SPC for Diameters - Parts not Perfectly round so charts are useless

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RMedrano

#21
Yes, the OD tolerance is clearly outside of the capability of the process. The out-of-roundness is eating up over 50% of your tolerance!! The ID would have been OK, except for that blip at the end. Was it real? Anyway, now you can say without exception that you are stuck with sorting the OD - unless someone that knows your process can make some improvements. If that ID had not had the one bad data point, you could have SPC'ed the ID one very 10 parts or so and been done with it. Any theories why the ID is so much better than the OD? Any lessons learned there?

Do you feel as if you really know more about your process with this charting technique than the ones you have tried before?
Dont know, Im going to show this to a couple of the tooling engineers and see if it helps them. They are the ones trying to improve the process.

Yeah that flyer was real on the ID, at least as far as i know. I asked for confirmation of the part, but like I said before this process is currently in another facility 40 miles from here, I havent gotten a reply to my inquiry yet.

Above all else you have helped me understand the how/why its not really possible to get a true (normal) capability analysis on this.
 
C

cristo

#22
All you have to do is set up your control limits to 75% of your spec and run. [/B]
I haven't made a study of SPC and roundness, or all of the posts in this thread, so maybe I missed something.

But that statement up there sounds a whole lot like something that is NOT SPC as far as I've ever seen it.

Setting control limits based on a specification? What????
 

bobdoering

Stop X-bar/R Madness!!
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#23
That is because everything you have seen so far is based on the normal distribution, and precision machining is not. The issue is not roundness, it is precision machining that makes the difference. So yes, the rules are different - and it is true you likely have not seen this before. But, it is statistical...and it is process control. And, most importantly, it is correct. If you are in precision machining, I suggest you go through all of the postings in this thread. If not, then it may not apply to your processes at all.
 
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cristo

#24
The point I was making is that control limits are established based the amount of variation the process would be expected to exhibit if it were subject only to chance causes. This has nothing to do with specification limits.

So, setting control limits based on specifications...doesn't sound like statisitical process control. It might be a useful technique - I can't judge that. I'm not sure that the normal distribution has anything to do with it because control charts work with non-normal data.

Can you suggest a reference that explains the math behind setting a control limit based on a specification, and not on the natural variation of a process?
 

bobdoering

Stop X-bar/R Madness!!
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#25
That reference should be out by the end of the month. Until then, you need to know that in precision machining the most significant common cause is tool wear, it creates the sawtooth curve, that is the uniform distribution, and its controls are based on the probability of that rectangular distribution - which is much different than the normal distribution. The basics and the math have all been described in previous posts. "Chance causes only" only works for processes that exhibit natural variation - such as heat treat or the height of baked loaves of bread - and those processes can be controlled by normal distribution statistics. Precision machining does not fall into that category. In fact, if you get a chart that shows the normal distribution in precision machining, it is typically evidence of incorrect charting or a process out of control.
 
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cristo

#26
I assume here that you are using some type of chart that is based on the theory of a uniform distribution...

...in the same way that a p-chart uses binomial distribution theory, and a c-chart uses the poisson distribution theory for setting control limits.
 

bobdoering

Stop X-bar/R Madness!!
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#27
"control limits are established based the amount of variation the process would be expected to exhibit if it were subject only to chance causes"

I think it is more accurately put as: "control limits are established based the amount of variation the process would be expected to exhibit if it were subject only to common causes." Again, the "chance cause" is based on phenomena that have natural deviation, as described above. Practically all SPC texts are based on this, and it works for those types of processes. The particular issue of the correct SPC for precision machining and its unique distribution has not been dealt with well at all in the texts. We can not blame Dr. Shewhart for missing this condition, since the state of precision machining in the early 1930’s is not the same as today. Those that have applied these techniques understand clearly why this technique is correct, and that it works.
 
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bobdoering

Stop X-bar/R Madness!!
Trusted
#28
I assume here that you are using some type of chart that is based on the theory of a uniform distribution...

...in the same way that a p-chart uses binomial distribution theory, and a c-chart uses the poisson distribution theory for setting control limits.
Yes, the X hi/lo -R chart works very nicely. It is described in previous posts.
 

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