Dimensional Tolerancing
Dimensional Tolerancing
(OP)
If you saw a "2.0" on a mechanical drawing, and wanted to attach a tolerance to that number, (assuming of course that your "Tolerancing Data Block" is like ours and doesn't typically include a tolerance for a ".0" number), what tolerance would you go to, by default? ±.010 (or higher)?
-Soupy1957
-Soupy1957
Telepathy only enhances the world I live in!!





RE: Dimensional Tolerancing
How big of a tolerance can you functionally allow? Obviously the bigger the tolerance the easier to create the part, but you can only make the tolerance wide enough so as not to disturb form fit or function. There is no "standard" tolerance.
V
Mechanical Engineer
"When I am working on a problem, I do not think of beauty, but when I've finished, if the solution is not beautiful, I know it is wrong."
- R. Buckminster Fuller
RE: Dimensional Tolerancing
Aaron A. Spearin
ASQ CSSBB
Engineering Six-S'$
www.Engineering6ss.com
"The only constant in life is change." -Bruce Lee
RE: Dimensional Tolerancing
V
Mechanical Engineer
"When I am working on a problem, I do not think of beauty, but when I've finished, if the solution is not beautiful, I know it is wrong."
- R. Buckminster Fuller
RE: Dimensional Tolerancing
Aaron A. Spearin
ASQ CSSBB
Engineering Six-S'$
www.Engineering6ss.com
"The only constant in life is change." -Bruce Lee
RE: Dimensional Tolerancing
Because default drawing tolerances can give a more appropriate tolerance for the work.
Our drawings, for example, give a tolerance of +/-.005" for three-figure dimensions. This is a reasonable amount for general, non-NC machine work like tapped and clearance holes that have to line up between parts. The sig-fig tolerance would be .001", which is finer than necessary and is more difficult to produce by a machinist, especially if he is just scribing and center-punching by hand. And to show the dimension with two decimals would indicate a sig-fig tolerance of .010", which could mean that two holes could be off by as much as .020" relative to each other, which is a little too sloppy.
I think engineers give tolerances that are too tight more often than they do ones that are too loose, which can greatly increase the manufacturing cost of a part.
Don
Kansas City
RE: Dimensional Tolerancing
The drawing block does not contain a specification for the tolerance range of a 0 .X callout. I agree that one has no other option than to logically resort to sig-figs. But realistically there is a narrow likelihood of an inspector or machinist of recalling sig fig rules with enough confidence to make a pass fail decision. I think the best solution would be to figure out how much slop is allowed.
If the drawing is not controlled by your organization then I would think you have as much tolerance as sig fig rules allow for. If you do have jurisdiction and influence over the drawing block standards then modify the drawing block to specify tolerances for such callouts.
Failure is a prerequisite of successful design
RE: Dimensional Tolerancing
Kenneth J Hueston, PEng
Principal
Sturni-Hueston Engineering Inc
Edmonton, Alberta Canada
RE: Dimensional Tolerancing
If say the block tol is +-.010 for 2 decimal place and +-.005 for 3 decimal place and has nothing for 1 decimal place then I'd probably go to the +-.010.
At the end of the day though it's an incomplete drawing so you should probably ask whoever the drawing came from.
As to the sig fig stuff, unless somehow invoked by a note or similar on the drawing I'm not sure how you'd justify invoking it.
KENAT, probably the least qualified checker you'll ever meet...
RE: Dimensional Tolerancing
Matt
CAD Engineer/ECN Analyst
Silicon Valley, CA
sw.fcsuper.com
Co-moderator of Solidworks Yahoo! Group
RE: Dimensional Tolerancing
KENAT, probably the least qualified checker you'll ever meet...
RE: Dimensional Tolerancing
Additionally I could not even begin to guess an appropriate rule of thumb without knowing the material of the part and the process used to put it in spec.
a 2x4 cut to 2.0 feet with hand tools ... I would likely use +/- .25"
On the other hand a 2.0" long 1/2" diameter aluminum rod on a lathe +/- .005 is acheivable if nearby features call for it.
RE: Dimensional Tolerancing
KENAT, probably the least qualified checker you'll ever meet...
RE: Dimensional Tolerancing
If it was produced to 2.06, then for one significant figure the dimension would be 2.1, which does not meet the 2.0 stated. Similarily, if the part was produced to 1.94, this would be 1.9 in one place form, which doesn't meet 2.0 dimesion.
-- MechEng2005
RE: Dimensional Tolerancing
Fot the OP my idea is to evaluate the needs of the part or the assembly it goes into and use the largest tolerance that will result in a working part. You are the only expert on the use of your part involved in this discussion. Every idea presented here will produce a part, will it work?