Lecture Introduction to operations management - Chapter 13: Scheduling operations. In this chapter we will discuss: Batch scheduling, gantt charting, finite capacity scheduling, theory of constraints, priority dispatching rules, planning and control systems.
Operations Management Contemporary Concepts and Cases Chapter Thirteen Scheduling Operations McGrawHill/Irwin Copyright © 2011 by The McGrawHill Companies, Inc. All rights reserved Chapter 13 Outline Batch Scheduling Gantt Charting Finite Capacity Scheduling Theory of Constraints Priority Dispatching Rules Planning and Control Systems 132 Synonyms Shop Floor Control Scheduling Operations Production Activity Control (PAC) Detailed Planning and Scheduling (DPS) 133 Scheduling Operations Last and most constrained decision in the capacity planning hierarchy Results in a timephased plan Time frame ranges from a few hours to a few months Has conflicting objectives – – – High efficiency Low inventories Good customer service 134 Batch Scheduling Very complex scheduling environment Can be thought of as “Network of Queues” Customers or jobs spend most of their time at work stations waiting to be processed Typical for actual work to be 20 percent or less of the total throughput time Closely related to MRP (see Chapter 16) 135 Batch Processing Move-queue-work-wait-move move move queue move WS wait move queue WS wait move move Work is done according to work orders 136 Difficulties Of Batch/Job Shop Scheduling Variety of jobs processed Different routing and processing requirements of each job Number of different orders in the facility at any one time Competition for common resources 137 Responsibilities of Production Control Department Loading – Check availability of material, machines & labor Sequencing – Release work orders to shop & issue dispatch lists for individual machines Monitoring – Maintain progress reports on each job until it is complete 138 Gantt Charting Proposed by Henry Gantt in 1917 Machine performance measures: – Makespan – total time to complete a set of jobs – Machine utilization – percent of make span time a machine (or person) is used Used primarily to monitor progress of jobs One technique for executing the production plan 139 Job Data for Scheduling Example Job Work center/ Machine Hours A/2, B/3, C/4 Due Date C/6, A/4 B/3, C/2, A/1 4 C/4, B/3, A/3 A/5, B/3 1310 Scheduling Example In what sequence should the jobs be done? Job 5 Process A Job 1 A Job 2 Job 4 Process C C Process B Job 3 B 1311 Where is the bottleneck? Total machine times for the five jobs: – – – Machine A: 15 hours Machine B: 12 hours Machine C: 16 hours C appears to be the bottleneck But! A is used for every job; C is not Either one could determine makespan 1312 Gantt Chart for Example 1313 Conclusions about Batch Scheduling Performance is highly sequence dependent Waiting time depends upon job interference in the schedule and available capacity Finding optimal schedules is not practical, but good heuristics are available 1314 Finite Capacity Scheduling (1) Finite capacity scheduling loads jobs onto work stations being careful not to exceed the capacity of any given station Done at the detailed planning and scheduling (DPS) level Part of the loading responsibility for executing the production plan 1315 Finite Capacity Scheduling (2) Can be used to identify bottlenecks A bottleneck is a work center whose capacity is less than the demand placed on it, and is less than the capacities of all other resources A bottleneck will constrain the capacity of the entire shop (or office) 1316 Theory of Constraints (TOC) Proposed by Goldratt in The Goal (1983) Goal is to make money Key elements of “goals” according to TOC: – – – Throughput—sales minus cost of goods sold Inventory—raw materials Operating expenses—cost of conversion including overhead Production does not count until it is sold! 1317 Theory of Constraints (TOC) A constraint is anything that is slowing down production—a bottleneck – – – A machine or workstation The market Procurement system The bottleneck determines the capacity of the system Implication: The operations manager should focus on the bottleneck to increase capacity and throughput (and make more money) 1318 Theory of Constraints (TOC) The bottleneck should be scheduled to achieve maximum throughput Nonbottlenecks should be scheduled to keep the bottleneck busy A workinprocess queue should be in front of the bottleneck Nonbottleneck resources may be idle Find ways to relieve or reduce the bottleneck 1319 Theory of Constraints (TOC) Ways of reducing the bottleneck or constraint: Increase capacity Divert work that doesn’t need to go through the bottleneck Catch rejects before they get to the bottleneck Increase output at the bottleneck – Larger batches – Reduced setup times 1320 Priority Dispatching Rules What are priority dispatching rules? – If you have more than one job waiting at a work station, how do you select which one to process next? The criterion you use for selecting the next job is your dispatching rule In front office services, the most common rule is “first come, first served” (except for emergency services) Part of the sequencing responsibility 1321 Priority Dispatching Rules Commonly used in services: – FCFS (First Come, First Served) Commonly used in manufacturing: – CR (Critical Ratio, Minimizes average lateness) – MINPRT (Minimum Processing Time or SPT, shortest processing time) This rule minimizes total waiting time 1322 Planning and Control Systems What delivery date do I promise? How much capacity do I need? When should I start on each particular activity or task? How do I make sure that the job is completed on time? Sometimes referred to as Advanced Planning & Scheduling (APS) 1323 Summary Batch Scheduling Gantt Charting Finite Capacity Scheduling Theory of Constraints Priority Dispatching Rules Planning and Control Systems 1324 End of Chapter Thirteen 1325 ... Customers or jobs spend most of their time at work stations waiting? ?to? ?be processed Typical for actual work? ?to? ?be 20 percent or less of the total throughput time Closely related? ?to? ?MRP (see? ?Chapter? ?16)... Batch Processing Move-queue-work-wait-move move move queue move WS wait move queue WS wait move move Work is done according? ?to? ?work orders 136 Difficulties Of Batch/Job Shop Scheduling Variety of jobs processed... Time frame ranges from a few hours? ?to? ?a few months Has conflicting objectives – – – High efficiency Low inventories Good customer service 134 Batch Scheduling Very complex? ?scheduling? ?environment Can be thought of as “Network of Queues”