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

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

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

(OP)
(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?

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

No, it is not out of specification as long as it meets the tolerances & specifications shown on the drawing.

You used the OD as the datum and then took a circular runout of the ID with the result of .060" TIR (or FIM) but there was nothing on the drawing requiring this.

Dave D.
www.qmsi.ca

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

(OP)
If this is the case, it's interesting to note that the drawing is essentially missing a specification.

Maybe we should address this by making a note within our standard title block.

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

Per ASME stds no there is not an implied runout/concentricity in the absence of GD&T.

See ASME Y14.5M-1994 section 2.7.3. or equivalent section in other versions.

Per the standard one option is to add a note such as "PERFECT ORIENTATION (OR COAXIALITY OR LOCATION OF SYMMETRICAL FEATURES) AT MMC REQUIRED FOR RELATED FEATURES.

I suppose even here, you could fall foul of ambiguity in what counts as 'related features' so arguably you may be better explicitly specifying it with GD&T.

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

The Y14.5 standard states: "The limits of size do not control the orientation or location relationship between individual features. Features shown perpendicular, coaxial, or symmetrical to each other must be toleranced for location or orientation to avoid incomplete drawing requirements."

I have seen drawings in the past with a note that was something like this: "PERFECT COAXIALITY REQUIRED AT MMC." This is not a good practice, in my opinion. It leaves too many open questions and does not identify datum features.

Why not take 15 seconds and specify a datum feature and a positional or runout tolerance?

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

Lifttrucks - You said "Why not take 15 seconds and specify a datum feature and a positional or runout tolerance?"

The simple answer to that is that there appears to be less people trained in GD&T especially since 2008. Training is not on the high priority list for many manufacturers since most are concerned about survival.

If Designers have not taken a GD&T course of some sort, it may not be wise applying it. In other words, if you don't really understand GD&T, maybe you shouldn't use it. Go for the note.

Dave D.
www.qmsi.ca

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

(OP)
I agree with dingy here. The primary problem I find with GD&T is it requires a Designer/Drafter who can write/read it, a fabricator who can read it and an inspector/quality engineer who can read it.

GD&T seems to work best if you keep it simple; some of the extreme GD&T examples in the book are difficult for everyone in the process to comprehend, so the meaning tends to get lost (there are exceptions).


In this case, if a supplier sent me parts that were .060" TIR and I told him to fix them or send me new parts; and he refused because "it wasn't specified"; I'd be looking for another supplier.

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

And if I were the supplier, who had parts rejected based on a requirement not specified on the drawing, then I might look for a new customer.  Or at least, next time the customer wants a 'favor' of some kind, may be less accommodating.

Assuming it's a small part then yeah .06 is pretty far off nominally aligned, but if you didn't spec it then how was the vendor to know it wasn't acceptable?

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

joay11,

- Since only two diameter dimensions are shown and there is nothing else on a drawing that could somehow imply any relationship between the features, lifttrucks is correct saying that "The limits of size do not control the orientation or location relationship between individual features. Features shown perpendicular, coaxial, or symmetrical to each other must be toleranced for location or orientation to avoid incomplete drawing requirements". Such drawing shows that something probably should be coaxial, but actually it does not specify how much tolerance is allowed for this relationship.

- The drawing is not defining any datum feature so assigning inner diameter as such may or may not be correct. One could take ID as datum feature, but someone else could use OD and they both might be correct. Noone will be able to judge this as the drawing simply seems to be incomplete.

- It is not specified anywhere that runout tolerance is the geometrical characteristic you should measure. Again, the drawing does not give any directions on this. Even assuming that someone is aware that the mutual relationship between ID & OD is missing (like you are) two different persons could go for two different methods of checking it. Circular runout, total runout or maybe position tolerance?

- Runout tolerance value does not have to be equal or smaller than size tolerance. If the tolerance for diameter is +/.005, it does not mean that for instance maximum total runout value is .010. Since total runout is a composite tolerance of cylindricity, orientation and axis offset of a diameter its value can be greater than .010. So your assumption on maximum runout value is unfortunately wrong.    

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

"GD&T seems to work best if you keep it simple; some of the extreme GD&T examples in the book are difficult for everyone in the process to comprehend, so the meaning tends to get lost (there are exceptions)."

I would say having one datum feature symbol and a runout or position tolerance is as simple as you can get.

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

"The primary problem I find with GD&T is it requires a Designer/Drafter who can write/read it, a fabricator who can read it and an inspector/quality engineer who can read it."

That's the primary problem with English, algebraic expressions, surface roughness requirements, heat treat requirements, weld symbols, etc.

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

Lifttruck - You said "I would say having one datum feature symbol and a runout or position tolerance is as simple as you can get."

Should the Designer place a circular runout on the ID relative to the OD? This controls the off center and roundness of the ID but not bend of the ID along its length. There is still of possibility that a mating part would not fit completely along the length of the ID.

Maybe the Designer should place a total runout which includes the attributes of circular runout but also the bend of the feature along its length. Many shop floor people really don't know the meaning of total runout and how one should confirm it.

Then there is concentricity and now we have an argument on what it means and how one should confirm it.

Of course, positional with a diametrical tolerance zone could be applied but should it be at MMC or RFS. Now at MMC, it is applicable to a hard gauge or checking fixture while at RFS one must measure it in various positions along its length perpendicular to its axis if appropriate measuring equipment was available.

Should one place a diametrical tolerance zone as part of the tolerance or leave it out?

We could also reference the datum feature which is an OD in either RFS or MMC.

It all depends upon the actual relationship between the OD and ID during assembly or usage. I just don't think that it is quite a simple application and applying the wrong requirement may not help.  

Dave D.
www.qmsi.ca

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

Applying no requirement at all and assuming the part will function properly obviously did not help either...

How do you know the note is applying the correct requirement? What's the difference between applying the wrong requirement in note form or symbol form?

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

It is great to have this discussion, because it is clearly showing that GD&T is indispensible whenever any relationship (geometrical and/or functional) between features exists.

This example with seemingly very simple bushing or something similar should be used in every GD&T training to visualize implications of no GD&T on a drawing and limitations of classical coordinate dimensioning. Simple part but so many issues, caused by incomplete drawing.

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

I agree liftrucks that is why I think people need a little training in this subject before applying anything. Unfortunately, that is not happening today.

One could also note a minimum wall thickness which should, to a degree, would control the off centre condition and everyone understands its meaning. It still does not control the straightness of the ID.
 

Dave D.
www.qmsi.ca

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

Dave, I think GDT trainers, such as yourself, are indispensable since many colleges do a poor job training engineers in technical drawing. They must think we still get our own draftsman to make the drawings for us. :)

I guess the point I'm trying to make is that at some point it becomes beyond impractical to try to spell everything out on a drawing because someone may not be able to read it.

Don Day says it better than I could: http://www.tec-ease.com/gdt-tips-view.php?q=250
 

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

A Designer should place all the requirements needed on a drawing and the tube with the ID off center to the OD is an example of all the needs not placed on the drawing. Which geometrical control to use is not so simple though.

Does that mean that all dimensions on a drawing should be covered by GD&T? I don't think so. I looked at the 2 drawings that you reference on the tec-ease web site and question the top drawing along with the bottom one. The 2009 standard still recognizes linear & angularity tolerances and, in my opinion, they should be used in conjunction with GD&T. We have had this discussion here before.
 

Dave D.
www.qmsi.ca

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

I did not communicate that linear or angular tolerances have no place (nor did Tech-Ease).

We make drawings using line conventions, orthographic projections, title block information, bills of material/raw material specs, surface texture requirements, weld symbols, local and general notes, linear and angular tolerances, etc. and require engineers and manufacturing personnel to be able to read and understand these requirements. At some point in history I'm sure someone said: "the problem with surface texture symbols is..."

Good luck putting a geometric tolerance on a drawing in note form and making it clearer than the ASME Y14.5 standard.

From my experience, the vast majority of mechanical drawings cannot be fully or properly defined without GDT. That does not mean that every dimension has to be controlled with GDT - but many do. And if someone is making mechanical drawings and is not using GDT, then the vast majority of their drawings are under-defined.

My stance is that I will not make an incomplete drawing just because someone may not completely understand it.

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

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.

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

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
www.d3w-engineering.com
 

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

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
Lorono's SolidWorks Resources & SolidWorks Legion
Follow me on Twitter

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

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." - Robert Hunter
 

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

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
www.d3w-engineering.com
 

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

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.
www.qmsi.ca

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

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

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

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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What is Engineering anyway: FAQ1088-1484: In layman terms, what is "engineering"?

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