Scheduling in a Job Shop
A schedule is a timetable for performing activities, using resources, or allocating facilities. The purpose of operations scheduling in a job shop is to disaggregate the master production schedule (MPS) into time-phased weekly. daily, and/or hourly activities-in other words. to specify in precise terms the planned workload on the production process in the very short run. Operations control focuses on job-order progress and. where necessary, expediting orders and/or adjusting system capacity to make sure that the MPS is met. In designing a scheduling and control sy-tern. provision must he made for efficient performance of the following functions:I. Allocating orders. equipment. and personnel to work centers or other specified locations, Essentially. this short-run planning. 2. Determining the sequence of order performance: that is, establishing job priorities. 3. initialing performance of the scheduled work. This is commonly termed the dispatching of orders.
4. Shop-floor control (or production activity control), which involves n. Reviewing the status and controlling the progress of orders as they are being worked on.
b. Expediting late and critical orders. I
5. Revising the schedule to reflect recent changes in order status. 6. Assuring that quality control standards are being met. A simple job shop-scheduling process is shown in Exhibit 12.1. At the start of the day, he job dispatcher (in this case. a production control person assigned to this department) selects and sequences the available jobs to be run at individual workstations. The dispatcher's decisions would be based on the operations and routing requirements of each job,status of existing jobs on the machines. the queue of work before each machine, job priorities. material availability. anticipated job orders to be released later in the day, and worker and machine capabilities. To help organize the schedule, the dispatcher would draw on shop-floor information from the previous day and external information provided by central production control. process engineering. and so on. The dispatcher also would meet with the foreman or supervisor of the department to discuss the feasibility of the schedule, especially with respect to workforce considerations and identifying potential bottlenecks. Visual schedule boards provide an-efficient and simple way to communicate the priority and status of work. What makes scheduling each individual job so difficult? Consider the following factors:
• This good service may never haw been done before, so the estimates of the expected length of time for completion of the various components may be quite different from the actual time.
• The sequence of operations is extremely flexible and, with a cross-trained workforce, the number of possible sequence can be huge. Trying to assess the expected results of different sequences with a goal of finding the best sequence is usually very difficult.
• For different operations, the determination of the "best" sequence may vary-for one case it may De the minimization of waste, for another it may be the minimization of idle facilities, for a third it may be the maximization of throughput. and 0 on. Thu even with extensive research into the job shop scheduling problem. it i~ hard to find quantitative or mechanical algorithm that are always appropriate for all situations.