Tool Approval Process - Should we be using Cmk for tool validation

#1
I am not sure if this is the right forum, but here goes anyway. I have been asked to look at a procedure for a Tool Approval Process. There is one point in particular that I would like advice on. For critical characteristics the requirement is that a CMk value of 1.67 is achieved. The question I am asking is should we be using CMk at all? To my mind, this tells us nothing about the process, only about the machine. If the tool is moved to another machine, the whole thing needs to be done again! I would have thought it better to do a process evaluation, i.e. CPk or PPk. Two, is the value of 1.67 sufficient? Do you think that this is a very good way to go?

Any advice and guidance appreciated.
 
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#2
DJN said:
I am not sure if this is the right forum, but here goes anyway. I have been asked to look at a procedure for a Tool Approval Process. There is one point in particular that I would like advice on. For critical characteristics the requirement is that a CMk value of 1.67 is achieved. The question I am asking is should we be using CMk at all? To my mind, this tells us nothing about the process, only about the machine. If the tool is moved to another machine, the whole thing needs to be done again! I would have thought it better to do a process evaluation, i.e. CPk or PPk. Two, is the value of 1.67 sufficient? Do you think that this is a very good way to go?

Any advice and guidance appreciated.
If there are other folks here who've used CMk to good advantage I'd like to hear about it as well. Personally, I think it's an evil plot hatched by a statistician with too much time on his hands:lol: .
If you take the process approach, understanding the variability inherent in the indivdual components of the system is important, but most of the time I think it's intuitive (as to machine capability) or a matter of experience, and not worth a lot of effort. If you've done your APQP due-diligence and you find that your process doesn't yield at the expected level, your improvement efforts might lead to machine issues, but in my experience those are rare, or represent something that should have been anticipated.
Like I said, if someone has had different experience, please tell us about it.
 
#3
Thanks for the reply. I too have serious doubts as to the validity of CMk. This procedure is for a general tool validation process, not just for automotive. I think I will press for a change of direction, i.e. process validation. Machine capability studies may have a place in determining the boundaries, but any 'dodgy' dimensions should be followed up with a process study, at least that is my humble opinion. Could I say that machine studies have largely been superseded by process studies?
 

Evelyn7E

Starting to get Involved
#4
I am not sure if this is the right forum, but here goes anyway. I have been asked to look at a procedure for a Tool Approval Process. There is one point in particular that I would like advice on. For critical characteristics the requirement is that a CMk value of 1.67 is achieved. The question I am asking is should we be using CMk at all? To my mind, this tells us nothing about the process, only about the machine. If the tool is moved to another machine, the whole thing needs to be done again! I would have thought it better to do a process evaluation, i.e. CPk or PPk. Two, is the value of 1.67 sufficient? Do you think that this is a very good way to go?

Any advice and guidance appreciated.
DJN if you use the Cmk which is the machine capabilty, a good machine capability does not equal to good product capability which is Ppk. You have to validate the product capability Ppk which is base on individual measurement. Ensure it is normally distributed and represent the population
 

Johnny Quality

Quite Involved in Discussions
#5
DJN if you use the Cmk which is the machine capabilty, a good machine capability does not equal to good product capability which is Ppk. You have to validate the product capability Ppk which is base on individual measurement. Ensure it is normally distributed and represent the population
Evelyn,

What should we do if the data is not normally distributed?
 

Evelyn7E

Starting to get Involved
#6
Evelyn,

What should we do if the data is not normally distributed?
Johnny ,

1. You can request for bigger sample size, maybe about 50 or 100 and check again
2. Check if you the characteristic you are measuring if it is suppose to be normally distributed, almost all mechanical measurement should normal
3. Check if there is some screening to get the sample for measurement
 
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