iSixSigma

How to calculate Cpk

Six Sigma – iSixSigma › Forums › Old Forums › General › How to calculate Cpk

Viewing 9 posts - 1 through 9 (of 9 total)
  • Author
    Posts
  • July 2, 2007 at 2:12 pm #47433

    Murray
    ★ 10 Years ★
    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

    0
    July 2, 2007 at 2:33 pm #158116

    Omashi Sabachi
    ★ 10 Years ★
    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) 

    0
    July 2, 2007 at 3:14 pm #158120

    Makiwari
    ★ 10 Years ★
    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?

    0
    July 2, 2007 at 6:13 pm #158124

    MM
    ★ 20 Years ★
    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?

    0
    July 2, 2007 at 7:24 pm #158128

    Omashi Sabachi
    ★ 10 Years ★
    Participant

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

    0
    July 9, 2007 at 2:26 pm #158380

    MrKid
    ★ 20 Years ★
    Participant

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

    0
    July 9, 2007 at 5:45 pm #158390

    annon
    ★ 10 Years ★
    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.

    0
    July 12, 2007 at 2:24 pm #158527

    k.bhadrayya
    ★ 20 Years ★
    Participant

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

    0
    July 12, 2007 at 9:06 pm #158559

    Omashi Sabachi
    ★ 10 Years ★
    Participant

    Thank You

    0
  • Author
    Posts
Viewing 9 posts - 1 through 9 (of 9 total)

The forum ‘General’ is closed to new topics and replies.