1. Sequencing
Sequencing: Determine the order in which jobs at a work center will be processed.
Workstation: An area where one person works, usually with special equipment, on a specialized job.

2. Sequencing
Job time: Time needed for setup and processing of a job.

3. Priority Rules FCFS - first come, first served
Jobs are processed in the order in which they arrive at a machine or work center.
SPT - shortest processing time
Jobs are processed according to processing time at a machine or work center, shortest job first.
DD - due date
Jobs are processed according to due date, earliest due date first.

4. Priority Rules CR - critical ratio S/O - slack per operation
Jobs are processed according to smallest ratio of time remaining until due date to processing time remaining.
S/O - slack per operation
Jobs are processed according to average slack time (time until due date minus remaining time to process). Compute by dividing slack time by dividing slac time by number of remaining operations including the current one.
Rush – emergency
Emergency or Preferred Customers first.

5. Assumptions to Priority Rules
Setup time is deterministic.
The set of jobs is known, no new jobs arrive after processing begins and no jobs are canceled.
Processing times are deterministic rather than variables.
There will be no interruptions in processing such as machine breakdowns , accidents or worker illnesses.

6. Definitions Job Flow Time Job Lateness
The length of time a job is in the shop at a particular workstation or work center.
Job Lateness
This is the length of time the job completion date is expected to exceed the date the job was due or promised to a customer.

7. Definitions Makespan Average Number of Jobs
This is the total time needed to complete a group of jobs. It is the length of time between the start of the first job in the group and the completion of the last job in the group.
Average Number of Jobs
Jobs that are considered in a shop are considered to be work in process inventory. Mathematically
Average Number of Jobs= Total Flow Time / Makespan

8. Example
Determine the sequence of jobs, average time flow, average days late and average number of jobs at the work center, for each of these rules:
FCFS
SPT DD CR

9. Example Data JOB Processing Time Due Date A 2 7 B 8 16 C 4 D 10 17 E 515 F 12 18

10. FCFS Assume Jobs arrived in the following order: A-B-C-D-E-F JOB
Sequences
Processing Time (1)
Flow
Time ( cumulative processing time)
(2)
Due Date
(3)
2-3 A 2 7 B 8 10 16 C 4 14 D 24 17 E 5 29 15 F 12 41 18 22
120 54

11. FCFS Average Flow time= Total Flow Time/Number of Jobs=120/6=20 days
Average Tardiness=54/6=9
The makespan =41 days
Average Number of Jobs at workstation=
120/41=2.93 jobs per workstation

12. SPT rule SPT, the sequence is A-C-E-B-D-F
JOB
Sequences
Processing Time
(1)
Flow
Time ( cumulative processing time)
(2)
Due Date
(3)
2-3 A 2 7 C 4 6 E 5 11 15 B 8 19 16 3 D 10 29 17 12 F 41 18 23
108 40

13. SPT rule Average Number of Jobs at workstation=
108/41=2.63 jobs per workstation
The makespan =41 days
Average Tardiness=40/6=6.67days
Average Flow time= Total Flow Time/Number of Jobs=108/6=18 days

14. Summary Part A,B,C and D JOB Sequences Rule Average Flow Time (Days)
Average Lateness
Average Number of Jobs of the Work Center
FCFS
20.00
9.00
2.93
SPT
18.00
6.67
2.63 DD
18.33
6.33
2.68 CR
26.67
14.17
3.9

15. Two Work Center Sequencing
Johnson’s Rule: Technique for minimizing completion time 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. Johnson’s Rule Conditions
Job time must be known and constant.
Job times must be independent of sequence.
Jobs must follow same two-step sequence.
Job priorities cannot be used.
All units must be completed at the first work center before moving to second.