Sum of Cpk - Updating Product Tolerances

O

Oskar_S

#1
Hi

Has an interesting problem.

I'm updating the height tolerances on one of our products. The product is build up of plates and the tolerances are based on a linear asumption.

NoP=Number of plates

Height=2.24*NoP +4 ,with tolerances LL -1.5% UL +0.05%

I have measured the height of 300 units with 8 different number of plates (30,36,40,50,60,80,100 and 110 plates) and calculated a cpk for each NoP. Now I want to pool the 8 different Cpk together so I have one Cpk for the product for all NoP.
 
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#2
Now I want to pool the 8 different Cpk together so I have one Cpk for the product for all NoP.
Report the lowest Cpk. Also note that it will not accommodate between lot variation, so you will likely be reporting a Cpk you can not meet over time.
 
K

kaikai

#4
Is there a tendency at the Cpk in the series of 8 different number of plates ?

If there are sufficient amount of data, it's highly likely that there is confirmable trend.
 

Miner

Forum Moderator
Staff member
Admin
#5
why would you use different numbers of plates?
Products like electric motors will vary the number of laminations (plates) to change the electrical performance of the motor. This may be a similar situation.

Oskar_S,
I would consider other options. In your situation the tolerance and the variation are both increasing as you increase the number of plates.

Try plotting the Cpk against the number of plates. You may be able to establish a regression equation (with confidence limits) for the Cpk based on the number of plates.
 

Bev D

Heretical Statistician
Staff member
Super Moderator
#6
Products like electric motors will vary the number of laminations (plates) to change the electrical performance of the motor. This may be a similar situation.
yes that's why I asked - it gives other responders a better understanding of the OP's situation.

*I* would also like to know more about why is performing a Cpk study; what is the goal? that too helps us better answer the real question the OP has...we do see posters here who ask a qustion that seems a little odd and upon further investigation we find that the poster has started down a path that can't really get him to his desired end state.
 
O

Oskar_S

#7
Hi

Thanks for your answers. Now to the questions:

We are producing plate heatexchangers (capacite is proportional to number of plates, one model can have from 10 to 120 plates)

We have today a very tight tolences on the hight (nominal value +1,5% -0.5%) due to our design department in the past just took tolences without checking our capability.

I have plotted the cpk for different number of plates for different % tolerances and proposed a new set of tolerances based on the number of plates with lowest cpk.
 
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