Hardness Testing - Low Cpk's using Minitab

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Curtis317

#1
Our supplier had provided there test results. Using Mini tab I calculated the Cpk's. Of course with hardness testing the only specification limit is a minimum. But the Cpk's are .96, .7 and .710. that seems extremely low. These are 3 measurements taken on different portions of the part. If we tried asking the supplier to improve the consistency of the hardness would it increase the hardness overall to get a better Cpk? Notes: yes the distribution is normal for all 3 measurements. The Cpk was done on 95 measurements over time.
 
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sitapaty

#2
Re: Hardness testing low Cpk's

Calculation of cpk for a single limit tolerance ie only + side or only minus is theoretically misleading.
 
C

Curtis317

#3
Re: Hardness testing low Cpk's

Sitapaty, Your comment has no validity. If I redo the calculation with an upper limit the Cpk stays the same.
 
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Miner

Forum Moderator
Staff member
Admin
#4
Our supplier had provided there test results. Using Mini tab I calculated the Cpk's. Of course with hardness testing the only specification limit is a minimum. But the Cpk's are .96, .7 and .710. that seems extremely low. These are 3 measurements taken on different portions of the part. If we tried asking the supplier to improve the consistency of the hardness would it increase the hardness overall to get a better Cpk? Notes: yes the distribution is normal for all 3 measurements. The Cpk was done on 95 measurements over time.
Most likely the supplier will just increase the hardness.

Reducing variation would be difficult. Have they done an R&R study on the tester? Depending on the type of equipment they might be able to improve (e.g., go from hand held to bench top), but if they are using the latest equipment will be limited.

There is probably considerable within part variation. If there are critical locations, they might improve by specifying a test location. Finally there is usually a lot of part to part variation from the location in the HT oven.
 
C

Curtis317

#6
The material is steel that is case hardened. The people who are measuring it specialize heat treatment. So I would think they know what they are doing but do not really appreciate statistics and how it can show you that with a low Cpk you have a potential for 20,000 PPM under specification.
 

ScottK

Not out of the crisis
Staff member
Super Moderator
#7
You're crunching the data from a process you don't control.
If they don't care about the statistics and only care if they meet a minimum hardness I think it's going to be tough to get the number you're looking for.
Especially if their data say "we've never had a complaint for under spec material so why should I care about this Cpk value?"
You might have to train them and contractually agree, or find another supplier who gets it.
 

Steve Prevette

Deming Disciple
Staff member
Super Moderator
#8
The material is steel that is case hardened. The people who are measuring it specialize heat treatment. So I would think they know what they are doing but do not really appreciate statistics and how it can show you that with a low Cpk you have a potential for 20,000 PPM under specification.
One thing - have you looked at the time series SPC chart to see if the data are stable and predictable prior to calculation of the Cpk.
 

Bev D

Heretical Statistician
Staff member
Super Moderator
#9
First remember the goal- no nonconforming parts. The goal isn't to get a high Cpk. Cpk is a silly number that requires many theoretical things to be met to have real world validity...

Most people don't understand statistics because they are conceptual. You need to make them concrete...graphs of the data do this. Can you provide the data you used with the appropriate labeling so we can plot it in time series with the within piece data identified and maybe even the oven locations and lot numbers? A multi vari chart is the best start for analyzing the capability of a process.
 

Ron Rompen

Trusted Information Resource
#10
Capability on a one-sided specification (where it is possible to go outside of that limit) is very difficult to accomplish. As you know, Cpk is derived (in part) from Cp, and if you do not have an upper and lower limit, you can't calculate Cp. Introducing an artificial limit doesn't help, as it can influence the results without really being meaningful. I do not know of a statistical model which can take something like that into account and still calculate Cp and Cpk accurately.

Have you tried plotting your suppliers measurements as a run chart, with precontrol limits (based on historical data from them) to demonstrate that their process is within control? That may be the easier way to do this.
 
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