PROCESS CAPABILITY – A SIMPLE ILLUSTRATION

21 April 2016: Wolfgang Schultz

Process capability describes the ability of a (production) process to produce characteristics meeting the specifcation. You usually conduct a process capability analysis to establish this capability. Process capability analysis is even used as a generic term for machine performance studies and process capability analyses, i.e. a two-stage approach.
 
In order to perform a machine performance study, a machine or production facility normally produces 50 parts in one continuous production run over a short period of time. The influencing factors remain quite stable while the values of the characteristic to be inspected are measured. In many cases, the values approach a normal distribution function. Now you determine average and standard deviation s. You need them to calculate capability indices C_m and C_mk. When the values are normally distributed, the capability index C_m is the ratio between the tolerance range T and the sixfold standard deviation 6s. It thus refers to the relation between the variation range and the tolerance. The capability index C_mk is calculated from the ratio between the shorter (“critical“) distance from the average to the upper or lower specifcation limit USL / LSL and the threefold standard deviation 3s. C_mk considers the variation range and the centred process average within the specifcation. Non-normally distributed values require a different calculation method as defned in the ISO 22514 series of standards. The result of a machine performance study is also referred to as short-term capability. As an example, ISO 22514-3 recommends to establish machine performance based on 100 observations. ISO standards apply the symbols P_m and P_mk to short-term capability indices where P refers to performance.

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