1. CHAPTER 4 : SCHEDULING Presented by: HAMKA BIN TAIP
MOHD SAFUAN BIN ANUAR
NUR AMIRA BINTI KHAMIS
PUVENDRAN A/L SUNDARAM

2. OUTLINE
4.1 Introduction 4.2 Job Shop Scheduling Terminology 4.3 Sequencing Rules FCFS SPT EDD CR 1

3. 4.1 INTRODUCTION What is Scheduling ?
Last stage of planning before production occurs
Specifies when labor, equipment, facilities are needed to produce a product or provide a service 2

4. Operations Scheduling
Scheduling is an allocation decision
Scheduling decisions allocate resources over relatively short time periods: a few months, weeks, or hours.
The results are time allocations of production resources to different jobs
All the orders can be completed while all production resources are utilized with their loads being balance 3

5. Production Management4

6. Objectives of Operations Scheduling
1. Meet customer due dates
2. Minimize job lateness
3. Minimize completion time
4. Minimize idle time
5. Minimize WIP inventory
6. Minimize the average flow time through the system
7. Reduce setup cost 5

7. Typical Scheduling and Control Functions
The following functions must be performed in scheduling and controlling a shop floor:
Allocating orders, equipment, and personnel to work centers or other specified location-short term capacity planning
Determining the sequence of order performance
Initiating performance of the scheduled work, commonly termed the dispatching of jobs
Shop-floor control, involving
Reviewing the status and controlling the progress of orders as they are being worked on
Expediting the late and critical orders 6

8. Scheduling Operations
Operations schedules are short-term plans designed to implement the master production schedule.
Operations scheduling focuses on how best to use existing capacity.
Often, several jobs must be processed at one or more workstations. Typically, a variety of tasks can be performed at each workstation. 7

9. Cont… Scheduling Operations
1. Scheduling in High-Volume System – To obtain a smooth rate of flow of goods or customers through the system in order to get a high utilization of labor and equipment. Often referred as flow systems; scheduling refer to flow shop scheduling 8

10. Cont… Scheduling Operations
Scheduling in Intermediate-Volume System – Typically produce standard outputs. The run sizes are relatively small compared to flow shop.
Scheduling in Low Volume System – Job shop scheduling 9

11. Low-Volume System
Low-volume, job shop (batch) operations, are designed for flexibility.
Products are made to order and orders usually differ considerably in terms of processing requirements, materials needed, processing time and sequence and setup.
Each product or service may have its own routing (scheduling is much more difficult)
Many service operations have similar issues – how to distribute the workload among work centers and what job processing sequence to use 10

12. Job Shop
A job shop is organized by machines which are grouped according to their functions. 11

13. Sequencing Jobs
Operations schedules are short-term plans designed to implement the sales and operations plan
An operation with divergent flows is often called a job shop
Low-to medium-volume production
Utilizes job or batch processes
No similar pattern of movements of jobs from one machine to the next
Difficult to schedule because of the variability in job routings and the continual introduction of new jobs to be processed 12

14. Job Shop Sequencing
Shipping Department
Raw Materials
Legend:
Batch of parts
Workstation
Diagram of a Manufacturing Job Shop Process
Not all jobs are assumed to require exactly the same number of operations, and some jobs may require multiple operations on a single machine.
Each job may have a different required sequencing of operations.
Operations scheduling of shop floor usually means job shop scheduling; 13

15. Cont…. Sequencing Jobs
An operation with line flow is often called a flow shop
Medium- to high-volume production
Utilizes line or continuous flow processes
All jobs follow the same paths from one machine to the next
Tasks are easier to schedule because the jobs have a common flow pattern through the system 14

16. Flow Shop Sequencing 15

17. Method of Shop Floor Planning and Control for Job Shop
1. Gantt Chart
Used for variety of purposes related to loading and Scheduling
The purpose is to organize and visually display the actual or intended use of resources in a time framework. 16

18. Cont……Method of Shop Floor Planning and Control for Job Shop
2. Loading
Refer to the assignments of jobs to processing (work) centers.
Loading decisions involve assigning specific jobs to work centers and to various machines in the work centers.
3. Scheduling
Forward scheduling and Backward scheduling 17

19. Cont……Method of Shop Floor Planning and Control for Job Shop
1. Gantt Charts
Developed in the early 1900’s by Henry Gantt
1.1 Load charts
- Illustrates the workload relative to the capacity of a resource
- Shows job schedule by employee 18

20. Cont……Method of Shop Floor Planning and Control for Job Shop
1.2 Progress charts:
- Illustrates the planned schedule compared to actual performance
- Brackets show when activity is scheduled to be finished. Note that
design and pilot run both finished late and feedback has not started yet. 19

21. Cont……Method of Shop Floor Planning and Control for Job Shop
2. Loading
2.1 Infinite loading:
Ignores capacity constraints, but helps identify bottlenecks in a proposed schedule to enable proactive management
2.2 Finite loading:
Allows only as much work to be assigned as can be done with available capacity – but doesn’t prepare for inevitable slippage 20

22. Cont……Method of Shop Floor Planning and Control for Job Shop
3. Scheduling
3.1 Forward Scheduling
Scheduling ahead from a point in time
If the issue is “How long will it take to complete this job?”
3.2 Backward Scheduling
Scheduling backward from a due date
If the issue is “When the latest the job can be started and still be completed by the due date?” 21

23. Start processing when order is received regardless of due date
Schedule the job’s last activity so it is finished right before the due date 22

24. 4.2 Job Shop Scheduling Terminology
Parallel processing versus sequential processing
Sequencing Processing: the m machines are distinguishable, and different operations are performed by different machines.
Parallel processing: The machines are identical, and any job can be processed on any machine. M1 M2 M3 M4
Job A
Job B
Job A M1 M2
Job B M3 M4
M1, M2, M3, and M4 are different;
Job A has 2 operations which should be processed on different Machines: M1and M2;
Job B has 3 operations which should be processed on different Machines: M3, M2 and M4;
M1, M2, M3, and M4 are identical;
Jobs A and B can be processed on any one of the 4 machines 23

25. Cont… Job Shop Scheduling Terminology
2 Flow time
The flow time of job i is the time that elapses from the initiation of that job on the first machine to the completion of job i.
The mean flow time, which is a common measure of system performance, is the arithmetic average of the flow times for all n jobs
Mean Flow Time=(F1+F2+F3)/3
Machines M1
Job 1
Job 2
Job 3 M2
Job 1
Job 2
Job 3
F1: FT of Job 1
Time
F2: FT of Job 2
F3: FT of Job 3 24

26. Cont… Job Shop Scheduling Terminology
3. Make-span
The make-span is the time required to complete a group of jobs (all n jobs).
Minimizing the make-span is a common objective in multiple-machine sequencing problems.
Job 1
Job 2
Job 3
Machines M1 M2
Time
F1: FT of Job 1
F2: FT of Job 2
F3: FT of Job 3
Make-span of the 3 jobs 25

27. Cont… Job Shop Scheduling Terminology
4. Tardiness and lateness
Tardiness is the positive difference between the completion time and the due date of a job.
Lateness refers to the difference between the job completion time and its due date and differs from tardiness in that lateness can be either positive or negative.
If lateness is positive, it is tardiness; when it is negative, it is earliness
Tardiness
of Job i
Due date
Completion
time of Job i
Lateness>0---Tardiness
Due date
of Job i
Completion
time of Job i
Lateness<0---Earliness
When the completion of Job is earlier than due date, the tardiness is 0 26

28. 4.3 Sequencing Rules FCFS (first come-first served)
Jobs are processed in the sequence in which they entered the shop;
The simplest and nature way of sequencing as in queuing of a bank
SPT (shortest processing time)
Jobs are sequenced in increasing order of their processing time;
The job with shortest processing time is first, the one with the next shortest processing time is second, and so on;
EDD (earliest due date)
Jobs are sequenced in increasing order of their due dates;
The job with earliest due date is first, the one with the next earliest due date is second, and so on; 27

29. Cont… Sequencing Rules
CR (Critical ratio)
Critical ratio is the remaining time until due date divided by processing time;
Scheduling the job with the smallest CR next;
Current time
Remaining time of Job i
Due date of Job i
Processing time of Job i
CRi=Remaining time of Job i/Processing time of Job i
=(Due date of Job i-current time)/Processing time of Job i
CR provides the balance between SPT and EDD, such that the task with shorter remaining time and longer processing time takes higher priority;
CR will become smaller as the current time approaches due date, and more priority will given to one with longer processing time;
For a job, if the numerator of its CR is negative ( the job has been already later), it is naturally scheduled next;
If more than one jobs are later, higher priority is given to one that has shorter processing time (SPT). 28

30. Cont… Sequencing Rules
Example 5.1
A machine center in a job shop for a local fabrication company has five unprocessed jobs remaining at a particular point in time. The jobs are labeled 1, 2, 3, 4, and 5 in the order that they entered the shop. The respective processing times and due dates are given in the table below.
Sequence the 5 jobs by above 4 rules and compare results based on mean flow time, average tardiness, and number of tardy jobs
Job number
Processing Time
Due Date 1 2 3 4 5 11 29 31 61 45 33 32 29

31. 1. First Come First Serve (FCFS)
Mean Flow time=268/5=53.6
Average tardiness=121/5=24.2
No. of tardy jobs=3.
1. First Come First Serve (FCFS)
Job number
Processing Time
Due Date 1 2 3 4 5 11 29 31 61 45 33 32
Job
Completion Time
Due Date
Tardiness
Totals 30

32. 2. Shortest Processing Time (SPT)
Mean Flow time=135/5=27.0
Average tardiness=43/5=8.6
No. of tardy jobs=1.
2. Shortest Processing Time (SPT)
Job number
Processing Time
Due Date 1 2 3 4 5 11 29 31 61 45 33 32
Job
Processing Time
Completion Time
Due Date
Tardiness
Totals 31

33. 3. Earliest Due Date (EDD)
Mean Flow time=235/5=47.0
Average tardiness=33/5=6.6
No. of tardy jobs=4.
3. Earliest Due Date (EDD)
Job number
Processing Time
Due Date 1 2 3 4 5 11 29 31 61 45 33 32
Job
Processing Time
Completion Time
Due Date
Tardiness
Totals 32

34. Current time should be reset after scheduling one job
4. Critical Ratio (CR)
Current time: t=0
Job number
Processing Time
Due Date
Critical Ratio 1 2 3 4 5 11 29 31 61 45 33 32
61/11(5.545)
45/29(1.552)
31/31(1.000)
33/1 (33.00)
32/2 (16.00)
Current time should be reset after scheduling one job
Current time: t=31
Job number
Processing Time
Due Date-Current Time
Critical Ratio 1 2 4 5 11 29 30 14
30/11(2.727)
14/29(0.483)
2/1 (2.000)
1/2 (0.500) 33

35. Cont … 4. Critical Ratio (CR)
Mean Flow time=289/5=57.8
Average tardiness=87/5=17.4
No. of tardy jobs=4.
Cont … 4. Critical Ratio (CR)
Current time=60
Job number
Processing Time
Due Date-Current Time
Critical Ratio 1 4 5 11 2
-27
-28
1/11(0.0909)
-27/1<0
-28/2<0
Both Jobs 4 and 5 are later, however Job 4 has shorter processing time and thus is scheduled first; Finally, job 1 is scheduled last.
Job number
Processing Time
Completion Time
Tardiness 3 2 4 5 1 31 29 11 60 61 63 74 15 28 13
Totals
289 87 34

36. Sequencing Rules——Summary
Mean Flow Time
Average Tardiness
Number of
Tardy Jobs
FCFS
SPT
EDD CR
53.6
27.0
47.0
57.8
24.2
8.6
6.6
17.4 3 1 4
Discussions
SPT results in smallest mean flow time
EDD have the minimum tardiness (42, 43, 18, and 31 for the 4 different rules) 35

37. Figure 1: Jobs are assigned as below:
Exercise
There are five sheet metal jobs are waiting to be assigned at a company work centre.Their work (processing) time and due dates are given below. Then, determine the sequence of processing according to (FCFS,SPT,EDD and CR)
 Figure 1: Jobs are assigned as below:
Job
Processing Time
Due Date A 6 8 B 2 C 18 D 3 15 E 9 23 36