Operations Operations ManagementManagement

Short-Term SchedulingShort-Term Scheduling短期批程短期批程Chapter 15Chapter 15

Week #16-2

OutlineOutline GLOBAL COMPANY PROFILE: DELTA AIRLINES

THE STRATEGIC IMPORTANCE OF SHORT-TERM

SCHEDULING ( 短期批程在策略上的重要性 ) SCHEDULING ISSUES ( 批程問題 )

Forward and Backward Scheduling Scheduling Criteria

SCHEDULING PROCESS-FOCUSED WORK CENTERS

Outline - ContinuedOutline - Continued LOADING JOBS IN WORK CENTERS

Input-Output Control Gantt Charts Assignment Method

SEQUENCING JOBS IN WORK CENTERS Priority Rules for Dispatching Jobs Critical Ratio Sequencing N Jobs on Two Machines: Johnson’s

Rule

Outline - ContinuedOutline - Continued LIMITATIONS OF RULE-BASED DISPATCHING

SYSTEMS FINITE SCHEDULING THEORY OF CONSTRAINTS BOTTLENECK WORK CENTERS REPETITIVE MANUFACTURING SCHEDULING FOR SERVICES

Scheduling Service Employees with Cyclical Scheduling

Learning ObjectivesLearning Objectives

When you complete this chapter, you should be able to :

Identify or Define: Gantt charts Assignment method Sequencing rules Johnson’s rules Bottlenecks

Learning Objectives - continuedLearning Objectives - continued

When you complete this chapter, you should be able to :

Describe or Explain: Scheduling Sequencing Shop loading Theory of constraints

Delta AirlinesDelta Airlines

10% of Delta’s flights disrupted per year - half of those by weather

Cost: $440-million in: lost revenue overtime pay food and lodging vouchers

$33-million hi-tech nerve center 18 staff $35-million savings

Strategic Implications of Strategic Implications of Short-Term SchedulingShort-Term Scheduling

By scheduling effectively, companies use assets more effectively and create greater capacity per dollar invested, which, in turn, lowers cost

This added capacity and related flexibility provides faster delivery and therefore better customer service

Good scheduling is a competitive advantage which contributes to dependable delivery

Deals with timing of operations Short run focus: Hourly, daily, weekly Types

TodayToday Due Date

BB EE

Forward SchedulingForward Scheduling

TodayToday Due Date

BB EE

Backward SchedulingBackward Scheduling

Short-Term SchedulingShort-Term Scheduling

Hospital Outpatient treatments Operating rooms

University Instructors Classrooms

Factory Production Purchases

© 1984-1994 T/Maker Co.

Short-Term Scheduling ExamplesShort-Term Scheduling Examples

Organization Managers Must ScheduleOperating room usePatient admissionNursing, security, maintenance staffsOutpatient treatments

Classrooms and audiovisual equipmentStudent and instructor schedulesGraduate and undergraduate courses

Production of goodsPurchase of materialsWorkers

Scheduling DecisionsScheduling Decisions

Mount Sinai Hospital

Indiana University

Lockheed-Martin Factory

Organization Managers Must Schedule

Chefs, waiters,bartendersDelivery of fresh foodsEntertainersOpening of dining areas

Maintenance of aircraftDeparture timetablesFlight crews, catering, gate, and ticketing personnel

Scheduling DecisionsScheduling Decisions

Hard Rock Cafe

Delta Airlines

Capacity Planning, Aggregate Scheduling, Master Capacity Planning, Aggregate Scheduling, Master Schedule, and Short-Term SchedulingSchedule, and Short-Term Scheduling

Capacity Planning 1. Facility size 2. Equipment procurement

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

Master Schedule 1. MRP 2. Disaggregation of master plan

Long-term

Intermediate-term

Short-term

Intermediate-term

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

Forward and Backward Forward and Backward SchedulingScheduling

Forward scheduling: begins the schedule as soon as the requirements are known jobs performed to customer order schedule can be accomplished even if due date is

missed often causes buildup of WIP

Backward scheduling: begins with the due date of the final operation; schedules jobs in reverse order used in many manufacturing environments, catering,

scheduling surgery

The Goals of Short-Term The Goals of Short-Term SchedulingScheduling

Minimize completion time Maximize utilization (make effective use of

personnel and equipment) Minimize WIP inventory (keep inventory levels

low) Minimize customer wait time

Qualitative factors Number and variety of jobs Complexity of jobs Nature of operations

Quantitative criteria Average completion time Utilization (% of time facility is used) WIP inventory (average # jobs in system) Customer waiting time (average lateness)

Choosing a Scheduling MethodChoosing a Scheduling Method

Level Use Methods

Repetitive-Focused

Product-Focused

Process-Focused

Variety of Methods

Scheduling Methods Differ by Scheduling Methods Differ by ProcessProcess

High variety, low volume systems Products made to order Products need different materials and processing Complex production planning and control Production planning aspects

Shop loading Job sequencing

Process-Focused Work CentersProcess-Focused Work Centers

Requirements for Scheduling Requirements for Scheduling Process-Focused Work CentersProcess-Focused Work Centers

Schedule incoming orders without violating capacity constraints of individual work centers

Check availability of tools and materials before releasing an order to a department

Establish due dates for each job and check progress against need dates and order lead times

Check work-in-progress as jobs move through the shop Provide feedback on plant and production activities Provide work-efficiency statistics and monitor operator times for

payroll and labor distribution analyses

Types of Planning FilesTypes of Planning Files

Item master file - contains information about each component the firm produces or purchases

Routing file - indicates each component’s flow through the shop

Work-center master file - contains information about the work center such as capacity and efficiency

Process-Focused Planning SystemProcess-Focused Planning System

Forecast &Firm Orders

MaterialRequirements

Planning

AggregateProduction

Planning

ResourceAvailability

MasterProductionScheduling

ShopFloor

Schedules

CapacityRequirements

PlanningRealistic?

Yes

No, modify CRP, MRP, or MPS

Assigning jobs to work centers Considerations

Job priority (e.g., due date) Capacity

Work center hours available Hours needed for job

Approaches Gantt charts (load & scheduling) - capacity Assignment method - job to specific work center

Loading Jobs in Work CentersLoading Jobs in Work Centers

Week Ending 6/6 6/13 6/20 6/27 7/4 7/11

Planned input 280 280 280 280 280

Actual input 270 250 280 285 280

Cumulative Deviation -10 -40 -40 -35

Planned Output 320 320 320 320

Actual Output 270 270 270 270

Cumulative Deviation -50 -100 -150 -200

Cumulative Change in Backlog

0 -20 -10 -+5

Input-Output ControlInput-Output Control

Explanation: 270 input, 270 output, implies 0 change

Explanation: 250 input, 270 output implies –20 change (20

standard hours less in the work center)

Order Release Begins Shop LoadingOrder Release Begins Shop Loading

No

Release Order

Hold Release

Priority & Capacity OK?

Gross Requirements

Scheduled Receipts

Available

Net Requirements

Planned Order Receipt

Planned Order Release

Yes

1 2

100

100

3

Options for Managing Facility Options for Managing Facility Work FlowWork Flow

Correcting performance Increasing capacity Increasing or reducing input to the work center by:

routing work to or from other work centers increasing or decreasing subcontracting producing less (or more)

Shows relative workload in facility Disadvantages

Does not account for unexpected events Must be updated regularly

Work Center M T W Th F

Metal Works Job 349 Job 350Mechanical Job D Job GElectronics Job B Job HPainting Job C Job E Job I

Gantt Load ChartGantt Load Chart

Gantt Load Chart for Week of March 6Gantt Load Chart for Week of March 6 Shows relative workload in facility Disadvantages

Does not account for unexpected events Must be updated regularlyDay

Work Center Monday Tuesday Wednesday Thursday Friday

Metalworks Job 349 Job 350

Mechanical Job 349 Job 406

Electronics Job 408 Job 349

Painting Job 295 Job 408 Job 349

Processing Unscheduled

Center not available (for example, maintenance time, repairs, shortages)

Gantt Scheduling ChartGantt Scheduling ChartStart of an activity

Scheduled activity time allowed

Point in time when chart is reviewed

S T W T F SDay

Job

Job A

Job B

Job C

Maintenance

Now

End of an activity

Actual work progress

Non-production time

Day 1

Day 2

Day 3

Day 4

Day 5

Day 6

Day 7

Assigns tasks or jobs to resources Type of linear programming model

Objective Minimize total cost, time etc.

Constraints 1 job per resource (e.g., machine) 1 resource (e.g., machine) per job

Assignment MethodAssignment Method

Assignment Method - Four StepsAssignment Method - Four Steps Subtract the smallest number in each row from every number in that row; then

subtract the smallest number in every column from every number in that column

Draw the minimum number of vertical and horizontal straight lines necessary to cover all zeros in the table If the number of lines equals either the number of rows or the number of

columns, then you can make an optimal assignment (Step 4) Otherwise:

Subtract the smallest number not covered by a line from every other uncovered number. Add the same number to any number(s) lying at the intersection of any two lines. Return to Step 2

Optimal assignments will always be at the zero locations of the table

Assignment Method – Type Setter Assignment Method – Type Setter ExampleExample

TypesetterJob

A B C

R-34 $11 $14 $ 6

S-66 $ 8 $10 $11

T-50 $9 $12 $7

Initial set-up

Step 1a & 1bStep 1a & 1b

TypesetterJob

A B C

R-34 5 8 0

S-66 0 2 3

T-50 2 5 0

TypesetterJob

A B C

R-34 5 6 0

S-66 0 0 3

T-50 2 3 0

Step 1a Step 1b

Step 2Step 2

TypesetterJob

A B C

R-34 5 6 0

S-66 0 0 3

T-50 2 3 0

Smallest uncovered number

Step 3Step 3

TypesetterJob

A B C

R-34 3 4 0

S-66 0 0 5

T-50 0 1 0

Make assignmentsR-34 →C, S-66S → B, T-50 →A

Production Control Production

Which job do I run next?

Dispatch ListOrder Part Due QtyXYZ 6014 123 100ABC 6020 124 50

Job PacketJob XYZ

Order release

Sequencing ChallengeSequencing Challenge

Specifies order jobs will be worked Sequencing rules

First come, first served (FCFS) Shortest processing time (SPT) Earliest due date (EDD) Longest processing time (LPT) Critical ratio (CR) Johnson’s rule

SequencingSequencing

Priority Rules for Dispatching JobsPriority Rules for Dispatching Jobs First come, first served The first job to arrive at a work center is processed first Earliest due date The job with the earliest due date is processed first

Shortest processing time The job with the shortest processing time is processed first

Longest processing time The job with the longest processing time is processed first Critical ratio The ratio of time remaining to required work time remaining is

calculated, and jobs are scheduled in order of increasing ratio.

FCFS

EDD

SPT

LPT

CR

Process first job to arrive at a work center first Average performance on most scheduling criteria Appears ‘fair’ & reasonable to customers

Important for service organizations Example: Restaurants

First Come, First Served RuleFirst Come, First Served Rule

Shortest Processing Time RuleShortest Processing Time Rule

Process job with shortest processing time first. Usually best at minimizing job flow and minimizing

the number of jobs in the system Major disadvantage is that long jobs may be

continuously pushed back in the queue.

Longest Processing Time RuleLongest Processing Time Rule

Process job with longest processing time first. Usually the least effective method of sequencing.

Process job with earliest due date first Widely used by many companies

If due dates important If MRP used

Due dates updated by each MRP run

Performs poorly on many scheduling criteria

Earliest Due Date RuleEarliest Due Date Rule

Ratio of time remaining to work time Ratio of time remaining to work time remainingremaining

Process job with smallest CR first Performs well on average lateness

CRTime remaining

Work days remainingDue date - Today's date

Work (lead) time remaining

=

=

Critical Ratio (CR)Critical Ratio (CR)

Advantages of the Critical RatioAdvantages of the Critical RatioScheduling RuleScheduling Rule

Use of the critical ratio can help to: determine the status of a specific job establish a relative priority among jobs on a common

basis relate both stock and make-to-order jobs on a

common basis adjust priorities and revise schedules automatically for

changes in both demand and job progress dynamically track job progress and location

Criteria to Evaluate Priority RulesCriteria to Evaluate Priority Rules

Jobs#times ΣFlow

time completion Average

times Flowtimes Process

nUtilizatio

times Processtimes Flow

system the in jobs of number Average

jobs of Numbertimes Late

lateness job Average

Job Sequencing ExampleJob Sequencing Example

Job Job WorkProcessing time in days

Job Due Date (day)

A 6 8

B 2 6

C 8 18

D 3 15

E 9 23

FCFSFCFS

Parameter Value

Average completion time

15.4 days

Utilization 36.4%

Average number of jobs in the system

2.75 jobs

Average job lateness

2.2 days

Sequence

A

B

C

D

E

SPTSPT

Parameter Value

Average completion time

13 days

Utilization 43.1%

Average number of jobs in the system

2.32 jobs

Average job lateness

1.8 days

Sequence

B

D

A

C

E

EDDEDD

Parameter Value

Average completion time

13.6 days

Utilization 41.2%

Average number of jobs in the system

2.43 jobs

Average job lateness

1.2 days

Sequence

B

A

D

C

E

LPTLPT

Parameter Value

Average completion time

20.6 days

Utilization 27.2%

Average number of jobs in the system

3.68 jobs

Average job lateness

9.6 days

Sequence

E

C

A

D

B

SummarySummary

Rule Average Completion Time (days)

Utilization (%)

Average Number of Jobs in the

System

Average Lateness (Days)

FCFS 15.4 36.4 2.75 2.2SPT 13.0 43.1 2.32 1.8EDD 13.6 41.2 2.43 1.2LPT 20.6 27.2 3.68 9.6

Critical Ratio (CR)Critical Ratio (CR)

Job Job WorkProcessing time in

days

Job Due Date (day)

Critical Ratio

A 6 8 0.75

B 2 6 0.33

C 8 18 0.44

D 3 15 0.20

E 9 23 0.39

Sequence

A

C

E

B

D

Used to sequence N jobs through 2 machines in the same order

© 1995 Corel Corp.

© 1995 Corel Corp.

Saw Drill

Job AJob A

Job BJob B

Job CJob C

Jobs (N = 3)

Johnson’s RuleJohnson’s Rule

Johnson's Rule - Scheduling N Johnson's Rule - Scheduling N Jobs on Two MachinesJobs on Two Machines

All jobs are to be listed, and the time each requires on a machine shown.

Select the job with the shortest activity time. If the shortest time lies with the first machine, the job is scheduled first; if with the second machine, the job is scheduled last.

Once a job is scheduled, eliminate it.Apply steps 2-3 to the remaining jobs, working

toward the center of the sequence.

List jobs &activity times

Select job withshortest time

Machine?

ScheduleFIRST

ScheduleLAST

Eliminate jobfrom list

Jobs left?Breakarbitrarily

Ties?

YesYes

11

22

YesYesStop

NoNo

NoNo

Johnson’s Rule StepsJohnson’s Rule Steps

Johnson’s Rule - ExampleJohnson’s Rule - Example

Job Work Center 1(Drill Press)

Work Center 2(Lathe)

A 5 2

B 3 6

C 8 4

D 10 7

E 7 12

Johnson’s Rule - ExampleJohnson’s Rule - ExampleAStep 1

B AStep 2

B C AStep 3

B D C AStep 4

B E D C AStep 5

Graphical Depiction of Job FlowGraphical Depiction of Job Flow

B E D C A

B E D C A

Work center 1

Work center 2

0 3 10 20 28 33

0 3 9 10 20 22 28 29 33 35Time =>

Time =>

B E D C A

= Job completed= Idle

Limitations of Rule-Based Limitations of Rule-Based Dispatching SystemsDispatching Systems

Scheduling is dynamic; therefore, rules need to be revised to adjust to changes in process, equipment, product mix, etc.

Rules do not look upstream or downstream; idle resources and bottleneck resources in other departments may not be recognized

Rules do not look beyond due dates

Finite Scheduling SystemFinite Scheduling System

Deals with factors limiting company’s ability to achieve goals

Types of constraints Physical

Example: Machines, raw materials Non-physical

Example: Morale, training

Limits throughput in operations

Theory of ConstraintsTheory of Constraints

Theory of ConstraintsTheory of ConstraintsA Five Step ProcessA Five Step Process

Identify the constraintsDevelop a plan for overcoming the identified

constraintsFocus resources on accomplishing the constraints

identified in step 2Reduce the effects of the constraints by off-

loading work or by expanding capabilityOnce one set of constraints is overcome, return to

the first step and identify new constraints

Bottleneck work centers have less capacity than prior or following work centers

They limit production output © 1995 Corel Corp.

Bottleneck Work CentersBottleneck Work Centers

Techniques for Dealing With Techniques for Dealing With BottlenecksBottlenecks

1. Increase the capacity of the constraint2. Ensure well-trained and cross-trained employees

are available to operate and maintain the work center causing the constraint

3. Develop alternate routings, processing procedures, or subcontractors

4. Move inspections and tests to a position just before the constraint

5. Schedule throughput to match the capacity of the bottleneck

The 10 Commandments for The 10 Commandments for Correct SchedulingCorrect Scheduling

1. Utilization of a non-bottleneck resource is determined not by its own capacity but by some other constraint in the system

2. Activating a resource is not synonymous with utilizing a resource

3. An hour lost at a bottleneck is an hour lost of the whole system

4. An hour saved at a non-bottleneck is a mirage5. The transfer batch may not, and many times

should not, be equal to the process batch

The 10 Commandments for The 10 Commandments for Correct SchedulingCorrect Scheduling

6. The amount processed should be verifiable and not fixed

7. Capacity and priority need to be considered simultaneously, not sequentially

8. Damage from unforeseen problems can be isolated and minimized

9. Plant capacity should not be balanced

10. The sum of the local optimums is not equal to the global optimum

Repetitive Manufacturing - Repetitive Manufacturing - Advantages of Level Material UseAdvantages of Level Material Use

Lower inventory levels, releasing capital for other uses

Faster product throughput Improved component quality and hence improved

product quality Reduced floor space requirements Improved communication among employees

because they are closer together Smoother production process because large lots

have not “hidden” the problems

Scheduling for ServicesScheduling for Services

Appointment systems - doctor’s office Reservations systems - restaurant, car rental First come, first served - deli Most critical first - hospital trauma room

Cyclical SchedulingCyclical Scheduling

Plan a schedule equal in weeks to the number of people being scheduled

Determine how many of each of the least desirable off-shifts must be covered each week

Begin the schedule for one worker by scheduling the days off during the planning cycle (at a rate of 2 days per week on average)

Cyclical Scheduling - ContinuedCyclical Scheduling - Continued

Assign off-shifts for the first worker Repeat this pattern for each other worker, but

offset by one week from the previous Allow each worker to pick his/her “slot” or “line” in

order of seniority Mandate that any changes from the chosen

schedule are strictly between the personnel wanting to switch