Sequencing rules, progress milestones and product structure in a multistage job shop

Manufacturing products which include the assembly of components requires knowledge of both the resources required to manufacture that part, as defined in the product routing sheet, and the relationship of components to be assembled at successive stages as defined in the product structure, or Bill of Materials (BOM). Job shops in which this type of manufacturing is done are generally referred to as multistage assembly job shops. Scheduling a multistage assembly job shop requires the proper coordination of materials flow through the various stages necessary to complete a product. It requires considering the time a product may have to wait for its parallel components before the required assembly operation can begin, as well as the waiting time required for a particular machine. Consequently, multistage assembly job shop scheduling is a much more complex problem than that of scheduling a single‐stage job shop. This paper examines several important issues in shop floor control in a multistage assembly job shop, namely: 1. Which sequencing rules are most appropriate for a multistage assembly job shop? 2. Do progress milestones improve the performance of the sequencing rules? 3. Are the various sequencing rules and progress milestones sensitive to product structure? Although an MRP system is not used specifically in this research, questions related to MRP implementation, such as the practice of using component or assembly due dates, are examined. The objectives of the study are to determine the appropriate combinations of sequencing rules and progress milestones for multistage assembly job shops and to evaluate the sensitivity of sequencing rules and progress milestones to the complexity of product structure. This research examines the performance of fourteen due date oriented sequencing rules in a simulated ten‐machine multistage job shop having one assembly work center. Due date based sequencing rules are utilized since previous research has indicated that such rules generally resulted in better shop performance than other types of rules tested. Although the focus of this research is on multistage job shops, the sequencing rules tested are inclusive of those due date oriented rules found to be successful in both single‐stage and multistage job shops. Each sequencing rule is tested using three methods of setting due date milestones to control the progression of a job toward completion. These methods are: (1) job due dates, (2) assembly due dates, and (3) operation due dates. Again, the use of these three methods of setting progress milestones is based on the findings of previous research. Both the sequencing rules and progress milestones are evaluated with respect to the complexity of the product's structure. Herein, the product structures used in the study represent flat products (many components at few levels) and tall products (few components at many levels). Five separate BOMs are employed for each of the two product structures, to examine the sensitivity of the sequencing rules and type of progress milestones to product structure. Also, the logical extremes of flat and tall BOMs are selected to better assess the impact of product structure on sequencing rule performance. The findings indicate that the practice of using component due dates to sequence jobs as done in many MRP environments may not be appropriate. In fact, the use of progress milestones worsened the performance of the fourteen dispatching rules for all performance criteria collected. Three of the fourteen dispatching rules tested were found to result in consistently poorer performance than the remaining eleven rules. Very little difference was found between the performance of these other eleven dispatching rules. Also, the sequencing rules were found to perform equally well in a shop characterized by tall BOMs as in a shop characterized by flat BOMs.