The Politics Of GD&T
The Politics Of GD&T
(OP)
I am starting this thread to remove this discussion from kasabis's "TP & FCF Basics".
dingy2: I do not know if you are or are not the person I am referencing. I really did not mean to single you out, I disagree with the whole politics of "make it easier for them and they will come".
The old system does not work. The new will not be followed until that fact is generally understood. The 2009 standard finally says it in the forward: " ...stronger admonition than in the past that the fully defined drawing should be dimensioned using GD&T with limit dimensioning reserved primarily for the size dimensions for features of size".
Anyone who understands zero tolerancing at MMC or LMC should know we are not making the part harder to make. We are trying to provide manufacturing options. The fact the educators have not gotten this across and seem to pussy foot around management is what keeps us in the current state. It may not be popular, it may not be what management wants to here, sorry. I had hoped here I was speaking mostly to true believers and people who want to learn about GD&T not how to avoid it.
"Do not use perpendicularity on small land pilots", (because we can't check it and it probabally made it anyway?) why shouldn't I say it then, I don't care if they check it or not, we do not reall check every point on flatness either guys (and girls, I hope?). "Don't specify finishes on 125 on greater" (because we will get it anyway?) How about in the future when almost everything is rapid manufactured except for fine machining. I think it is our job to state known requirements on the drawing so people don't need to guess.
I believe, other than the validity of the design itself, it's proper documentation is engineering's second most important function, I mean no offence to anyone in particular but I do want to battle this whole culture of lets avoid GD&T.
dingy2: I do not know if you are or are not the person I am referencing. I really did not mean to single you out, I disagree with the whole politics of "make it easier for them and they will come".
The old system does not work. The new will not be followed until that fact is generally understood. The 2009 standard finally says it in the forward: " ...stronger admonition than in the past that the fully defined drawing should be dimensioned using GD&T with limit dimensioning reserved primarily for the size dimensions for features of size".
Anyone who understands zero tolerancing at MMC or LMC should know we are not making the part harder to make. We are trying to provide manufacturing options. The fact the educators have not gotten this across and seem to pussy foot around management is what keeps us in the current state. It may not be popular, it may not be what management wants to here, sorry. I had hoped here I was speaking mostly to true believers and people who want to learn about GD&T not how to avoid it.
"Do not use perpendicularity on small land pilots", (because we can't check it and it probabally made it anyway?) why shouldn't I say it then, I don't care if they check it or not, we do not reall check every point on flatness either guys (and girls, I hope?). "Don't specify finishes on 125 on greater" (because we will get it anyway?) How about in the future when almost everything is rapid manufactured except for fine machining. I think it is our job to state known requirements on the drawing so people don't need to guess.
I believe, other than the validity of the design itself, it's proper documentation is engineering's second most important function, I mean no offence to anyone in particular but I do want to battle this whole culture of lets avoid GD&T.





RE: The Politics Of GD&T
thread1103-192933: GD&T part costing more thread1103-152475: Fully Employing Positional Tolerancing Over Rectangular Tolerancing?.. thread1103-160822: GD&T and Shipbuilding
What is Engineering anyway: FAQ1088-1484: In layman terms, what is "engineering"?
RE: The Politics Of GD&T
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.
www.qmsi.ca
RE: The Politics Of GD&T
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...
- You applied dimensions and tolerances to every feature of the drawing.
- You specified and followed correctly a drawing standard such that we can all be in agreement on what your drawing means.
- Your tolerances are achievable by manufacturing.
- Your part can be inspected to verify that it meets tolerances.
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.
RE: The Politics Of GD&T
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.
RE: The Politics Of GD&T
What is Engineering anyway: FAQ1088-1484: In layman terms, what is "engineering"?
RE: The Politics Of GD&T
RE: The Politics Of GD&T
John-Paul Belanger
Certified Sr. GD&T Professional
http://www.gdtseminars.com
RE: The Politics Of GD&T
What is Engineering anyway: FAQ1088-1484: In layman terms, what is "engineering"?
RE: The Politics Of GD&T
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.
RE: The Politics Of GD&T
What is Engineering anyway: FAQ1088-1484: In layman terms, what is "engineering"?
RE: The Politics Of GD&T
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
RE: The Politics Of GD&T
Yes you are correct about straightness at MMC and not RFS. Thanks.
Dave D.
www.qmsi.ca
RE: The Politics Of GD&T
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.
RE: The Politics Of GD&T
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
RE: The Politics Of GD&T
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.
RE: The Politics Of GD&T
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".
RE: The Politics Of GD&T
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.
www.qmsi.ca
RE: The Politics Of GD&T
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?
RE: The Politics Of GD&T
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.
RE: The Politics Of GD&T
Circular or total runout should only be applied on small diametrical features relative to a centre line created by another cylindrical feature. If one cannot place the part in a divider head (chuck), then it is too big for either runout. It is not conducive to confirm runouts using a CMM since they can only estimate the runout rather than give a concrete number.
Many years ago, I was a Quality Consultant in high precision screw machine company producing tiny cylindrical parts for the automotive companies. We had tolerances in microns and the only way we could control the off centre and roundness of a feature was with either circular or total runout. Positional was not finite enough to discern movement while runout 100% contacts the cylindrical surface while it is being rotated.
If the product get a bit larger with corresponding increase in tolerances, then positional could be used and one would have a choice of circular, total runout and positional. I would never use concentricty.
Runouts take skill to confirm while positional at MMC is confirmed by the use of a checking fixture on the shop floor. I have an example of total runout at www.qmsi.ca/runout.html with checking method.
Dave D.
www.qmsi.ca
RE: The Politics Of GD&T
Back in the day, I used to subscribe to the "give as much tolerance as possible, while maintaining function" philosophy. You know, the ivory tower ideal. I would advocate using any Y14.5 tools, no matter how arcane or obscure, in order to open up tolerances. 3 or 4 FCF's for one feature? No problem. If the people using the drawing are intimidated or are not able to understand it, then too bad for them. It's all in the book, look it up. That was my attitude.
Here's an example. There was a feature that had a Total Runout of .005 to datum A. I determined that the function only required Circularity within .005 and Cylindricity within .010. Position at MMC within .020 was adequate for the coaxiality requirement. Way more tolerance for manufacturing! I'm a hero, right? Of course when I suggested that, everyone went ape. "Oh, the shop will never understand that! They wouldn't be able to just use a dial anymore, we'd have to get an expensive roundness tester! This is going to drive the part cost through the roof! Let's just stick with the Total Runout of .005, thank you". Sigh.
This type of scenario repeated itself so many times that I eventually began to soften my stance. I realized that, in (too) many cases, the costs of the extra drawing complexity outweigh the benefits of the extra tolerance. Sad but true. I have found that it is rare that one can get away with using the most sophisticated Y14.5 tools without severe consequences. Particularly in certain industries.
These days, I try to use the function-based dimensioning and tolerancing as a starting point, as one of several possible options. Then simplify the characteristics and tighten the tolerances (the two always go together) until the result is palatable in the client's situation. Occasionally, the client is fully comfortable with the function-based callouts. Sometimes it gets simplified all the way to doing away with GD&T altogether and just specifying very tight plus/minus tolerances instead (when one of the constraints is that the inspection guy has to be able to verify everything with calipers).
Evan Janeshewski
Axymetrix Quality Engineering Inc.
www.axymetrix.ca
RE: The Politics Of GD&T
that coordinated every tool required or associated with that Major, & Sub Assy's. that way it was assured it would fit together. GD&T is now that Plaster Master.
The problem is the left hand not talking to the right hand.
some one must make sure that all designs are coordinated.
I belive most Engineers & Designer Drafters do a good job.
the checkers must know what he or she is checking for!!
As a manufacture I like GD&T tolerancing. I also like to know the fit, form, & function. also Is it a very critical componet or not? So that more steps can be taken to insure
no mistakes are made.
RE: The Politics Of GD&T
RE: The Politics Of GD&T
Checker, what's that?
What is Engineering anyway: FAQ1088-1484: In layman terms, what is "engineering"?
RE: The Politics Of GD&T
I don't disagree that our perspective gives us different approach to GD&T. My perspective is this, ever since I became the GD&T, guy quite a few years ago, 95% of my time is geting called to the shop or inspection because the .001 runout, or whatever it is, has not been made. So I see the opposite side, some one cut corners, the part didn't make it and now it needs sorting out, generally the part will work, which really angers managment.
I am realistic, otherwise I would not be a supporter of ISO's general tolerancing principle, from my perspective we just cut it too short is all. If your tolerances are such that 99% (you put in your number) pass that is probabally good enough.
RE: The Politics Of GD&T
When I produce a drawing of a sheet metal component or a weldment, I take much more time thinking through the drawing than I do if the part is machined. Your runout case is a perfect example of a waste of time in a machine shop, where the original .005" is easily achieved. It would take more time and effort for the machinist to take advantage of your .020" positional tolerance, which is the exact opposite of what you wanted.
When you start trying to locate things within Ø.005" on a sheet metal part or a weldment, you have to understand your requirements, and the fabrication process. Everyone needs to understand the GD&T you are going to use to make everything fabricatable.
RE: The Politics Of GD&T
Now in non-aerospace, GD&T is rarely used. Interesting how many I come across either don't understand, or don't want to, or simply refuse to use it.
I'm slowly forgetting a lot of it because I have not used much of it for the past 4 years.
Chris
SolidWorks 09 SP4.1
ctopher's home
SolidWorks Legion
RE: The Politics Of GD&T
It was manditiory? wow, was it a big company? We are a relitivly small family owned company now part of a huge corporation, so the thinking is very small. I agree that some areas might not be well served by GD&T (yet??), say sheet metal. In my area the drawingss are asuming way too much.
RE: The Politics Of GD&T
One company I worked at was medium size, the other was a major large aerospace company.
Chris
SolidWorks 09 SP4.1
ctopher's home
SolidWorks Legion
RE: The Politics Of GD&T
What is Engineering anyway: FAQ1088-1484: In layman terms, what is "engineering"?
RE: The Politics Of GD&T
Sorry, I know the history, I have a copy of MIL-STD-8. I don't mean use it, I mean believe in it, not because someone dictates. I would not say we use it correctly and we are defiantly in the defense business. We use it at about MIL-8 mentality too. Most of the people around me would prefer it say that way.
I have always worked at places where they had in house manufacturing, I'll bet that is the problem, "we don't need it on the drawing we can just talk to them".
RE: The Politics Of GD&T
We had this battle, though not specific to GD&T, with our manufacturing people back in the UK defence company I used to work for all the time. The drawings have to be to a standard than any competant supplier can build to them. Just because we designed a certain product, and up to now had always won the manufacturing contracts, didn't mean it would never go to someone else.
What is Engineering anyway: FAQ1088-1484: In layman terms, what is "engineering"?
RE: The Politics Of GD&T
I want to thank you guys for the insights I have gotten in the forums the past few months I have been here. I will be on at home, so sorry, you won't really lose me. I am even taking my copy of 2009 home just in case, but, it is the season we think about these things.
RE: The Politics Of GD&T
RE: The Politics Of GD&T
I have worked in all spectrums of these industries from Military\Aerospace in large companies to "ma and pa" job shops. Here is what I have found to be the pros and cons of GD&T amongst them.
In Military and aerospace it is mostly a liability issue. The drawing must contain all design intent, and leave no gray area, and still be producible. I have sat in conference rooms with high end Engineers of major aerospace companies that would review drawings one dimension at a time. I had a tolerance on one part that was +/- .0005. The question was asked where did that come from? Would +/-.0007 be acceptable. Every dimension, and every tolerance had to be justified.
For the smaller shops it comes down to ecenomics. There is no way they want the time to a do a drawing outweigh the time to make the part. The ecenomics are also prohibitive in the fact they do not have the tools to measure many of the GD&T requirements. That is were the "make it as perfect as possible" comes in. They simply cannot measure with the tools they have. Many have the machinist checking their own parts, or do a fit test with something supplied by the customer. Management of these companies also do not want or cannot afford to invest in the training. They have high turn around, and many of the machinist/fabricators/welders know how to run a machine, but not read GD&T.
I am sure there are exceptions to the above but in general I believe the use comes down to liability verses ecenomics.
Just my 2 cents.
Marshall
Marshell Design
http://sites.google.com/site/marshelldesign/
RE: The Politics Of GD&T
If you are required to produce drawings of Level_III quality, you need to work to a standard. This can be ASME Y14.5. This can be the ISO standard. This be the Billy Bob and Cousin Elmo standard, if it is documented and available. GD&T is only relevant because ASME Y14.5 recommends it. ASME Y14.5M allows you to interpret non-GD&T drawings.
The whole point of Level_III drawings is that the military or government or other customer can hand those drawings over to their favorite manufacturing facility and expect to get working product.
If you are a manufacturer producing proprietary product, you can sub-contract to a small group of fabricators who understand your stuff, or you can manufacture in-house. As I noted above, you can produce horrible drawings, and your fabricator can cope with it. As noted in Joe Osborn's article Tips on Designing Cost Effective Machined Parts, your fabricator is going to try to deliver good looking, working parts, no matter how bad the drawings are. This allows your crappy drawings to produce working parts, but note Joe Osborn's remarks about bad drawings.
The other advantage of ASME Y14.5 is that you can find designers, drafters, checkers and fabricators who have been trained in it. If it takes a couple of years to master your in-house standard, you cannot quickly replace or augment employees and fabricators. Note how none of our Eng-Tips GD&T experts claim to have mastered GD&T quickly. Don't re-invent the wheel unless you have to.
RE: The Politics Of GD&T
I agree with you completely that all drawings SHOULD follow some standard, be it military, commercial, or internal. I was refering mostly to the use of GD&T symbology, and the implentation of its use into any of those standards, and why many companies do not use them.
My post was mostly an observation, and not an opinion. If you were to ask my opinion, I would push companies that are small, and do not use it, to start the training, and start implementing it. My reasoning would be that the goal of most small companies is to become large, and it is much easier to make the jump now, then to wait until you are required by customers to use it, and not be prepared as you grow your customer base.
Marshall
Marshell Design
http://sites.google.com/site/marshelldesign/
RE: The Politics Of GD&T
RE: The Politics Of GD&T
A good designer will know how to define the part so that it can be understood by the fabricator and using a common standard makes this much easier to accomplish. Similar to the way we are using english to communicate on these fora as opposed to multiple languages.
"Good to know you got shoes to wear when you find the floor." - Robert Hunter
RE: The Politics Of GD&T
Evan Janeshewski
Axymetrix Quality Engineering Inc.
www.axymetrix.ca
RE: The Politics Of GD&T
Usually the optimum is somewhere between the 2 extremes. Sometimes parts get redesigned to better suit manufacturing capabilities etc. I'm all for making sure your required tolerances are realistic and within process capability etc.
However, if you don't worry about function first isn't there a worry you'll make useless parts?
What is Engineering anyway: FAQ1088-1484: In layman terms, what is "engineering"?
RE: The Politics Of GD&T
I will vote for you as dictator.
Well said, a star for you.
Standing
SolidWorks Pro 2009 x64, SP3.0, PDMWorks Workgroup, SolidWorks BOM,
HP xw8600, 64-bit Windows Vista Business, Service Pack 1
Intel Xeon CPU, 3.00 GHz, 16 GB RAM, Virtual memory 166682 MB, nVidia Quadro FX 4600
RE: The Politics Of GD&T
Theoretically, production should be able to take my drawings and send them out to any shop that suits their fancy, be it machine shop, sheet metal shop, foundry, blacksmith or anything else you can think of. The reality is that when I complete my drawings, I know how the part is going to be fabricated. I do not know how this works for you, but I would be very surprised if there was much flexibility.
If my requirement is to place a bend in my part to ±.002", a sheet metal shop cannot do it. The part will have to be machined. The bend radii will be irrelevant. The material will have to change from Aluminium_5052-H32 to Aluminium_6061-T6. Now, I can change some of the wall thicknesses.
The overwhelming probability is that you know what the fabrication procedure is. Your tolerances have to be feasible for this.
There is a very minor issue of you telling a machine shop to drill a 10mm hole and get the diameter to, say, within ±2mm, somehow. You can seriously clutter up a machining drawing in attempt to provide maximum allowance. It will be a complete waste of time.
RE: The Politics Of GD&T
What is Engineering anyway: FAQ1088-1484: In layman terms, what is "engineering"?
RE: The Politics Of GD&T
This is a statement that I have heard, used, and been perplexed about for years.
On one hand you dont really care if they carve the metal from a billet, using a toothpick and butterknife as long as it meets the drawing. When we spec a thread, we dont care if its roll tapped or tap and drilled.
On the other hand, you do know how they are going to build it. If we have a 4'x4'x 1/2" thick plate with a 1/2" high boss in the middle, do we show it that way or do we show a sheared plate, with the boss welded on? We knew how it was to be made. If later its decided machining would be better, we would make a product or cost improvement change. If we went a step further, why even spec a specific material, why not just the desired range of properties it has to meet.
When we open our cadd program, we have to decided... hmmm, assembly, part, weldment or sheet metal. We have an idea of what production will do.
Though ASME Y14.5 does not want you to specify specific processes, such as tap drill size, etc. They do supply tools that apply more to one process than another. If you are machining you would specify primary, secondary and tertiary planes to help the machinist. The same part, if molded or cast may contain datum points. So in a sense the process has been defined.
Thank you
Marshall
Marshell Design
http://sites.google.com/site/marshelldesign/
RE: The Politics Of GD&T
I'm not sure you are actually contradicting the sentiment of the statement you question.
Matt Lorono
Lorono's SolidWorks Resources & SolidWorks Legion
RE: The Politics Of GD&T
RE: The Politics Of GD&T
"The drawing should be a contract for inspection, not a recipe for manufacture"
Unfortunately, in many companies, the operation would grind to a halt if designers actually followed this.
Evan Janeshewski
Axymetrix Quality Engineering Inc.
www.axymetrix.ca
RE: The Politics Of GD&T
There are shades of gray/grey. For very high volume stuff or difficult to manufacture parts or some other situations, it can really pay dividends to go through the manufacturing process and taking great care to be sure to make the drawing suit it. However even then you want to avoid tying down anything to a specific process unnecessarily incase someone else has a bright idea on how to improve run rates etc. You don't want to be changing drawings every time someone on the shop floor or in manufacturing has a bright idea, do you?
For a relatively low volume, not too complex part, you can probably waste more money spending time fully optimizing the part for production etc. than you save.
It's not that you pay no attention to how a part is made as a designer, it's that you want to give the manufacturers the maximum freedom to manufacture it however best suits them while meeting your functional requirements.
Axyms quote reflects the prevalent attitude at least in the US & UK from what I've seen. Then again, maybe there's a link to the perceived decline of manufacturing in those countries
What is Engineering anyway: FAQ1088-1484: In layman terms, what is "engineering"?
RE: The Politics Of GD&T
I agree with this. Manufacturing procedures should not be described on the drawing. The fact remains that at completion of design, you know how your part is going to be fabricated. Your tolerances should reflect this.
Example:
You have just designed the new super-duper handy-dandy widget. You have designed the chassis as a part CNC machined from billet.
You make one and then a couple more, and everyone is happy with it. Your customer says "Cool! Now make another 10,000."
Now, you want to send the chassis out for investment casting. You have options.
- Send your current drawings out as is. The foundry figures out the tooling and subsequent machining procedures needed to reproduce the machined part.
- Go back and review your design in context of a casting. Casting is less accurate than machining, and produces an inferior surface finish. Casting quickly and easily produces weird, complex shapes, allowing you to integrate components (DFMA), and make the thing look cool. If you can eliminate all the accurate tolerances, you eliminate the machining step and you save serious money.
None of this affects the need for GD&T or standards compliance. It does affect the organization of your drawing package, and the configuration and tolerances on your drawings.RE: The Politics Of GD&T
Your story involves designing a part with a certain fabrication process in mind, and basing the tolerances on what that process is capable of.
The intent of the statements in Y14.5, and (what I believe to be) the intent of the quote I gave, is the opposite of that.
The idea is that at the completion of design, you know what tolerances are required for the function of the part. The process chosen to fabricate the part should reflect this.
Evan Janeshewski
Axymetrix Quality Engineering Inc.
www.axymetrix.ca
RE: The Politics Of GD&T
It is probably not possible replace a well designed, functioning machined part with an economical casting. You will have to redesign your product to work around the limitations of the technology, and to take advantage of it.
Example:
In a message above, someone mentioned a boss that might be machined in place, or added by welding. Machining the boss in place means paying for a large, expensive billet, and machining away most of the material. Welding the boss in place is not completely simple with mild steel, and quite a bit more complicated with aluminium. Casting the boss in place is dead easy, as is hollowing it out from the opposite side. You no longer pay for the extra part, or for the fasteners, or for the assembly procedure.
There is no problem drilling and tapping holes in a machine shop so that something can be assembled using small machine screws. On a casting, I would look for a way to lose this process. A solution would be fewer, larger fasteners, with some sort of retained nut.
If we go back and redesign the product around a welded sheet metal chassis, again we have a series of challenges and opportunities that result in re-design.
RE: The Politics Of GD&T
The engineering print should be a complete part specification; material, finish, feature sizes and tolerances, without having to reference another document to determine a nominal size or tolerance (my soapbox item) such as referencing ISO 2768.
The argument that GD&T isn't needed because a manufacturing process is in place to make sure you get good (functional) parts regardless of the drawing is biting a lot of nay-sayers in the butt these days as their companies move further towards outsourcing (domestically or offshore) and the quality "miraculously" drops. If they had a product specification which was complete and accurate for the part functionality then tribal knowledge would not be a crippling factor.
That a startling number of companies pay only lip service to GD&T is not the fault of trainers, it is due to what I call "management by magazine" mentality. Ever come across a manager who reads an article & decides to implement the changes he just read about ... of course his people are brighter & faster & more professional and dedicated, so they can do it in 1/10th the time and 1/100th the cost ... ? That's management by magazine. I enjoy having engineering managers sitting in the class, and putting them on the spot just like everyone else in class. If they're good leaders, their people already know that they are not gods and they relax and enjoy the class. If they're bureaucratic managers, they at least get a good view of the efforts being made by their people and how much there is to absorb and process.
On the item of issuing only what a shop can handle as far as drawing content is concerned, I offer this thought ... who here would like to write using Egyptian hyroglyphics instead of the modern English language? Things evolve as a fact of nature, or else they become extinct. Manufacturing businesses, shops, engineers (designers & draftspeople too), machinists and inspectors that can't or won't evolve will drive their own extinction.
Btw, interesting debate. Tks for restarting it!
Jim Sykes, P.Eng, GDTP-S
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