Control Chart out of control? Cpk - Process Out of Control

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C

CMfgT

#22
If you have any further questions after reviewing that thread, I will be more than happy to discuss it.


I would like to take you up on your offer. I am going to try the high and low chart on one of our grinding part numbers. How do I collect the data?

From what I have read on the post you mentioned. It seems i need 20 consecutive measurements, I need the high and low measurement. (Say the part is 12 inches long. Do I measure the high spot and low spot, i guess I am thinking of taper here, say my high spot is at 3 inches length wise, and low at 9 inches...just as an example.

OR am I way off, are you meaning measure the same spot but rotate the part a few degrees and measure again. Going back to my example, measure at 3 inches length wise, rotate the part a few degrees and measure again.

The reason why i say this is the example is of a washer. The part is not long, so minimal taper exist.

If I am not clear let me know. :frust:

OK once I am ready to collect, do I just set the control limits at 75% of the tolerance and tell the operator not to adjust the machine until the high hits the UCL and tell him to adjust the high down to the LCL once the high hits the UCL. AM I understanding this right???????

Thanks
 
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bobdoering

Stop X-bar/R Madness!!
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#23
From what I have read on the post you mentioned. It seems i need 20 consecutive measurements, I need the high and low measurement. (Say the part is 12 inches long. Do I measure the high spot and low spot, i guess I am thinking of taper here, say my high spot is at 3 inches length wise, and low at 9 inches...just as an example.

OR am I way off, are you meaning measure the same spot but rotate the part a few degrees and measure again. Going back to my example, measure at 3 inches length wise, rotate the part a few degrees and measure again.

The reason why i say this is the example is of a washer. The part is not long, so minimal taper exist.
You can do taper, but that is a little more advanced. The simple approach is to pick a particular diametric slice (which should be easy on a washer!). It has to be the same diametric slice on every part you measure. For example, on a longer part, if you measure the diametric slice at 3.000" from the end, you must do that measurement at the same location on every part you measure. That is because you are trying to control the diameter variation from the tool wear - and you do not want to influence that variation with taper, etc. So you are correct, if using a micrometer, every 45 degrees will likely pick up the roundness variation. For more data, air gages work great, and indicators can be a little quicker, too.

OK once I am ready to collect, do I just set the control limits at 75% of the tolerance and tell the operator not to adjust the machine until the high hits the UCL and tell him to adjust the high down to the LCL once the high hits the UCL. AM I understanding this right???????
Yes, that is correct! 20 parts should at least give you an idea of the slope (tool wear rate). If you have not made an adjustment within 20 parts, you can extrapolate the slope to predict how many parts you can make before adjustment. It may be 25 parts - it may be a weeks worth. It depends... When you have an idea of how many parts you make between adjustments, divide that number by 5 to get your sampling rate. For example, if you have 125 parts between adjustments, your sample rate is one part every 25 parts.

You will be amazed at what you find!:cool:
 
C

CMfgT

#24
Thanks for the quick response. I might have been unclear, but I am not doing a washer, I was referencing the thread you mentioned to read. Most of my grinding parts are between 4 inches long up to 12 -13 inches long.

But I think i understand it. I will try and get some data on a key feature job tomorrow or Monday and post. Thank you, i am really exicted to get some REAL results.
 

bobdoering

Stop X-bar/R Madness!!
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#25
Great! Like I said, grinding is the gold standard for the uniform distribution. I hope you can share your data! :cool:
 
C

CMfgT

#26
Well, I am not really seeing a big difference in roundness on our parts so it is hard to see the hi and low aspect. A very qualified centerless grinder operator has been helping out other operators with roundness, so this is making the data collection for the hi low difficult:mad:,but is actually a good thing for our quality.
 

bobdoering

Stop X-bar/R Madness!!
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#27
Well, I am not really seeing a big difference in roundness on our parts so it is hard to see the hi and low aspect. A very qualified centerless grinder operator has been helping out other operators with roundness, so this is making the data collection for the hi low difficult:mad:,but is actually a good thing for our quality.
I agree, that is a very good thing. If you are satisfied with the resolution of your measurement, and the roundness is insignificant (less than 10% of your tolerance or so), then it will collapse to a single point, and your roundess will be zero. If so, no problem - you should still see the sawtooth curve, you should still be using the 75% of tolerance control limits, and you should still not make any adjustments to the machine (other than for surface finish issues) until the OD reaches the upper control limit, and then adjust down to the lower control limit (not the mean). Are you seeing the sawtooth curve? :cool:
 
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