Confused about the machine capability index Cmk

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qualityengineer

Dear All,
I have seen different commetns in the forums about machine capability index (Cmk). Some of these say that it is calculated just as the Cpk, the only difference being you keep all conditions but the machine itself the same (which is very difficult). Some other say that it is calculated by dividing the upper/lower tolerance minus the process average by four sigma (unlike three sigma when calculating cpk) because it containes less variation. Both sounds reasonable but is there any generally accepted approach in automotive industry (for ppap process)?
Thanks
 
Miner

Miner

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See Cmk - Machine Capability Index

The formula is the same as for Cpk with one distinction. Cpk uses sigma derived from Rbar/d2 of subgrouped samples, while Cmk uses the RMS sigma (same sigma as on your calculator) of a single sample of consecutive parts.
 
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