Minimally dimensioned CAD drawings GD&T 3

[swisscheese]

On minimally dimensioned CAD drawings I hear Y14.41 is not user friendly, not particularly useful (and not inexpensive). I've been searching and searching for anything that provides a good set of rules for how to manufacture and inspect against CAD drawings that include only a single general tolerance. Does anyone know of or have any internal documents or web sites they would share (publicly or privately)? Thanks.

 

[KENAT]

I know he's the fabricator.

I'm not saying he doesn't aim for tighter tolerances.

However, if his customers can't even be bothered to properly request certain tolerances, why offer tight ones?

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[fcsuper]

Customers do what customers want to do.

Matt Lorono
Lorono's SolidWorks Resources & SolidWorks Legion

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[swisscheese]

Kenat - Once again I see an error I made - I meant a position tolerance zone of diameter 2T. And unless you have a strong objection I think this type of position is more natural and easier to grasp. Therefore (with some more polishing) we have:

====

Based on a specified numeric value T (e.g. .005") the manufactured part shall be considered in conformance (governed by ASME Y14.5 2009) if:

1) FOS are within tolerance +/- T; and
2) The position* of circular features (at MMC) are within tolerance zone of diameter 2T; and
3) A surface profile* of 2T applied overall is met for non-circular features; and
4) Any explicit GD&T and comments are satisfied and shall take precedence over the above.

*Best fit will apply if no datum is specified.

=====

Do you see any serious problems or ambiguities with this?

 

[321GO]

swisscheese,

aren't your terms confusing?

ASME Y14.5 2009 does not support one single tolerance, the tolerance is dependent on the size of the feature, or i'm i wrong?

So, if you accept one single T, you by fact do not confirm to ASME Y14.5 2009.


To me, it seems you are ignoring the complexity of this issue.





"If you have an important point to make, don't try to be subtle or clever. Use a pile driver. Hit the point once. Then come back and hit it again. Then hit it a third time - a tremendous whack."
Winston Churchill

 

[swisscheese]

321GO - What in particular do you feel is confusing? Y14.5 does not have the policy we are developing but that does not mean we can't develop a policy for our customers who don't want (and in some cases don't have the skill) to explicitly specify GD&T. And we are using Y14.5 concepts in the details of the policy.

It's true that looser tolerances are often used for larger features and a customer is welcome to specify that, but the policy is intended to give a simple rule that covers model aspects that don't have explicit GD&T. We have been working this way for years.

Certainly GD&T *can* and often is complex, but it does not *have* to be. As long as the policy is clear, customers will know what to expect and inspection will know how to inspect.

 

[KENAT]

swisscheese, I can't say I really like it, as I find the whole idea a bit concerning, but maybe it's not too terrible.

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[pmarc]

If I can add some words to the discussion...

I was taught that the fundamental reason of using GD&T on drawings is precise and unambiguous description of designer's intent, so as a result clear and full definition of part's geometry.

Maybe this will sound brutal, but I believe such RDD ideas are in total oposition to GD&T philosophy. Sorry to be so general, but it would require writing quite a long essay to describe all ambiguities that stand behind this kind of policies. And you would have to write even longer text to make this vague areas clear. And in the consequence this short policy would have to be at least a few page document.

 

[fcsuper]

As long as your legal department can defend the policy and you reference the policy on every drawings, you should be OK, though expect some pushback from some who may not want to read it. Quotes and quote requests notoriously dismiss special requirements by the other party.

Matt Lorono
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[swisscheese]

Pmarc - I welcome any specific examples of situations where you believe there is ambiguity in latest version of the policy discussed in this thread.

 

[PeterStock]

Pmarc, swisscheese is producing parts, not drawings. He is being given models, or models and incomplete drawings, and a single tolerance value. His policy is a description of how he is interpreting the intent of the customer.

Peter Stockhausen
Senior Design Analyst (Checker)
Infotech Aerospace Services

http://www.infotechpr.net

 

[MechNorth]

Sorry, but did I miss any reference to a GD&T standard? Which one applies? To which scenario? It's a critical difference and distinguishing concern. Within Y14.5, position of a feature of size isn't established by +/- tolerances; there's not a single example or discussion of that method in the standard, so +/- to locate a feature of size doesn't work (Dave, I know you disagree, but that's an ongoing discussion); the center isn't reproducible without invoking GD&T, not to mention that it's actually a 3-D location that you're inspecting, not just at one end of the hole. Next, how are you even measuring size of a hole; at the end(s) only? What happens in between? Without invoking GD&T (specifically Y14.5), you don't get size controlling form, so you can have a bowed hot dot configuration and the part is ok as long as its individual cross-sections are within the size spec.

As for Y14.41, the '03 release was the first kick at the can; it was needed as a bridge to guide CAD companies on how to set up their software for a common set of functionalities, and start the transition over to model-based GD&T application. Work has started on the next revision, and we'll see what comes of it now that at least some of the original requirements have been implemented in CAD (though most CAD is way behind as of today). For those that see it as a useless document, that's from a users' perspective; look at it from an overview perspective of CAD and engineering, and you'll see that it was a foundation stone.

Many companies have been using a note "ALL OVER" or "GENERAL TOLERANCE" with a surface profile control for better than a decade, with great success. Some, unfortunately, don't understand how to use it effectively and therefore make it a burden rather than a useful tool ... then again, from this very forum we know that observation is equally true of many GD&T implementations. "ALL OVER" symbol is new in '09; previously, all we had for symbology was the "ALL AROUND".





Jim Sykes, P.Eng, GDTP-S
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[dingy2]

Jim:

I absolutely agree with you that "position of a feature of size is not established with +/-". They must be shown in basic dimensions.

Now, the location of a feature of size may be different. I know of many examples in the newest standard that show this without positional but, as you stated before, you found a disclaimer of some sort.

I would suggest just looking at the cover of the 2009 standard and note that not all of the features of size have their location shown in positional tolerances. Let's see, the diameter of 99 - 100 and also the diameter 56.6 - 57.6 on the cover are not shown in positional - mmmmmmm.



Dave D.

http://www.qmsi.ca

 

[MechNorth]

To be more accurate, Dave, they're not shown at all. Does that, then, mean that they have to be perfectly located, or is it an incomplete drawing (See 1.1.4 Figures)? (Note also that the datum features are not indicated on the image... what horrible implications that may lead to!) Many people read too much into the absence of something in the figures and don't consider that the figure is cited as a specific illustration to a specific point being made in the text.

Please, Dave, list for us some of the examples you suggest are in the standard wherein "locations" of features of size are controlled by +/- tolerances.

I've taught a number of people, including several other instructors, who initially held a similar perspective to yours. Many think that "preferred" and "should", etc. give you carte-blanche to do anything because it isn't specifically precluded. Those who have worked to establish voluntary standards know that those are about the strongest words that can be used outside of a federally mandated standard. Federal standards can use the words "shall", "will", "must", etc. as specific directives, but voluntary standards cannot because they have specific meaning under the law. ASME produces voluntary standards in that you are free to adopt them, and there is typically no specific legal action (other than contractual and civil liability) for twisting them out of shape. I understand that many companies have done /are doing /will do it as you endorse, however that is not supported in the Y14.5 standard or any other published standard, voluntary or mandated. Just because some people drink & drive without having had a terrible incident, it doesn't mean that it's acceptable or appropriate. Society moved beyond acceptance of DUI, hopefully industry will accept that GD&T has some distinctly right and wrong methods, per whatever standard is adopted.



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

"Sorry, but did I miss any reference to a GD&T standard?"

Yes MechNorth you did. When giving suggestions to the OP on how to come up with a way of interpreting the meaningless +- tol with no dimensions I referred to ASME Y14.5M-1994 several times.

The OP is more or less forced to accept drawings this way. He's trying to come up with a moderately robust way of interpreting it, explicitly detailing this way, and referencing it on all quotes/contracts.

I still don't like it, but I'm not completely without sympathy for the OP. I suspect refusing to take these drawings, or charging to redraw them etc. may be a business limiting approach. Certainly you can try and educate the other party but when that fails, or risks alienating them, is the OP's basic idea - though problematic - so unreasonable?

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[MechNorth]

I caught your reference, Kenat, but not the OP's to '09. I understand SwissCheese's plight, and empathize; I suspect most of us have been there / are there; I certainly have.

That being said, however, I can't see it standing up in court as it's not by any means clear. I've been stuck in pissing contests before about who's right, and nobody wins in the absence of a defined & prescribed standard. Otherwise, it's all "best effort". I don't have a better solution, either, but I would hate to see SwissCheese walk away from this with a warm & fuzzy that this is going to be effective and protective.

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

Well it's a step in the right direction isn't it?

If SwissCheese comes up with a reasonable document detailing how +- tolerances will be applied to nominal geometry by his organization, and that document is referenced on contracts then so long as the document is reasonably robust it should be some use.

Surely it's the fact that it's in (or referenced by) his contract that gives it some standing - not that it is or isn't correct to 14.5 or the like.

As someone mentioned, it may be worth talking to a lawyer, though of course the chances of them knowing what's going on with a topic like this are slim.

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[dingy2]

Jim:

Are you stating that the front page example of the 2009 ASME Y14.5 is wrong?

Dave D.

http://www.qmsi.ca

 

[KENAT]

Or perhaps he was suggesting to keep 1.1.4 in mind.

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[MechNorth]

Right on both counts, Kenat. Often something documented as a baseline is better than nothing, and at least gives you a starting point for dialog. And yes, I was suggesting to keep 1.1.4 in mind when considering the figure on the cover. Other than being incomplete, I don't see anything that doesn't meet the standard; again, I'm concerned about people reading something into the absence of information.

Dave, if you're going to make a statement about +/- tolerances being used in the standard ('09) to locate features of size, you really need to back it up, or you should reasonably retract that statement to sift some of the mud from the waters.

Jim Sykes, P.Eng, GDTP-S
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[dingy2]

Jim:

You didn't answer my question on whether the front page example was wrong. Maybe the example should have deleted the features of size not shown in positional?? I would think that ASME should at least have the front cover example correct.

There is not a statement anywhere that mandates that features of size must be shown using position. There is a recommendation in the 94 & 2009 standard though. There is also a recommendation of not using concentricity but it does not say it is wrong to use it.

There are so many examples in the latest ASME revision on cylindrical products where the feature is shown with an angularity tolerance and no positional. Fig. 4-38 is an example. Is this figure wrong?

Fig. 3-30 reflect many features of size without positional tolerances. Is this figure wrong or maybe we can use the disclaimer you found a little while ago.

Must angularity tolerances on features of size be a refinement of positional? I can't find that in the standard either. How would one show the location of a hole where only perpendicularity is shown?

I realize that you promote default positional and profile of a surface tolerancing. Does that really reflect the design intent of the part? Should the features that truly have a function or mating relationship be specified separately or is everything lumped in to the default?





Dave D.

http://www.qmsi.ca