Operations Operations ManagementManagement

Short-Term SchedulingShort-Term Scheduling

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 through dependable delivery

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

TodayToday Due Date

BBBB EEEE

Forward SchedulingForward Scheduling

TodayToday Due Date

BBBB EEEE

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-Short-Term Scheduling Examples

Scheduling DecisionsScheduling Decisions

Mount Sinai Hospital

Indiana University

???

Delta Airlines

Operating room use

Patient admissionNursing, security, maintenance staffsOutpatient treatments

Classrooms and audiovisual equipment

Student and instructor schedulesGraduate and undergraduate courses

Production of goodsPurchase of materialsWorkers

Maintenance of aircraft

Departure timetables

Flight crews, catering, gate, and ticketing personnel

Organization Managers Must Schedule

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

Capacity 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

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

Long-term

Intermediate-term

Short-term

Intermediate-term

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 The Goals of Short-Term Scheduling

minimize completion time minimize WIP inventory (keep inventory

levels low) maximize utilization (make effective use

of personnel and equipment) 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

A Production Planning and A Production Planning and Control System ShouldControl System Should

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 - containing information about each component the firm produces or purchases

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

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

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

Forecast &Firm Orders

MaterialRequirements

Planning

AggregateProduction

Planning

ResourceAvailability

MasterProductionScheduling

ShopFloor

Schedules

CapacityRequirements

PlanningRealistic?

Yes

No, modify CRP, MRP, or MPS

Process-Focused Planning SystemProcess-Focused Planning System

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) Assignment method

Shop LoadingShop Loading

Release Order

Release Order

Hold Release

Hold Release

Priority & Capacity OK?

1 2 3

Gross Requirements

Scheduled Receipts 100

Available

Net Requirements

Planned Order Rec.

Planned Order Rel. 100100100100

YesYesYesYes

NoNoNoNo

Order Release Begins Shop LoadingOrder Release Begins Shop Loading

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

Gantt Load ChartGantt Load Chart

Shows relative workload in facility Disadvantages

Does not account for unexpected events Must be updated regularly

Work Center M T W Th F

Sht. Metal Job A Job F

Mechanical Job D Job G

Electrical Job B Job H

Painting Job C Job E Job I

Gantt Load ChartGantt Load Chart

S M T W T F S

DayJob

Job A

Job B

Job C

Repair

Used to monitor job progress

Today

Gantt Scheduling ChartGantt Scheduling Chart

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

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

Production Control Production

Which job do I run next?

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

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

Job PacketJob XYZ

Order release

Sequencing ChallengeSequencing Challenge

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

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 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

Avg. completion timeFlow time

# Jobs

Utilizatio nProcess ti me

Flow time

Avg. # jobs in sSystemFlow time

Processing time

Avg. job lateness =Late time

# Jobs

=

=

=

Criteria to Evaluate Priority RulesCriteria to Evaluate Priority Rules

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 Jobs on Johnson's Rule - Scheduling N Jobs on Two MachinesTwo 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?

YesYesYesYes

1111

2222

YesYesYesYes

StopNoNoNoNo

NoNoNoNo

Johnson’s Rule StepsJohnson’s Rule Steps

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

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 constraints Develop a plan for overcoming the identified

constraints Focus resources on accomplishing the previous

step Reduce the effects of the constraints by off-

loading work or by expanding capability Once one set of constrains is overcome, return to

the first step and identify new constraints

Have less capacity than prior or following work centers

Limit production output Remedies

Increase capacity of bottleneck Develop other routings or subcontract Schedule throughput to match bottleneck

© 1995 Corel Corp.

Bottleneck Work CentersBottleneck Work Centers

Techniques for Dealing With Techniques for Dealing With BottlenecksBottlenecks

Increase capacity of the constraint Ensuring well-trained and cross-trained employees

are available to operate and maintain this constraint

Developing alternate routings, processing procedures, or subcontractors

Moving inspections and tests to a position just before the constraint

Scheduling throughput to match the capacity of the bottleneck

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

The 10 Commandments for The 10 Commandments for Correct SchedulingCorrect Scheduling

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

activating a resource is not synonymous with utilizing a resource

an hour lost at a bottleneck is an hour lost of the whole system

an hour saved at a non-bottleneck is a mirage the transfer batch may not, and many times should

not, be equal to the process batch

The 10 Commandments for Correct Scheduling

the amount processed should be verifiable and not fixed

capacity and priority need to be considered simultaneously, not sequentially

Damage from unforeseen problems can be isolated and minimized

plant capacity should not be balanced the sum of the local optimums is not equal to

the global optimum

The Advantages of Level Material UseThe Advantages of Level Material Use

lower inventory levels faster product throughput improved component quality and

product quality reduced floor-space requirements a smoother production process because

large lots have not "hidden" the problems