SPC on Multiple Cavities - Customer wants SPC done on each notch - Cylindrical part

J

jager

#1
SPC on multiple cavities, new topic

We have a symmetrical part which has 4 notches. After the part is stamped and tumbled to remove burrs, it is impossible to tell which notch is which. The customer wants SPC done on each notch. The tolerance is .070 ± .0005 inches wide. Since we cannot tell the notches apart, is there an acceptable method where we can just combine all notches into one big study? I have be measuring them as left, right, top and bottom, but all the readings are mixed since I don't really know which is left, right, etc. Thanks.
 
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Wes Bucey

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#2
jager said:
We have a symmetrical part which has 4 notches. After the part is stamped and tumbled to remove burrs, it is impossible to tell which notch is which. The customer wants SPC done on each notch. The tolerance is .070 ± .0005 inches wide. Since we cannot tell the notches apart, is there an acceptable method where we can just combine all notches into one big study? I have be measuring them as left, right, top and bottom, but all the readings are mixed since I don't really know which is left, right, etc. Thanks.
Sound to me like another case for CONTRACT REVIEW.

Talk to the customer and explain the situation.

From a technical standpoint, SPC of the notches should probably be done prior to tumbling, deburring, and subsequent mixing of part orientation. Otherwise, the concept of order of manufacture is completely lost. I presume the customer is concerned about whether the die or tooling setup is a source of special variation when he asks for SPC on each separate notch. Is this a valid concern? If not, you should be able to present a case to the customer for some charting which answers his concerns.

Alternately, agree with customer on method of unobtrusive mark on part prior to tumbling to orient the piece if he is just looking to establish overall rate on a "batch."
 
Last edited:
J

jager

#3
spc on notches

The notches are hard to separate even before tumble and the specs are very tight for flatness, burr, etc.. It is a tiny part and only .010" thick. I don't think we could put an ID mark on the part anyplace. We are going to discuss this today and probably contact the customer. Thank you for your reply. You've been very helpful.
 
B

ben sortin

#4
Measure all four notches on one part and record the minimum and the maximum. Chart the range on a cusum chart. Make sure your measurement system is stable. Verify set up, intensify your PM out of the shoot, and ask your customer my favorite question...
 
B

Bill Ryan - 2007

#5
How many cavities are you talking about? Are they identified on the part? I guess my question is - What do you do if just one notch on one of the cavities is causing a problem?

Apparently you have already done a bunch of measuring - if you throw all the data together what does it look like?

Hope your customer can shed some light for you. Let us know how you make out.

Bill
 
J

Jay Sturgeon

#6
Addition

Would it be possible to add a punch that would identify the cavities after they exit the die path. You could then take the selection any way you wanted. Another suggestion would be (if there is room) to put a seperate exit shoot for the caivites and have them run into their own containers. If you had to certify product for any reason and you only had a problem with one cavity you'd have to look at 100% of the lot wouldn't you. I see a number of reasons to keep the parts seperate. That's what we do here and then we tumble the cavities serperately also.

Just some thoughts. I don't have the last word on the subject that's for sure.

Good luck.

Wes is pretty big on Contract review isn't he. Just kidding.

:bigwave:
 
J

jager

#7
notches

It is one part, but 4 notches in it. It is round with a notch every 90°. The machine can spit out about 10,000 an hour! We are to provide SPC data on each notch at PPAP and annual PPAP. I'll let you know how it comes out. Thanks.
 
J

Jay Sturgeon

#8
Same as Before

Once again, it may be the same part, but it is four seperate cavities. Idenify the cavities or seperate the cavities. Cavities 1,2,3 &4.
 
J

jager

#9
4 notches, symmetrical part

Just asked the customer about lumping all of the notch widths into one cap study so there would be all the sizes represented. He said that would be a good plan. They will evaluate our studies and decide how much extra tolerance we will be allowed (print change). Thanks again for your ideas.
 
R

Rob Nix

#10
IMH (statistically backgrounded) O, that idea violates the whole central limit theorem on which it is based. Your data will contain 4 separate distributions in one, and it is no longer normal - which means the main premise for control & capability is gone.

If all they are interested in is conformity, then just do GO/NO GO checks. I have a lot more to add about this but I gotta go.
 
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