Master of Business Administration (MBA)
GBS 660
Production and Operations
Management
Shopfloor Planning and Control
1
Shopfloor Planning and Control
1. Scheduling Process-Focused Manufacturing
2. Scheduling and Control in the Job Shop
3. Elements of the Job-Shop Scheduling
Problem
4. Priority Rules and Techniques
1.
Scheduling Process-Focused
Manufacturing
❑ Process-focused factories are often called job
shops
❑ A job shop’s work centers are organized around
similar types of equipment or operations
❑ Workers and machines are flexible and can be
assigned to and reassigned to many different
orders
❑ Job shops are complex to schedule because of
the small volume and large variety of tasks
2
Pre-Production Planning
❑ Design the product in customer order
❑ Plan the operations the product must pass
through ..... this is the routing plan
❑ Work moves between operations on a move
ticket
Objectives of Scheduling
1. Meeting due dates
2. Minimising late completion of jobs
3. Minimising completion time
4. Minimising response time
5. Minimising time in system
6. Minimising overtime
7. Minimising idle time
8. Minimising work-in-progress inventory
9. Maximising machine or labour utilisation
3
2.
Scheduling and Control in the Job Shop
❑ Scheduling is the last stage of planning before
production begins
❑ Functions performed in scheduling and controlling a
job shop:
1. Allocating orders, equipment, and personnel to work
centers
2. Establishing job priorities and determining the sequence in
which the jobs will be performed
3. Initiating performance of the scheduled work (dispatching
of orders)
4. Reviewing the status and controlling progress of orders, and
expediting late and critical orders (Shop-floor control)
5. Revising the schedule in light of changes in order status
Input-Output Control
❑ Input-output control identifies problems such as
insufficient or excessive capacity, bottlenecks or
any issues that prevents the order from being
completed on time
Input
Work
Center
Output
❑ Planned input should never exceed planned
output
4
Input-Output Control Report (Week -1)
Week:
-1
1
2
3
4
Planned input: labor-hrs
Actual input: labor-hrs
Cumulative deviation
100
50
40 100
Planned output: labor-hrs
Actual output: labor-hrs
Cumulative deviation
120
70
50 100
Planned ending WIP: l-h
Actual ending WIP: l-h
50
30
20
20
70
Input-Output Control Report (Week 1)
Week:
-1
1
2
3
Planned input: labor-hrs
Actual input: labor-hrs
Cumulative deviation
100
50
-50
50
40 100
Planned output: labor-hrs
Actual output: labor-hrs
Cumulative deviation
120
110
-10
70
50 100
Planned ending WIP: l-h
Actual ending WIP: l-h
50
10
30
20
70
4
20
5
Input-Output Control Report (Week 4)
Week:
-1
1
2
3
4
Planned input: labor-hrs
Actual input: labor-hrs
Cumulative deviation
100 50 40 100
50 40 30 80
-50 -60 -70 -90
Planned output: labor-hrs
Actual output: labor-hrs
Cumulative deviation
120 70 50 100
110 50 20 70
-10 -30 -60 -90
Planned ending WIP: l-h
Actual ending WIP: l-h
50
10
70
30
0
20
10
20
20
Assigning Jobs to Work Centers:
How Many Jobs/Day/Work Center
❑ Infinite loading
– Assigns jobs to work centers without regard to
capacity
– Unless excessive capacity exists, long queues
occur
❑ Finite loading
– Uses work center capacity to schedule orders
– Popular scheduling approach
– Integral part of Capacity Requirement Planning
(CRP)
6
Assigning Jobs to Work Centers:
Which Job Gets Built First?
❑ Forward scheduling
– Jobs are given earliest available time slot in
operation
– excessive WIP usually results
❑ Backward scheduling
– Start with promise date and work backward through
operations reviewing lead times to determine when a
job has to pass through each operation
– Less WIP but must have accurate lead times
Forward Scheduling
B
Today
Backward Scheduling
E
B
Due Date
Today
E
Due Date
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3.
Elements of the Job-Shop Scheduling
Problem
1. Job Arrival Patterns:
– Jobs arrive in a batch (static arrival), that is jobs are
scheduled at one time (say, once a week).
– Jobs arrive over a time interval according to some
statistical distribution (dynamic arrival), that is jobs
are dispatched as they arrive.
2. Number and Variety of Machines:
– The number of machines in the shop affects the
scheduling process
– Scheduling problem is simplified if there is one
machine, or if a group of machines can be treated
as one
3. Ratio of Workers to Machines:
– If there are more workers than machines or an
equal number of workers and machine, the shop is
referred to as a machine-limited system
– If there are more machines than workers, the shop
is referred to a labour-limited system
4. Flow Patterns of Jobs:
– Flow shop, where all the jobs follow the same path
from one machine to the next
– Randomly routed job shop, where there is no
similar pattern of movement of jobs from one
machine to the next
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5. Job Sequencing:
– The process of determining which job is started
first on some machine or work center from among
the waiting jobs
– Priority Rules are the rules used in obtaining a job
sequence
6. Priority Rule Evaluation Criteria:
1) Meeting due dates of customers or downstream
operations
2) Minimising flow time (the time a job spends in the
shop)
3) Minimising work in process
4) Minimising idle time of machines and workers
4.
Priority Rules and Techniques
❑ We want to determine the sequence in which
we will process a group of waiting orders at a
work center
❑ Many different sequencing rules (priority
rules) can be followed in setting the priorities
among orders
❑ There are numerous criteria for evaluating the
effectiveness of the sequencing rules
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Sequencing Rules
• First-Come First-Served (FCFS)
Next job to process is the one that arrived first
among the waiting jobs
• Shortest Processing Time (SPT)
Next job to process is the one with the shortest
processing time among the waiting jobs
• Earliest Due Date (EDD)
Next job to process is the one with the earliest due
(promised finished) date among the waiting jobs
• Least Slack (LS)
Next job to process is the one with the least [time to
due date minus total remaining processing time]
among the waiting jobs
• Critical Ratio (CR)
Next job to process is the one with the least [time to
due date divided by total remaining processing time]
among the waiting jobs
• Least Changeover Cost (LCC)
Sequence the waiting jobs such that total machine
changeover cost is minimized
10
Evaluating Sequencing Rules
❑ Average flow time - average amount of time
jobs spend in shop
❑ Average number of jobs in system
❑ Average job lateness - average amount of time
job’s completion date exceeds its promised
delivery date
❑ Changeover cost - total cost of making
machine changeovers for group of jobs
4.1 Scheduling n Jobs on One Machine
(n/1)
Use the FCFS, SPT, and Critical Ratio (CR) rules to
sequence the five jobs below. Evaluate the rules on the
bases of average flow time, average number of jobs in
the system, and average job lateness.
Job Processing Time Time to Promised Completion
A
6 hours
10 hours
B
12
16
C
9
8
D
14
14
E
8
7
11
FCFS Rule
A>B>C>D>E
Processing Promised
Flow
Job
Time
Completion Time Lateness
A
6
10
6
0
B
12
16
18
2
C
9
8
27
19
D
14
14
41
27
E
8
7
49
42
49
141
90
FCFS Rule Performance
– Average flow time:
141/5 = 28.2 hours
– Average number of jobs in the system:
141/49 = 2.88 jobs
– Average job lateness:
90/5 = 18.0 hours
12
SPT Rule
Job
A
E
C
B
D
A>E>C>B>D
Processing Promised
Flow
Time
Completion Time Lateness
6
10
6
0
8
7
14
7
9
8
23
15
12
16
35
19
14
14
49
35
49
127
76
SPT Rule Performance
– Average flow time:
127/5 = 25.4 hours
– Average number of jobs in the system:
127/49 = 2.59 jobs
– Average job lateness:
76/5 = 15.2 hours
13
Critical Ratio Rule
E>C>D>B>A
Processing Promised
Flow
Job
Time
Completion Time Lateness
E (.875)
8
7
8
1
C (.889)
9
8
17
9
D (1.00) 14
14
31
17
B (1.33) 12
16
43
27
A (1.67)
6
10
49
39
49
148
93
Critical Ratio Rule Performance
– Average flow time:
148/5 = 29.6 hours
– Average number of jobs in the system:
148/49 = 3.02 jobs
– Average job lateness:
93/5 = 18.6 hours
14
Comparison of Rule Performance
Rule
Average
Flow
Time
Average
Number of Jobs
in System
Average
Job
Lateness
FCFS
SPT
CR
28.2
25.4
29.6
2.88
2.59
3.02
18.0
15.2
18.6
SPT rule was superior for all 3 performance criteria.
4.2 Scheduling n Jobs on Two Machines
(n/2)
❑ When two or more jobs must be processed on
two machines or work centers in a common
sequence, we may want to select a sequence
that must hold for both machines or work
centers
❑ Johnson’s rule can be used to find the
sequence that minimizes the total production
time through both machines or work centers
15
Johnson’s Rule
1. List the processing time for each job on both
machines or work centers
2. Select the shortest processing time in either machine
or work center
3. If the shortest time is at the first machine or work
center, put the job in the first unassigned slot in the
schedule. If the shortest time is at the second
machine or work center, put the job in the last
unassigned slot in the schedule.
4. Eliminate the job assigned in step 3
5. Repeat steps 2-4 for each remaining job until the
schedule is complete
There are five jobs which must go through two
machines A and B. Processing time in hours is
given below. Determine the optimal sequence and
the elapsed time.
Job
Machine A
Time (hrs.)
Machine B
Time (hrs.)
C124
B113
L501
P658
I621
2.0
2.1
1.9
1.8
1.5
2.5
2.4
2.2
1.6
1.4
16
Johnson’s Rule:
Least
Time
1.4
1.6
1.9
2.0
2.1
Job
Machine
Type
Schedule
Slot
I621
P658
L501
C124
B113
B
B
A
A
A
5th
4th
1st
2nd
3rd
Elapsed Time-Gantt Chart
0
1.9
3.9
Machine A L501 C124
Machine B Idle
0
6.0
B113
L501
1.9
9.3
12.0
P658 I621
Idle
B113
P658 I621
C124
4.1
7.8
6.6
9.0
10.6 12.0
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4.3 Scheduling for Least Changeover Costs
❑ Changeover costs - costs of changing a
processing step in a production system over
from one job to another:
– Changing machine settings
– Getting job instructions
– Changing material
– Changing tools
❑ Usually, jobs should be processed in a
sequence that minimizes changeover costs
Minimising Changeover Costs:
Job Sequencing Heuristic
1. First, select the lowest changeover cost
among all changeovers (this establishes the
first two jobs in the sequence)
2. The next job to be selected will have the
lowest changeover cost among the
remaining jobs that follow the previously
selected job
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Example 1: Minimizing Changeover Costs
Hardtimes Heat Treating Service has 5 jobs waiting
to be processed at work center #11. The job-to-job
changeover costs are listed below. What should the
job sequence be?
A
Jobs B
That C
Follow
D
E
Jobs That Precede
A
B C
D
E
-- 65 80 50 62
95 -- 69 67 65
92 71 -- 67 75
85 105 65 -95
125 75 95 105 --
Job sequence with minimum changeover cost:
A follows D ($50 is the least c.o. cost)
C follows A ($92 is the least following c.o. cost)
B follows C ($69 is the least following c.o. cost)
E follows B (E is the only remaining job)
Job sequence is
D–A–C–B–E
Total changeover cost = $50 + 92 + 69 + 75
= $286
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Example 2: Minimizing Changeover Costs
A printing company does custom printing jobs for
local firms and schools. The operations manager is
currently developing a weekly printing schedule
for the printing press. He has developed
changeover costs for six waiting jobs. All jobs
carry equal priority, so the deciding factor in
selecting a job sequence is the total changeover
cost for the six jobs.
What is the preferred job sequence?
Jobs with Changeover Costs
A
B
Jobs
C
That
Follow D
E
F
Jobs That Precede
A
B C
D
E
-- 12 15 10 35
25 -- 20 20 25
27 15 -- 12 20
16 30 10 -25
35 20 25 30
-20 25 15 25 30
F
20
20
15
30
30
--
20
Job sequence with minimum changeover cost:
There is a tie on the starting lowest changeover cost
between (D-A) and (C-D). Therefore develop two
sequences and choose one with least total cost.
Sequence 1: D – A – F – C – B – E
Total changeover cost = 10 + 20 + 15 + 20 + 20
= $85
Sequence 2: C – D – A – F – B – E
Total changeover cost = 10 + 10 + 20 + 20 + 20
= $80
In this case sequence C-D-A-F-B-E is the preferred
sequence.
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