Question

Scheduling involves the timing of operations to achieve the efficient movement of units through a system.  This material addressed the issues of short-term scheduling in process-focused and service environments.  

Answer the following question:

1. What is the overall objective of scheduling?  List the 4 criteria for determining the effectiveness of a scheduling decision.  How do these criteria relate to the 4 criteria for sequencing decisions?

Answer 1

ANS.Scheduling is the process of prescribing “When” each operation in a production process is to be executed. Scheduling is “The determination of time that is required to perform each operation and also the time required to perform the entire series of operations as routed.”

The fundamental objective of scheduling is to arrange the manufacturing activities in such a way that the cost of production is minimized and the goods produced are delivered on due dates.

(i) In order to meet the delivery dates the sequence of operations is properly planned.

(ii) To have minimum total time of production by having better resources utilisation.

(iii) For having maximum capacity utilization and reducing the labour cost by minimization of idleness of machines and manpower.

(iv) To avoid unbalanced allocation of work among the various departments and workstations.

SEQUENCING

Sequencing is concerned with determining the order in which jobs are processed. Not only must the order be determined for processing jobs at work centers, but also for work processed at individual work stations. When work centers are heavily loaded and lengthy jobs are involved, the situation can become complicated. The order of processing can be crucial when it comes to the cost of waiting to be processed and the cost of idle time at work centers.

• Random (R). Pick any job in the queue with equal probability. This rule is often used as a benchmark for other rules.
• First come/first served (FC/FS). This rule is sometimes deemed to be fair since jobs are processed in the order in which they arrive.
• Shortest processing time (SPT). The job with the shortest processing time requirement goes first. This rule tends to reduce work-in-process inventory, average throughput time, and average job lateness.
• Earliest due date (EDD). The job with the earliest due date goes first. This seems to work well if the firm performance is judged by job lateness.
• Critical ratio (CR). To use this rule, one must calculate a priority index using the formula (due date - now)/(lead time remaining). This rule is widely used in practice.
• Least work remaining (LWR). An extension of SPT, this rule dictates that work be scheduled according to the processing time remaining before the job is considered to be complete. The less work remaining in a job, the earlier it is in the production schedule.
• Fewest operations remaining (FOR). This rule is another variant of SPT; it sequences jobs based on the number of successive operations remaining until the job is considered complete. The fewer operations that remain, the earlier the job is scheduled.
• Slack time (ST). This rule is a variant of EDD; it utilizes a variable known as slack. Slack is computed by subtracting the sum of setup and processing times from the time remaining until the job's due date. Jobs are run in order of the smallest amount of slack.
• Slack time per operation (ST/O). This is a variant of ST. The slack time is divided by the number of operations remaining until the job is complete with the smallest values being scheduled first.
• Next queue (NQ). NQ is based on machine utilization. The idea is to consider queues (waiting lines) at each of the succeeding work centers at which the jobs will go. One then selects the job for processing that is going to the smallest queue, measured either in hours or jobs.
• Least setup (LSU). This rule maximizes utilization. The process calls for scheduling first the job that minimizes changeover time on a given machine.

  GANTT CHARTS

  Gantt charts are named for Henry Gantt, a management pioneer of the early twentieth century. He proposed the use of a visual aid for loading and scheduling. Appropriately, this visual aid is known as a Gantt chart. This Gantt chart is used to organize and clarify actual or intended use of resources within a time framework. Generally, time is represented horizontally with scheduled resources listed vertically. Managers are able to use the Gantt chart to make trial-and-error schedules to get some sense of the impact of different arrangements.

  SCHEDULING SERVICE OPERATIONS

  The scheduling of services often encounters problems not seen in manufacturing. Much of this is due to the nature of service (i.e., the intangibility of services and the inability to inventory or store services and the fact that demands for services are usually random.) Random demand makes the scheduling of labor extremely difficult as seen in restaurants, movie theaters, and amusement parks. Since customers do not like to wait, labor must be scheduled so that customer wait is minimized. This sometimes requires the use of queuing theory or waiting-line theory. Queuing theory uses estimate arrival rates and service rates to calculate an optimum staffing plan. In addition, flexibility can often be built into the service operation through the use of casual labor, on-call employees, and cross-training.