J-1 OPERATIONS SCHEDULING SUPPLEMENT J J O PERATIONS S CHEDULING LEARNING GOALS T his supplement focuses on operations After reading this supplement, you should scheduling , which involves assigning jobs to be able to: workstations or employees to jobs for speci- fied time periods. Effective scheduling helps man- 1. Define new performance measures (beyond agers achieve the full potential of their supply flow time and past due) for evaluating a chains. Chapter 14, “Operations Planning and schedule. Scheduling,” covers the basics of scheduling—Gantt 2. Describe the decision rules (beyond FCFS charts, workforce scheduling, two rules (FCFS and and EDD) to sequence jobs. EDD) for sequencing work at a single workstation, 3. Determine schedules for single and and two commonly used performance measures multiple workstations. (flow time and past due). Here we deepen your understanding with additional performance mea- sures and priority sequencing rules, a discussion of scheduling multiple workstations, and a discussion of scheduling a two-station flow shop. my om lab and the Companion Website at www.pearsonhighered.com contain many tools, activities, and resources designed for this supplement.
J-2 SUPPLEMENT J OPERATIONS SCHEDULING operations scheduling Scheduling Service and Manufacturing A type of scheduling in which jobs are Processes assigned to workstations or employees are assigned to jobs for specified time The scheduling techniques we discuss in this supplement cut across the various process periods. types found in services and manufacturing. Many service firms are characterized by a front-office process with high customer contact, divergent work flows, customization, and, consequently, a complex scheduling environment. Often customer demands are difficult to predict, which puts a high premium on scheduling employees to handle the varied needs of customers. At the other extreme in the service industry, a back-office process has low customer involvement, uses more line work flows, and provides stan- dardized services. Inanimate objects are processed; these processes take on the appear- ance of manufacturing processes. Manufacturing processes also benefit from operations scheduling techniques. Our discussion of the operations scheduling techniques in this supplement has application for job, batch, and line processes in services as well as in manufacturing. Schedules for continuous processes can be developed with linear programming (see Supplement E, “Linear Programming”). Although the scheduling techniques in this chapter provide some structure to the selection of good schedules, many alternatives typically need to be evaluated. We begin by looking at the performance measures managers use to select good schedules. Performance Measures We already covered two important performance measures in Chapter 14, “Operations Planning and Scheduling.” Flow time is the time a job spends in the service or manufactur- ing system, and past due (tardiness) is the amount of time by which a job missed its due date. In this regard, a job is the object receiving service or being manufactured. For example, a job may be a customer waiting for service at a state licensing bureau or it may be a batch of pistons waiting for a manufacturing process. These two performance measures can be insufficient, depending on the competitive priorities of a process. Additional performance measures follow: � Makespan. The total amount of time required to complete a group of jobs is called makespan makespan . Minimizing makespan supports the competitive priorities of cost (lower inventory) and time (delivery speed). The total amount of time required to complete a group of jobs. Makespan = Time of completion of last job - Starting time of the first job � Total Inventory. This performance measure is used to measure the effectivness of schedules for manufacturing processes. The sum of scheduled receipts and on-hand inventories is the total inventory . total inventory The sum of scheduled receipts and on- Total inventory = Scheduled receipts for all items + On-hand inventories of all items hand inventories. Minimizing total inventory supports the competitive priority of cost (inventory hold- ing costs). � Utilization. The degree to which equipment, space, or the workforce is currently being used, measured as the ratio of the average output rate to maximum capacity. Maximizing the utilization of a process supports the competitive priority of cost (slack capacity). These performance measures often are interrelated. For example, minimizing the average flow time tends to increase utilization. Minimizing the makespan for a group of jobs tends to increase utilization. Understanding how flow time, makespan, past due, and utilization interact can make the selection of good schedules easier. Sequencing Jobs Operations schedules are short-term plans designed to implement the sales and opera- tions plan. Often, several jobs must be processed at one or more workstations. Typically, a variety of tasks can be performed at each workstation. If schedules are not carefully planned to avoid bottlenecks, waiting lines may develop. For example, Figure J.1 depicts
J-3 OPERATIONS SCHEDULING SUPPLEMENT J the complexity of scheduling a manufacturing process. When a job order is received for a part, the raw materials are collected and the batch is moved to its first operation. The colored arrows show that jobs follow different routes through the manufacturing process, depending on the product being made. At each workstation, the next job to process is a decision because the arrival rate of jobs at a workstation often differs from the processing rate of the jobs at a workstation, thereby creating a waiting line. In addi- tion, new jobs can enter the process at any time, thereby creating a dynamic environ- ment. Such complexity puts pressure on managers to develop scheduling procedures that will handle the workload efficiently. In this section, we focus on scheduling approaches used in two environments: (1) diver- gent flow processes and (2) line flow processes. A manufacturer's operation with divergent flows is often called a job shop , which specializes in low- to medium-volume production and utilizes job or batch processes. The front office would be the equivalent for a service job shop provider. Jobs in divergent flow processes are difficult to schedule because of the variability A manufacturer's operation that specializes in job routings and the continual introduction of new jobs to be processed. Figure J.1 depicts in low- to medium-volume production and a manufacturer’s job shop. A manufacturer's operation with line flows is often called a flow utilizes job or batch processes. shop , which specializes in medium- to high-volume production and utilizes line or flow shop continuous flow processes. The back office would be the equivalent for a service provider. Tasks are easier to schedule because the jobs have a common flow pattern through the sys- A manufacturer's operation that tem. Nonetheless, scheduling mistakes can be costly in either situation. specializes in medium- to high-volume production and utilizes line or continuous flow processes. Job Shop Sequencing Just as many schedules are feasible for a specific group of jobs at a particular set of workstations, numerous methods can be used to generate schedules. They range from straightforward manual methods, such as manipulating Gantt charts, to sophisticated computer models for developing optimal schedules. One way to generate schedules in job shops is by using priority sequencing rules , which allows the schedule for a workstation to evolve over a period of time. The decision about which job to process next is made with simple priority rules whenever the workstation becomes available for further processing. One advantage of this method is that last-minute information on operating conditions can be incorporated into the schedule as it evolves. We already covered two important sequencing rules in Chapter 14, “Operations Planning and Scheduling.” The first-come, first-served (FCFS) rule gives the job arriving at the workstation first the highest priority. The earliest due date (EDD) rule gives the job with the earliest due date based on assigned due dates the highest priority. Such rules can be applied by a worker or � FIGURE J.1 Diagram of a Manufacturing Job Shop Process Shipping department Raw materials Legend: Batch of parts Workstation
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