Scheduling Operations Chapter 13

Chapter 13 Outline Batch Scheduling Gantt Charting Finite Capacity Scheduling Theory of Constraints Priority Dispatching Rules Infinite Capacity Loading Planning and Control Systems

Synonyms Shop Floor Control Scheduling Operations Production Activity Control (PAC) Detailed Planning and Scheduling (DPS)

Batch Scheduling Very complex scheduling environment Can be thought of as “Network of Queues” Customers spend most of their time waiting Closely related to MRP (See chapter 16)

Batch Processing Move-queue-work-wait-move WS 1 WS 2 move queue wait wait queue move move Work is done according to work order

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

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

Gantt Charting Developed by Henry Gantt in 1917 Related concepts: 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

Job Data for Scheduling Example

In what sequence should the jobs be done? Scheduling Example In what sequence should the jobs be done? Job 2 Job 4 Job 5 Process A Process C Job 1 A C Process B Job 3 B

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.

Gantt Chart for Example

Finite Capacity Scheduling 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.

Theory of Constraints (TOC) Proposed by Goldratt in The Goal (1983) Goal is to make money. Key elements of “goals” according to TOC: Throughput—what is made and sold Inventory—raw materials Operating expenses—cost of conversion Production does not count until it is sold!

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).

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.” Part of the sequencing responsibility

Priority Dispatching Rules Commonly used in manufacturing: MINPRT (Minimum Processing Time or SPT, shortest processing time) This rule minimizes total waiting time. Critical Ratio (Minimizes average lateness) Commonly used in services: FCFS (First Come, First Served)

Infinite Capacity Loading “Infinite capacity loading” loads jobs onto work centers without regard for the total capacity of the work center. If the capacity for any given work center has been exceeded, the schedule must be changed. This is generally done at the MRP level before detailed scheduling and planning

Infinite capacity loading example: time lines Time line for job 1 Due date A (2 hrs) Move/Wait (4 hrs) B (3 hrs) Move/Wait (4 hrs) C (4 hrs) Day 1 Day 2 Day 3 Time line for job 2 Due date C (6 hrs) Move/Wait (4 hrs) A (4 hrs) Day 1 Day 2 Day 3

Infinite Capacity Loading example Work center A Work center B Work center C Hours scheduled 1 2 3 Day 1 2 3 Day 1 2 3 Day

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? Advanced Planning & Scheduling (APS)

Summary Batch Scheduling Gantt Charting Finite Capacity Scheduling Theory of Constraints Priority Dispatching Rules Infinite Capacity Loading Planning and Control Systems

End of Chapter Thirteen