Pre-Control Chart to guide in-process inspection / adjustment decisions

bobdoering

Stop X-bar/R Madness!!
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#41
After inspecting the dataset, it appears to be numbers that are trending upwards, then starts over.
That is correct!

SPC is not a valid tool in this case because there is no variability hence no need for statistics. If we know one number, we will also know the next.
There is variability, or it would be a straight, flat line. The variability has a rate - just happens to be relatively constant. There is a need for statistics, to ensure that the data remains within control limits, there are not special causes (we know what number to expect next, but it might not occur). Statistical Process Control (although not X-bar and R) is used to ensure the process remains in control, and the signals for adjustment are detected and the adjustments are made to the correct value. You are right in that it shows predictability, and therein lies a great deal of its power.

What you showed was the difference between discrete uniform distribution (your example) and continuous uniform distribution (my example - and the distribution found in precision machining). Nice example! :cool:
 

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Dan Kenkel

#42
Bob,

You had asked what type of processes we are wanting to monitor with the "pre-control" tool. This first phase will be for a machining department, aluminum automotive parts with fairly tight tolerances. Parts are ran on both multi-axis CNC machining centers and automatic line with with gang-head machining line. By the way, these processes have been reviewed/studied fairly recently and meet the 1.33 Cpk requirement, so we are starting with stable machining processes. Actually, many of these part features are 1.67 and better.

Currently, we are looking at some software options. We may see if Atul's company, Symphony Tech, can help develop a software program, adding the "pre-control" chart functions to their existing "SPC Workbench" software, not sure at this point.

Let me know if you know of a software option, we will be also looking into "QC Calc," made by ProLink which we now know does include this tool.
 

bobdoering

Stop X-bar/R Madness!!
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#43
You had asked what type of processes we are wanting to monitor with the "pre-control" tool. This first phase will be for a machining department, aluminum automotive parts with fairly tight tolerances. Parts are ran on both multi-axis CNC machining centers and automatic line with with gang-head machining line. By the way, these processes have been reviewed/studied fairly recently and meet the 1.33 Cpk requirement, so we are starting with stable machining processes. Actually, many of these part features are 1.67 and better.

Currently, we are looking at some software options. We may see if Atul's company, Symphony Tech, can help develop a software program, adding the "pre-control" chart functions to their existing "SPC Workbench" software, not sure at this point.

Let me know if you know of a software option, we will be also looking into "QC Calc," made by ProLink which we now know does include this tool.
Do these machines have auto tool compensation? I am glad you have identified that your processes are held to 1.33 to 1.66, but I am curious how the data was collected and how the capability was calculated. Often I have seen very high capabilities, but they were really sampling error or overcontrol. Even without those errors, chances are your capability is probably much greater if evaluated correctly. If you have what I define as precision machining - that is your primary variation (when measurement error is removed) is tool wear, then precontrol will appear to work. But, at that point, you might as well just set up I-MR with fixed control limits at 75% of the tolerance and run. It is the closest thing in software available to the X hi/lo-R chart.

Unfortunately I am not aware of any software that has X hi/lo-R. Those that I have discussed it with were A) unaware of the uniform distribution in precision machining and B) claimed precision machining was a special case that was not significant enough to generate code for. Do you feel that special? Anyway, using I-MR you will just miss out on the benefits of tracking roundness. I have found it particularly helpful with steel, titanium, etc. Not sure if aluminum will give the same stark results.

Were the capability studies run using data plotted in time sequence prior to preparing the histogram? That is a key analysis for precision machining. Are the multiple heads shared for each part, or do some parts run on some heads, and other parts run on other heads? If a hard only chucks some of the parts, then they should be charted separately, or the head to head variation will confound the results (as on a screw machine).

Would you be willing to share one of your capability studies? :cool:
 

bobdoering

Stop X-bar/R Madness!!
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#44
In fact by subgrouping, the Central Limit Theorem kicks in making them normal, besides a little deviation from normal will hurt us much.
Interesting note:

The central limit theorem (CLT) states that the re-averaged sum of a sufficiently large number of identically distributed independent random variables each with finite mean and variance will be approximately normally distributed (Rice 1995).

The continuous uniform distribution that arises from tool wear is neither random nor independent. So, CLT must not be assumed to apply to every distribution. :cool:
 
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