Semi-interchangeable machines: implications for workload control

Workload control (WLC) is developed as a production planning and control concept for make-to-order job shops. Effective WLC requires a profound decision regarding order release to allow balancing the workload across capacity groups. WLC design has to reflect the company characteristics, especially machine characteristics. In practice, machines that perform the same type of operations are generally not completely identical but semi-interchangeable. The importance of the load balancing function requires careful consideration of semi-interchangeability within WLC. This paper develops different control alternatives to deal with semi-interchangeability and tests them in a simulation study. The results of the study indicate that the most intuitive control option—grouping semi-interchangeable machines into a single capacity group and making the routeing decision at dispatching—does not give the best performance, despite the advantages of pooling synergy. For a low degree of interchangeability it is more attractive to place semi-interchangeable machines in separate capacity groups and to make a routeing decision at order release. This enables more detailed load balancing resulting in shorter throughput times. More generally, it is shown to be advantageous to postpone the final routeing decision until dispatching, though considering separate capacity groups for a preliminary routeing decision at the time of release.