operations management short-term scheduling operations management short-term scheduling
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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
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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