B
J Oliphant said:
ok, that thread was helpful.
Basically, SPC plays no role in our finished process. We ship and approve intermediate product based on whether the test (one per batch) meets specifications. Therefore we have a system that promotes operator tampering. (see recent conversations about the funnel experiment). We do not reject/alter finshed/intermediates based on control limits.
Therefore If I understand the terminology-- we have lots of special cause variation and our process ISN'T in satical control.
Since the process data lacks rational subgroups and statistical control, we must use Pp and Ppk as the thread states, correct??
Basically, SPC plays no role in our finished process. We ship and approve intermediate product based on whether the test (one per batch) meets specifications. Therefore we have a system that promotes operator tampering. (see recent conversations about the funnel experiment). We do not reject/alter finshed/intermediates based on control limits.
Therefore If I understand the terminology-- we have lots of special cause variation and our process ISN'T in satical control.
Since the process data lacks rational subgroups and statistical control, we must use Pp and Ppk as the thread states, correct??
I am also confused by your statement that "SPC plays no role in our finished product" but on the intermediate product which you "ship and approve". Is an intermediate product one that you ship from one department or plant to another and then they make it into a finished product? No matter what the answer is, SPC can be used to monitor finished product final testing, intermediate final testing or even the process controls such as temperature, pressure, reflux ratios etc. while you are making the product. Actually, there would be more value in learning what process variables have the most effect on your intermediate or final product and control chart those variables with the intent that you would find problems early and correct them before you get out of control or out of spec on your final product tests.
Also, control charts are used to determine if your process has changed and is not related to specification or product rejection. As a matter of fact, G&L Chap 9 caution practitioners about the confusion that can be caused when mixing control limits and specification limits in a discussion of control chart theory. It is possible to have a process that is not in control and still make product that passes final testing. Depending on how out of control the process is, I would not recommend that you wait too long to bring it back in control or you may find yourself with an entire batch that needs to be re-run or even worse, disposed of.
Rational subgroups is related to sampling methodology and must be done correctly if you want a meaningful control chart. G&L Chap 9 has information on this topic also.
Don't be shy about asking questions. In spite of what you may have read or taught in a class, control chart theory is not that easy and takes time to learn. If you are wondering about my references to Grant & Leavenworth, my book is liberally marked with post it notes on this subject and plenty of written notes in the margins. I was a "real" control chart user and a "real" engineer at one time and you have brought back some memories that are deeply buried but still there. When people talk about putting out fires at work, I always note that I may be the only one that actually did put out real fires.
Bill Pflanz