13-2 Chapter 13 Outline Batch Scheduling Gantt Charting Finite Capacity Scheduling Theory of Constraints Priority Dispatching Rules Planning and Control Systems
13-3 Synonyms Shop Floor Control Scheduling Operations Production Activity Control (PAC) Detailed Planning and Scheduling (DPS)
13-4 Scheduling Operations Last and most constrained decision in the capacity planning hierarchy Results in a time-phased plan Time frame ranges from a few hours to a few months Has conflicting objectives –High efficiency –Low inventories –Good customer service
13-5 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)
13-6 Batch Processing Move-queue-work-wait-move WS 1WS 2 Work is done according to work orders wait move queue move waitqueue move
13-7 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
13-8 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
13-9 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
13-11 Scheduling Example Process AProcess C Job 1 A C B Process B Job 3 In what sequence should the jobs be done? Job 5 Job 2Job 4
13-12 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.
13-14 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.
13-15 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.
13-16 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).
13-17 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!
13-18 Theory of Constraints (TOC) A constraint is anything that is slowing down production—a bottleneck. 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).
13-19 Theory of Constraints (TOC) The bottleneck should be scheduled to achieve maximum throughput. Non-bottlenecks should be scheduled to keep the bottleneck busy. A work-in-process queue should be in front of the bottleneck. Non-bottleneck resources may be idle. Find ways to relieve or reduce the bottleneck.
13-20 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 set-up times
13-21 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
13-22 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.
13-23 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)
13-24 Summary Batch Scheduling Gantt Charting Finite Capacity Scheduling Theory of Constraints Priority Dispatching Rules Planning and Control Systems