1. 1 1 Slide Short – Term Scheduling Professor Ahmadi

2. 2 2 Slide 1.Minimize completion time 2.Maximize utilization of facilities 3.Minimize work-in-process (WIP) inventory 4.Minimize customer waiting time Optimize the use of resources so that production objectives are met The Goals of Short-Term Scheduling

3. 3 3 Slide Strategic Implications of Short-Term Scheduling n n By scheduling effectively, companies use assets more effectively and create greater capacity per dollar invested, which, in turn, lowers cost n n This added capacity and related flexibility provides faster delivery and therefore better customer service n n Good scheduling is a competitive advantage which contributes to dependable delivery

4. 4 4 Slide Scheduling Issues Forward and Backward Scheduling n Forward scheduling: begins the schedule as soon as the requirements are known jobs performed to customer orderjobs performed to customer order schedule can be accomplished even if due date is missedschedule can be accomplished even if due date is missed often causes buildup of WIPoften causes buildup of WIP n Backward scheduling: begins with the due date of the final operation; schedules jobs in reverse order used in many manufacturing environments, catering, scheduling surgeryused in many manufacturing environments, catering, scheduling surgery

5. 5 5 Slide n Assigning jobs to work centers n Considerations Job priority (e.g., due date)Job priority (e.g., due date) CapacityCapacity  Work center hours available  Hours needed for job n Approaches Input-Output ControlInput-Output Control Gantt charts (load & scheduling) - capacityGantt charts (load & scheduling) - capacity Assignment method - job to specific work centerAssignment method - job to specific work center Loading Jobs in Work Centers

6. 6 6 Slide Week Ending 6/66/136/206/277/47/11 Planned input 280280280280280 Actual input 270250280285280 Cumulative Deviation -10-40-40-35 Planned Output 320320320320 Actual Output 270270270270 Cumulative Deviation -50-100-150-200 Cumulative Change in Backlog 0-20-10+5 Input-Output Control Example (See page 595 of your textbook)

7. 7 7 Slide Gantt Scheduling Chart (See page 596 of your textbook) Start of an activity Scheduled activity time allowed Point in time when chart is reviewed STWTFS Day Job Job A Job B Job C Maintenance Now End of an activity Actual work progress Non- production time Day 1 Day 2 Day 3 Day 4 Day 5 Day 6 Day 7

8. 8 8 Slide n Assigns tasks or jobs to resources n Type of linear programming model ObjectiveObjective  Minimize total cost, time etc. ConstraintsConstraints  1 job per resource (e.g., machine)  1 resource (e.g., machine) per job Assignment Method

9. 9 9 Slide Assignment Problem n Four persons are to be assigned to four different jobs. The following table represents the cost of assigning various people to different jobs. Assign each person to a job so that the costs will be minimized. What is the minimum cost of assignment? Indicate the assignment. n JOBS PERSON IIIIIIIV A11748 A11748 B314126 B314126 C971310 C971310 D611514 D611514

10. 10 Slide n Specifies the order in which jobs should be performed at work centers n Priority rules are used to dispatch or sequence jobs  Sequencing (Priority) rules First come, first served ( FCFS )First come, first served ( FCFS ) Shortest processing time ( SPT )Shortest processing time ( SPT ) Earliest due date ( EDD )Earliest due date ( EDD ) Longest processing time ( LPT )Longest processing time ( LPT )  Critical ratio ( CR )  Johnson’s rule Sequencing Jobs

11. 11 Slide Priority Rules for Dispatching Jobs First come, first served (FCFS) The first job to arrive at a work center is processed first Earliest due date (EDD) The job with the earliest due date is processed first Shortest processing time (SPT) The job with the shortest processing time is processed first Longest processing time (LPT) The job with the longest processing time is processed first Critical ratio (CR) The ratio of time remaining to required work time remaining is calculated, and jobs are scheduled in order of increasing ratio.

12. 12 Slide Criteria to Evaluate Priority Rules

13. 13 Slide Job Sequencing Example (See page 600 of your textbook) Job Job Work Processing time in days Job Due Date (day) A68 B26 C818 D315 E923

14. 14 Slide FCFS (first come, first served) Job Sequence Job Work (Processing) Time Flow Time Job Due Date Job Lateness A6680 B2862 C816180 D319154 E928235 287711 Sequence A-B-C-D-E

15. 15 Slide FCFS (first come, first served) Sequence A-B-C-D-E Average completion time = = 77/5 = 15.4 days Total flow time Number of jobs Utilization = = 28/77 = 36.4% Total job work time Total flow time Average number of jobs in the system = = 77/28 = 2.75 jobs Total flow time Total job work time Average job lateness = = 11/5 = 2.2 days Total late days Number of jobs

16. 16 Slide Shortest Processing Time (SPT) Job Sequence Job Work (Processing) Time Flow Time Job Due Date Job Lateness B2260 D35150 A61183 C819181 E928235 28659 Sequence B-D-A-C-E

17. 17 Slide Shortest Processing Time (SPT) Sequence B-D-A-C-E Average completion time = = 65/5 = 13 days Total flow time Number of jobs Utilization = = 28/65 = 43.1% Total job work time Total flow time Average number of jobs in the system = = 65/28 = 2.32 jobs Total flow time Total job work time Average job lateness = = 9/5 = 1.8 days Total late days Number of jobs

18. 18 Slide Earliest Due Date (EDD) Job Sequence Job Work (Processing) Time Flow Time Job Due Date Job Lateness B2260 A6880 D311150 C819181 E928235 28686 Sequence B-A-D-C-E

19. 19 Slide Earliest Due Date (EDD) Sequence B-A-D-C-E Average completion time = = 68/5 = 13.6 days Total flow time Number of jobs Utilization = = 28/68 = 41.2% Total job work time Total flow time Average number of jobs in the system = = 68/28 = 2.43 jobs Total flow time Total job work time Average job lateness = = 6/5 = 1.2 days Total late days Number of jobs

20. 20 Slide Longest Processing Time (LPT) Job Sequence Job Work (Processing) Time Flow Time Job Due Date Job Lateness E99230 C817180 A623815 D3261511 B228622 2810348 Sequence E-C-A-D-B

21. 21 Slide Longest Processing Time (LPT) Sequence E-C-A-D-B Average completion time = = 103/5 = 20.6 days Total flow time Number of jobs Utilization = = 28/103 = 27.2% Total job work time Total flow time Average number of jobs in the system = = 103/28 = 3.68 jobs Total flow time Total job work time Average job lateness = = 48/5 = 9.6 days Total late days Number of jobs

22. 22 Slide Sequencing Example Rule Average Completion Time (Days) Utilization (%) Average Number of Jobs in System Average Lateness (Days) FCFS15.436.42.752.2 SPT13.043.12.321.8 EDD13.641.22.431.2 LPT20.627.23.689.6 Summary of Rules

23. 23 Slide 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  But SPT moves long jobs to the end which may result in dissatisfied customers  FCFS does not do especially well (or poorly) on any criteria but is perceived as fair by customers  EDD minimizes lateness

24. 24 Slide  Ratio of time remaining to work time remaining n Process job with smallest CR first n Performs well on average lateness CR Time remaining Work daysremaining Due date- Today's date Work (lead) time remaining = = Critical Ratio (CR)

25. 25 Slide Johnson's Rule - Scheduling N Jobs on Two Machines 1.All jobs are to be listed, and the time each requires on a machine shown. 2.Select the job with the shortest activity time. If the shortest time lies with the first machine, the job is scheduled first; if with the second machine, the job is scheduled last. 3.Once a job is scheduled, eliminate it. 4.Apply steps 2-3 to the remaining jobs, working toward the center of the sequence.

26. 26 Slide List jobs & activity times Select job with shortest time Machine? Schedule FIRST Schedule LAST Eliminate job from list Jobs left? Break arbitrarily Ties? Yes 1 2 Yes Stop No No Johnson’s Rule Steps

27. 27 Slide Johnson’s Rule Example (See Page 603 0f your textbook) Job Work Center 1 (Drill Press) Work Center 2 (Lathe) A52 B36 C84 D107 E712

28. 28 Slide Johnson’s Rule Example Job Work Center 1 (Drill Press) Work Center 2 (Lathe) A52 B36 C84 D107 E712 DABCE

29. 29 Slide Techniques for Dealing With Bottlenecks 1.Increase the capacity of the constraint 2.Ensure well-trained and cross-trained employees are available to operate and maintain the work center causing the constraint 3.Develop alternate routings, processing procedures, or subcontractors 4.Move inspections and tests to a position just before the constraint 5.Schedule throughput to match the capacity of the bottleneck