U-chart Sanity/File Review - Is the chart constructed properly?

P

Phil Fields

#1
Hello,
I am using a U-Chart for the 1st time (actually I have not used statistics much in about 10 years), and would like to have someone review my file before I go much further.
The issue is that at receiving we are checking the torque of terminal screws on an electrical contactor, there are 6 screws per assembly we are checking. The minimum torque value is 45 in-lbs. We are using a torque wrench set @ 45 in-lbs with a flat head screw driver bit. If the Torque wrench “clicks” as soon as force is applied, than that terminal passes. If the there is any movement of the wrench before it "clicks", than the terminal fails. I am trying to determine and chart defects per unit per sample lot inspected.
Is the chart constructed properly?
Is this the best chart method to use?

Thank you,
Phil
 

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Tim Folkerts

Trusted Information Resource
#2
Phil,

I ran your numbers through Minitab and here is what it gave:

Date Defects Units PPOI1 CENL1 CONL1 CONL2
13-Jun-06 10 6 1.66667 1.76667 0.138785 3.39455
15-Jun-06 13 8 1.62500 1.76667 0.356879 3.17645
23-Jun-06 9 6 1.50000 1.76667 0.138785 3.39455
17-Jul-06 21 10 2.10000 1.76667 0.505715 3.02762

It is a little tough to read, but hopefully you can decipher it.

The control limits are tighter for larger sets, but your limits seem to just get tighter with time. And you don't have control limits for the first date, so I think there is something fundamentally different about the way you are getting your control limits compared to Minitab. You might want to double check how you did your calculations.


Tim F

P.S. One other thing I almost always suggest in cases like this is to consider is finding a way to measure the actual number for the torque, rather than just pass/fail. Pass/fail is clearly easier and more fool-proof (certainly important considerations), but variable data allows more robust data analysis.
 
P

Phil Fields

#3
Tim,
Thank you for your response. I ran my numbers with QI Marco (Excel Based Stat program) and came up with the results that Minitab gave you. Now I know that I am on the right path, and can work with our supplier to track their corrective action.

I appreciate your help,
Phil
:thanks:
 
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