Ops MGT
Scheduling
McGraw-Hill/Irwin
Copyright © 2012 by The McGraw-Hill Companies, Inc. All rights reserved.
- You should be able to:
Explain what scheduling involves and the importance of good scheduling
Describe scheduling needs in high-volume and intermediate-volume systems
Describe scheduling needs in job shops
Use and interpret Gantt charts, and use the assignment method for loading
Give examples of commonly used priority rules
Summarize some of the unique problems encountered in service systems, and describe some of the approaches used for scheduling service systems
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- 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 is constrained by multiple system design decisions
- System capacity
- Product and/or service design
- Equipment selection
- Worker selection and training
- Aggregate planning and master scheduling
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- Flow 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
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- Few 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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- The 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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Scheduling
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Scheduling
- Outputs 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 production
- Rather, it is more economical
to produce intermittently
- Work centers periodically
shift from one product to
another
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- Three basic issues:
- Run size of jobs
- The timing of jobs
- The sequence in which jobs will be produced
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- Important considerations
- Setup cost
- Usage is not always as smooth as assumed in the economic lot size model
- Alternative scheduling approach
- Base production on a master schedule developed from customer orders and forecasted demand
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- Job shop scheduling
- Scheduling for low-volume systems with many variations in requirements
- Make-to-order products
- Processing requirements
- Material requirements
- Processing time
- Processing sequence and steps
- A complex scheduling environment
- It is impossible to establish firm schedules until actual job orders are received
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- Loading
- 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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Scheduling
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Scheduling
- Load chart
- A Gantt chart that shows the loading and idle times for a group of machines or list of departments
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- 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
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Infinite loading
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Finite loading
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Scheduling
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Scheduling
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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- Schedule chart
- A Gantt chart that shows the orders or jobs in progress and whether they are on schedule
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- Assignment model
- A linear programming model for optimal assignment of tasks and resources
- Hungarian method
- Method of assigning jobs by a one-for-one matching to identify the lowest cost solution
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Row reduction: subtract the smallest number in each row from every number in the row
Enter the result in a new table
Column reduction: subtract the smallest number in each column from every number in the column
Enter the result in a new table
Test whether an optimum assignment can be made
Determine the minimum number of lines needed to cross out all zeros
If the number of lines equals the number of rows, an optimum assignment is possible. Go to step 6
Else, go to step 4
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If the number of lines is less than the number of rows, modify the table:
Subtract the smallest number from every uncovered number in the table
Add the smallest uncovered number to the numbers at intersections of cross-out lines
Numbers crossed out but not at intersections of cross-out lines carry over unchanged to the next table
Repeat steps 3 and 4 until an optimal table is obtained
Make the assignments
Begin with rows or columns with only one zero
Match items that have zeros, using only one match for each row and each column
Eliminate both the row and the column after the match
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- Sequencing
- Determine the order in which jobs at a work center will be processed
- Priority 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
- Time needed for setup and processing of a job
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Scheduling
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Scheduling
- FCFS - first come, first served
- SPT - shortest processing time
- EDD - earliest due date
- CR - critical ratio
- S/O - slack per operation
- Rush - emergency
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Scheduling
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Scheduling
- The set of jobs is known; no new orders arrive after processing begins and no jobs are canceled
- Setup time is independent of processing time
- Setup time is deterministic
- Processing times are deterministic
- There will be no interruptions in processing such as machine breakdowns or accidents
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Scheduling
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Scheduling
- Local 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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- 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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- Johnson’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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Scheduling
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Scheduling
- Job time must be known and constant for each job at the work center
- Job times must be independent of sequence
- Jobs must follow same two-step sequence
- All jobs must be completed at the first work center before moving to second work center
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Scheduling
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Scheduling
List the jobs and their times at each work center
Select the job with the shortest time
If the shortest time is at the first work center, schedule that job first
If the shortest time is at the second work center, schedule the job last.
Break ties arbitrarily
Eliminate the job from further consideration
Repeat steps 2 and 3, working toward the center of the sequence, until all jobs have been scheduled
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- Variability 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 science
- It is an ongoing task for a manager
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- Set realistic due dates
- Focus 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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- Service scheduling often presents challenges not found in manufacturing
- These are primarily related to:
The inability to store or inventory services
The random nature of service requests
- Service scheduling may involve scheduling:
Customers
Workforce
Equipment
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Scheduling
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Scheduling
- 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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Scheduling
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Scheduling
- In many environments, the scheduling requirements are fairly similar
- e.g., hospitals, police/fire departments, restaurants, supermarkets
- Rotating schedules
- Set a scheduling horizon
- Identify the work pattern
- Develop a basic employee schedule
- Assign employees to the schedule
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- If 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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