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:
What is Engineering anyway: faq1088-1484
