## EXPANDED MEASUREMENT UNCERTAINTY IN 100% INSPECTIONS

2019-06-19 - **How to apply the second edition of VDA Volume 5 in practice**

*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.