you should be able to: 1. explain what scheduling involves and the importance of good scheduling 2....
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You should be able to:1. Explain what scheduling involves and the importance of good
scheduling2. Describe scheduling needs in high-volume and intermediate-
volume systems3. Describe scheduling needs in job shops4. Use and interpret Gantt charts, and use the assignment
method for loading5. Give examples of commonly used priority rules6. Summarize some of the unique problems encountered in
service systems, and describe some of the approaches used for scheduling service systems
Chapter 16: Learning Objectives
16-1
Scheduling Scheduling:
Establishing the timing of the use of equipment, facilities and human activities in an organization
Effective scheduling can yield Cost savings Increases in productivity Other benefits
16-2
Scheduling ContextScheduling is constrained by multiple
system design decisionsSystem capacityProduct and/or service designEquipment selectionWorker selection and trainingAggregate planning and master scheduling
16-3
Scheduling Hierarchies
16-4
High Volume SystemsFlow System
High-volume system in which all jobs follow the same sequence
Flow system scheduling Scheduling for flow systems The goal is to achieve a smooth rate of flow of
goods or customers through the system in order to get high utilization of labor and equipment
Workstation 1
Workstation 2
Output
16-5
High-Volume: Scheduling DifficultiesFew flow systems are entirely dedicated to a
single product or service Each product change requires
Slightly different inputs of parts Slightly different materials Slightly different processing requirements that must
be scheduled into the line Need to avoid excessive inventory buildup Disruptions may result in less-than-desired output
16-6
High-Volume Success FactorsThe following factors often dictate the
success of high-volume systems:• Process and product design• Preventive maintenance• Rapid repair when breakdowns occur• Optimal product mixes• Minimization of quality problems• Reliability and timing of supplies
16-7
Intermediate-Volume SystemsOutputs fall between the standardized type of
output of high-volume systems and the make-to-order output of job shops
Output rates are insufficient to warrant continuous productionRather, it is more economical
to produce intermittentlyWork centers periodically
shift from one product to another
16-8
Intermediate-Volume SystemsThree basic issues:
Run size of jobsThe timing of jobsThe sequence in which jobs will be produced
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16-9
Intermediate-Volume SystemsImportant considerations
Setup costUsage is not always as smooth as assumed in
the economic lot size modelAlternative scheduling approach
Base production on a master schedule developed from customer orders and forecasted demand
16-10
Low-Volume SystemsJob shop scheduling
Scheduling for low-volume systems with many variations in requirementsMake-to-order productsProcessing requirementsMaterial requirementsProcessing timeProcessing sequence and steps
A complex scheduling environmentIt is impossible to establish firm schedules until actual
job orders are received
16-11
Low-Volume Systems: LoadingLoading
the assignment of jobs to processing centers Gantt chart
Used as a visual aid for loading and scheduling purposes
Purpose of the Gantt chart is to organize and visually display the actual or intended use of resources in a time framework
Managers may use the charts for trial-and-error schedule development to get an idea of what different arrangements would involve
16-12
Gantt ChartsLoad chart
A Gantt chart that shows the loading and idle times for a group of machines or list of departments
16-13
Loading Approaches Infinite loading
Jobs are assigned to workstations without regard to the capacity of the work center
Finite loading Jobs are assigned to work centers taking into account the work
center capacity and job processing times
1 2 3 4 5 6
over overCapacity
Infinite loading
1 2 3 4 5 6
Capacity
Finite loading
16-14
Scheduling Approaches· Forward scheduling
· Scheduling ahead from some point in time.· Used when the question is:
· “How long will it take to complete this job?· Backward scheduling
· Scheduling backwards from some due date· Used when the question is:
· “When is the latest this job can be started and still be completed on time?”
16-15
Gantt ChartsSchedule chart
A Gantt chart that shows the orders or jobs in progress and whether they are on schedule
16-16
Managing Work FlowsInput/Output (I/O) control
Managing work flow and queues at work centers Without I/O control:
o If demand exceeds processing capacity, a work center overload is created
o If work arrives more slowly than a work center can handle, work center underutilization results
The goal is to strike a balance between input and output rates in order to minimize queues and maximize utilization
16-17
I/O Chart
16-18
AssignmentAssignment model
A linear programming model for optimal assignment of tasks and resources
Hungarian methodMethod of assigning jobs by a one-for-one
matching to identify the lowest cost solution
16-19
Sequencing
Sequencing Determine the order in which jobs at a work
center will be processedPriority rules
Simple heuristics used to select the order in which jobs will be processed
The rules generally assume that job setup cost and time are independent of processing sequence Job time
o Time needed for setup and processing of a job
16-20
Priority RulesFCFS - first come, first servedSPT - shortest processing timeEDD - earliest due dateCR - critical ratioS/O - slack per operationRush - emergency
16-21
Priority Rules: AssumptionsThe set of jobs is known; no new orders arrive
after processing begins and no jobs are canceledSetup time is independent of processing timeSetup time is deterministicProcessing times are deterministicThere will be no interruptions in processing such
as machine breakdowns or accidents
16-22
Priority Rules: Local v. GlobalLocal priority rules:
Focus on information pertaining to a single workstation when establishing a job sequence
Global priority rules:Incorporate information from multiple
workstations when establishing a job sequence
16-23
Sequence: Performance Metrics Common performance metrics:
Job flow time This is the amount of time it takes from when a job arrives until it is complete It includes not only processing time but also any time waiting to be
processed
Job lateness This is the amount of time the job completion time is expected to exceed the
date the job was due or promised to a customer
Makespan The total time needed to complete a group of jobs from the beginning of the
first job to the completion of the last job
Average number of jobs Jobs that are in a shop are considered to be WIP inventory
16-24
Two Work Center SequencingJohnson’s Rule
Technique for minimizing makespan for a group of jobs to be processed on two machines or at two work centers. Minimizes total idle time Several conditions must be satisfied
16-25
Johnson’s Rule ConditionsJob time must be known and constant for
each job at the work centerJob times must be independent of
sequenceJobs must follow same two-step sequenceAll jobs must be completed at the first
work center before moving to second work center
16-26
Johnson’s Rule: Optimum Sequence1. List the jobs and their times at each work center
2. Select the job with the shortest timea. If the shortest time is at the first work center, schedule that
job firstb. If the shortest time is at the second work center, schedule
the job last.c. Break ties arbitrarily
3. Eliminate the job from further consideration
4. Repeat steps 2 and 3, working toward the center of the sequence, until all jobs have been scheduled
16-27
Scheduling DifficultiesVariability in
Setup times Processing times Interruptions Changes in the set of jobs
Except for small job sets, there is no method for identifying an optimal schedule
Scheduling is not an exact scienceIt is an ongoing task for a manager
16-28
Minimizing Scheduling DifficultiesSet realistic due datesFocus on bottleneck operations
First, try to increase the capacity of the operations If that is not possible
Schedule bottleneck operations first Then, schedule non-bottleneck operations around the
bottleneck operations
Consider lot splitting of large jobs Often works best when there are large differences in job
times
16-29
Theory of constraintsProduction planning approach that emphasizes
balancing flow throughout a system, and pursues a perpetual five-step improvement process centered around the system’s currently most restrictive constraint. Bottleneck operations limit system output
o Therefore, schedule bottleneck operations in a way that minimizes their idle times
Drum-buffer-ropeo Drum = the scheduleo Buffer = potentially constraining resources outside of the
bottlenecko Rope = represents synchronizing the sequence of
operations to ensure effective use of the bottleneck operations
Theory of Constraints
16-30
Theory of Constraints (contd.)Varying batch sizes to achieve greatest output of
bottleneck operations Process batch
The economical quantity to produce upon the activation of a given operation
Transfer batch The quantity to be transported from one operation to
another, assumed to be smaller than the first operation’s process batch
16-31
Theory of Constraints (contd.)Improving bottleneck operations:
1. Determine what is constraining the operation2. Exploit the constraint (i.e., make sure the constraining
resource is used to its maximum)3. Subordinate everything to the constraint (i.e., focus on
the constraint)4. Determine how to overcome (eliminate) the constraint5. Repeat the process for the next highest constraint
16-32
Theory of Constraints: MetricsThree important theory of constraints
metrics:Throughput
The rate at which the system generates money through sales
Inventory Inventory represents money tied up in goods and
materials used in a processOperating expense
All the money the system spends to convert inventory into throughput; including, utilities, scrap, depreciation, and so on
16-33
Service Operation ProblemsService scheduling often presents
challenges not found in manufacturing These are primarily related to:
1. The inability to store or inventory services2. The random nature of service requests
Service scheduling may involve scheduling:1. Customers2. Workforce3. Equipment
16-34
Scheduling Service Operations Scheduling customers: Demand
Management Appointment systems
Controls customer arrivals for service Reservation systems
Enable service systems to formulate a fairly accurate estimate demand on the system for a given time period
Scheduling the workforce: Capacity Management Cyclical Scheduling
Employees are assigned to work shifts or time slots, and have days off, on a repeating basis
16-35
Operations StrategyIf scheduling is done well:
Goods and services can be made or delivered in a timely manner
Resources can be used to best advantage Customers will be satisfied
It is important to not overlook the importance of scheduling to strategy and competitive advantage
16-36
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