Machine capability
Machine capability is a term from production technology that characterises the stability and reproducibility of a production step on a machine in production.
It allows a statement to be made about the proportion of rejects and rework to be expected when operating this machine.
The machine capability can be calculated during quality assurance via a CAQ system. Machine capability is closely related to the process capability indices Cp and CpK of a production process.
In order to determine the machine capability, a sample of workpieces (at least 50 pieces), previously determined according to statistical criteria, is produced under specified conditions and then the critical variables for the subsequent function of the part are measured. The external framework conditions and influencing factors must not change or must change only slightly. These include, among many other influencing factors, the operator personnel, the production process, the material (identical starting material, if possible identical batch), measuring equipment, temperatures and operating pressures.
The statistical distribution that applies to the sample is then determined; a normal distribution is often assumed for this purpose. In the next step, the position of the mean value and the dispersion of the measured variable are determined. Based on these two values, the machine capability is finally calculated, which is given as a dimensionless number. The larger this numerical value is, the more stable the production process is.
Basically, two different figures can be determined for machine capability:
- CmK gives a statement about how the mean of the sample lies to the given tolerance
- Cm shows how large the spread of the size is.
Ideally, Cm = CmK; this means that the mean value lies symmetrically in the tolerance field. In practice, there is often a deviation between the mean value and the nominal value of the size, so that the machine capability CmK is usually smaller than Cm.
The minimum machine capability value for a process to be suitable for series production depends on the specific company. Depending on the industry, there are standard specifications, such as those in the automotive industry set by the German Association of the Automotive Industry or the IATF. Common minimum requirements are values of 1.33 (corresponds to four times the standard deviation σ in a normal distribution) or 1.67 (corresponds to 5 σ).
In addition to this, there are requirements for process capability, whereby the minimum value for process capability must be the same as or slightly lower than that for machine capability, as the determination of process capability is subject to further realistic influencing factors (e.g. deviations of the material between different batches, different ambient temperature and humidity, different plant operators).