operations management - stikom...

Transparency Masters to accompany Heizer/Render –Principles of Operations Management, 5e, and Operations Management, 7e

© 2004 by Prentice Hall, Inc., Upper Saddle River, N.J. 0745815-1

Operations Operations ManagementManagement

ShortShort--Term SchedulingTerm SchedulingChapter 15Chapter 15



OutlineOutline♦GLOBAL COMPANY PROFILE: DELTA

AIRLINES♦THE STRATEGIC IMPORTANCE OF SHORT-

TERM SCHEDULING♦SCHEDULING ISSUES

♦ Forward and Backward Scheduling♦Scheduling Criteria

♦SCHEDULING PROCESS-FOCUSED WORK CENTERS



Outline Outline -- ContinuedContinued♦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 Outline -- ContinuedContinued♦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 methods♦ Sequencing rules♦ Johnson’s rules♦ Bottlenecks



Learning Objectives Learning Objectives -- continuedcontinued

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 ShortShort--Term SchedulingTerm 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 that 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

ShortShort--Term SchedulingTerm Scheduling



♦Hospital♦ Outpatient treatments♦ Operating rooms

♦University♦ Instructors♦ Classrooms

♦Factory♦ Production♦ Purchases

© 1984-1994 T/Maker Co.

ShortShort--Term Scheduling ExamplesTerm 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 ScheduleChefs, 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 ShortSchedule, and Short--Term SchedulingTerm Scheduling

Capacity Planning1. Facility size2. Equipment procurement

Aggregate Scheduling1. Facility utilization2. Personnel needs3. Subcontracting

Master Schedule1. MRP2. Disaggregation of master plan

Long-term

Intermediate-term

Short-term

Intermediate-term

Short-term Scheduling1. Work center loading2. 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,

and scheduling surgery



The Goals of ShortThe Goals of Short--Term 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 (percentage of time facility is used)♦ WIP inventory (average number of 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

ProcessProcess--Focused Work CentersFocused Work Centers



Requirements for Scheduling Requirements for Scheduling ProcessProcess--Focused Work CentersFocused 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



ProcessProcess--Focused Planning SystemFocused Planning System

Forecast andFirm Orders

MaterialRequirements

Planning

AggregateProduction

PlanningResourceAvailability

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 and scheduling)—capacity♦Assignment method—job to specific work center

Loading Jobs in Work CentersLoading Jobs in Work Centers



-+5-10-200Cumulative Change in Backlog

-200-150-100-50Cumulative Deviation

270270270270Actual Output

320320320320Planned Output

-35-40-40-10Cumulative Deviation

280285280250270Actual input

280280280280280Planned input

7/117/46/276/206/136/6Week Ending

InputInput--Output ControlOutput 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 and 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 WorkflowWorkflow

♦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 FMetal 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 regularly

Job 349Job 408Job 295PaintingJob 349Job 408ElectronicsJob 406Job 349MechanicalJob 350Job 349Metalworks

FridayThursdayWednesdayTuesdayMondayDayWork Center

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 CMaintenance

Now

End of an activity

Actual work progressNon-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 MethodAssignment Method——Four StepsFour Steps1 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

2 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:

3 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

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



Assignment MethodAssignment Method——Type Setter Type Setter ExampleExample

$7$12$9T-50

$11$10$ 8S-66

$ 6$14$11R-34

CBATypesetterJob

Initial set-up



Step 1a & 1bStep 1a & 1b

052T-50

320S-66

085R-34

CBATypesetterJob

032T-50

300S-66

065R-34

CBATypesetterJob

Step 1a Step 1b



Step 2Step 2

032T-50

300S-66

065R-34

CBATypesetterJob

Smallest uncovered number



Step 3Step 3

010T-50

500S-66

043R-34

CBATypesetterJob

Make assignments



♦ 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’ and 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

Transparency Masters to accompany Heizer/Render –Principles of Operations Management, 5e, and Operations


♦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

Management, 7e



♦♦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

CR Time remainingWork days remaining

Due date – Today's dateWork (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 ΣFlowtime completion Average =

times Flowtimes Process nUtilizatio

ΣΣ

=

times Processtimes Flow system the in jobs of number Average

ΣΣ

=

jobs of Numbertimes Late lateness job Average Σ

=



Job Sequencing ExampleJob Sequencing Example

239E153D188C62B86A

Job Due Date (day)

Job WorkProcessing time in days

Job



FCFSFCFSValueParameter

2.2 daysAverage job lateness

2.75 jobsAverage number of jobs in the system

36.4%Utilization

15.4 daysAverage completion time

E

D

C

B

A

Sequence



SPTSPTValueParameter



43.1%Utilization

13 daysAverage completion time

E

C

A

D

B

Sequence



EDDEDDValueParameter



41.2%Utilization


E

C

D

A

B

Sequence



LPTLPTValueParameter



27.2%Utilization


B

D

A

C

E

Sequence



SummarySummary

Average Lateness (Days)

Average Number of Jobs in the

System

Utilization (%)

Average Completion Time (days)

Rule

9.63.6827.220.6LPT1.22.4341.213.6EDD1.82.3243.113.0SPT2.22.7536.415.4FCFS



Critical Ratio (CR)Critical Ratio (CR)

0.390.200.440.330.75

Critical Ratio

239E153D188C62B86A

Job Due Date (day)

Job WorkProcessing

time in days

Job

D

B

E

C

A

Sequence



♦ 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 RuleJohnson's Rule——Scheduling N Scheduling N Jobs on Two MachinesJobs on Two Machines

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

➋ 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.

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

toward the center of the sequence.



List jobs andactivity 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 RuleJohnson’s Rule——ExampleExample

710D127E

48C63B25A

Work Center 2(Lathe)

Work Center 1(Drill Press)

Job



Johnson’s RuleJohnson’s Rule——ExampleExampleAStep 1

ABStep 2

ACBStep 3

ACDBStep 4

ACDEBStep 5



Graphical Depiction of Job FlowGraphical Depiction of Job Flow

A

C ADEB

CDEBWork 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 RuleLimitations of Rule--Based Based Dispatching SystemsDispatching Systems

♦Scheduling is dynamic; therefore, rules need to be revised to adjust to changes in process, equipment, product mix, and so on.

♦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

➀ Identify the constraints➁ Develop a plan for overcoming the identified

constraints➂ Focus resources on accomplishing the constraints

identified in step 2➃ Reduce the effects of the constraints by off-

loading work or by expanding capability➄ Once 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

Transparency Masters to accompany Heizer/Render –Principles of Operations Management, 5e, and Operations


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

Management, 7e



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 balanced10. The sum of the local optimums is not equal to

the global optimum



Repetitive ManufacturingRepetitive 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 and 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 two days per week on average)



Cyclical Scheduling Cyclical Scheduling -- ContinuedContinued♦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

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