OPM 533 8-1 Operations Operations Management Management Short-Term Scheduling Short-Term Scheduling Chapter 8 Chapter 8
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Operations Operations ManagementManagement
Short-Term SchedulingShort-Term SchedulingChapter 8Chapter 8
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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
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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
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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
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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
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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
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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
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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
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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
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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)
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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
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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
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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
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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
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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.
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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
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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
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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
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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
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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.
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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
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Finite Scheduling SystemFinite Scheduling System
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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
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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
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Bottleneck work centers have less capacity than prior or following work centers
They limit production output
© 1995 Corel Corp.
Bottleneck Work CentersBottleneck Work Centers
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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
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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
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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
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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
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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