【转帖】using spreadsheet for tolerance stackup

yang686526

using spreadsheet for tolerance stackup
we manufacture pressure vessels, dfma reports following critical parts,
1. neck
2. top & bottom head
3. shell
4. stud
i would like to have a good understanding on how to implement tolerance stackup using data from spreadsheet. for starter we are only concerned in 1-d (height only) analysis for poc.
i have seen the excel file from kenat, its just i am new to statistical tolerancing or even tolerancing, i need a basic work around which is simple and easy.
any help will be appreciated.
thanks
irfan, simple tolerance stacks are explained in asme y14.5m-1994 section 2.6.
in your case lets say:
1. neck 1+-.01
2. top & bottom head 2+-.01 (is this one part or 2, i've assumed 1?)
3. shell 5+-.01
4. stud .25+-.01
so 1 + 2 + 5 + .25 =8.25 nominal
you can add the tolerances in the same way.
(+-.01) + (+-.01) + (+-.01) + (+-.01) = +-.04
in cases where it's not equal bilateral +- tolerance you can have two tolerance columns in your spreadsheet.  one for plus one for negative.
also, you need to consider if your part is geometrically positve or negative, a simple stack is always positive but when you go up to one 'face' and then back away from it, the 'backing away' dimensions are negative.
any help at all?
kenat,
i am following "traditional approaches to analyzing mechanical tolerance stacks" by paul drake. i do understand loop diagram (somewhat) and its just i don't want to get tangle in statistical lingo too much.
irfan, there is no "statistical lingo" in my above post.  it's a simple 'worst case' analysis.
in fact, as regards the excel spreadsheet of mine you reference, there is no statistical lingo there (assuming you're refering to my holes tolerance calculator).
is this link any help.
oh i didn't mean in your post, its just my own read and research i am doing. my case is for statistical and not worst case.
i can now see how loop diagram is formed. thank you very much.
let me give you a brief idea what i am trying to do here is, to achieve a scenario, where i can mock up different tolerances on variable parts(not fixed) to get a desired stack up height of my vessel.
take a look at this.
i didn't create it but have tidied it up and used it a few times.
it's error prone when you insert rows etc so be careful.
i wouldn't necessarily use as it but it may give you some ideas.
kenat,
good spreadsheet kenat.   stack analysis is probably me least favorite task but one of the most essential.

that's great. is "geom factor" same as sensitivity?
what is the importance of process capability indices, like cpk on statistical tolerancing for stackup? is it too far stretched of analysis if i include cpk. any easy reading reference?
thanks
geom factor is whether it's positive or negative contributer to the stack.
hopefully the attatched sketch helps clarify (sorry the terminology isn't quite right).
have you searched on this site about cpk, there was a thread fairly recently as i recall.
kenat,
thanks for clearing it up, now i fully understand.
what is the importance of tol^2 (normal) in the wokrsheet you have uploaded previously. how will this feed back to my tolerance.

have you heard of the term "root sum squared" or "root mean squared"?
if not, then frankly, you may be in over your head - or at least have some serious reading to do.
kenat,