# How to calculate Cpk

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Viewing 9 posts - 1 through 9 (of 9 total)
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• #47433

Murray
Participant

I have some confusion in calculating Cpk of one of my grinding process. It is mentioned in the drawing that surface roughness should not be less than 0.6 Rz. Now we have minimum limit here but of course if one gets better than that like 0.5, 0.4 etc, then it is more better. So in this case upper specs limit is not mentioned. Now  in the formula of Cpk, we have USL and LSL. My question is, we have lower Specs but not upper. What to do now, how to calculate Cpk.
Thanks for any comments.
AL

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#158116

Omashi Sabachi
Participant

Try  to  use  the  formula
(X-bar) – LSL divided by 3s (SD) (it  could  be  used  when  the   specification is  a  minimum  tolerance (unilateral tolerance)

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#158120

Makiwari
Participant

How many fomulae does Cpk have? Is it one or two? Why does it have two?
Now that you’ve thought about that how do you interpret Cp?
What other measures can you use?

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#158124

MM
Participant

hi Omashi – wanted to understand here and as well try to answer Mak….
There are two formulas to find out (in my easier terminology) outliers from USL and LSL.
Per me, Omashi has given the formula for LSL cos u have specified that there is only LSL identified in your process and there is no USL.
As far as I remember, formula for USL is USL-x bar divided by 3SD.
Omashi am I correct?

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#158128

Omashi Sabachi
Participant

You calculate  both  figures  ,but  you  consider  the  smaller  one only

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#158380

MrKid
Participant

Allen,
If you have a one-sided lower spec, then Cpk=(Xbar-LSL)/3s.

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#158390

annon
Participant

Just to kick this horse one more time, a simpler definition for Cpk would be to use the distance between your mean and nearest specifcation limit divided by (3) short term standard deviations  (ie remember to subgroup).  Also, you will need to consider stability and normality.

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#158527

Participant

Dear Allen
Go with the formula given by Mr.Omashi. . That is correct.

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#158559

Omashi Sabachi
Participant

Thank You

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