Is there an implied runout/concentricity in the absence of GD&T? 3

[joay11]

(I will form my question using a hypothetical drawing):

Envision a simple drawing of a bushing. The only view is a cross section (with centerline) which has some length Dim, an outer diameter Dim of .500(+/-.005) and an inner diameter Dim of .250(+/-.005). The drawing is completely absent of any GD&T and the title block contains no information about concentricity (or roundness).

The bushing arrives; I measure the part and find the runout is as much as .060" from the OD with respect to the ID (way off). Is this bushing out of spec.?

(Please note: this is NOT a question on how to GD&T, but rather a question of what can be implied in the absence of GD&T)



Taking a stab at my own question:

The centerline in the view implies a theoretical perfect center axis for both the inner and outer diameter Dim's (I may be wrong about this). Therefore, a theoretical cylindrical tolerance band exists for both the ID and OD with respect to the same centerline; since the tolerance for both diameter's is +/-.005, the maximum implied allowable runout is .010".....so, I would say the part is out of spec. Am I wrong?

 

[KENAT]

lifttrucks - the note I suggested came straight from ASME Y14.5M-1994, though as I noted there could still be room for debate on what 'related features' means, and without reference to as standard, questions about what MMC means.

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[DeanD3W]

joay11,
Several responses here are very good regarding the "no GD&T" drawing you're speaking of. Yes, you may not be happy with a supplier who provides the bushings with a relatively huge coaxiality error, and while they're at it maybe they manage to make curvy bent bushings too. I hope this OEM/customer (you apparently) orders parts only in small quantities... If you order a large enough batch of expensive enough parts you could run your business out of business because you absolutely have to buy every one of those unusable parts for full price, or plan to pay later along with the supplier's court costs. The drawing you're describing is of no use to you if you plan to have the legal authority to reject parts.

Operating without GD&T means you're relying on the goodwill and long term sense of your suppliers, rather than providing them with good specifications. Sort of like a professional sports team operating with no contracts for the players.

GD&T was not invented by geeky folks with nothing better to do. The language was developed to address very real problems with +/- tolerancing. Yes, there is a problem with the academic world thinking GD&T is too easy and industry thinking it's too hard. Without GD&T though, we have utter ambiguity.

Some engineering schools do have some decent GD&T courses. RIT, at least in their ME Technology program, has about three courses and Oregon State University has one course. These courses I'm speaking of need to be at least Junior or, much better, Senior level courses. Many schools have "this is the name of this symbol" as part of general courses in the first or second year which don't provide enough experience to be helpful. This would be a good topic for another thread... Which engineering schools have decent GD&T courses?

Dean

http://www.d3w-engineering.com

 

[KENAT]

But Dean, creating detail drawings is below most engineers, so why spend time teaching it at uni?

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[fcsuper]

KENAT, sarcasm noted. The "drafter" is a dying breed as 3D CAD software makes it so much easier to push drafting duties on to engineers instead. Why pay a drafter half the money to finish a drawing in half the time, when you can pay an engineer 100K+ to take twice as long to make a drawing that is only half as good? For some reason, this works for many companies.

Matt Lorono, CSWP
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[KENAT]

In fairness, a lot of the drafters around don't understand the GD&T etc. as well as they should - and this includes some more experienced folks.

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[ewh]

Old discussion...
I agree with dingy that a wall thickness could have been used and this problem would not have occurred.
I also feel strongly that some conservative use of GD&T would better solve the issue.
As far as the current lack of GD&T training, I also don't see it getting any better but actually getting worse.
Of course, we don't manufacture much (other than debt) here in the states, so it won't matter pretty soon.

"Good to know you got shoes to wear when you find the floor." - [small]Robert Hunter[/small]

 

[DeanD3W]

KENAT and fcsuper,
You've both seen the same thing that I've encountered... At my former full time job, they once laid off all the drafters, then after they came to their senses they brought a couple in on contract basis.

Just as with large outsourcing projects, it doesn't seem like most managers ever look back to determine the real cost of their great "efficiency programs".

Another disadvantage of engineers doing their own drawings is the lack of consistency... A small group of drafters will generally produce more consistent drawings than a larger group of engineers.

In my opinion (based upon experience, that is) I think a GD&T adviser/consultant will always be helpful for most engineers. The engineers need the background knowledge to understand and decide upon the datum feature selection and tolerancing approach, but they're also occupied with other things and only think explicitly about GD&T maybe one or two months out a year. The common lack of a GD&T adviser helping during review meetings, then verifying that the mark-up is turned into correct revisions, is part of the problem. With good drafters, engineers who understand, and a GD&T consultant helping, then the an important step, a good drawing, can be achieved... Then we get to talk about the quality of dimensional measurement data from suppliers :0).

The idea that you can graduate with a mechanical (or industrial, or manufacturing) engineering degree without sufficient GD&T coursework is absurd... Sort of like a business major graduating without sufficient accounting knowledge, or an English major with one class with 20% of the time spent covering grammar and writing as their sole grammar and writing education. GD&T is one topic for which the engineering technology programs generally do much better than the typical "higher level", normal engineering curriculum.

Dean

http://www.d3w-engineering.com

 

[dingy2]

ewh - I agree that I don't see much GD&T training in the future especially since the 2008 meltdown. Companies are struggling to exist and training in not high priority.

In Canada, most of our automotive companies (ones that are still existing) are paid in US dollars and now with our dollar hovering around $1.03 compared to the US dollar, I receive very few contacts about training.

I also agree with Dean that graduating people with an engineering degree should be thoroughly competent in GD&T but it is not happening today. I have in the past had many, many graduate engineers taking basic GD&T with me and there was minimal if any training given in our Canadian universities.

GD&T may seem like an easy subject but I don't think it is. It did take me about 10 years of training to be comfortable.

Dave D.

http://www.qmsi.ca

 

[MikeHalloran]

GD&T was just getting a foothold in the auto industry when I graduated; nobody else had heard of it. It's a tool for communicating design intent. It's not the only way to do so. It's not a religion; some people seem confused about that.

Should it be in the engineering curriculum? I don't think so, but I don't decide. The people who do decide seem to be suffering a serious disconnect from what's going on in the marketplace for engineers.

They've dropped drafting, even as their real customers were dumping draftsmen and coercing engineers into doing the drafting work, however badly.

They've emphasized computer aided engineering, even as their real customers were dumping degreed engineers and promoting designers who happened to be sitting at the CAD computers into doing the engineering work, however badly.


They seem to occasionally, maybe cyclically, attempt to include some practical material, e.g. manufacturability and such, but their teams don't have manufacturing skills in depth, so it always fizzles out, and they go back to teaching ivory tower skills, which they do have in depth.




Mike Halloran
Pembroke Pines, FL, USA

 

[KENAT]

I suppose the question might be, what gets dropped to fit in the GD&T?

Now in the US system with all it's prereqs, one might suggest that the 'history of basket weaving 101' could be dropped in favor of 'Drafting & Tolerancing 101' but heaven forbid the graduates aren't well rounded.

In the UK system, it's a bit trickier to determine what to drop since, at least when I went, pretty much all subjects were technical.

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