A
GR&R Issues in Automated Testing
Ok, this is all well and good, however what in the world does this have to do with qualifying an automated TEST machine that has no operator influence? I fail to see how the standard AIAG methods apply to this situation. The controversy surrounding what is needed to accept a machine for shipment seems to be widespread. Typical GR&R software requires 2 or more appraisers. What if there are no appraisers? I found the following links useful:
http://www.symphonytech.com/freeware.htm
http://www.qualitymag.com/CDA/ArticleInformation/features/BNP__Features__Item/0,6425,100281,00.html (Invalid URL)
http://www.mathoptions.com/grandron.htm (Invalid URL)
I don't want to create an encyclopedia of data and meaningless charts. All I want is to demonstrate my machine will meet EV requirements. Therefore, why wouldn't the Instantaneous method be acceptable and more practical for short term analysis on automated test equipment than complicated Cpk/Ppk calculations? IMO, until the machine is in production where data can be collected over long periods, the more simplified method as described by John Raffaldi should suffice.
Also, what about uncertainty of the measurement equipment when doing GR&R? Is it a simple matter of subtracting the uncertainty of each device based on the supplier's data from the total tolerance of the test being performed? Uncertainty is another area that to me is neither well understood or explained.
I'm open for constructive criticism and instruction. Thanks.
P.S. The customer is just as confused.
Rob Nix said:
Welcome QeSmithy.
IMHO
Juran's QC Handbook (4th & 5th ed.) calls Cpk a "performance index". The AIAG SPC Manual (1995) calls Ppk a "performance index", and Cpk a "capability index for a stable process". So it gets a little muddied after a while (and let me say, not all that important).
Cp and Cpk are at least as old as Ishikawa's 1982 "Guide to Quality Control" and are simply the inverse of capability ratios. They applied to short and long term measures of capability. Ppk was first used in the context of new product submissions (ISIR/PPAP) and still exists on some company's warrant forms for their initial (and obviously short term) estimates of process potential. Ford Motor company was one of the first. Ford did not invent Cpk & Ppk, but they DID champion their use - with the same understanding that Cpk = long term and Ppk = short term. Ppk was used also in other earlier PPAPs for Short Term estimates of capability. However, somehow, in the last decade the roles have reversed (especially by the 6 sigma people). This is partly due to the AIAG's SPC Manual defining Ppk as a nebulous "performance index". Does that definition imply performance over time? Who knows. Somehow the issue got confused.
Interestingly, the 1999 printing of the AIAG PPAP Manual speaks of using Cpk when historical (long term) data is available - enough to divide into subgroups, and using Ppk when it is not. In any event, the general calculations are the same (sure, the estimate or calculation for standard deviation varies, to which I say, blah blah blah
), and since it is all really a matter of semantics, if everyone agrees (especially you and your customer) then that is all that matters.
I stand by the older Cpk = long term, Ppk = short term understanding. Cpk is a more valid index for on-going process capability measurement.
PS: You'll find tons of debate on this if you do a Google search on "cpk vs ppk".
IMHO
Juran's QC Handbook (4th & 5th ed.) calls Cpk a "performance index". The AIAG SPC Manual (1995) calls Ppk a "performance index", and Cpk a "capability index for a stable process". So it gets a little muddied after a while (and let me say, not all that important).
Cp and Cpk are at least as old as Ishikawa's 1982 "Guide to Quality Control" and are simply the inverse of capability ratios. They applied to short and long term measures of capability. Ppk was first used in the context of new product submissions (ISIR/PPAP) and still exists on some company's warrant forms for their initial (and obviously short term) estimates of process potential. Ford Motor company was one of the first. Ford did not invent Cpk & Ppk, but they DID champion their use - with the same understanding that Cpk = long term and Ppk = short term. Ppk was used also in other earlier PPAPs for Short Term estimates of capability. However, somehow, in the last decade the roles have reversed (especially by the 6 sigma people). This is partly due to the AIAG's SPC Manual defining Ppk as a nebulous "performance index". Does that definition imply performance over time? Who knows. Somehow the issue got confused.
Interestingly, the 1999 printing of the AIAG PPAP Manual speaks of using Cpk when historical (long term) data is available - enough to divide into subgroups, and using Ppk when it is not. In any event, the general calculations are the same (sure, the estimate or calculation for standard deviation varies, to which I say, blah blah blah
), and since it is all really a matter of semantics, if everyone agrees (especially you and your customer) then that is all that matters. I stand by the older Cpk = long term, Ppk = short term understanding. Cpk is a more valid index for on-going process capability measurement.
PS: You'll find tons of debate on this if you do a Google search on "cpk vs ppk".
Ok, this is all well and good, however what in the world does this have to do with qualifying an automated TEST machine that has no operator influence? I fail to see how the standard AIAG methods apply to this situation. The controversy surrounding what is needed to accept a machine for shipment seems to be widespread. Typical GR&R software requires 2 or more appraisers. What if there are no appraisers? I found the following links useful:
http://www.symphonytech.com/freeware.htm
http://www.qualitymag.com/CDA/ArticleInformation/features/BNP__Features__Item/0,6425,100281,00.html (Invalid URL)
http://www.mathoptions.com/grandron.htm (Invalid URL)
I don't want to create an encyclopedia of data and meaningless charts. All I want is to demonstrate my machine will meet EV requirements. Therefore, why wouldn't the Instantaneous method be acceptable and more practical for short term analysis on automated test equipment than complicated Cpk/Ppk calculations? IMO, until the machine is in production where data can be collected over long periods, the more simplified method as described by John Raffaldi should suffice.
Also, what about uncertainty of the measurement equipment when doing GR&R? Is it a simple matter of subtracting the uncertainty of each device based on the supplier's data from the total tolerance of the test being performed? Uncertainty is another area that to me is neither well understood or explained.
I'm open for constructive criticism and instruction. Thanks.
P.S. The customer is just as confused.
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