While scheduling the job shop type of operations system in addition to deciding which job should go to which work center, it is also necessary to decide on the sequence in which the jobs are to be processed in any work center. This relative prioritization determines the actual time schedule of the individual jobs. The priorities can be decided based upon or a number of criteria. What these criteria can be, and their relative usefulness shall be discussed in the coming paragraphs.
How does any job priority rule influence the production output performance? Given a situation where there is one machine and a number of jobs waiting to be processed on that machine, the total time to process all the jobs is the same irrespective of the priority rule. However, if other characteristics of performance such as the average waiting time per job or average number of jobs waiting in line (and therefore the amount of work in process inventory) are relevant, then the priority rule does matter.
Now, imagine a set of jobs requiring the use of two machines, which are to be used one after the other. Depending upon the sequencing rule, even the total time for that set of jobs will vary. This is so because the jobs interfere with another; and while one job is being performed on a machine, the other job/jobs may have to wait. In actual factory or any operations situation a job involves a number of operations with the use of a number of machines. Therefore, the priority rules will have a significant effect on the shop performance. A job shop can be thought of as complex queuing system with probabilistic times, multiple channels, multiple servers, with different jobs requiring the use of different sets of servers. The priority rule or queue discipline has a very important role to play in a queuing system. In addition to being a complex a queuing system, job shop is a dynamic system where the relative urgency of the jobs is forever changing. This makes the situation even more complicated and less amendable to the use of analytical techniques of Queuing Theory of the scheduling of jobs and operations.
Research has concentrated on the priority rules by simulating the production shop and determining the relative worth of the different rules. In addition to simulation, there are some optimization methods available (such as Johnson’s rule) and also visual aids in the form of charts(such as the Gantt Chart).
The basic questions to ask would be: Why do we need all these aids to find better ways of work flow planning and control? The reasons are:
There is pressure on completing the jobs by the promised due date to the customer (or as required by the customer) under the constraints of available machinery, manpower and materials capacity, The objective is to meet the demands with the least cost. If the cost was no problem and the machinery capacity was large there would be no need to bother about sequencing. However, this is never the case capacity was large there would be no need to bother about sequencing. However this is never the case and we need to achieve the best utilizations of machines and manpower, and keep as small work in process inventories as possible. The work-in-process is necessary to keep the utilization of machinery and manpower at a high level.
Thus, if one cost component is lowered, the other cost component goes up. Therefore, a balance needs to be struck between them so as to have lowest total costs while meeting the market demands. And for this strategies available to us are that of assignment and sequencing of jobs in the shops.