Operations Management Ch 16

Scheduling
Establishing the timing of the use of equipment,
facilities and human activities in an organization. Last step before actual output occurs.
Scheduling Operations:
Scheduling manufacturing operations, Scheduling service operations
Scheduling manufacturing operations:
High-volume systems
Intermediate-volume systems
Low-volume systems
Scheduling in High-Volume Systems
High-volume flow systems utilize standardized
equipment and activities.
Continuous operations.
High-Volume Scheduling
Success Factors:
Process and product design
Preventive maintenance
Rapid repair when breakdown occurs
Minimization of quality problems
Reliability and timing of supplies
Scheduling in
Intermediate-Volume Systems
Outputs are between standardized highvolume
systems and made-to-order job shops.
Intermittent (not continuous) operations
Job-shop scheduling:
Scheduling for low-volume systems with many
variations in requirements
Orders differ in processing requirements, materials
needed, processing time, processing sequence and
setups.
Two basic issues in Job-shop scheduling:
Loading: Assigning jobs to processing (work) centers
Sequencing: Determining the order in which jobs will
be processed
Loading context:
A number of work centers can perform a job.
A work center can perform multiple jobs.
Gantt Charts:
Charts used as a visual aid for loading and
scheduling. Need to be updated frequently.
Gantt load charts:
Show the loading and idle times of departments,
machines, or facilities
Display relative workloads over time
Infinite loading:
Jobs are assigned to work centers without regard to
the capacity of the work center.
Can lead to overloads and underloads
Form queues in some work centers
Finite loading:
Jobs are assigned to work centers taking into account
the work center capacity and job processing times.
Need to be updated frequently due to possible delays,
addition and cancellation of jobs
Forward scheduling
Scheduling ahead from some point in time
Backward scheduling
Scheduling by working backwards in time from the due date
Assignment Method:
There are a number of jobs and a number of work
centers.
The processing cost or time that occurs for each work
center to process each job varies.
Assign each job to a work center, with only one job
assigned to each work center, in order to minimize
total processing cost or time.
Specific method: Hungarian method
Assignment Method Math Formula:
For example, the number of ways to assign 3
jobs to 3 work centers is: 3! = 3 * 2* 1 = 6
Sequencing
Determining the order in which jobs at a work
center will be processed
Priority Sequencing Rules:
FCFS: First come, first served
SPT: Shortest processing time
EDD: Earliest due date
CR: Critical ratio
S/O: Slack per operation
Rush: Emergency
Critical ratio
Jobs are processed according to smallest ratio of
time remaining until due date to processing time
remaining.
Assumptions of Priority Rules
The set of jobs is known, with no addition or
cancellation of jobs.
Setup and processing times are independent of
processing sequence.
Setup and processing times are deterministic.
There are no interruptions in processing such as: machine breakdowns, accidents, worker illness
Job flow time
Time between job arrival and job departure at a work center
Job lateness
Difference between actual completion time and the due date
Tardiness
When only record positive differences and treat
all other differences as zero
Makespan
Total time needed to complete a group of jobs
Average number of jobs in a work center:
Total flow time รท Makespan
Comparison of Sequencing Rules:
No one sequencing rule excels on all criteria.
SPT does well on minimizing flow time and number of
jobs in the system.
SPT moves long jobs to the end which may result in
dissatisfied customers.
EDD minimizes lateness.
FCFS does not do especially well (or poorly) on any
criteria but is perceived as fair by customers.
Johnson’s Rule
Technique for minimizing completion time for a
group of jobs to be processed on two subsequent
machines or work centers.
Johnson’s Rule Optimum Sequence:
1. List the jobs and their times at each work
center.
2. Select the job with the shortest time.
3. Eliminate the job from further consideration.
4. Repeat steps 2 and 3 until all jobs have been
scheduled.
Scheduling Difficulties:
Variability in processing times and setup times.
Interruptions
Changes in the set of jobs
No method for identifying optimal schedule
Scheduling is not an exact science
Minimizing Scheduling Difficulties:
Set realistic due dates
Focus on bottleneck operations
Consider lot splitting of large jobs
Yield Management:
When capacity is fixed, the application of pricing
strategies to allocate capacity among various categories
of demand in order to maximize the overall revenue.
Examples of Fixed capacity:
Hotel, motel rooms, airline seats, unsold rooms and seats can’t be carried over.