OPM 5338-1

Operations Operations ManagementManagement

Short-Term SchedulingShort-Term SchedulingChapter 8Chapter 8

OPM 5338-2

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

Scheduling IssuesScheduling Issues

Scheduling deals with the timing of operationsScheduling deals with the timing of operations The task is the allocation and prioritization of The task is the allocation and prioritization of

demanddemand Significant issues areSignificant issues are

The type of scheduling, forward or backwardThe type of scheduling, forward or backward The criteria for prioritiesThe criteria for priorities

OPM 5338-4

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

Hospital

University

Factory

OPM 5338-5

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

Airlines

OPM 5338-6

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

OPM 5338-7

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

Scheduling CriteriaScheduling Criteria

1.1. Minimize completion timeMinimize completion time

2.2. Maximize utilization of facilitiesMaximize utilization of facilities

3.3. Minimize work-in-process (WIP) inventoryMinimize work-in-process (WIP) inventory

4.4. Minimize customer waiting timeMinimize customer waiting time

Objective of scheduling : To optimize the use Objective of scheduling : To optimize the use of resources so that production objectives are of resources so that production objectives are

metmet

OPM 5338-9

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

OPM 5338-10

Requirements for Scheduling Process-Requirements for Scheduling Process-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 & check progress against need dates & order lead times

Check work-in-progress as jobs move through the shop

Provide feedback on plant & production activities

Provide work-efficiency statistics & monitor operator times for payroll & labor distribution analyses

Planning and Control FilesPlanning and Control Files

1.1. An item master file contains information about each An item master file contains information about each componentcomponent

2.2. A routing file indicates each component’s flow through A routing file indicates each component’s flow through the shopthe shop

3.3. A work-center master file contains information about the A work-center master file contains information about the work centerwork center

Planning FilesPlanning Files

Control FilesControl Files

Track the actual progress made against the planTrack the actual progress made against the plan

OPM 5338-12

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

Input-Output ControlInput-Output Control

Identifies overloading and underloading Identifies overloading and underloading conditionsconditions

Prompts managerial action to resolve Prompts managerial action to resolve scheduling problemsscheduling problems

Can be maintained using ConWIP cards that Can be maintained using ConWIP cards that control the scheduling of batchescontrol the scheduling of batches

OPM 5338-14

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)

OPM 5338-15

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

OPM 5338-16

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

OPM 5338-17

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

OPM 5338-18

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

OPM 5338-19

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.

OPM 5338-20

Longest Processing Time RuleLongest Processing Time Rule Process job with longest processing time first. Usually the least effective method of sequencing.

Earliest Due Date RuleEarliest Due Date Rule 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

Critical Ratio (CR)Critical Ratio (CR)

An index number found by dividing the time An index number found by dividing the time remaining until the due date by the work time remaining until the due date by the work time

remaining on the jobremaining on the job Jobs with low critical ratios are scheduled ahead of Jobs with low critical ratios are scheduled ahead of

jobs with higher critical ratiosjobs with higher critical ratios Performs well on average job lateness criteriaPerforms well on average job lateness criteria

CR = =CR = =Due date - Today’s dateDue date - Today’s date

Work (lead) time remainingWork (lead) time remainingTime remainingTime remaining

Workdays remainingWorkdays remaining

OPM 5338-22

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

OPM 5338-23

Criteria to Evaluate Priority RulesCriteria to Evaluate Priority Rules

Jobs#

timesΣFlow timecompletion Average

times Flowtimes Process

nUtilizatio

times Processtimes Flow

system the in jobs of number Average

jobs of Numbertimes Late

lateness job Average

OPM 5338-24

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

OPM 5338-25

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.

OPM 5338-26

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 Capacity SchedulingFinite Capacity Scheduling

Overcomes disadvantages of rule-based systems by Overcomes disadvantages of rule-based systems by providing an interactive, computer-based graphical providing an interactive, computer-based graphical

systemsystem May include rules and expert systems or simulation May include rules and expert systems or simulation

to allow real-time response to system changesto allow real-time response to system changes Initial data often from an MRP systemInitial data often from an MRP system

FCS allows the balancing of delivery needs and FCS allows the balancing of delivery needs and efficiencyefficiency

OPM 5338-28

Finite Scheduling SystemFinite Scheduling System

OPM 5338-29

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

OPM 5338-30

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

OPM 5338-31

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

They limit production output

© 1995 Corel Corp.

Bottleneck Work CentersBottleneck Work Centers

OPM 5338-32

Techniques for Dealing With Techniques for Dealing With BottlenecksBottlenecks

1. Increase the capacity of the constraint

2. 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

OPM 5338-33

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 mirage

5. The transfer batch may not, and many times should not, be equal to the process batch

OPM 5338-34

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

Drum, Buffer, RopeDrum, Buffer, Rope

The drum is the beat of the system and provides the The drum is the beat of the system and provides the schedule or pace of productionschedule or pace of production

The buffer is the inventory necessary to keep The buffer is the inventory necessary to keep constraints operating at capacityconstraints operating at capacity

The rope provides the synchronization necessary to The rope provides the synchronization necessary to pull units through the systempull units through the system

Scheduling Repetitive FacilitiesScheduling Repetitive Facilities

Level material use can help repetitive facilities Level material use can help repetitive facilities Better satisfy customer demandBetter satisfy customer demand Lower inventory investmentLower inventory investment Reduce batch sizeReduce batch size Better utilize equipment and facilitiesBetter utilize equipment and facilities

OPM 5338-37

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 ServicesScheduling Services

Service systems differ from manufacturingService systems differ from manufacturing

ManufacturingManufacturing ServicesServices

Schedules machines Schedules machines and materialsand materials

Schedule staffSchedule staff

Inventories used to Inventories used to smooth demandsmooth demand

Seldom maintain Seldom maintain inventoriesinventories

Machine-intensive and Machine-intensive and demand may be smoothdemand may be smooth

Labor-intensive and Labor-intensive and demand may be variabledemand may be variable

Scheduling may be bound Scheduling may be bound by union contractsby union contracts

Legal issues may Legal issues may constrain flexible constrain flexible schedulingscheduling

Few social or behavioral Few social or behavioral issuesissues

Social and behavioral Social and behavioral issues may be quite issues may be quite importantimportant

OPM 5338-39

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

Scheduling Service Employees With Scheduling Service Employees With Cyclical SchedulingCyclical Scheduling

Objective is to meet staffing requirements with Objective is to meet staffing requirements with the minimum number of workersthe minimum number of workers

Schedules need to be smooth and keep Schedules need to be smooth and keep personnel happypersonnel happy

Many techniques exist from simple algorithms Many techniques exist from simple algorithms to complex linear programming solutionsto complex linear programming solutions

Cyclical Scheduling ExampleCyclical Scheduling Example

1.1. Determine the staffing requirementsDetermine the staffing requirements

2.2. Identify two consecutive days with the lowest total Identify two consecutive days with the lowest total requirements and assign these as days offrequirements and assign these as days off

3.3. Make a new set of requirements subtracting the Make a new set of requirements subtracting the days worked by the first employeedays worked by the first employee

4.4. Apply step 2 to the new row Apply step 2 to the new row

5.5. Repeat steps 3 and 4 until all requirements have Repeat steps 3 and 4 until all requirements have been metbeen met

Cyclical Scheduling ExampleCyclical Scheduling Example

MM TT WW TT FF SS SS

Employee 1Employee 1 55 55 66 55 44 33 33

CapacityCapacity

Excess CapacityExcess Capacity

Cyclical Scheduling ExampleCyclical Scheduling Example

MM TT WW TT FF SS SS

Employee 1Employee 1 55 55 66 55 44 33 33

Employee 2Employee 2 44 44 55 44 33 33 33

CapacityCapacity

Excess CapacityExcess Capacity

Cyclical Scheduling ExampleCyclical Scheduling Example

MM TT WW TT FF SS SS

Employee 1Employee 1 55 55 66 55 44 33 33

Employee 2Employee 2 44 44 55 44 33 33 33

Employee 3Employee 3 33 33 44 33 22 33 33

CapacityCapacity

Excess CapacityExcess Capacity

Cyclical Scheduling ExampleCyclical Scheduling Example

MM TT WW TT FF SS SS

Employee 1Employee 1 55 55 66 55 44 33 33

Employee 2Employee 2 44 44 55 44 33 33 33

Employee 3Employee 3 33 33 44 33 22 33 33

Employee 4Employee 4 22 22 33 22 22 33 22

CapacityCapacity

Excess CapacityExcess Capacity

Cyclical Scheduling ExampleCyclical Scheduling Example

MM TT WW TT FF SS SS

Employee 1Employee 1 55 55 66 55 44 33 33

Employee 2Employee 2 44 44 55 44 33 33 33

Employee 3Employee 3 33 33 44 33 22 33 33

Employee 4Employee 4 22 22 33 22 22 33 22

Employee 5Employee 5 11 11 22 22 22 22 11

CapacityCapacity

Excess CapacityExcess Capacity

Cyclical Scheduling ExampleCyclical Scheduling Example

MM TT WW TT FF SS SS

Employee 1Employee 1 55 55 66 55 44 33 33

Employee 2Employee 2 44 44 55 44 33 33 33

Employee 3Employee 3 33 33 44 33 22 33 33

Employee 4Employee 4 22 22 33 22 22 33 22

Employee 5Employee 5 11 11 22 22 22 22 11

Employee 6Employee 6 11 11 11 11 11 11 00

CapacityCapacity

Excess CapacityExcess Capacity

Cyclical Scheduling ExampleCyclical Scheduling Example

MM TT WW TT FF SS SS

Employee 1Employee 1 55 55 66 55 44 33 33

Employee 2Employee 2 44 44 55 44 33 33 33

Employee 3Employee 3 33 33 44 33 22 33 33

Employee 4Employee 4 22 22 33 22 22 33 22

Employee 5Employee 5 11 11 22 22 22 22 11

Employee 6Employee 6 11 11 11 11 11 11 00

Employee 7Employee 7 11

CapacityCapacity

Excess CapacityExcess Capacity

Cyclical Scheduling ExampleCyclical Scheduling Example

MM TT WW TT FF SS SS

Employee 1Employee 1 55 55 66 55 44 33 33

Employee 2Employee 2 44 44 55 44 33 33 33

Employee 3Employee 3 33 33 44 33 22 33 33

Employee 4Employee 4 22 22 33 22 22 33 22

Employee 5Employee 5 11 11 22 22 22 22 11

Employee 6Employee 6 11 11 11 11 11 11 00

Employee 7Employee 7 11

CapacityCapacity 55 55 66 55 44 33 33

Excess CapacityExcess Capacity 00 00 00 00 00 11 00