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

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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

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Scheduling ContextScheduling is constrained by multiple

system design decisionsSystem capacityProduct and/or service designEquipment selectionWorker selection and trainingAggregate planning and master scheduling

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Scheduling Hierarchies

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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

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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

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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

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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

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Intermediate-Volume SystemsThree basic issues:

Run size of jobsThe timing of jobsThe sequence in which jobs will be produced

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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

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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

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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

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Gantt ChartsLoad chart

A Gantt chart that shows the loading and idle times for a group of machines or list of departments

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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

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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?”

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Gantt ChartsSchedule chart

A Gantt chart that shows the orders or jobs in progress and whether they are on schedule

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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

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I/O Chart

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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

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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

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Priority RulesFCFS - first come, first servedSPT - shortest processing timeEDD - earliest due dateCR - critical ratioS/O - slack per operationRush - emergency

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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

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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

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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

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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

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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

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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

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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

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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

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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

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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

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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

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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

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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

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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

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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

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