Process Capability Index – Cpk
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 This topic has 9 replies, 9 voices, and was last updated 15 years, 6 months ago by ssstudent.

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October 5, 2006 at 5:50 am #44795
Ron EllisonMember@RonEllison Include @RonEllison in your post and this person will
be notified via email.Hi Guys
Calculating a single Cpk value for an upper or lower specification is generally straight forward – apply the formula
When the process has more than one specification element, say two or three, the object has a height, a diameter and weight specification each with a upper and lower acceptable value.
Is it possible to calculate one Cpk value for the product or are they expressed individually, this is – a height capability, a weight capability and a diameter capability.
What is the formula for combining Cpk values
Appreciate any help on this issue
Ron0October 6, 2006 at 5:03 am #144338Cpk per characteristic should be calculated. If this is a machining process where multiple characteristics are generated on a single part, each chara. should be studied & capability determined according to standard statistical methods. ( even if on a mutliple station machine) Any chara., not in control, is adjusted and restudied.
Cpk of differing characteristics are not combined to get an overall Cpk. The process is no better than the worst Cpk of the like status (importance of) characteristics.
apples & oranges
0October 6, 2006 at 9:04 pm #144378Need to have a Cpk for each because each is considered a defect opportunity.
0October 7, 2006 at 2:23 am #144379Cpk is the index measure your process capability to achieve certain target. Therefore, when you perform a Cpk estimation using Minitab, you only need to key in the following
1. USL
2. LSL
3. Target
4. Historical mean (optional)
5. Known std dev (optional)Various specification integrated in the process will affect capability to achieve the “Out Come” You need to perform a DOE to optimize the process parameter setting, then only you collect data for Cpk stduy.0October 9, 2006 at 1:33 am #144405Thanks all for your comments – much appreciated
0October 12, 2006 at 5:52 pm #144638
k.bhadrayyaParticipant@k.bhadrayya Include @k.bhadrayya in your post and this person will
be notified via email.Dear Ron Ellison
If I can get your mail ID , I can attach my document where in the defects are measured by mesurement and count of defects for various caharacteristisc and calculated the overall Cpk value of the product. This will be self expalinatory. How to account the individual caharacteristic Cpk values to obtain an overall value of a product.
Thanks
K.Bhadrayya
0October 13, 2006 at 10:34 pm #144743Hi K
If you can post a fax number I can reach you on or fax your email to me on 61 7 47470445
THANKS0October 14, 2006 at 5:06 pm #144777Wrong!
This is as dumb and Praveen’s conjured sigma score.0October 14, 2006 at 10:26 pm #144782
Eric MaassParticipant@poetengineer Include @poetengineer in your post and this person will
be notified via email.In terms of its effect on composite yield, the composite Cpk could be approximated by min(Cpk1, Cpk2, …, Cpkn)
Skip Creveling’s company, PDSS, came up with a Capability Growth Index for Cp and for Cpk; if I recall correctly, the CGI for Cpk is something like:
CGI = 100% * Sum from i to n of { Min(Cpki, 1.5)/1.5 } / n where n is the number of critical parameters.
So, CGI ranges from 0 to 100%, and is clamped by 1.5 at the upper end (and presumably clamped by 0 at the lower end).0November 30, 2006 at 4:02 am #148179
ssstudentMember@ssstudent Include @ssstudent in your post and this person will
be notified via email.Hi Ron,
Cpk whenever calculated has to be done seperately for each specification element. It is the measure of capability of process to produce part within specfication w.r.t that particular element.
Ans these cannot be combined together.
0 
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