chapter seventeen scheduling chapter 17 scheduling

CHAPTER SEVENTEENSCHEDULING

Chapter 17

Scheduling

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Overview of Production Planning HierarchyCapacity Planning 1. Facility size 2. Equipment procurement

Aggregate Planning 1. Facility utilization 2. Personnel needs 3. Subcontracting

Master Schedule 1. MRP 2. Disaggregation of master plan

Long-term(Chapter 5)

Intermediate-term(Chapter 12)

Short-term(Chapter 17)

Intermediate-term(Chapters 12 and 14)

Short-term Scheduling 1. Work center loading 2. Job sequencing

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• Have the correct amount of resources at the right time and in the right place

• Meet the job’s due date - customer service

• Utilization of the organization’s resources - productivity

The Scheduling Problem

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Build A

A Done

Build B

B Done

Build C

C Done

Build D

Ship

JAN FEB MAR APR MAY JUN

On time!

Scheduling Manufacturing Operations

• High-volume

• Intermediate-volume

• Low-volume

• Service operations

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Work Center #1 Work Center #2 Output

High-Volume Systems• Flow system: High-volume system with

standardized equipment and activities• Flow-shop scheduling: Scheduling for

high-volume flow system– line balancing (Chapter 6)– design of work systems (Chapter 7)

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High-Volume Success Factors

• Process and product design

• Preventive maintenance

• Rapid repair when breakdown occurs

• Optimal product mixes

• Minimization of quality problems

• Reliability and timing of supplies

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• Outputs are between standardized high-volume systems and made-to-order job shops

• Three basic issues– run size of jobs– timing of jobs– sequence in which jobs should be

processed

Intermediate-Volume Systems (1 of 2)

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Intermediate-Volume Systems (2 of 2)

• Economic run size

• Base production on a master schedule developed from customer orders and forecasts of demand

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Low-Volume Systems• A variety of unique (customized) jobs arrive at the shop• Someone must analyze each job regarding its routing

(which work centers it will be required to visit), and the required amount of resources needed at each work center

• Typically the customer is told when the job will be finished (due date)

• The jobs are moved through the required work centers as each becomes available according to a scheduling system

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Scheduling Low-Volume Systems

• Loading - assignment of jobs to process centers

• Sequencing - determining the order in which jobs will be processed

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Loading Approaches (1 of 2)

• Infinite loading– assumes capacity of a work center is infinite– priority sequencing rule then used– managers may need to respond to capacity

overload conditions

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Loading Approaches (2 of 2)

• Finite loading – projects the starting and stopping times of

each job at each work center– takes into account capacities of each work

center and the processing times of jobs so that capacity is not exceeded

• In special cases, an algorithm such as the assignment method may be used

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Everything is#1 Priority

Sequencing

• Determine the order in which jobs waiting at a work center will be processed

• One approach is to use priority rules (simple heuristics) to select the order in which jobs waiting in line will be processed

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Examples of Priority Rules

• FCFS - first come, first served

• SPT - shortest processing time

• DD - due date

• CR - critical ratio

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Examples of Performance Measuresfor a Job Shop Scheduling System (1 of 2)

• Job flow time – the length of time a job is in the shop at a particular workstation; includes processing time, wait time, transportation time

• Average job flow time – the sum of the flow time for each job in a group divided by the number of jobs

• Makespan – total time needed to complete a group of jobs; sum of processing times

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Examples of Performance Measuresfor a Job Shop Scheduling System (2 of 2)

• Job lateness – the length of time the job completion time is expected to exceed the time the job is due

• Average lateness – the sum of the lateness for each job in a group divided by the number of jobs

• Average number of jobs – average work-in-process for a group of jobs; total flow time/makespan

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Two Work Center Sequencing

• Johnson’s Rule– technique for minimizing completion

time for a group of jobs to be processed on two processes or at two work centers.

– minimizes total idle time of the two processes

• Several conditions must be satisfied

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Johnson’s Rule Conditions

• Job time must be known and constant

• Job times must be independent of sequence

• Jobs must follow same two-step sequence (flow shop)

• Job priorities cannot be used

• All units must be completed at the first work center before moving to second

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Example Problem

Job DataEntry

Verify

A 2.5 1.7

B 3.8 2.6

C 1.9 1.0

D 1.8 3.0

first last

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• Used as a visual aid for loading and scheduling

• Purpose is to organize and clarify the actual or intended use of resources in a time framework

• Two most commonly used types– load chart– schedule chart

Gantt Chart

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WorkCenter

Mon. Tues. Wed. Thurs. Fri.

1 Job 3 Job 42 Job 3 Job 73 Job 1 Job 6 Job 74 Job 10

Figure 17-2Gantt Load Chart

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Example ProblemJob DataEntry

Verify

A 2.5 1.7

B 3.8 2.6

C 1.9 1.0

D 1.8 3.0

first last

CAD B

DataEntry

Verify

time

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Scheduling Approaches• Forward scheduling

– scheduling ahead, from some point in time

• Backward scheduling– scheduling by working backward from

the job’s due date

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Input/Output Control

• Used to manage work flow so that queues and waiting times are kept under control

• Input to a work center is compared to the output of that work center for some time period

• An imbalance indicates management should investigate

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Service Operation Problems

• Cannot store or inventory services

• Customer service requests are random

• Scheduling service involves– Customers– Workforce– Equipment

Duh! Where’sDilbert?

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Scheduling Service Operations

• Appointment systems– Controls customer arrivals for service

• Reservation systems– Estimates demand for service

• Scheduling the workforce– Manages capacity for service

• Scheduling multiple resources– Coordinates use of more than one resource