# When to perform SPC study? (Relation with cp,cpk)

Six Sigma – iSixSigma Forums Old Forums General When to perform SPC study? (Relation with cp,cpk)

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• #50861

Onur Öncül
Participant

Dear Experts,
I know that capability and stability of a process are completely different concepts but I wonder if cp and cpk values can be accepted as a measure for deciding to start a SPC study or not? If yes, then what cp-cpk value is enough to be in safe region. (especially for automotive industry)
For example, if my cp and cpk values are 3, is it really necessary to perform a SPC study of that process?
Best Regards
Onur Oncul

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#175406

Participant

I don’t exactly know what you mean by “SPC Study”.
SPC is a control tool. If your CpK is 3, then you still need to control the process. How much, how often, SPC or another tool?  You have not given enough information for a good evaluation.

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#175408

Onur Öncül
Participant

Ok you’re right. “SPC” is enough. There is no need to say “SPC Study”.
I aggree that SPC is a control tool which gives us information about stability of the process. As all we know, LCL and UCL are computed by using measured values only. That means LCL and UCL is not related with the tolerance. Result: SPC examines the process only within itself.
I quoted a phrase from isixsigma.com below. Ken describes the situation perfectly
“The value itself can be thought of as the amount the process (car) can widen before hitting the nearest spec limit (garage door edge).Cpk=1/2 means you’ve crunched nearest the door edge (ouch!) Cpk=1 means you’re just touching the nearest edge Cpk=2 means your width can grow 2 times before touching Cpk=3 means your width can grow 3 times before touching” Ken K.
If you have a hangar, does size of your car really matter by means of parking difficulty? (Hangar: Specification limits, Size: variation within the process)

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#175410

Severino
Participant

The short answer to your question is no.  The reason is because your Cp, Cpk values are meaningless unless you have shown your process is in control (stable).

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#175415

annon
Participant

Jsev607 is correct:  your process will be in only one of four states – stable & capable, unstable and capable, stable and incapable, unstable and incapable.  A capability assessment of unstable data means you are looking at data that is unpredictable….which means the data you are using in your capability assessment will change….hence, your capabilty measures will change.  That is why LSS teaches that you prove stability first, capability second.

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#175446

Onur Öncül
Participant

Really helpful explanations, thank you all.
Second Case: Let’s assume that my process was proven to be stable in the long term. And also pp and ppk values has reached 10. is it still necessary to continue SPC?
I ask these questions to utilize my spc team’s workforce efficiently

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#175447

Eddy
Participant

1. Is it possible Cpk and Ppk value give a negative value?
2. What does this means? Is it wrong calculations or spec issue?

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#175448

KPK
Participant

This is quite possible to have a Cpk or Ppk value as negative. This means that process is in bad shape as average of the data against capability study falls beyond specification limit.When cpk is zero this means that process average and one of the specification limits are same i.e these are getting superimposed.
KPK

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#175463

annon
Participant

The more capable your process (and I would call a process with a Cpk of 10 extremely capable) the less SPC is required.  What is sufficient a sufficient level of control will be a business decision:  what is the cost of SPC efforts vs the result of a shift in your process mean?  Go from there.

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#175473

Six Sigma Shooter
Member

Michael,
Deming and Shewhart are quite clear on the subject that SPC is not just a monitoring / control tool – it is a continual process improvement tool.  If we take the limited view that is is just for process control and monitoring, we miss out its their true power:  variation reduction and improvement.
Just my \$0.02 worth,
Shooter

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#175478

Brian M
Participant

The process needs to be shown to be in control before the CPk has any meaning. I have seen data that shows an acceptable CPk value that is making parts out of spec (the process was out of control btw). The problem comes from small within variation and large between variation giving you a unrealistically low std deviation. Bottom line, you should chart your process, demonstrate that it is stable and then report its capability.

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#175492

Rajesh Verma
Participant

If Cp & Cpk is 3 then there is no need to study.

Regards
Rajesh Verma

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#175507

Nik
Participant

Excellent point “Six Sigma Shooter!”
Having a stable process is one of the assumptions that goes into your process capability assessment.
Now SPC is not just a stability/control tool, it can also be used for root cause analysis and evaluating solutions independently of assessing the capability of the process.
It’s all part of using the tools to solve problems.

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#175519

Rhineg
Member

SPC should not be a yes / no question. Do you ever want to give up control? Your real issue should be: when Cpk shows excellent capability, what adjustments to sampling rate can I tolerate to reduce control costs without jeopardizing quality of delivery. With a Cpk of 3 you might make some small reductions in sampling frequency, while with Cpk of 10 you might make some larger reductions. HOWEVER, make sure you establish some signal points to return to higher sampling rates when Cpk decreases.

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#175576

Onur Öncül
Participant

Best Regards
Onur Öncül

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#182276

Participant

We have so many machines with less cp & cpk , so what are rhe steps are to be taken to improve these values?  this problem regards to grinding machines

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#182282

Craig
Participant

Just curious as to the percentage of out-of-spec material you are producing across all grinding machines? Do any of the grinding machines have acceptable cpk values?
Some basic first steps:
Look at your data and see what the major sources of variation are ( within piece, piece to piece, etc)  If you didn’t collect your data to show this….design a multi-vari study and run it.
I could just say “Apply six sigma methodologies!”  I assume the M phase is complete and you know that your measurement system is capable!

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#182283

Ron
Member

Onur,
A process or machine capability study should be performed upon initial purchase of a piece of equipment and after any significant maitenance operation perfomre oh that equipment prior to putting it back into production service.
As a general rule I try to run a study each quarter and monitor the process with SPC chartage in the periods between running the studies.
If you follow this refgimen you will be continuously running someform of “SPC” .

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#182295

Chris Seider
Participant

Are you serious?  If the Cpk, or more appropriately Ppk, was 3ish, then why run SPC on that process parameter?  Obviously the process has enough controls–unless it’s a case of overly lax standards by the customer.
I’ve successfully eliminated wallpaper SPC on processes for those with Ppk above 2 if reasons for success were known–the control system being in place.

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#182296

Fen
Participant

Just my 2c, it still depends on the RPN. If you’re controlling a safety critical component, ie severity 9 or 10, you may want to continue SPC.

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#182300