Lecture Introduction to operations management - Chapter 13: Scheduling operations

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”