CPK <--------

[Underspec]

Someone please explain the following about CPK:

- The origin and history.
- What it actually stands for, the C, P, K?
- Anything else that might involve it.

Thanks...

[ag1979]

Cpk - Process Capability Index
Cp - Process Repeatability

Cp - this value is based on the range of measured values and your tolerances. It basically tells you whether or not your process is repeatable. High value good low value bad. As a standard we need a Cp of 1.67 or higher.

Cpk - this value is based on the range of measured values, the average of measured values, and your tolerances. It basically tells you the odds of your parts being out of spec. High value good low value bad. As a standard we need a Cpk of 1.67 or higher.

Generally you need a minimum of 25 values (parts) in order to achieve an accurate value. For the most part you need a good Cp value. If you don't, usually this tells you that there is a problem with your process. Something is making your process unstable.

If you want more detailed info just re-post. This can be quite the lengthly topic.

[rebeldude]

This study was actually invented the the -----. But we had to start using it also to stay in competition. For most parts of the trade, this study is a waste of time, but I get paid by the hour

[ufo_pilot]

Should you use an excel sheet to calculate this formula, I believe it goes something like this:

=IF(USL-AVE>AVE-LSL,(AVE-LSL)/(Stdev*3),(LSL-AVE)/(Stdev*3))

[craiger_ny]

in Excel Cpk = MIN(((USL-AVERAGE)/(3*STDEV)),((AVERAGE-LSL)/(3*STDEV)))

[John Kingston]

I believe manufacturing industries began using Cp and Cpk in the 1920's. I've looked through a number of books I have that cover Quality Tools. The only one that mentions any date and people, is the AT&T Statistical Quality Control Hanbook. In the preface it states "The techniques described are essentially those which have been used in all types of industry since their development during the 1920's by Dr. Shewhart."

I hope this answers the history and origin part of your question. I know there haev been articles covering the history of Quality Control in Quality Progress magazine from The ASQ. I didn't have time to look through those. I will in the next few days/weeks, as you piqued my interest.

[Underspec]

I remember reading somewhere that the (k) stands for something that is actually latin. What it is, I can't remember...maybe someone can ?

[craiger_ny]

The best I could come up with is that the k comes from "katayori" which means deviation, inclination or offset in Japanese. I have only undug this from on-line searches, nothing verifiable.

Craig

[A Disho]

Something abour - cp & cpk values:

Process Capability :

·Cp : This is the Capability index which is define as tolerance width divided by the Process Capability, irrespective of process centering.


Specification Range USL - LSL
Capability Ratio = --------------------------- = -------------------- = Cp
Process Capability 6 σ




·Cpk : This is the Capability index which accounts for process centering and is defined as the minimum of Upper Capability Index ( defined as the upper tolerance spread divided by the actual upper process spread) or lower Process Capability ( Lower tolerance spread divided by the actual lower process spread ).

* Useful for measuring continual improvement using trends over time.
·Determining whether or not a process is capabile of meeting customer requirement.











--------------------------------------------- USL / UCL Marginal process
when NO room for any error
A - ----------------------------------------------- X process centered and NOT SHIFTED
6 σ
---------------------------------------------- LSL / LCL CP = ------------ = 1
6 σ





------------------------------------------------ UCL

________________________________ USL Process Incapable

Actual Variation Not good process

B - ------------------------------------------------X CP Less than 1

Allowed Variation
_________________________________ LSL


------------------------------------------------- LCL





_________________________________ USL
1 σ
---------------------------------------------------UCL Good Process

Actual Variation
2σ
C - -------------------------------------------------- X CP Greater than 1

2σ

----------------------------------------------------LCL
Allowed Variation 1σ
___________________________________LSL




Understanding Process Capability and Process Performance :

The output of statisticlly stable ( in control) process can be described by its distibution ,
A-if the distribution is not properly located to the center(they are not close enogh to the target value
in such case ,some parts may even be out of cpecification ( process Incapable of meeting the customer’s needs.
B-If the distribution has too much spread,regardless of where the distribution is located( process Incapable)-charasteristics of the distribution are not known exactly,data must be gathered to estimate them.

· Indices of process variation only,relative to specifications: Cp and Pp .


UCL - LCL
CP = ---------------------
6 σ



* Indices of process variation and centering combined ,relative to specifications: Cpk and Ppk .


= =
USL – X X - USL
Ppk = ------------------------- OR ------------------
3 σ 3 σ