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ISO GD&T vs. ASME GD&T 8

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pmarc

pmarc

Mechanical
Sep 2, 2008
3,227
What do you think about discussing more in details about differences between ISO and ASME GD&T? What are the most important discrepancies between the two in your opinion?
 
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Guys,

This thread was created to COMPARE ISO with ANSI/ASME.
Did anyone notice that there is no general tolerance ASME standard AT ALL?
So, there in nothing to compare. There is a "bad" standard vs. no standard.

"The outcome being that the drawing no longer defines what is an acceptable part"
No, drawings don't define it, engineers do. If due to laziness or lack of professionalism someone forgets to attach tolerance and/or geometry requirement to critical feature, 2768 will come back and bite him in the leg. I have no problem with that.

Sorry for all the bitterness, it's a bad case of Mondays.

And on the lighter note:
"How 'bout the fact that the ASME standard is one volume with a cost of US$170, but for ISO you have to buy scads of separate standards where the total cost gets into several hundred dollars!"
Have you ever seen commecial for Stella Artois beer? "Perfection has its price"

For years on this forum ISO criticism was limited to 2768 and price of Y14 book. How 'bout something new for a change?

ISO 1101 declares Independence principle by default. It's a small standard - about 7 pages. 4 of them are dedicated to explaning Envelope principle. How much of Y14 is dedicated to explaining Independence? Could that be the reason ANSI/ASME guys fail to understand ISO guys more often than the other way around?
 
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I don't think that the amount of pages dedicated for explaining Envelope or Independency rules has anything to do with their understanding. This logic can rather lead to an opposite conclusion that ISO guys should have serious difficulties in understanding of most of GD&T concepts because majority of ISO standards are really short and showing general cases only.
 
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I am talking about open-minded approach here.

ISO allows you to invoke Y14.5 on your drawing and be compliant still. Is thre any place in ASME that allows use of "foreign" standard?

And speaking about difficulties of understanding: Apparently you don't fully understand Y14.5 unless you buy Y14.5.1 on top of $170 yo've already spent. How's that for double-speak?
 
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Why would somebody like to invoke ISO or any other "foreign" GD&T standards on ASME prints? I do not see a reason. If you are thinking specifically about invoking independency rule into ASME drawings, I do not think there is something incorrect in placing a general note like: "UNLESS OTHERWISE SPECIFIED PERFECT FORM AT MMC NOT REQUIRED". This would be a switch between default tolerancing principles, wouldn't it?
 
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Or of course, use the new "I" modifier.

John-Paul Belanger
Certified Sr. GD&T Professional
Geometric Learning Systems
 
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So, first off fsincox, as I've already said I have a copy of 2768 and back when I had my first run in with it spent a bunch of time studying it thank you very much. As an idea of a standard to suggest what are realistic manufacturing tolerances or similar it arguably has some merit. However, as a complete and comprehensive solution to the never ending problem of most designer not being willing/able to spend enough time & effort to properly tolerance drawings/do so most efficiently I'd say it's a miss.

Say whatever you want about checkers, our place 'saw the light' a couple of years ago and I no longer get that miserable task, except when someones drawings are so bad that even management can't divert the blame. Now I even get to do some project management so I could play the other side of the fence if I really wanted.

I believe there was an ASME equivalent to 2768, which was indeed a din originally. I think it's been brought up before, or maybe I'm imagining it.

I've already said that using 2768 incorrectly is comparable to using the typical inch block tols incorrectly. Though, one might incorrectly think they were doing a better job simply by referencing a 'standard' than relying on some fairly arbitrary numbers on a drawing template or similar.

The first few years of my career were in the UK to iso standards, so any implications of nationalism are questionable.

Finally, the implications that using 2768 is analogous to the whole 'critical dimension' debacle is scary stuff - enough to make me like it even less.

Posting guidelines faq731-376 (probably not aimed specifically at you)
What is Engineering anyway: faq1088-1484​
 
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Ken, Frank,

"complete and comprehensive solution to the never ending problem of most designer not being willing/able to spend enough time & effort to properly tolerance drawings" is to fire the bastards and replace them with knowledgeable skilled professionals.

Bad drawings cannot be improved by magically applying some "fairy dust" to them, be it genius-written Standard to invoke, or Tolerance sticker made by Elves.

At the end of the day everything is made by imperfect people. And guess what? I spent number of years in a machine shop. We were producing new part every day (or night), so I had literally hundreds of prints going thru my hands. In all those years one single drawing made by some government sub-sub-contractor would qualify as "standard-compliant". The rest was Creative drafting, or complete fubar which is fashionable word on the forum today. There was never one unsatisfied customer. Period.

The truth is: One can make a living by producing good parts from the bad prints, and at the end of the day its all that counts.
Don't try to save the world by application of general tolerances.

I am old and I am tired. Next time let me offer Ken my help - I will be holding people that he will be punching.
:)


 
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Ken,
You should know the above statement is not correct as written in the ISO standard. It is an exaggeration for dynamic effect.
You are quite correct: ANSI B4.3 General Tolerances for Metric Dimensioned Produts.
The German reference was not in reference to you and your heritage, but from the fact that, we both know, this standard is basically derived from the DIN (German standards) version.

Jim,
The problems you see in the general application of the ISO standard are in my opinion related to the same problems I see with the ASME standard. While ASME does not cover general tolerances like ISO in the same way, they do try to cover the meaning of how to interpret implied general tolerances. We seem to see a progression of statements in the standard over the years to less certainty of intent of the old implied tolerance dimensions. They basically are recommending using the "new GD&T" system for better coverage in stronger and stronger language but refusing to pull out the rug completely. So I think the real problem is they are retroactively trying to give a clear definition to what is an already an acknowledged imperfect system.
Frank
 
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Interesting, Frank. There are many conversations about withdrawing some older standards, or significantly overhauling them to remove some historically less-useful bits. There tends to be an erring on the side of caution by generally diluting things and redirecting. C'est la vie. There is gross imperfection in both systems and it really comes down to which one you were initially trained in for most people.

Jim Sykes, P.Eng, GDTP-S
Profile Services TecEase, Inc.
 
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Jim,
I agree, again we are really working in an imperfect world. I am not suprised at all about the caution, either. I believe it would only cause more resistance to adopting the standards than there already is currently. We still live in 1982.
Frank
 
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No Jim,

It will be fools like me who think latest and greatest stuff will get them somewhere.
 
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CH,
I am all for keeping up, myself, I have been pushing for it. In my years of experiance change does not come easy to bureaucracies. I would venture to say it even scares them.
Are you the boss?
Frank
 
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I am the guy lucky enough to have the boss who is listening.

Given current state of economy no-one will retire at 65, so there is metric ton of time ahead.

:)


 
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So I've been thinking about this issue a bit, especially as regards a drawing defining an acceptable part (based on function) per ASME, and Iso 2768 para 6/7 about function not the drawing defining it.

Maybe part of my concern is that much of early GD&T as I understand it came out of the need/desire to ensure interchangeability, any part 'A' would assemble to any part 'B' and this is still a major factor to me today. This is still what ASME with it's emphasis on worst case & drawing defining an acceptable part seems to lean towards.

The thing with not relying on the functional needs as summarized in an ASME drawing is what happens if it's a spare part for instance. If function is defined as being able to be assembled, well because you aren't assembling it straight away how can you tell if it's a good part?

If function is based on some strength requirement, then you potentially end up having to run some calculations, doing FEA, or maybe even proof tests... Now I'm sure many of us have been involved with approving deviations/waivers/concessions etc. which required some of the above, however the iso 2768 para 6/7 forces you to do it as I see it.

Of course, then there's the statistcal tolerancing approach which is already a deviation from true interchangeability but widely practiced and accepted.

Just some incoherent musings, sorry.

Posting guidelines faq731-376 (probably not aimed specifically at you)
What is Engineering anyway: faq1088-1484​
 
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The simultaneous requirement is not treated the same, either.
Frank
 
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Kenat, you just have to let it go.
All the parts have Fit, Form and Function requirements, but every feature is "biased" towards one of them.
If you are in America you probably have a car. Think of the wheel.
The interchangeability is achieved thru fit and form requirement.
Wheel has mounting holes that have to "fit" over the studs (by diameter), there is a position requirement for wheel to fit on certain model hub. The cone countersinks on the holes probably controlled by really crazy GD&T (everybody hates cones, right?)
And then there are vent holes with the sole "function" to let the air to the brake rotors. There is no fitting part.
The dreadful clause only applies to vent holes. In fact, there are many wheels that look different, but are actually interchangeable.
Disclaimer: not a wheel designer. Things are probably more complicated (balance, etc.) Just trying to make a distinction between things that have and don't have to physically "fit", but both have a "function".
 
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Kenat said:
...and Iso 2768 para 6/7 about function not the drawing defining it
Click to expand...

I believe that is incorrect. The drawing per 1011, 5459, & 8015 defines an acceptable part; not the "tolerance unless otherwise specified." I think you are taking that part of 2768 way out of context.

Joe
SW Premium 2011 SP3.0
Dell T3500 Xeon W3505 2.4Ghz
6.0GB Win7 Pro x64
ATI FirePro V5800
 
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CH,
Not everygody hates cones ... they are actually a specialty of mine due to my mold-design origins. I actually had one instructor tell me that it was impossible to use cones as datum features and that we would have to redesign our molds to use flats & cylinders ... don't think so. Cones can be a challenge for those that don't know/like/understand profile of a surface, but they're easy once you go that way.

Jim Sykes, P.Eng, GDTP-S
Profile Services TecEase, Inc.
 
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