Process Capability Training and Explanation 1

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Purpose and Contents 3 Purpose: To get a basic feeling on capability studies. Contents: Normal Distribution Scattering Process Potential Machine Capability Process Capability Preliminary Process Capability Formulas for capability and potential Customer and internal requirements

The Normal distribution 4 Sigma = 5  = 50 Assume you have a chicken farm and you measure the weight of the eggs. Their weight is around 50g – means some eggs have only 45g but some have 55g. If you make a distribution curve where you show how many eggs have what weight, you will get following curve.

Capability 5 Tolerance Scattering The Capability is the ratio of Tolerance to Scattering

How to measure scattering? 6 +-Sigma +-2 Sigma +-3 Sigma Assume you have a normal distribution – then 99% of all data lie within +- 3 sigma. The capability is 1 if ~99% of the data lie within the tolerance (+- 3 sigma). The capability is better than 1 if more than ~99% of the data lie within the tolerance.

Potential versus capability 7 The better the data lie in the middle of the tolerance the better the capability. The potential is what we could achieve if the scattering is the same but the data would lie in the middle.

Different capability types 8 Cmk - Machine capability Short term study. Take a sample of e.g. 50 parts as they are produced, measure the characteristic you are interested and calculate the Cmk. Cpk - Process capability Long term study. Take samples over a long time (e.g. 3 month; e.g. each week 5 parts), measure the characteristic you are interested and calculate the Cpk. Ppk – Preliminary Process capability Mid term study. Take samples over a mid time (e.g. 3 weeks; e.g. each day 3 parts), measure the characteristic you are interested and calculate the Ppk.

Capability formulas 9 The formulas for Cpk and Ppk are the same as for Cmk.

Potential formulas 10 Cm = USL - LSL 6 s See visualisation of capabilityvisualisation of capability The formulas for Cp and Pp are the same as for Cm. A comprehensive article about capabilities can be found at

Capability requirements 11 Our Customers’ requirements are: Cpk > 1.00 Cpk for safety items > 1.33 Our requirements should be: Cpk > 1.33 Cpk for safety items > 1.67 SigmaCpkppm 31, , ,670,5 62,000,002