Oddities of Cpk - Chicken parts

R

Ravi Khare

#11
I too am foxed with processes with 5+ Cpk not being within specs.

I totally agree with what Spaceman Spiff says. Most processes are subject to a systematic drift due to tool wear. It pays to set the process not at the centre, but closer to one end of the spec. The systematic tool wear will then gradually take the process to the other end of the spec zone. That way you can take the best benefit by producing maximum quantity at one tool setup.

However you need to be careful to see that the process already has a Cp of 1.33+, so that wide random variations and outliers do not result in out of spec work. If Cp is lesser than 1.33, you will still have to set the proces at the spec centre and keep fine tuning the centering more often in the run.
 
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V

vasilist

#12
Cpk continued...

At first i want to thank you all for your replies.

After one month searching (and the advices from you of course) i understood i was wrong about my approaching in the measurements i have done.

From an e-article i read that we use Cpk when the process is under statistical control. When the process isn't under statistical control we use Ppk.

I also want to thank God because if my supplier had a Quality Assurance Manager with experience in statistics i would have been searching for new job propably (...!)

Well the task was and is the following : the supplier gives chicken pieces. I gave him the weight specifications (in grammars). So i expect from him certain numbers between a range. I weight periodically pieces (100 each time in groups of 5) and i use MINITAB to check if he is following the specs.

That's what i have done without considering that hiw process was not under statistical control. So i think that until he reaches that point i will use Ppk.

I would be more than happy to read any other suggestions to measure his capability of following the specifications.

Vasilis
 
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