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McGraw-Hill/Irwin The McGraw-Hill Companies, Inc. 2007, All Rights Reserved
Scheduling Operations
Chapter 13
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Chapter 13 Outline
Batch Scheduling
Gantt Charting
Finite Capacity SchedulingTheory of Constraints
Priority Dispatching Rules
Infinite Capacity LoadingPlanning and Control Systems
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Synonyms
Shop Floor Control
Scheduling OperationsProduction Activity Control (PAC)
Detailed Planning and Scheduling(DPS)
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Batch Scheduling
Very complex scheduling environment
Can be thought of as Network of Queues
Customers spend most of their time waitingClosely related to MRP (See chapter 16)
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Batch Processing
Move-queue-work-wait-move
WS 1 WS 2
Work is done according to work order
waitmove
queue
move
movewaitqueue
move
movemove
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Difficulties Of Batch/Job Shop
Scheduling
Variety of jobs processed
Different routing and processingrequirements of each job
Number of different orders in the
facility at any one time
Competition for common resources
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Responsibilities of Production
Control Department
Loading
Check availability of material, machines& labor
Sequencing
Release work orders to shop & issuedispatch lists for individual machines
Monitoring
Maintain progress reports on each jobuntil it is complete
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Gantt Charting
Developed by Henry Gantt in 1917
Related concepts:
Makespantotal time to complete a set of jobsMachine utilizationpercent of make span time a
machine (or person) is used.
Used primarily to monitor progress of jobs
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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
Process A Process C
Job 1 AC
B
Process B
Job 3
In what sequence should the jobs be done?Job 5 Job 2 Job 4
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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.
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Gantt Chart for Example
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Finite Capacity Scheduling
Finite capacity scheduling loads jobs ontowork stations being careful not to exceedthe capacity of any given station.
Done at the detailed planning andscheduling (DPS) level
Part of the loading responsibility.
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Theory of Constraints (TOC)
Proposed by Goldratt in The Goal (1983)
Goal is to make money.
Key elements of goals according to TOC:
Throughputwhat is made and sold
Inventoryraw materials
Operating expensescost of conversion
Production does not count until it is sold!
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Theory of Constraints (TOC)
A constraint is anything that is slowingdown productiona bottleneck.
A machine or workstation
The market
Procurement system
The bottleneck determines the capacity ofthe system.
Implication: the operations manager shouldfocus on the bottleneck to increase capacityand throughput (and make more money).
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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 forselecting the next job is your dispatching rule.
In front officeservices, the most common
rule is first come, first served.
Part of the sequencing responsibility
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Priority Dispatching Rules
Commonly used in manufacturing: MINPRT (Minimum Processing Time orSPT, shortest
processing time) This rule minimizes total waiting time.
Critical Ratio(Minimizes average lateness)
Commonly used in services: FCFS (First Come, First Served)
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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
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Infinite capacity loading example: time lines
A
(2 hrs)
Move/Wait
(4 hrs)
B
(3 hrs)
Move/Wait
(4 hrs)
C
(4 hrs)
Day 1 Day 2 Day 3
Due date
Move/Wait
(4 hrs) A (4 hrs)
Day 1 Day 2 Day 3
Due date
C (6 hrs)
Time line for job 1
Time line for job 2
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Infinite Capacity Loading example
6
5
4
3 Job 2
2
1
Job 1 Job 1Hoursschedule
d
Work center A
6
5
4
3
2
1 Job 1
Work center B
6
5
4
3
2
1 Job 1
Job 2
Work center C
1 2 3Day
1 2 3Day
1 2 3Day
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Planning and Control Systems
What delivery date do I promise?
How much capacity do I need?
When should I start on each particular activityor task?
How do I make sure that the job is completed
on time?Advanced Planning & Scheduling (APS)
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Summary
Batch Scheduling
Gantt Charting
Finite Capacity SchedulingTheory of Constraints
Priority Dispatching Rules
Infinite Capacity LoadingPlanning and Control Systems
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End of Chapter Thirteen