1. 8 Job Sequencing & Operations Scheduling CHAPTER Arranged by
Dr Sh Salleh bin Sh Ahmad
Originated from: Operations Management, Eighth Edition, by William J. Stevenson
Copyright © 2005 by The McGraw-Hill Companies, Inc. All rights reserved.

2. Learning Outcomes
After completing the chapter, students should be able to:
Explain what scheduling involves and the importance of good scheduling
Discuss scheduling needs in high-volume and intermediate-volume systems
Discuss scheduling needs in job shops
Use and interpret scheduling Gantt charts

3. Learning Outcomes (continue)
After completing the chapter, students should be able to:
Discuss and give examples of commonly used priority rules
Describe some of the unique problems encountered in service systems, and describe some of the approaches used for scheduling service systems

4. 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

5. High-Volume Systems
Flow system: High-volume system with Standardized equipment and activities
Flow-shop scheduling: Scheduling for high-volume flow system
Work Center #1
Work Center #2
Output

6. Scheduling Manufacturing Operations
High-volume
Intermediate- volume
Low-volume
Service operations
Build A
A Done
Build B
B Done
Build C
C Done
Build D
Ship
JAN
FEB
MAR
APR
MAY
JUN
On time!

7. High-Volume Success Factors
Process and product design
Preventive maintenance
Rapid repair when breakdown occurs
Optimal product mixes
Minimization of quality problems
Reliability and timing of supplies

8. Intermediate-Volume Systems
Outputs are between standardized high-volume systems and made-to-order job shops
Run size, timing, and sequence of jobs
Economic run size:

9. Scheduling Low-Volume Systems
Loading - assignment of jobs to process centers
Sequencing - determining the order in which jobs will be processed
Job-shop scheduling
Scheduling for low-volume systems with many variations in requirements

10. Gantt Load Chart
Figure 15.2
Gantt chart - used as a visual aid for loading and scheduling

11. Loading
Infinite loading – Jobs are assigned to work centers without regard to the capacity of the work centre
Finite loading – Jobs are assigned to work centers taking into account the work centre capacity and the processing times
Forward scheduling – Scheduling ahead, from some point in time
Backward scheduling – Scheduling by working backwards from the due dates
Schedule chart

12. 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.

13. Sequencing
Priority rules: Simple heuristics used to select the order in which jobs will be processed.
Job time: Time needed for setup and processing of a job.
Everything is #1 Priority

14. Priority Rules FCFS - first come, first served
Table 15.2
FCFS - first come, first served
SPT - shortest processing time
EDD - earliest due date
CR - critical ratio
S/O - slack per operation
Rush - emergency
Top Priority

15. Simple Sequencing Rules
PROCESSING DUE
JOB TIME DATE
A 2 7
B 8 16
C 4 4 D
E 5 15 F

16. Simple Sequencing Rules: FCFS
Sequence: A-B-C-D-E-F
Job Processing Flow DUE TARDINESS
SEQUENCE TIME TIME DATE Flow – dd
(0 if negative) A B C D E F
Average flow time = 120/6 = 20 days Utilization = 41/120 = 34%
Average Tardiness = 54/6 = 9 days Makespan = 41 days
Average number of jobs at the work center = 120/41 = 2.93

17. Simple Sequencing Rules: SPT
Sequence: A-C-E-B-D-F
Job Processing Flow DUE TARDINESS
SEQUENCE TIME TIME DATE Flow – dd
(0 if negative) A C E B D F
Average flow time = 108/6 = 18 days Utilization = 41/108 = 38%
Average Tardiness = 40/6 = 6.67 days Makespan = 41 days
Average number of jobs at the work center = 108/41 = 2.63

18. Simple Sequencing Rules: EDD
Sequence: C-A-E-B-D-F
Job Processing Flow DUE TARDINESS
SEQUENCE TIME TIME DATE Flow – dd
(0 if negative) C A E B D F
Average flow time = 110/6 = days Utilization = 41/110 = 37%
Average Tardiness = 38/6 = 6.33 days Makespan = 41 days
Average number of jobs at the work center = 110/41 = 2.68

19. Sequencing Rules: Critical Ratio
Job Processing DUE Critical Ratio
SEQUENCE TIME DATE Calculation
A (7-0)/2 =3.5
B (16-0)8 =2.0
C (4-0)/4 =1.0
D (17-0)/10=1.7
E (15-0)/5=3.0
F (18-0)/12=1.5
The lowest CR is Job C, therefore it is selected as the first sequenced job

20. Sequencing Rules: Critical Ratio
At day 4 [Job C completed], the CR are
Job Processing DUE Critical Ratio
SEQUENCE TIME DATE Calculation
A (7-4)/2 =1.5
B (16-4)8 =1.5 C
D (17-4)/10=1.3
E (15-4)/5=2.2
F (18-4)/12=1.17
The lowest CR is Job F, therefore it is selected as the 2nd sequenced job

21. Sequencing Rules: Critical Ratio
At day 16 [Job C and F completed], the CR are
Job Processing DUE Critical Ratio
SEQUENCE TIME DATE Calculation
A (7-16)/2 =-4.5
B (16-16)8 =0.0 C
D (17-16)/10=0.1
E (15-16)/5=-0.2 F
The lowest CR is Job A, therefore it is selected as the 3rd sequenced job

22. Sequencing Rules: Critical Ratio
At day 18 [Job C, F and A completed], the CR are
Job Processing DUE Critical Ratio
SEQUENCE TIME DATE Calculation A
B (16-18)8 =-0.25 C
D (17-18)/10=-0.1
E (15-18)/5=-0.6 F
The lowest CR is Job E, therefore it is selected as the 4th sequenced job

23. Sequencing Rules: Critical Ratio
At day 23 [Job C, F, A and E completed], the CR are
Job Processing DUE Critical Ratio
SEQUENCE TIME DATE Calculation A
B (16-23)8 =-0.88 C
D (17-23)/10=-0.6 E F
The lowest CR is Job B, therefore it is selected as the 4th sequenced job.
The last sequence is set to Job D.
Therefore the final sequence is C-F-A-E-B-D

24. Sequence: C-F-A-E-B-D
Sequencing Rules: CR
Sequence: C-F-A-E-B-D
Job Processing Flow DUE TARDINESS
SEQUENCE TIME TIME DATE Flow – dd
(0 if negative) C F A E B D
Average flow time = 133/6 = days Utilization = 41/133 = 31%
Average Tardiness = 58/6 = 9.67 days Makespan = 41 days
Average number of jobs at the work center = 133/41 = 3.24

25. Comparison of the four sequence rules
3.24
9.67
22.17 CR
2.68
6.33
18.33
EDD
2.63
6.67
18.00
SPT
2.93
9.00
20.00
FCFS
Average Number of Jobs at the Work Center
Average Tardiness (days)
Average Flow Time (days)
Rule

26. Sequencing Jobs Through Multiple Serial Process
Example
In a manufacturing process, there are three type of components that must go through the machines of A, B and C respectively. Recommend the best components sequencing that will minimize the idle and makespan.
Components Processing Time (min)
Machines C1 C2 C3 A 2 4 B 8 3 C 6

27. Components Processing Time (min)
Machines C1 C2 C3 A 2 4 B 8 3 C 6
With 3 jobs (components) to be schedule, we can arrange the sequence with:
C1-C2-C3 or C1-C3-C2 or C2-C1-C3 or …..
Total possible sequences is equal to n!, n = number of jobs.
n! = 3! = 3 x 2 x 1 = 6

28. Components Processing Time (min)
Machines C1 C2 C3 A 2 4 B 8 3 C 6
Scheduling sequence of C1-C2-C3: makespan = 22 min C1 C2 C3 2 4 8 10 12
Idle 15 14 20 22 A B C

29. Components Processing Time (min)
Machines C1 C2 C3 A 2 4 B 8 3 C 6
Ask the student to complete the task of solving for scheduling sequence of C2-C1-C3

30. Components Processing Time (min)
Machines C1 C2 C3 A 2 4 B 8 3 C 6
Scheduling sequence of C2-C1-C3: makespan = 18 min C2 C1 C3 2 4 8 12
Idle 15 10 16 18 A B C

31. 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

32. 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

33. Sequencing Jobs Through Two Serial Process
Johnson’s Rule
List time required to process each job at each machine. Set up a one-dimensional matrix to represent desired sequence with # of slots equal to # of jobs.
Select smallest processing time at either machine. If that time is on machine 1, put the job as near to beginning of sequence as possible.
If smallest time occurs on machine 2, put the job as near to the end of the sequence as possible.
Remove job from list.
Repeat steps 2-4 until all slots in matrix are filled and all jobs are sequenced.

34. Johnson’s Rule A B C D E JOB PROCESS 1 PROCESS 2 A 6 8 B 11 6 C 7 3

35. Johnson’s Rule (cont.) E A D B C JOB PROCESS 1 PROCESS 2
(sanding) 5 11 20 31 38
Process 2
(painting) 15 23 30 37 41
Idle time
Completion time = 41
Idle time = =10

36. Scheduling Difficulties
Variability in
Setup times
Processing times
Interruptions
Changes in the set of jobs
No method for identifying optimal schedule
Scheduling is not an exact science
Ongoing task for a manager

37. Minimizing Scheduling Difficulties
Set realistic due dates
Focus on bottleneck operations
Consider lot splitting of large jobs

38. Scheduling Service Operations
Appointment systems
Controls customer arrivals for service
Reservation systems
Estimates demand for service
Scheduling the workforce
Manages capacity for service
Scheduling multiple resources
Coordinates use of more than one resource

39. Cyclical Scheduling
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

40. Service Operation Problems
Cannot store or inventory services
Customer service requests are random
Scheduling service involves
Customers
Workforce
Equipment