Measurement System Artificially Creates a Skewed Distribution

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C

clarked3

#12
I agree that a micrometer, ball-anvil or otherwise (i.e. a curved surface on the inside of the tube), would be a more accurate measurement. As RSEGRIGGY points out though it is the method we agreed with the supplier. In fact, they suggested it, we had reservations but they assured us they could perform the measurement accurately.

I have suspicions they may be using a caliper attachment of some sort or their method is slightly different and this has been lost in translation.

They have begun the Gage R&R so hopefully we will find out soon.
 

Ninja

Looking for Reality
Staff member
Super Moderator
#13
If you already have the customer-directed equipment in house, you might consider doing your own GRR so that you have your own real data.

The customer passing GRR that you didn't see performed by their operators, and you passing GRR at your site with your operators are two different things.

Having your own data allows you to push back in the negotiation with data in hand.

As a best case, perhaps you find out that their method works perfectly well and that your (and our) concerns are unfounded...
 

TWA - not the airline

Trusted Information Resource
#14
The question is why there is such a difference between your results and those of your vendor though you are doing it the same way. Especially when considering that calipers are not perfectly suited for such tasks (as George and RSEGRIGGY correctly pointed out) you'd expect that due to the higher inaccuracy systematic differences would rather be masked than become pronounced. But if the tube get more oval over time then you'd expect a distortion of your results because the actual curvature of the place where you measure the wall thickness would be different. This effect would be much less if you'd use an anvil micrometer...
 

bobdoering

Stop X-bar/R Madness!!
Trusted Information Resource
#15
First: If you are measuring a circular feature, you should always measure the high and the low for each circular slice - since there are an infinite number of diameters, and one diameter measurement is statistically insignificant to represent the diameter. Oval parts may actually be better represented by the average of the high and the low, if it is a flexible material.

Second: The comments about what happens to the material after it leaves the manufacturer is also important considerations

Third: What is the effect of their die, die life, etc. on the dimensions over time? Extrusion dies wear, causing the dimension (ID) to increase. So, long term the variation would not be expected to be normal or centered.
 

kuyakut

Involved In Discussions
#16
Did your supplier do a shrinkage study. It may fine during the time your supplier measure the parts,but after it was stored and sent to your plant it may shrink and cause dimenion to skewed.
 
C

clarked3

#17
[FONT=&quot]So here are the results of the GRR (and a bit of a rant!*Disclaimer*)[/FONT]

[FONT=&quot]Two-Way ANOVA Table With Interaction [/FONT]
[FONT=&quot] [/FONT]
[FONT=&quot]Source DF SS MS F P[/FONT]
[FONT=&quot]Part no 74 0.0226890 0.0003066 2.5508 0.000[/FONT]
[FONT=&quot]Operator 3 0.0059907 0.0019969 16.6131 0.000[/FONT]
[FONT=&quot]Part no * Operator 222 0.0266843 0.0001202 1.1980 0.073[/FONT]
[FONT=&quot]Repeatability 300 0.0301000 0.0001003[/FONT]
[FONT=&quot]Total 599 0.0854640[/FONT]
[FONT=&quot] [/FONT]
[FONT=&quot] [/FONT]
[FONT=&quot]Alpha to remove interaction term = 0.25[/FONT]
[FONT=&quot] [/FONT]
[FONT=&quot] [/FONT]
[FONT=&quot]Gage R&R [/FONT]
[FONT=&quot] [/FONT]
[FONT=&quot] %Contribution[/FONT]
[FONT=&quot]Source VarComp (of VarComp)[/FONT]
[FONT=&quot]Total Gage R&R 0.0001228 84.05[/FONT]
[FONT=&quot] Repeatability 0.0001003 68.68[/FONT]
[FONT=&quot] Reproducibility 0.0000224 15.36[/FONT]
[FONT=&quot] Operator 0.0000125 8.56[/FONT]
[FONT=&quot] Operator*Part no 0.0000099 6.80[/FONT]
[FONT=&quot]Part-To-Part 0.0000233 15.95[/FONT]
[FONT=&quot]Total Variation 0.0001461 100.00[/FONT]
[FONT=&quot] [/FONT]
[FONT=&quot] [/FONT]
[FONT=&quot] Study Var %Study Var[/FONT]
[FONT=&quot]Source StdDev (SD) (6 * SD) (%SV)[/FONT]
[FONT=&quot]Total Gage R&R 0.0110805 0.0664831 91.68[/FONT]
[FONT=&quot] Repeatability 0.0100167 0.0600999 82.88[/FONT]
[FONT=&quot] Reproducibility 0.0047376 0.0284253 39.20[/FONT]
[FONT=&quot] Operator 0.0035371 0.0212228 29.27[/FONT]
[FONT=&quot] Operator*Part no 0.0031517 0.0189102 26.08[/FONT]
[FONT=&quot]Part-To-Part 0.0048271 0.0289627 39.94[/FONT]
[FONT=&quot]Total Variation 0.0120863 0.0725178 100.00[/FONT]
[FONT=&quot] [/FONT]
[FONT=&quot] [/FONT]
[FONT=&quot]Number of Distinct Categories = 1[/FONT]


[FONT=&quot]Turns out the reason the Results were different in the 1st place is because the operators didn't measure the parts correctly on my side.:mad: These parts are from a new supplier so they were meant to perform the measurements in the same way as they did the parts from the old supplier. I assumed that they had been trained to do this correctly...how wrong was I! I found this out when I sent them the protocol for the GRR.[/FONT]
[FONT=&quot]So once I'd re-trained them we performed the GRR and the results were equivalent. There was a minor discrepancy, however... [/FONT]
[FONT=&quot]As you can see the caliper method is inadequate. The method is as follows: measure the wall thickness at 4 points 2 in the X plane and 2 in the Y plane, i.e. at 12, 3, 6 & 9 on a clock face. Calculate the average and report that. My operators had only been taking 1 value. I was going to see whether, if they took the measurements in the same tube orientation everytime would the variation decrease.[/FONT]
[FONT=&quot]I started to think up a 2nd quick study to see if I could decrease the gage variation when I notice the tubes have 4 points marked on the tip. Knowing full well what they'd done I asked why they'd marked the tubes. "Well we realised that if we both took the 4 measurements in different places it might increase the variation so we marked them all up and then both measured them at the same point" therefore artificially decreasing the variation in the operator reproducibility... :frust:[/FONT]
[FONT=&quot]Moral of the story: Just because people are highly educated does not mean they have an ounce of sense! Watch them at all times! [/FONT]
[FONT=&quot]The supplier maintains that they don't have any issues with any other customers when they use this technique so we should look at widening the tolerances (I think this is a bit of a joke but nevertheless I won't be signing off on it!)[/FONT]
 
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