www stangerweb de Process Capability Training and Explanation
www. stangerweb. de Process Capability Training and Explanation 1
www. stangerweb. de Purpose of Easy. Tools There are lots of Quality tools around that can help improving many situations of the daily quality life. The problem with many tools is that you need training and experience to use them effectively. Easy. Tools is a collection of easy to use and mostly self explaining tools that everybody can use. No need for weeks of training or years of experience. 2
www. stangerweb. de Purpose and Contents 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 3
The Normal distribution www. stangerweb. de Assume you have a chicken farm and you measure the weight of the eggs. Their weight is around 50 g – means some eggs have only 45 g but some have 55 g. If you make a distribution curve where you show many eggs have what weight, you will get following curve. m = 50 Sigma = 5 4
www. stangerweb. de Capability The Capability is the ratio of Tolerance to Scattering Tolerance 5
www. stangerweb. de How to measure scattering? 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). +-Sigma +-2 Sigma +-3 Sigma The capability is better than 1 if more than ~99% of the data lie within the tolerance. 6
Potential versus capability www. stangerweb. de 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. 7
Different capability types www. stangerweb. de 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. 8
www. stangerweb. de Capability formulas The formulas for Cpk and Ppk are the same as for Cmk. 9
Cm = www. stangerweb. de Potential formulas USL - LSL 6 s The formulas for Cp and Pp are the same as for Cm. See visualisation of capability A comprehensive article about capabilities can be found at http: //www. itl. nist. gov/div 898/handbook/pmc/section 1/pmc 16. htm 10
www. stangerweb. de Capability requirements 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 Sigma Cpk ppm 3 1, 00 2700 4 1, 33 64 5 1, 67 0, 5 6 2, 00 0, 002 11
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