OPRATIN management assignment ( uploaded all related chapters ) &

Operations Management in the Supply Chain Decisions and Cases Seventh Edition

Chapter 12

Scheduling Operations

© McGraw-Hill Education. All rights reserved. Authorized only for instructor use in the classroom. No reproduction or further distribution permitted without the prior written consent of McGraw-Hill Education.

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Learning Objectives

12.1 Describe the concept of batch scheduling.

12.2 Construct a Gantt chart.

12.3 Create work schedules using forward and backward scheduling.

12.4 Explain the implications of the theory of constraints for scheduling.

12.5 Compare various dispatching rules.

12.6 Describe the important factors to consider when designing a scheduling system.

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Scheduling Operations

• Most detailed and most constrained decisions in capacity planning hierarchy

• Results in a time-phased plan (by hour, week)

• Allocation of resources (workers, machines) to tasks

• Trade-offs (conflicting objectives) – High efficiency

– Low inventories

– Good customer service

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Batch Scheduling

• Network of queues, as job moves from work station to work station

– job = manufacturing parts, customer, paperwork

– work station = machine, room, facility, worker

• Customers or jobs spend most of their time at work stations waiting to be processed

• Typical for actual work to be 5-20% of total throughput time

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Batch Processing

move  queue  process  wait  move…

WS = work station

“move” = movement of work: parts, customers, paperwork, etc.

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Challenges of Batch/Job Shop Scheduling

• Variety of jobs processed

• Different routing and processing requirements of each job

• Number of different job orders in the facility at any one time

• Competition for common (constrained) resources

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Gantt Charting

• Scheduling multiple jobs thru a set of work centers, minimizing completion time

• Machine performance measures:

– Makespan = total time to complete a set of jobs

– Machine utilization = percent of makespan time machine (or person) is used.

• Used to monitor progress of jobs

• Optimal schedule can be computationally intensive for multiple jobs/multiple machines

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Job Data for Scheduling Example

Job Work center / Machine hours Due date

1 A/2, B/3, C/4 3

2 C/6, A/4 2

3 B/3, C/2, A/1 4

4 C/4, B/3, A/3 4

5 A/5, B/3 2

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Scheduling Example

In what sequence should the jobs be done?

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Gantt Chart for Example (1 of 2)

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Gantt Chart for Example (2 of 2)

Machine idle (hr)

A 5

B 8

C 4

17

Makespan = 20 hr

Job Job waiting time (hr) Delivery time (hr)

1 0 9

2 9 19

3 14 20

4 1 11

5 3 11

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Shop Performance Measures

• Machine Efficiency: Makespan or machine utilization

• Customer: Delivery times of the jobs (minimizing job waiting time is complementary measure)

Change the job sequence from 1, 4, 5, 2, 3 to improve efficiency and/or delivery times to the customer.

Machine(s) could be added to improve both efficiency and delivery times.

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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 challenging, but good heuristics are available.

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Finite Capacity Scheduling

• Scheduling jobs onto workstations, but not exceeding the capacity of any given resource.

• Used to identify bottlenecks.

• Forward scheduling – To determine completion date for all orders.

• Backward scheduling – Work backward from due date to determine

start dates for orders.

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Theory of Constraints (TOC) (1 of 3)

• Proposed by Goldratt in The Goal (1983)

• Goal is to make money from operations

• Production does not have value until it is sold!

• Key elements: – Throughput = sales minus cost of raw

materials

– Inventory = raw materials value

– Operating expenses = cost of labor and overhead

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Theory of Constraints (TOC) (2 of 3)

• Constraint is anything slowing down production … a bottleneck. – Machine or workstation

– Market

– Procurement system

• The bottleneck determines the capacity of the system.

• Implication: Operations manager should focus on bottleneck to increase capacity and throughput (and make more money).

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Theory of Constraints (TOC) (3 of 3)

• 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 always be in front of the bottleneck.

• Non-bottleneck resources may be idle.

• Find ways to relieve or reduce the bottleneck.

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Priority Dispatching Rules

• Which job should be processed next?

– Rule for selecting the next job from the queue

• Common in services:

– First come, first served

– Priority rule (first-class customers first)

– Preemptive rule (most severe patient treated next)

• Common in manufacturing:

timeprocessingremaining

datedueuntiltimeremaining =ratioCritical

– Shortest processing time (quickest job)

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Planning and Control Systems

• What delivery date should be promised?

• Where is the bottleneck?

• When should each activity or task be started?

• How is on-time job completion ensured?

• Sometimes referred to as:

– Advanced Planning & Scheduling (APS)

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Summary

12.1 Describe the concept of batch scheduling.

12.2 Construct a Gantt chart.

12.3 Create work schedules using forward and backward scheduling.

12.4 Explain the implications of the theory of constraints for scheduling.

12.5 Compare various dispatching rules.

12.6 Describe the important factors to consider when designing a scheduling system.

© McGraw-Hill Education. All rights reserved. Authorized only for instructor use in the classroom. No reproduction or further distribution permitted without the prior written consent of McGraw-Hill Education.

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