Number five should have the following text added, IMHO: "are useful theoretical concepts, but, manufacturing does not like them and since they tell us what to do"
Frank

 

[pmarc]

Let me put some more comments:
#1. I would add something about implied 90 degree angles too [1.4(i), (j), 2.1.1.2].

#4. No remarks to the meaning of this point, however personally I am missing a statement like: "Coordinate dimensioning should not be used for locating features/features of size due to its ambiguity for product definition, manufacturing and inspection. Position tolerance (for FOS) or profile of surface tolerance (for non-FOS) shall be considered instead."

#5. Though I do not like to arbitrarily close the door to the applicability of existing Y14.5 definitions, my proposal would be: "Interpretation of Symmetry and Concetricity is not equivalent to the interpretation of Position of nominally coplanar/coaxial features. Whenever coplanarity or coaxiality is a design intent, Position shall be used."

#6. "Form tolerance, when applied to surface elements, must always be a refinement of size tolerance, unless Rule #1 is overridden on the print anyhow."

#14. I would not take your statement for granted, meaning that positional tolerance at LMC for hole and for corresponding pin could be used to assure press-fit equally well - if not better.

 

[powerhound]

Thanks for the additional comments.

I asked for feedback so I certainly am not going to rebut anyone's participation and opinion. I appreciate the help. I would like to address one bullet that almost everyone seemed to have an issue with though.

I flat out said to ignore symmetry and concentricity. This is not because I personally don't like them. It's because they are inferior controls to what the designer really wants.

Regarding symmetry, position is always better. I often get the argument that drawoh made and always with the same disclaimer at the front. "I wouldn't do it, but...". The point is that if symmetry went away, no one would miss it. It offers absolutely nothing that position doesn't and you can't use a modifier with it. If anyone has seen a use for symmetry that position wouldn't cover please let me know about it so I can stop hating on this poor little control.

Regarding Concentricity, the only argument I've ever heard for it is high RPM applications where balance is a concern. This may be enough to keep concentricity on the books. I would change #5 to state this but I don't know if the high RPM argument is valid. Does anyone have anything to offer regarding concentricity?

Again, thanks for your participation.

Powerhound, GDTP T-0419
Engineering Technician
Inventor 2010
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[Belanger]

For my comment regarding #6, I like pmarc's solution (something about "unless Rule #1 is overridden..."

It's wise to discourage concentricity and symmetry, but maybe it can be presumptuous to flat-out prohibit them. (The Detroit 3 car companies did prohibit them, over a decade ago.)

Rather than pigeon-holing concentricity to parts at high RPM, a more general way to say it is that the designer is concerned about equal distribution of mass around the datum axis. If the designer wants that, and understands the ramifications of concentricity and how tedious it can be to measure, then he's within rights to use it ... if the company's guidelines haven't outlawed it.

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

http://www.gdtseminars.com

 

[powerhound]

Oh yeah, I actually meant to say that. I will add the "unless rule #1 is overridden." to #6. Thanks pmarc.

Powerhound, GDTP T-0419
Engineering Technician
Inventor 2010
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[Bouing]

#14 If you want go-nogo gages to check position, you need the MMC

I would add this one that I often see, especially on old timer drawings.

Do not write drill, tap, ream or any process unless as per Y14.5 2009 basic rules 1.4e:

The drawing should define a part without speci-
fying manufacturing methods. Thus, only the diameter
of a hole is given without indicating whether it is to be
drilled, reamed, punched, or made by any other opera-
tion.

 

[KENAT]

Sorry Power Hound, I can't seem to find anything so maybe I was imagining it - or it was on the 'wiki' that our former overlords kept when the sold us off.

Having been in your situation I can appreciate that you need to keep it short and to the point and this may mean over simplifying some things.

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[MechNorth]

Pls don't use "True Position", as the term doesn't exist in '94. "True" has come to mean the "Basic" or "Theoretically perfect" location.

#5 could use some fleshing out rather than "are useless" (though they are). In training, I ask what symmetry & concentricity mean, and I always get the high-school explanations back.

On #9, also add "TYP" not allowed. Don't know if your company has issues with quantities (one of my old companies put quantities in parentheses (10)), but you may want to outline the correct method of specifying quantities.

Nice list, but I suspect it will grow.

Jim Sykes, P.Eng, GDTP-S
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[DeanD3W]

powerhound,
I completely agree with your statement about concentricity and symmetry. I don't think concentricity addresses any balance consideration well enough for rotating parts... A three lobe shape, for instance, could be perfectly balanced but it will have terrible "concentricity" if measured as concentricity is supposed to be, or a part with very nice concentricity and varying material density could be poorly balanced.

In your #2 the term should be datum "feature" label/symbol/identifier, rather than datum identifier/symbol/label.

Dean

 

[weavedreamer]

Another common oversight is the omission of the PTZ in conjunction with threaded holes.
Many also by route, add MMC to the thread callout deriving its value from the pitch diameter, a practice Appendix B does not identify.

 

[TheTick]

If you're going to come down on concentricity, you should probably take the time to encourage use of runout or position in its place.