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1ãã 2002 South-Western/Thomson Learning2002 South-Western/Thomson Learning TMTM
Slides preparedSlides prepared
by John Loucksby John Loucks
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Chapter 16
Manufacturing Operations Scheduling
B2 [----------]
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Overview
Scheduling Process-Focused Manufacturing Scheduling Product-Focused Manufacturing
Computerized Scheduling Systems
Wrap-Up: What World-Class Companies Do
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Scheduling
Process-Focused
Manufacturing
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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.
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Scheduling and Shop-Floor Decisions
MaterialRequirementsPlan (MRP)
CapacityRequirementsPlan (CRP)
Order-Processing orRouting Plans
Planned
Order ReleasesReport
Work Center
Loading andOvertime Plan
Assignment of
Orders toWork Centers
MasterProductionSchedule (MPS)
Product DesignandProcess Plans
Day-to-Day Scheduling and Shop-Floor Decisions
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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
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Common Shop Floor Control Activities
The production control department controls andmonitors order progress through the shop.
Assigns priority to orders
Issues dispatching lists
Tracks WIP and keeps systems updated
Controls input-output between work centers
Measures efficiency, utilization, and productivity
of shop
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Shop Floor Planning and Control
Input-Output Control Gantt Chart
Finite and Infinite Loading
Forward and Backward Scheduling
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Gantt Charts
Gantt charts are useful tools to coordinate jobsthrough shop; graphical summary of job status and
loading of operations
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Gantt Charts
Machining
Fabrication
Assembly
Test
Tue. Wed. Thu. Fri. Sat.Work Centers Mon.
E F G
C D E
H C
F
ED
H C D
Scheduled Progress Setup, Maint.
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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 CRP
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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 backwardthrough operations reviewing lead times todetermine when a job has to pass through eachoperation
Less WIP but must have accurate lead times
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Order-Sequencing Problems
Sequencing Rules Criteria for Evaluating Sequencing Rules
Comparison of Sequencing Rules
Controlling Changeover Costs Minimizing Total Production Time
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Order-Sequencing Problems
We want to determine the sequence in which we will process a group of waiting orders at a work center.
Many different sequencing 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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Order-Sequencing Rules
First-Come First-Served (FCFS) Next job to process is the one that arrived firstamong 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 earliestdue (promised finished) date among the waiting jobs
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Order-Sequencing Rules
Least Slack (LS) Next job to process is the one with the least [timeto due date minus total remaining processingtime] among the waiting jobs
Critical Ratio (CR) Next job to process is the one with the least [timeto due date divided by total remaining processingtime] among the waiting jobs
Least Changeover Cost (LCC)
Sequence the waiting jobs such that total machinechangeover cost is minimized
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Evaluating the Effectiveness
of Sequencing Rules
Average flow time - average amount of time jobsspend 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
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Experience Says:
First-come-first-served Performs poorly on most evaluation criteria
Does give customers a sense of fair play
Shortest processing time
Performs well on most evaluation criteria But have to watch out for long-processing-time
orders getting continuously pushed back
Critical ratio
Works well on average job lateness criterion
May focus too much on jobs that cannot becompleted on time, causing others to be late too.
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Example: Sequencing Rules
Use the FCFS, SPT, and Critical Ratio rules tosequence 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 8D 14 14
E 8 7
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Example: Sequencing Rules
FCFS Rule A > B > C > D > E Processing Promised Flow
Job Time Completion Time Lateness
A 6 10 6 0
B 12 16 18 2C 9 8 27 19
D 14 14 41 27
E 8 7 49 42
49 141 90
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Example: Sequencing Rules
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
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Example: Sequencing Rules
SPT Rule A > E > C > B > D Processing Promised Flow
Job Time Completion Time Lateness
A 6 10 6 0
B 8 7 14 7
C 9 8 23 15
D 12 16 35 19
E 14 14 49 35
49 127 76
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Example: Sequencing Rules
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
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Example: Sequencing Rules
Critical Ratio Rule E > C > D > B > A
Processing Promised Flow
Job Time Completion Time Lateness
E (.875) 8 7 8 1C (.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
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Example: Sequencing Rules
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
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Example: Sequencing Rules
Comparison of Rule PerformanceAverage Average Average
Flow Number of Jobs Job
Rule Time in System Lateness
FCFS 28.2 2.88 18.0
SPT 25.4 2.59 15.2
CR 29.6 3.02 18.6
SPT rule was superior for all 3 performance criteria.
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Controlling Changeover Costs
Changeover costs - costs of changing a processingstep 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 thatminimizes changeover costs
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Controlling Changeover Costs
Job Sequencing Heuristic First, select the lowest changeover cost among all
changeovers (this establishes the first two jobs in
the sequence)
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: Minimizing Changeover Costs
Hardtimes Heat Treating Service has 5 jobswaiting to be processed at work center #11. The job-
to-job changeover costs are listed below. What
should the job sequence be?
Jobs That PrecedeA B C D E
A -- 65 80 50 62
B 95 -- 69 67 65
C 92 71 -- 67 75D 85 105 65 -- 95
E 125 75 95 105 --
Jobs
That
Follow
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Example: Minimizing Changeover Costs
Develop a job sequence: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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Minimizing Total Production Time
Sequencing n Jobs through Two Work Centers When several jobs must be sequenced through two
work centers, we may want to select a sequence
that must hold for both work centers
Johnson’s rule can be used to find the sequence
that minimizes the total production time through
both work centers
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Johnson’s Rule
1. Select the shortest processing time in either workcenter
2. If the shortest time is at the first work center, put the
job in the first unassigned slot in the schedule. If the
shortest time is at the second work center, put the jobin the last unassigned slot in the schedule.
3. Eliminate the job assigned in step 2.
4. Repeat steps 1-3, filling the schedule from the frontand back, until all jobs have been assigned a slot.
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Example: Minimizing Total Production Time
It is early Saturday morning and The FinestDetail has five automobiles waiting for detailingservice. Each vehicle goes through a thoroughexterior wash/wax process and then an interiorvacuum/shampoo/polish process.
The entire detailing crew must stay until the lastvehicle is completed. If the five vehicles aresequenced so that the total processing time isminimized, when can the crew go home. They willstart the first vehicle at 7:30 a.m.
Time estimates are shown on the next slide.
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Example: Minimizing Total Production Time
Exterior Interior
Job Time (hrs.) Time (hrs.)
Cadillac 2.0 2.5Bentley 2.1 2.4
Lexus 1.9 2.2
Porsche 1.8 1.6
Infiniti 1.5 1.4
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Example: Minimizing Total Production Time
Exterior
Interior
0 1.9 3.9 6.0 7.8 9.3 12.0
0 1.9 4.1 6.6 9.0 10.6 12.0
L C B
L
P I
Idle C B P I
Idle
It will take from 7:30 a.m. until 7:30 p.m. (notallowing for breaks) to complete the five vehicles.
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Scheduling
Product-Focused
Manufacturing
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Batch Scheduling
EOQ for Production Lot Size How many units of a single product should be
included in each production lot to minimize annual
inventory carrying cost and annual machine
changeover cost?
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Example: EOQ for Production Lots
CPC, Inc. produces four standard electronicassemblies on a produce-to-stock basis. The annualdemand, setup cost, carrying cost, demand rate, and
production rate for each assembly are shown on thenext slide.
a) What is the economic production lot size for eachassembly?
b) What percentage of the production lot of power
units is being used during its production run?c) For the power unit, how much time will pass
between production setups?
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Example: EOQ for Production Lots
Annual Setup Carry Demand Prod.
Demand Cost Cost Rate Rate
Power Unit 5,000 $1,200 $6 20 200
Converter 10,000 600 4 40 300
Equalizer 12,000 1,500 10 48 100
Transformer 6,000 400 2 24 50
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Example: EOQ for Production Lots
% of Power Units Used During Productiond/p = 20/200 = .10 or 10%
Time Between Setups for Power Units
EOQ/d = 1,490.7/20 = 74.535 days
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Batch Scheduling
Run-Out Method Attempts to use the total production capacity
available to produce just enough of each product
so that if all production stops, inventory of each
product runs out at the same time
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Example: Run-Out Method
QuadCycle, Inc. assembles, in batches, four bicycle models on the same assembly line. The
production manager must develop an assembly
schedule for March.
There are 1,000 hours available per month for bicycle assembly work. Using the run-out method
and the pertinent data shown on the next slide,
develop an assembly schedule for March.
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Example: Run-Out Method
Assembly March AprilInventory Time Forec. Forec.
On-Hand Required Demand Demand
Bicycle (Units) (Hr/Unit) (Units) (Units)
Razer 100 .3 400 400Splicer 600 .2 900 900
Tracker 500 .6 1,500 1,500
HiLander 200 .1 500 500
O
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Example: Run-Out Method
Convert inventory and forecast into assembly hours
Assemb. March MarchInvent. Time Forec. Invent. Forec.
On-Hand Req’d. Dem. On-Hand Dem.
Bicycle (Units) (Hr/Unit) (Units) (Hours) (Hours)
Razer 100 .3 400 30 120
Splicer 600 .2 900 120 180
Tracker 500 .6 1,500 300 900
HiLander 200 .1 500 20 50
Total 470 1,250
(1) (2) (3) (4) (5)
(1) x (2) (2) x (4)
E l R O M h d
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Example: Run-Out Method
Compute aggregate run-out time in monthsAggregate Run-out Time =
= [(Total Inventory On-Hand in Hours)
+ (Total Assembly Hours Available per Month)- (March’s Forecasted Demand in Hours)]
/ (April’s Forecasted Demand in Hours)
= (470 + 1,000-
1,250)/1,250 = .176 months
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C t i d S h d li
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Computerized Scheduling
Develops detailed schedules for each work centerindicating starting and ending times
Develops departmental schedules
Generates modified schedules as orders move
Many packages available.... select one most
appropriate for your business
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E d f Ch t 16
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End of Chapter 16