A comparative study of priority dispatching rules in a hybrid assembly/job shop

Abstract

SUMMARY This paper reports on research conducted on the use of priority dispatching rules in a hybrid assembly/job shop which manufactures both single-component and multiple-component products. A simulation model was constructed and a large stale experiment performed. Statistical analysis of the simulation results indicated significant impact of both the priority rules tested, and the product-mix considered on shop performance. Among the 12 priority rules tested, the SPT (shortest processing time) rule and the ASMF-SPT (assembly jobs first with SPT as tie-breaker) rule performed very well with respect to measures like lateness, flow time, tardiness, staging time, and percent of jobs tardy. These findings lead to further investigation of a combined priority rule, MIXED, which implements the ASMF-SPT rule at all machine centres that process components of assembly jobs, and the SPT rule at the remaining machine centres which process non-component jobs. The additional research results yielded evidence that the MIXED rule can reduce the staging time of the SPT rule, and yielded betrer results than the ASMF-SPT rule with regard to other performance measures. The most interesting finding, however, was the small variation in flow time distribution resulting from use of the MIXED rule when there were more assembly jobs. In an MRP environment, it is especially desirable to have a priority dispatching rule resulting in minimum variation in individual flow times which allows the replenishment lead times to be estimated with greater accuracy.