What is the value of Cpk for a new process?

Jim Wynne

Staff member
Admin
#11
austin999 said:
Hi Jim

my question was 'What is the value of Cpk when creating a new process?'
This was the question asked to me..
Asked to you by whom?? Did you ask that person what, exactly, it was that he/she wanted to know? Did he mean what should the target value be? Do you understand the basic concepts behind process capability indices? Does he? We want to help you, but you really need to give us more information. The question, as you've asked it, can't be answered.
 
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A

austin999

#12
Jim,


This was the question asked to me by an employer for a job position.
The question was 'What is the value of Cpk, when creating a new process'?
he e-mailed that question to me and asked me to reply him with answer.
He has not given any details regarding the new processs.
I know we can calculate Cpk with the help of data.

Could you please help me to figure out some possible solutions for this.
If we can't answer this question without any data provided, what are the questions i can ask him regrading this question?

Thanks in advance.
 

Miner

Forum Moderator
Staff member
Admin
#13
austin999 said:
Hi Jim

my question was 'What is the value of Cpk when creating a new process?'
This was the question asked to me..
Are you asking for the minimum acceptable Cpk value of a new process, or how to estimate the Cpk of a new process? Your question is still unclear.

If the question is "What is the minimum acceptable Cpk for a new process?", 1.67 is commonly used, but you may set your own requirement.

If you are asking how to estimate the Cpk of a new process, the only way that I have ever seen is to use the capability of a similar process.
 
A

austin999

#14
I believ he asked me 'what is the minimum acceptable Cpk value for new process?'

You said the minimum value is 1.67.
How did you get that?
What i know is Cpk=allowable range/6sigma

Could you please explain that to me?

Thank you very much.I really appreciate that.
 
R

ralphsulser

#15
A CPk of 1.67, and 1.33 are the standard automotive targets, or ask the customer what is expected if not automotive. If your customer does not have a specific requirement, then one thing I have done in the past was to set 70 or 80% of the specification for upper and lower control limits if you have no data on a new process. The when you get enough data calculate the actual CPk.and control limits. Depends on the characteristics if they are designated as special or regular.
 
A

austin999

#16
Thanks guys for your help.

If possible could you please explain how do you get 1.67 or 1.33.
Can you give brief description of Cpk.
What i know was Cpk=min(Cpu, Cpl).
when is Cp=Cpk?

I appreciate your help.
 
R

roland_lu

#17
austin999 said:
Thanks guys for your help.

If possible could you please explain how do you get 1.67 or 1.33.
Can you give brief description of Cpk.
What i know was Cpk=min(Cpu, Cpl).
when is Cp=Cpk?

I appreciate your help.

Hi, Pal

please do not be offended, I guess you are simply too lazy. Jim has pointed out good reference mat'l in this site. All you need is just spend little time going through it, it will help you understand the basics and the meanings behind the numbers.

Good luck with your study.
 
V

Vicman

#18
Cpk

New process? do you mean a new process for a product that you use to built with an old or actual process?
In this case there must be enough information for the target values of CPK´s of the new process, due to the early manufacturing experience.
Or, is it a process for a product with specific requirements from your customers?
Then there is no other choice to meet your customer requirements. Normally this is a agreement when the
job has been awarded, or when the partnership between supplier and customer started. Some companies are very specific on what the values of CPK´s should be in the short and long term.
Finally if you talking of a entirely new product that the company is manufacturing for the first time, then there is a need to run the process first, to get a estimate of what the CPK´s value are.

Vicman
 
K

knight_sirk

#19
Hi to all! :bigwave:

First of all i would like to restate that
"there has been no unified formal approach in the
automotive industry on statistical process control"
- AIAG SPC Manual

Cpk - The capability index for a stable process. The estimate of sigma is based on within subgroup variation.

Ppk - The performance index. The estimate of sigma is based on total variation.

For an entirely new process, your process is not stable since you are just evaluating/studying the process. Meaning Cpk should not be calculated and an initial process study should first be conducted.

The purpose of the initial process study is to understand the process variation, not just to achieve a specific index value.

After you understand the process variation, you should Identify, evaluate and, wherever possible, eliminate special causes of variation.

Compute for the Ppk to understand if your current process is capable to meet the customer's requirement, Cpk.

Think of this, if you achieved a Ppk > 1.67 from your initial process study,
what more it would be in a stable process to achieve Cpk > 1.33.

:truce:
 

Jim Wynne

Staff member
Admin
#20
knight_sirk said:
For an entirely new process, your process is not stable since you are just evaluating/studying the process. Meaning Cpk should not be calculated and an initial process study should first be conducted.
This is absolutely wrong. There is no mathematical reason that enough data can't be collected in a new process to confirm stability and calculate Cpk.

knight_sirk said:
The purpose of the initial process study is to understand the process variation, not just to achieve a specific index value.
You're quoting from the PPAP manual, and this is true as far as it goes, which is not very far if the customer requires a minimum value, in which case the goal is to understand the process and achieve the target Cpk value.
 
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