Cm and Cmk

machine capability

Cm = (USL − LSL) / (8 · s), Cmk = min(USL − x̄, x̄ − LSL) / (4 · s)

Machine capability is measured on a short, undisturbed run of usually 50 consecutive parts on one machine, for example at machine acceptance. Because the run excludes long-term influences, VDA volume 4 uses 8 and 4 standard deviations instead of 6 and 3, and typical requirements are Cmk ≥ 1.67 or 2.0.

Calculate it

Process Capability & Control Charts: Cp, Cpk, Pp, Ppk, PPM and confidence intervals with X-bar/R, X-bar/S or I-MR charts and all 8 Nelson rules.

Related terms

• Cpk: process capability index