AboutJoe Martin Expertise I can answer questions about Quality Systems (AS9100, AS9103, Boeing D6-82479, etc.) Particular areas of expertise are Statistical Process Control (SPC), sampling inspection, design for Six Sigma, and Six Sigma tools in general.
Experience Quality engineer in aerospace for 20+ years. ASQ Certified Quality Engineer (#8920) since 1982.
I'm a product design engineering student who currently working on a thesis of designing tolerance of rolled form and welded door sash car assembly. I wanted to set a new tolerance on a few areas of the door frame which I found are the critical areas and always out of the tolerance range.
My problem here is on how to develop a new tolerance after I had known the standard deviation from the capability histogram. PC and PCI also had already calculated. I mean in terms of what equation to use to develop a new tolerance and does the capability histogram must be at the center of the nominal value in order to use the standard deviation in any of equation related? (because the data I collected is within the tolerance spec. but slightly shifted to the left or right from the nominal value)
Lastly, is it possible to do stack up tolerance on the door sash on a joint part (two parts) where the joint are welded and how? Sorry to ask but I'm totally new in this are (SPC method) and still learning. Your patience and guidance are much appreciated.
ANSWER: It’s impossible for me, having never seen your door sash, to give good answers to your questions. Every situation is unique. You say that the 3 areas are not only critical, but “always out of the tolerance range.” Is your intent to improve the process, or simply to change the tolerance so that parts are no longer nonconforming? Are you reworking the parts which fall outside the tolerance range?
In order to give you a reasonably intelligent opinion, I would need (1) a better understanding of both the parts and the process used to make them, and (2) some actual data. If you want to pursue it, let’s “talk” more.
Joe
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QUESTION: Yes, the out of tolerance areas are the critical areas from all of the point measured. The operator in the factory did some reworking so that the parts are all in the tolerance range. Currently I am working on improving the process and tolerance at the critical areas as well as at other areas of the parts so that the variation of the critical areas are normally distributed. But that's not what I concerned the most. What am I concern most is that now I am developing a new tolerance of all of the points measured and at the same time doing a tolerance stack up at the welded area. My question here is what is the equation relate to develop a new tolerance and I need your opinion on how to do tolerance stack up at the welded area? Is it possible to implement the analysis?
The parts used are following the standard of sheet metal JIS 3141 and roll formed process. There are 32 points inspected at the door sash.
Can you suggest suitable equations to design a new tolerane and how to do tolerance stack up at the welded parts? Thank you for your time.
Answer When you say "tolerance stack", I presume you mean the stack of 2 or more parts BEFORE they are welded into the door sash.
In any case, I suggest you type "Statistical Tolerancing" into your favorite search engine. You will find plenty of web sites with applicable formulas. There a also a couple of books on the subject if you want to go into greater detail.
If you want to send me some actual tolerances and measurement data, I'll take a look.
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