The Politics Of GD&T 2

This, or variations come up a lot on here:

thread1103-192933 thread1103-152475 thread1103-160822

Yes, I am the person you have referenced unless there are other people on this forum who have both levels as a GD&T Professional although I have never reflected that in this forum. I also do not wink and nod at management as you stated.

I do come from the Quality/Manufacturing background and have seen some ugly drawings that absolutely do not make any sense - maybe dreamed in heaven. Most drawings seemed to have the GD&T copied from other drawings.

I have also been involved in Control Plans where a team would decide the importance of characteristics (features) and develop manufacturing and confirmation methods depending upon the feature's functional importance. Should one use a checking fixture and how often? Does the profile of a surface determine a functional feature? If it does, should we scan the surface on CMM - big $$ now. How often?

I will always remember when I was a Quality Manager in an automotive supply company many, many years ago and received a drawing without any GD&T. I didn't know the functional importance of part or its features so I phoned the Designer in the Detroit area and asked. He told me that every dimension was important (everything) and would not tell me how it functioned or how the part fits. The personnel in their Quality group were able to give assistance so that the developed Control Plan was practical. Is that the approach we should take in the future - everything is important?

Should all departments work together today or should be go back to the concept where each department would build walls and disregard other departments. That's is their problem if they don't understand the drawing.

You did stated correctly out of foreword of the 2009 edition. It also stated that we must "design more precisely state the functional requirement" on page vi.

Do you realize that if we use any orientation symbol alone, the feature must be shown in a +/-? See figure 6-18 on page 107. The seems to contradict the quoted statement.

Do you realize that we do not have to place straightness in RFS on any feature of size since rule #1 covers it? So, if there is a tube and it must fit inside a hole, let rule #1 cover the form. Too bad if they don't understand it on the shop floor. That's their problem. The Designer's butt is covered as long as the drawing states "complies with ASME Y14.5 - 2009". Does that help anyone?

There are a lot of areas that are contradictory, ambiguous and vague in the ASME Y14.5 standard. I would never suggest that "his whole culture of lets avoid GD&T" but let's use GD&T to reflect the function and relationship of the features and if there is a easier way to apply GD&T so it would be well understood, let's do it.

Dave D.

fsincox,

If I ever get elected dictator and I get to specify what a good drawing looks like, I will require it to be inspectable. This means...

[ol]
[li]You applied dimensions and tolerances to every feature of the drawing.[/li]
[li]You specified and followed correctly a drawing standard such that we can all be in agreement on what your drawing means.[/li]
[li]Your tolerances are achievable by manufacturing.[/li]
[li]Your part can be inspected to verify that it meets tolerances. [/li]
[/ol]

I think the first two points are self evident.

If your fabricator tells you he cannot meet your tolerances but he will make a best effort, you lose control over your process. You need to fix your design so that the manufacturable tolerances work.

You must be able to inspect your part. You, and your fabricator, should have access to inspection tools that can measure to your tolerances. You must be able to access critical surfaces for the purposes of measurement. You must be able to jig the part as per your drawing.

JHG

fsincox,

I re-read KENAT's thread about GD&T parts costing more.

With SolidWorks, I can produce most fabrication drawings in a matter of minutes. I drop the part into the drawing. Click. Click. Click. Click. Click. Click. Click. Click. Click. Click. Click. Click. Click. Done! Out to the shop.

The drawing is covered by default tolerances that or may not be achievable by the fabricator. Put yourself in the shoes of a welder who has been handed a drawing of a 6'x4'x2' frame with three decimal inch dimensions, ±.005" tolerances. Regardless, it is obvious the drafter put no thought or concern into it. There is no point in the fabricator worrying about it. Slap something together and ship it.

Someone converts the drawing to a GD&T standard. Do they think about what they are doing? Do they tighten tolerances, or do they open them up?

Again, put yourself in the shoes of the fabricator. The drawing now is covered in FCFs. Now it appears they mean it, and we have to meet all that stuff. The price goes up.

In the two or three years I have been aggressively using zero positions at MMC, I have been questioned by one fabricator, and they accepted my explanation. My clearance hole for an M4 screw usually is Ø4.8/4.2. This is much sloppier than a drilled hole tolerance because it allows for positional error as well as drilling error.

If you have a hole Ø4.6/4.4 positioned to Ø0.2mm, and you change the positional error to zero at MMC, you have made your hole much more accurate. This sort of precision might just be a challenge for a jobbing shop. This might account for some higher prices.

JHG

Hmm drawoh, your lists looks similar to what I used to check for;-).

I'm not entirely sure what this thread is about, but it's very interesting. Please, continue...

FYI -- Rule #1 doesn't cover straightness RFS, but rather MMC.

John-Paul Belanger
Certified Sr. GD&T Professional

Drawoh, in that thread, the guy applying the GD&T to an old drawing knew what he was doing wrt GD&T, so while once in a blue moon he made an error, it was pretty rare. Now whether he fully understood the function was more likely to be a source of error but he did used go to to some lengths to find it out. I thought I'd explained the circumstances in that thread but maybe not well enough.

KENAT,

Maybe the pre-GD&T part was a semi-functional piece of junk! Perhaps the GD&T part saved all sorts of assembly and re-work time, justifying the extra fabrication cost.

What can happen with old, crappy drawings is that the suppler works out a process that keeps the customer satisfied. I have seen things assembled from horribly screwed up drawings. After a few assembly attempts, assembly gets on the phone to the fabricator and some sort of fabrication/assembly procedure is worked out. Since the drawings are not being followed, it is not possible to switch fabricators without a great deal of debugging and telephone time.

Now, you correct the mistakes, fix the tolerances, and apply all sorts of GD&T to the drawing. You have wrecked a nice, comfortable process.

JHG

Drawoh, I thought I'd put in that thread that the situation you detail in your 17 Dec 09 17:55 was my perception of the situation. However, at face value, the GD&T part cost more and gave ammunition to the doubters/haters.

In my experience, many (most?) engineers don't take enough drafting courses in their school or during their careers to know how to detail a parts effectively on drawings or in 3D models. When they rely on drafters and designers to detail parts, they often do not understand GD&T well enough to know if the drafter has captured their design intent.

Additionally, many (most?) drafters/designers do not know GD&T well enough to take an engineer's general instructions and turn that into a drawing that details a part in a way that captures the engineer's design intent using GD&T. The drafter/designers often wing-it. The engineers don't know well enough to correct errors and will often opt to just remove GD&T when it is used.

Many (most?) drafters/designers/engineers are afraid of GD&T. I know I used to be. I know many others are as well.



Matt Lorono
Lorono's SolidWorks Resources & SolidWorks Legion

J-P:

Yes you are correct about straightness at MMC and not RFS. Thanks.

Dave D.

KENAT,
Thank you for those references they do look interesting, I need more time to look them over.
dingy2,
I am strongly opposed to the “GD&T is too hard and regular people need protection from it”, philosophy, it is their job we are talking about. An educator's job is to instruct people how to use something, not too avoid it. Do you really even believe GD&T is necessary? I do. You want every one to work together only if we can work at the lowest common denominator; I say all need to learn it so we all are speaking the same language.
I do not see your issue with fig 6-18 a feature of size (+/- OK’d, even by the caliper guys) with a refinement of tangent plane parallelism it doesn’t even control the feature itself only a tangent plane.
As far as poorly done fabrication drawings, I know your pain. The current American standard seems to +/-.03 or just ignore the tolerances, you get what you get. Is that what you want? I have a sand casting drawing now that causes inspection problems every time we receive new parts. The part envelope is about 30” X 30” X 30”. A few lugs have 3 place dimensions on them, I do not know what they were thinking but it was something we inherited and has been produced for years by the same fabricator. Our inspection people now imply +/-.010 to it, the title block says machining tolerances +/-.010 for three place and +/-.030 for two place. Ignoring these things only works when things are constant and nothing will change, which never really did happen and is less frequent now more than ever.
I am well aware of rule #1 unlike many of the shop people I have worked with. I am not its biggest fan and believe it is an obsolete concept as is the implied decimal tolerance method.
I agree the standard is not perfect only avoiding it is not going to get it fixed.
As for your 4 points I agree except I think you all take #4 too far. It is OK if you do not check every point on a surface for flatness, or location, but when I say I don’t care if they check a small length pilot for squareness to the face it is blaspheme.
People who say we do not need to specify surface finishes on 125 or greater are basing this on the assumption that the process will produce a finish good enough, why can’t we turn it around, if the finish says 125 then you can process it this way. You are implying you know the process will product it anyway why not have it stated formally.
I certainly wanted to draw people out to debate these issues but my intent was never to single out one person, I believe the majority are with you.
Finally the nod and wink refer to telling managements what they want to hear, which the statement does. GD&T should be avoided; it is not needed to go that far. I saw the need for the new concepts now given years ago (to align to a datum framework but offset the zero origin. The fact that they don’t understand it does not change that it was needed obviously, some one else must have though so, too. Now there is a way to actually say it. I see that as a good thing not something to be afraid of.
Fcsuper, what you say is true that is why it is all the more important we state the fact that it IS NEEDED.

I would say the opposite about rule #1, in that it is even more necessary now that it was when it was first penned near the turn of the 20th Century. With litigation where it is at in the US, I doubt removing an absolute like Rule #1 would be helpful to the industry at all. I suspect that ISO will eventually incorporate it more often as lawsuits start to become more common in Europe.
If a company feels they don't need Rule #1, they can just apply new (I) symbol to all dims on their drawing in a general "unless otherwise specified note".

Matt Lorono
Lorono's SolidWorks Resources & SolidWorks Legion

fcsuper,
My position is not that zero at MMC/the envelope is a bad, at all. I think it should not be the default/automatic. I really am coming to agree wuth the ISO use is all. I do not know if the ISO will change, and where you get that information. Basically, Just as ANSI had default MMC until 1973 and changed to RFS default. Now, we assume the worst case, no extra tolerance, and have to state if it is allowed. That is just consistant with engineering logic, I believe the actual history speaks for itself.

dingy2,
I hadn't noticed the MMC on surfaces in the new standard until I read your site, so I need to thank you for that. I was too excited by all the other good stuff. I have read a few posts KENAT, supplied in another thread. You seem to be one who is worried about too much GD&T confusing the shop, Isn't the job of educators to overcome exactly that? If you preach: "don't use it", can't you see the irony? It is no different were I work, but will avoiding it make things any better? There is another site that takes about establishing corporate addendums (limits) on the application of the standard, that is the kind of wink and nod I am referring to also. A bunch of uneducated (GD&T wise) managers determining arbitrarily what shall and shall not be allowed; with out looking at specific problems, yea right, that is a new idea.
My last job, in machine tool, at one point I was “the GD&T guy”. The company had outside educators come in and train us all, then, their were teams to work together, shop, manufacturing, inspection, and engineering. I was not chosen for any of this at the out set, but. I loved the new way and quickly I became the go to guy for GD&T because I understood it believed in it, most did not. I was younger than most of the current engineering checkers (sorry, KENAT you are not the most unqualified) and they really resented my intrusion into their area. They started to complain: “it was too hard”, “no on understands it”, “we never needed it before”. Some in the company were believers but they were few and far between. This was the 1982 version and as I have stated before there were issues involving composite tolerances, profile tolerance, etc. that we needed for our parts but were not well covered/spelled out like they are now. I could see were the standard was going but had no proof. The book coverage was very basic at that time. Just like the new datum specification tools in the 2009 standard, I could 've used them too. Your arguments are the same ones they used then, no different. Now composite tolerances and profile, it is just take it for granted here, now, but, then it was on the edge. There are many out there that still don’t understand it , now. A new VP of engineering came in. he heard his experienced checkers complaining and issued a directive on GD&T, I still have it. Thou shat not profile, a bunch of other stuff, basically going back to the GD&T he “understood”, albeit incorrectly, and before we were all trained.
People always resist change, it is human nature, They will never learn it until it becomes part of the job requirement then they will suddenly be interested, some may not, good riddance.
I disagree with the only use it if you are going to check it attitude. From an engineering point of view, I see GD&T as a way of describing how 3D geometry functions on 2D drawings. I believe it is valuable enough for that reason alone, functional gauging and the rest are certainly additional benefits. I want to start another thread on sheet metal parts, there I can see avoiding it, but not because “people don’t know it”.

fsincox:

Each company should have a design review where the application of tolerances and GD&T would be reviewed on a new drawing. That is the time to discuss with other departments the GD&T application. I wonder how many companies perform design reviews?

I agree that many managers do not get GD&T training and don't understand the subject. Having said that, it is a difficult for the said managers to take the training in-plant with their underlings so many just don't do it. As a trainer, I make sure that I never ask a Manager a question that could be embarrassing in front of the other trainees. I would suggest that managers either take the subject online or in a public seminar but they must know GD&T as well as anyone in the departments.

As far as avoiding some geometric symbols, there are only a couple that I might avoid. I would not suggest using concentricity myself but maybe circular runout for low volume production or positional at MMC for high volume.

Profile of a surface is a fundamental symbol and has a great value if used where there is a function or relationship. I do not agree with placing it in notes as a default since this does not reflect its function but it sure is easy for the Designer. I really wish that each Designer who applies GD&T would ask themselves this question. What is the function or relationship of this feature and why did I apply GD&T? If it cannot be answered, then maybe the GD&T application may not be warranted.

I know where you are coming from and I think that we may not be too far off on the topic.



Dave D.

Dave,
Thanks, I am surprised that people support runout as much as they do. In my view it is literally an inspection method not a functional thing, am I wrong about that? Doesn’t runout really hearken back to old inspection techniques, not function? I am so surprised at the “when in doubt use runout” message I get here vs. the shop I work with hates it and we are replacing it with position and concentricity (which is blaspheme here). I just don’t understand the discrepancy. They do use a lot of CMM’s, if that helps.
All our old drawings use runout. We do not make large quantities of parts so the pressure is to not throw away parts. When an inspector tells me a part did not make the runout tolerance the first thing I ask is how much is the cylindricity error and what is the eccentricity. It seems to be obsolete, to me, Sure it is OK to check that way but in the end don’t you want the other information to determine function?
When I was first trained in the 82 standard I was told concentricity and symmetry were going to be obsolete, but on the contrary it seems to have come back in a big way. Now it has been defined so it is clear that position RFS at RFS is not the same thing, one applies to an envelope and the other to the real feature. Do you ever hear anything like this?

PS,
An easy example is runout on a surface perpendicular to an axis, by our simplicity concept why not say perp, right? Does engineering need to tell an inspector it is OK to check this way? Circular runout is a little different and thats the one everyone wants.