Process capability or Process control - Which comes first?

[ranvir.jaryal]

Hi all,

I would like to understand that in case of new process , what comes first , process control or process capability?

 

[jrubio]

Re: Process capability or Process control,what comes first?

Hi all,

I would like to understand that in case of new process , what comes first , process control or process capability?

You have mentioned difference concept:

process control. You have an index wich measure the efficiency of the process. i.e Number of G8D send within Time.

process capability. You have a charasterist witch control your process and this charasterist have a constrains Limit and therefore must be under these Limits.Sometimes the charasterist of the process is not in the process itself alythough is in the part.

Therefore are complementary.

i.e: Process: Stamping

process control: Oil Temperature less than 15ºC
Process Capability: width of stell within Tolerence and Cp Cpk of 3.

When the process Control Charasterist is not attribute is continual and it could be fix up Tolerance Limits the Contol process and process capability would be the same.

 

[Tim Folkerts]

I have to disagree with Javier.

Control means that you produce a product consistently. That there are no "special causes" of variation. This is typically determined by a control chart, like x-bar or p chart.

Capability means that you product a product consistenly within the specifications. This is typically determined by calculating various capability indices, like Cp or Ppk.


First you should determine that the process is in control. Then you can calculate the appropriate capabilty indices. If the process is out of control, then the capability will be changing. There is no point in calculating and reporting a value for Cpk if you expect the value will changing unpredictably.


Tim F

 

[CarolX]

Maybe I am wrong here, but this is how I see it

Capability - does the process have the ability to make the part.

Control - what checks are in place to keep the process capable of producing part.

 

[Al Rosen]

You need to have a stable process before you can ascertain its capability.

 

[wmarhel]

You need to have a stable process before you can ascertain its capability.

I agree with Al. If you reference Measuring Process Capability by Davis R. Bothe, he even states as much. I can't quote the exact excerpt as I don't have my copy handy, but I would be willing to reference the page and paragraph when I get home later.

Wayne

 

[jrubio]

Tim:
Tell me what do you think about my comments in red

I have to disagree with Javier.

Control means that you produce a product consistently. That there are no "special causes" of variation. This is typically determined by a control chart, like x-bar or p chart.

Do you mean Statical Process Control?

Capability means that you product a product consistenly within the specifications. This is typically determined by calculating various capability indices, like Cp or Ppk.

Ppk (Un-stable) Cpk (Stable)

First you should determine that the process is under Statical control. (That menas that fits a statical distribution known) Then you can calculate the appropriate capabilty indices. If the process is out of control, then the capability will be changing. There is no point in calculating and reporting a value for Cpk if you expect the value will changing unpredictably.


Tim F

 

[Tim Folkerts]

Maybe I am wrong here, but this is how I see it

Capability - does the process have the ability to make the part.

Control - what checks are in place to keep the process capable of producing part.

I was thinking specifically about statistical process control when I was saying "control". As you note, it can also be used to mean the checks that are in place, as in "document control" or "control of non-conforming products".

For "capabilty", I would explicitly add "ability to make the part to the customer's specifications." It is almost implied that if you are making something, that you are making it to satisfy a particular expectation, but it never hurts to be overly clear.

Ppk (Un-stable) Cpk (Stable)

I have a feeling that Cpk and Ppk are among the most abused and confused terms in quality engineering. And I can't say that I am the best authority on the proper use and meaning of the terms.

In my understanding, Ppk and Cpk are two different measures of capability that tell you two similar but not identical things about a process. Both should only be calculated when the process is reasonably stable and "in control".


Tim F

 

[jrubio]

You need to have a stable process before you can ascertain its capability.

Generally I agree but if you have a none stable process you can calculate the capability index also the problem is that this capability is named ppk instead of Cpk

 

[jrubio]

I was thinking specifically about statistical process control when I was saying "control". As you note, it can also be used to mean the checks that are in place, as in "document control" or "control of non-conforming products".

For "capabilty", I would explicitly add "ability to make the part to the customer's specifications." It is almost implied that if you are making something, that you are making it to satisfy a particular expectation, but it never hurts to be overly clear.



I have a feeling that Cpk and Ppk are among the most abused and confused terms in quality engineering. And I can't say that I am the best authority on the proper use and meaning of the terms.

In my understanding, Ppk and Cpk are two different measures of capability that tell you two similar but not identical things about a process. Both should only be calculated when the process is reasonably stable and "in control".


Tim F

Tim I disagree

Ppk is the capability when the process is not stable. Please Refer to
PPAP Manual page 7 Third Edition.

When the process becomes stable the index must be Cpk.

Please read page 7