All tolerances are non-accumulative
All tolerances are non-accumulative
(OP)
I am performing a loop stackup tolerance study on a sheetmetal assembly that has some assembly issues and have encountered an interpretation conflict. The note in the subject line is on every drawing. Before the firestorm of criticism begins, I was a student of Alex Krulikowski's in Michigan so I know what needs to be done with these drawings. That refinement isn't going to happen at this company. My question is, is this note a crude attempt at basic dimensioning? I was instructed, in no uncertain terms, that the hole you examine has +/-.030 for location and is not influenced by the previous hole to hole dimensional location tolerances. One can see the problem with this logic in a stackup if I have to disregard the previous location tolerances. Has anyone dealt with this kind of situation or could recomend a good shrink? Perhaps a source noting how wrong this drawing note is?





RE: All tolerances are non-accumulative
Not sure about a good shrink but I've had vaguely similar issues before with drawings referencing iso2768 thread1103-196260: Tolerance analysis ISO2768 thread1103-197786: tolerance analysis to ISO 8015 & 2768 part 1 (again!).
What is Engineering anyway: FAQ1088-1484: In layman terms, what is "engineering"?
RE: All tolerances are non-accumulative
I would also question using "TYP" and the dimensioning of a 90° angle.
I may have been working in the sticks, but I have never heard of Alex Krulikowski.
"Good to know you got shoes to wear when you find the floor." - Robert Hunter
RE: All tolerances are non-accumulative
There are a couple of other odd things about this drawing. The block tolerance for two-place decimals is +/- .03 and for three-place decimals it's +/- .030. Hopefully these weren't intended to have different meanings. Also, there is an explicit 90 degree angle shown - I haven't seen that in a while.
Evan Janeshewski
Axymetrix Quality Engineering Inc.
www.axymetrix.ca
RE: All tolerances are non-accumulative
The example you show is not an attempt at basic dims. Although basic dims would solve the issues with this drawing. Axym's interpretation seems to be accurate.
Essentially, it seems they are trying to employ a very crude positional tolerancing scheme via the use of linear dims/tols. From this interpretation, and how the drawing is dimensioned, each hole has multiple tolerance zones (at least 9 from a quick count) within which it much fit all. A simple positional tolerance only has one tolerance zone. This attempt to make a simplified drawing (by not using GD&t) actually makes it much more complex.
Bottom line, there is no such thing as "non-accumulative" tolerances in any standard. If this is the method they wish to use, then they should define what this means on the drawing. In fact, there's a lot of defining that will be necessary, as I see no reference to any drafting standards.
Matt Lorono
CAD Engineer/ECN Analyst
Silicon Valley, CA
Lorono's SolidWorks Resources
Co-moderator of Solidworks Yahoo! Group
and Mechnical.Engineering Yahoo! Group
RE: All tolerances are non-accumulative
How are you even to reckon the tolerance? perpendicular to the top face of the part? The bottom? The mid-plane? Exactly where.
If you don't use The Standard, to be clear, you need to adopt another one. Merely putting a note of non-accumulation doesn't do this job.
If your organization wants the leave things fuzzy for the sake of simplicity, then so be it. But fuzzy drawings, are, well, fuzzy. When there's a fit problem, fuzzy drawings are a down payment on a technically unresolvable dispute.
RE: All tolerances are non-accumulative
GeneS.
RE: All tolerances are non-accumulative
What is Engineering anyway: FAQ1088-1484: In layman terms, what is "engineering"?
RE: All tolerances are non-accumulative
It sounds to me like you need to win an argument with someone.
If datums were clearly specified on the drawing, the concept of non-accumulative tolerances might mean something. In the absence of them, I would want to know what feature they were not accumulating from. Tolerance will accumulate from all the other features.
Could you take one drawing and re-do it with proper datums and GD&T FCFs? This gives you a chance to demonstrate how your features are controlled unambiguously.
RE: All tolerances are non-accumulative
Don't expect logic to win the day.
What is Engineering anyway: FAQ1088-1484: In layman terms, what is "engineering"?
RE: All tolerances are non-accumulative
RE: All tolerances are non-accumulative
Many vendors still use the sight of a FCF as an excuse to jack up the price on their quotes. You got to call them on it and let them know that you know what the FCF means and that you know they know as well. :)
Best of luck!!!!
Matt Lorono
CAD Engineer/ECN Analyst
Silicon Valley, CA
Lorono's SolidWorks Resources
Co-moderator of Solidworks Yahoo! Group
and Mechnical.Engineering Yahoo! Group
RE: All tolerances are non-accumulative
Of course, if baseline dimensions were used the 2.750 dimension could end up being 2.810 to 2.790 and still be acceptable.
The "simpliest" solution to me would be to use baseline dimensions and adjust the tolerance to achieve an acceptable amount of spacing for the "2.750" dimenions (i.e. maybe the first hole is located 1.410 +/-.01 from the end, the second hole is 4.160 +/- .010 etc...)
-- MechEng2005
RE: All tolerances are non-accumulative
*smacks forehead*
RE: All tolerances are non-accumulative
"Good to know you got shoes to wear when you find the floor." - Robert Hunter
RE: All tolerances are non-accumulative
Peter Stockhausen
Senior Design Analyst (Checker)
Infotech Aerospace Services
www.infotechpr.net
RE: All tolerances are non-accumulative
What is Engineering anyway: FAQ1088-1484: In layman terms, what is "engineering"?
RE: All tolerances are non-accumulative
As for standing up in a court of law, the company has long since faded from existence, so it is now a moot point.
"Good to know you got shoes to wear when you find the floor." - Robert Hunter
RE: All tolerances are non-accumulative
RE: All tolerances are non-accumulative
What you always hear is "it's just not necessary". When they say that, it means that they don't want to think very hard.
Remember one of the most important fundamental rules of the 14.5 Standard -- "Drawings shall be subject to one interpretation".
It would be nice if such people simply said, "I hate to think very hard. Let's just fight it out after it becomes a problem." At least the virtue of honesty would shine through.
Now that's an argument I can understand (even if I don't agree with it).
RE: All tolerances are non-accumulative
Matt Lorono
CAD Engineer/ECN Analyst
Silicon Valley, CA
Lorono's SolidWorks Resources
Co-moderator of Solidworks Yahoo! Group
and Mechnical.Engineering Yahoo! Group