Can CPK be a good manufacturing processor?s quality objective?

Tim Folkerts

Super Moderator
#11
Re: Can CPK be a good manufacturing processor’s quality objective?

I for one would be very interested and might send the author of such an article some good cigars or golf balls.
:lol:

Unfortunately I don't smoke and I don't play golf. :nope:

Now, if you sent some golf clubs, then I might think about taking up golf! ;)
 
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ScottK

Not out of the crisis
Staff member
Super Moderator
#12
Re: Can CPK be a good manufacturing processor’s quality objective?

This thread is pretty good fopr explaining the various indices: http://elsmar.com/Forums/showthread.php?t=1994

I was thinking about trying to write an article explaining the various capabiltiy indices. My thought was to start with run charts of various sets of hypothetical data that illustrate particular good and bad features in data. After showing if the data was in control, I would calculate the various indices and discuss how and why they are different. I haven't seen such an approach and I think it might be a good way to get more of a gut feeling for the indices. If this sounds interesting/valuable to people here, it might spur me to make this a little higher priority. :yes:


Tim F
I'd be interested in your article.

One thing I'm finding in many places I work is that people interchange Cpk and Ppk calculations. MOre specifically, I find that most spreadsheets that claim to be calculating Cpk are actually calculating Ppk because it's easier in Excel to just use STDEV than to build in the table needed for estimating sigma with the R-bar.
 

Tim Folkerts

Super Moderator
#13
Re: Can CPK be a good manufacturing processor’s quality objective?

One thing I'm finding in many places I work is that people interchange Cpk and Ppk calculations. More specifically, I find that most spreadsheets that claim to be calculating Cpk are actually calculating Ppk because it's easier in Excel to just use STDEV than to build in the table needed for estimating sigma with the R-bar.

One furhter "lazy" reason for mixing up Ppk & Cpk is that in most cases the results will be practically the same. I've started generating some hypothetical data for the paper, and in the first few simple sets, the two have always been close. In fact, if the data is truly random and in control, the two should always be about the same (give or take the expected statistical fluctuation).

I can foresee circumstances where the two will be different, but I think that typically this will involve some sort of out-of-control signal from the control charts. As I generate & analyze more hypothetical data sets, I think a clearer picture should emerge.


Tim F
 

Bev D

Heretical Statistician
Staff member
Super Moderator
#14
Re: Can CPK be a good manufacturing processor’s quality objective?

One furhter "lazy" reason for mixing up Ppk & Cpk is that in most cases the results will be practically the same. I've started generating some hypothetical data for the paper, and in the first few simple sets, the two have always been close. In fact, if the data is truly random and in control, the two should always be about the same (give or take the expected statistical fluctuation).

I can foresee circumstances where the two will be different, but I think that typically this will involve some sort of out-of-control signal from the control charts. As I generate & analyze more hypothetical data sets, I think a clearer picture should emerge.


Tim F

they will be essentially the same IF you have rationally subgrouped as the between subgroup variation will be very small (essentially sampling error). sigma total = sqrt(sigma within subgroup^2 + sigma between subgroup^2)

however, many people don't rationally subgroup...
 

bobdoering

Stop X-bar/R Madness!!
Trusted Information Resource
#15
Re: Can CPK be a good manufacturing processor’s quality objective?

Again, for precision machining processes Cpk is not a good ongoing target measureable. You set the capability to its best economic value by setting the control limits to about 75% of the specifications. Increasing the 'capability' without an economic need actually increases overcontrol and variation from operator intervention - so it is not an improvement. In fact, for the uniform distribution continuous improvement would be dictated by reducing the slope of the sawtooth curve rise (reducing tool wear rate).

Something to ponder if you are in that kind of business. :cool:
 
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