Statistical methods in process management - Capability and performance - Part 8: Machine performance of a multi-state production process
|Publication Date:||1 June 2014|
|ICS Code (Application of statistical methods):||03.120.30|
This part of ISO 22514 aims to define the evaluation method to quantify the short-term capability of a production process (capacity of the production tool, widely termed capability), i.e. the machine performance index, to ensure compliance to a toleranced measurable product characteristic, when said process does not feature any kind of sorting system
If the production process integrates a sorting system, then this one (clearing away nonconforming parts) should be analysed independently.
This part of ISO 22514 does not aim to define evaluation methods of the capability of a production process that is gauged through long-term observation (capability process or performance process indices).
This part of ISO 22514 defines
- the principles guiding the development of indicators for quantifying capability, and
- the statistical methods to be employed.
The characteristics used to evaluate production process capability have statistical distributions, and it is presumed, a priori, that at least one of these distributions is multi-modal. A distribution is presumed to be multimodal if it results from the marked effect of at least one cause inducing a significant difference between the produced items.
This part of ISO 22514 applies, for example, to characteristics generated by processes such as the following:
- multi-cavity casting: simultaneously producing several identical parts from a mould featuring several cavities. Since each cavity has its own geometry and its own position in the mould architecture, it can create a systemic difference on the output result;
- multi-fixture machining: a part produced at the same time, but the produced parts are positioned in relation to the production tool by different fixture systems. Since each fixture has its own geometry, mount clamps, etc., it can create a systematic difference on the output result;
- batch load treatments: heat treatment applied at the same time on a set of identical parts (the batch load), distributed within a pre-defined space of furnace. The position of an item of the batch relative to the furnace can influence the output result.
Each cavity, fixture, or position in the batch load corresponds to a different state. The multi-state process can be understood as the result of the combination of different states within the same process (e.g. cavity, fixture, position in the batch load).
NOTE it needs to be ensured that such systematic differences, if any, constitute only a very small proportion of permissible error so that their impact is harmless and do not affect the capabilities of the process.