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.
Question QUESTION: Hye Joe,
I got problem here....
I want to improve a tolerance for the width weld bead on the join of steel plate.Currently, the width bead tolerance is in range of (3-6mm) or (4.5 +- 1.5mm).
here i attached you a process capability sixpack that i've done using minitab as your reference.There are 3 points that i had measured which is at point A,B,C...
from the minitab software...i got the standard deviation, cp,cpk etc....and i know..the tolerance can be reduce...
but i don't know and not so sure about the method that need to be used.
the questions here are?
How to find the new tolerance?
what methods or formula need to be used and applied?
Can you recommend me a new tolerance?
ANSWER: When properly used, “Minitab” is wonderful tool. But like many other tools, it can cause a lot of damage if it’s used incorrectly. Obviously, I know nothing about the product or the weld process in question, and no “6 pack” by itself is going to give me—or anybody else—the answer to your problem. (By the way, the graphic is too small for me to really see clearly.)
I’m willing to help on this, but ultimately YOU have to come up with the answers. Let’s pretend that I am the chief engineer at your company. If you want me to approve any change in tolerance, you have to convince me it’s a good idea. Here are some basic questions that must be answered before we even think about making tolerance changes:
How is this going to save the company money? (If it doesn’t, what is your reason for seeking the tolerance change?)
Is this process automated or manual?
Is all the data from one machine?
Are multiple operators and/or part numbers involved?
What is the relationship between points A, B, and C?
Why did you choose these points?
What sort of measurement device was used?
Over what length of time was the data collected?
Regards,
Joe
PS—If you decide to go forward, I will ultimately need some raw data.
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QUESTION: It is not in terms of money, it is just for my knowledge. Actually, in this situation i just wondering why the company used a big tolerance (3-6mm) but it can be reduced to (3.5-5.5mm) i guess.
the process is automated, which is welding process.
the data is from more than 1 machine and multiple operators involved.
The measurement device used is vernier caliper. There are 5 samples measured everyday for 25 days.
here i attached you a picture of 3 points that i've measured.
is it possible if i want to improve the tolerance?
Answer If I understand everything correctly, you are mainly interested in learning something by using real data. That’s good. However, in the “real world” I cannot imagine why anyone would want to “shrink” tolerances just because it is feasible. Unless there’s some problem with assembly or product performance because of these tolerances, leave them alone and go work on a real problem!
But for the moment, let’s assume that your tolerances are a real problem. As I mentioned, I am not a big advocate of the “six pack”—to me, it’s just a Minitab marketing gimmick. However, the data you have collected, if properly analyzed, could tell you a lot about your welding process—and how well that process could perform against any given tolerance.
Unfortunately, about all I can see in the graphic is that Point A data is skewed heavily to the left. Points B and C are skewed slightly to the right, but not nearly so much.
If you want to go to the trouble of sending me your raw data, preferably in Microsoft “Excel” format or as a text file, I will take a look and suggest some avenues of further exploration. Send it to:
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