A Study On Process Control For Multiple Sources Of Variation

Global market competition among manufacturers has prompted the rapid developing of statistical process control(SPC), especially for 6σquality improvement. But for some process with several source of variation, traditional shewhart control charts monitoring single sources of variation typically produce an unacceptably high number of false alarms, sometimes rendering the whole control system useless. So control system for multiple source of variation must be considered.This dissertation researches on theory and the methodology for quality control for machining process due to various sources.1. The dissertation sums up and develops systematically theory and the methodology for quality control for process due to various sources. A scheme for multi-vari process control is presented including multi-vari analysis, the choice of control charts and the sample size selection.2. The dissertation firstly use exponentially weighted moving average(EWMA) control charts are used to monitor the process with various sources of variation. It is proved that the power of EWMA control charts is more sensitive than improved shewhart control charts.3. The operating characteristic (OC) curves of the control charts with various sources of variation were calculated, presented and compared to those of the traditional shewhart control charts. This OC curves can assist in the evaluation of the power of control charts.4. The dissertation discusses the theory and methods to calculate sample size of control chart monitoring various sources of variation based on the OC curve of the control charts. An acceptableβwill depend on the sample size from OC curve, so the appropriate choice of sample sizes will usually be made on economic grounds.5. The dissertation also proposes another propoach to estimate the sample size based on confidence interval of the variance component due to various sources. An acceptable confidence interval of the variance component will determine the choise of sample size on economic grounds6. The process control of axis machining provides the applications example of process control with multiple sources of variation.