Machine selection rules in a dynamic job shop

With the increasing use of multipurpose machining centres in job shops, the scheduling problem can no longer neglect multiple job-routes. Existing scheduling approaches seldom address flexibility in job routes and the aim of this paper is to demonstrate that significant improvements to the scheduling performance of dispatching rules can be achieved easily through the use of simple machine selection rules. Three such rules are proposed in this paper and their effectiveness is evaluated through a simulation study of a dynamic job shop. In addition, three dynamic conditions, namely, the tightness of due dates, the flexibility of the job routes and the reliability of the machines, are varied to ensure that the simulation is performed for significantly different job shop conditions. The results of the simulation study indicate that improvements to the performance of simple dispatching rules are significantly enhanced when used with machine selection rules.