A different perspective on ISO 2768 1

[fsincox]

I have seen some comments here concerning ISO 2768 and 8015 and it's disclaimer at the end. Personally, I think they are just being realistic. The standard basically says their intent at general tolerances is to not to specify extra large tolerances manufacturing theoretically doesn't need. Then this final statement is basically saying a part may not be bad if it is in one of these areas, the general toleranced areas, by that measure it may be true. We generally don't put extraordinary large tolerances on our prints either. I only see the good old (2) place +/-.03 and (3) place +/-.010 that is our general tolerance. I think that is lazier engineering. I believe both ANSI and ISO standard are works in progress and not final statements, they evolve.
Do you accept the concept that natural/default tolerances may need to grow as parts/features get bigger? I think it is more realistic after some 30 years of experience. Thanks, I look forward to discussing it with you and learning more about it.

 

[KENAT]

fsincox, over reliance on any default tol, without at least considering function can, or most likely will, lead to problems. This is true whether the ISO 2768; decimal varying block; single value block tolerance or even general surface profile is invoked.

If we decide to accept that we aren't going to bother thinking about tolerance most of the time and will just invoke some kind of default then maybe the ISO is a bit better if used properly. However, if used improperly, which is often the problem with other default tols, I'm not sure it would be any better. Might be a bit better for manufacturing but even then if an inappropriate class is chosen perhaps not.

I'm not sure I follow your argument about ISO 2768 having prevented the problem in your casting. It seems that generally the tolerances weren't properly considered, and if it's a hole pattern isn't this where you say you always use position tol? ISO 2768 also isn’t intended to cover castings, it's for 'removal of metal' (i.e. machining) and sheet metal per the intro. So I’m not sure I follow. I agree +-.030 for hole location would probably be excessive if I understand the size of your item correct, though this would also depend on mating part tols.

As regards some features being more important than others etc, functionally this is often true to a point. However, this criticality should be reflected by the relevant tolerance etc. If this is achieved adequately by using 2768 great. It reminds me of the idea of identifying 'critical dimensions or features' on parts. Implicitly this means others are 'non critical', does this then mean you don't need to worry about the tolerance on them at all, then why have a tolerance? Always seemed a slippery slope to me.

Just for the record, as you asked twice, our tol block for inch is .x +-.03, .xx +-.01 .xxx +-.005. We sometimes customize it for specific parts but I’m hesitant to do this as people tend to get used to the usual values and mistakes get made. Our parts tend to be small but we have some larger ones. Our parts range from simple sheet metal panels or covers, essentially a plate with a few holes, to very small but complex machined parts in exotic materials which more experienced folks than I have struggled to tolerance/dimension adequately.

However I fear we've descended to a point of nit picking each others comments, rather than making clear arguments, if I started it I apologize. I should also make it clear that I don’t think being an “engineer” makes one inherently better at Tolerancing etc, the best checkers/drafters I’ve known didn’t have Bachelors.


KENAT,

Have you reminded yourself of faq731-376 recently, or taken a look at posting policies:

http://eng-tips.com/market.cfm?

What is Engineering anyway: faq1088-1484

 

[fsincox]

"fsincox, over reliance on any default tol, without at least considering function can, or most likely will, lead to problems. This is true whether the ISO 2768; decimal varying block; single value block tolerance or even general surface profile is invoked."
We are in complete agreement.
"It reminds me of the idea of identifying 'critical dimensions or features' on parts. Implicitly this means others are 'non critical', does this then mean you don't need to worry about the tolerance on them at all, then why have a tolerance? Always seemed a slippery slope to me."
I was shocked the first time I saw that and I am in complete agreement with you on the first part, I always thought an educated mfg. engineer shold determine that based on tolerance and his knowledge of the processes. Disagree on the last, A good designer needs an idea of the tolerances to produce a good design, if the process can produce it, fine.
"I'm not sure I follow your argument about ISO 2768 having prevented the problem in your casting. It seems that generally the tolerances weren't properly considered, and if it's a hole pattern isn't this where you say you always use position tol? ISO 2768 also isn't intended to cover castings" DIN/ISO 8062 is the same concept for castings, I thought I mentioned it earlier.
KENAT, I hope we were just clarifing positions. I do not mean to imply anything else.
Can we now get to the dependancy/independancy issue and envelope principle, or no?

 

[KENAT]

I'm not sure we disagree about a designer needing a basic idea of process capability and keeping it in mind during design, didn't mean to imply otherwise and don't think I did.

Missed your ref on 8062, I'm not familiar with that spec though I have seen various casting & moulding design guides where tolerance increases with size, and some for processes/materials where +-.03 would be optimistic on almost any size part.

There have been other threads on differences/advantages of ISO V ASME, I suspect mostly in GD&T forum. You may be better off starting a new thread over in titled something like "GD&T ISO V ASME dimension/tolerancing standards" if you want to expand the scope of the discussion.

KENAT,

Have you reminded yourself of faq731-376 recently, or taken a look at posting policies:

http://eng-tips.com/market.cfm?

What is Engineering anyway: faq1088-1484

 

[fsincox]

Thanks for your time. I don't think you did either. I think I just try to re-enforce a point and it seems like I mean you didn't agree.

 

[fcsuper]

Since ASME is not a member of ISO, these "differences" may eventually be resolved anyway.

Matt Lorono
CAD Engineer/ECN Analyst
Silicon Valley, CA
Lorono's SolidWorks Resources
Co-moderator of Solidworks Yahoo! Group
and Mechnical.Engineering Yahoo! Group

 

[fcsuper]

correction, since ASME is NOW a member of ISO...

Matt Lorono
CAD Engineer/ECN Analyst
Silicon Valley, CA
Lorono's SolidWorks Resources
Co-moderator of Solidworks Yahoo! Group
and Mechnical.Engineering Yahoo! Group

 

[fsincox]

I want to make one thing very clear here. Particularlly for the uninformed who may read this thread. From my reading of the actual standards, of which I do have a copy, the statement everyone seems to fear will lead to the collapse of civilization as we know it, only applies to the general tolerances applied by the standard. I believe it does not apply to all of the other stated tolerances on the drawing. In the context of the statistical control of manufacturing processes it is based on I feel they are just being practical and applaud their admission that a part that works/fits is OK. I say the same thing a few times a week.

 

[KENAT]

"a part that works/fits is OK", maybe but a part 'A' that fits one part 'B' OK but doesn't fit another part 'B' can be a pain. Especially when it comes to spares etc.

The concept of interchangeability was a big push behind the development of current tolerancing standards. If not careful ISO 2768 could lead to degredation of that.

Posting guidelines faq731-376

http://eng-tips.com/market.cfm?

(probably not aimed specifically at you)
What is Engineering anyway: faq1088-1484​

 

[fsincox]

Agreed KENAT,
I would like to get your input on the my GD&T philosophy thread, particularly about the pilot dia.

 

[KENAT]

I started to think about it but it made my brain ache, maybe at lunch time.

Posting guidelines faq731-376

http://eng-tips.com/market.cfm?

(probably not aimed specifically at you)
What is Engineering anyway: faq1088-1484​

 

[fsincox]

KENAT,
I guess I really need to look at real examples. The features I invision this tolerancing method used on are not real mating part features, so I may have misspoke when I said "fit". I know you have had run-ins with other examples and I really haven't had an opportunity to work with it day to day.
I was hoping to draw out more people who do and get their input.