dimensional stackup
dimensional stackup
(OP)
can anyone recommend how to analysis dimensions in a assembly stackup to make sure each will fit?
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RE: dimensional stackup
This is simple arithmetic isn't it?
I suppose it is not so simple when you take tolerances into account. When I do this, I draw the thing out and the most methodical way possible. I use absolute coordinates for everything. Each individual arthithmetic operation is simple. There are just lots of them.
After that, it is primarily a matter of understanding the drawings and the tolerances.
JHG
RE: dimensional stackup
Here is a link to a helpful document I found on the web.
http://
Tolerance analysis can be as simple as looking at dimensions, tolerances, part datums, and assembly functional datums. Or you could use one of the many modern statistical analysis tools. I would also suggest using excel to do your stats and other math functions.
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Heckler
Sr. Mechanical Engineer
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RE: dimensional stackup
RE: dimensional stackup
I have not found a general formal technique that would allow unskilled labor to pick numbers off drawings and plug them into Excel.
I set up a few cells for each feature or problem space boundary to be evaluated, and label them so I can remember which is which and so I can maybe re-use the block for a similar situation in the future.
I have so far not evolved a block that can usually be copied without modification. If there exists a system of tolerancing that would enable such an activity, I haven't stumbled onto it.
Statistical techniques may be useful for characterizing the behavior of a factory, but they are not real useful to a guy who is trying to adjust a machine setup.
Mike Halloran
Pembroke Pines, FL, USA
RE: dimensional stackup
My experience with Statistical methods is extensive but to be affective the entire business/manufacturing process needs to buy into the concept otherwise the loop can't be closed.
RE: dimensional stackup
In an assembly involving several dimensions, add up all the tolerances and then adjust any clearances, gaps, or fits to accommodate this. I think a worst case stackup is a little extreme and a statistical one might be more realistic, but there is some argument here.
RE: dimensional stackup
If you're using GD&T, you can do a tolerance stack-up. It's a combination of graphical and mathematical work that accounts for worst cases (e.g. smallest dowel in the largest hole)of size, location, form, etc. It is quite comprehensive. This will give you the worst-case scenarios, not the probable case. Typically people will have some heart problems when they see how bad things could be, and rationalize that it never has been that bad, so the analysis is garbage. If you have hard statistical data on the manufacturing capabilities, you can greatly improve the results by including them in the calculation. There are a number of CAD analysis packages that do this, and you can always do it manually (that way you understand the process and are less likely to overlook something). My suggestion would be to take a good course from someone like Tec-Ease and work at it manually.
Hope that helps. Good luck!
RE: dimensional stackup
One case from my own experience: we had an assembly of several parts that had been made for many years but suddenly one day could not be put together. Management called out the Tiger Team and the logical decision was that it was a design error. I was picked to do a stack up, but all I did was a quick worse case plus/minus stackup and found nothing wrong. Unacceptable answer. They gave it to another guy who added in all the concentricities, parallels, perpendiculars, and probably more, but still could not explain it. When they were done blaming engineering, it occurred to them to measure the parts. All were within spec but one which was something like an eighth inch off.