The Application of Capability Analysis and Statistical Process Control to Photochemical Machining

Process Capability is concerned with assessing how well process variations are being controlled within tolerance limits that have been specified. This was dealt with in Part 1 (Reference 1). This second article is concerned with process control, which aims to control the distribution of process variables such that they will be stable over a period of time. This means that ‘special causes’ that result in process variations are eliminated. In most processes that involve chemical reactions, Statistical Process Control (SPC) while of great benefit, is essentially supplementary to normal regulatory control. SPC will provide warnings of long term drifts, or other irregularity, in some process variable which may affect the quality of the product but might otherwise go undetected. This article will consider aspects of quality management, statistics, and the requirements for instituting a quality management system.