Production Scheduling
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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


2
Johnson’s Rule
1. Select the shortest processing time in either work
center
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 job
in 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 front
and back, until all jobs have been assigned a slot.
3
Example: Minimizing Total Production Time
It is early Saturday morning and The Finest
Detail has five automobiles waiting for detailing
service. Each vehicle goes through a thorough
exterior wash/wax process and then an interior
vacuum/shampoo/polish process.
The entire detailing crew must stay until the last
vehicle is completed. If the five vehicles are
sequenced so that the total processing time is
minimized, when can the crew go home. They will
start the first vehicle at 7:30 a.m.
Time estimates are shown on the next slide.
4
Example: Minimizing Total Production Time
Job
Exterior
Time (hrs.)
Interior
Time (hrs.)
Cadillac
Bentley
Lexus
Porsche
Infiniti
2.0
2.1
1.9
1.8
1.5
2.5
2.4
2.2
1.6
1.4
5
Example: Minimizing Total Production Time

Johnson’s Rule
Least
Time
Job
1.4
1.6
1.9
2.0
2.1
Infiniti
Porsche
Lexus
Cadillac
Bentley
Work
Center
Interior
Interior
Exterior
Exterior
Exterior
Schedule
Slot
5th
4th
1st
2nd
3rd
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Example: Minimizing Total Production Time
0
1.9
3.9
Exterior
L
C
Interior
Idle
L
0
1.9
6.0
B
7.8
P
C
4.1
9.3
I
B
6.6
Idle
P
9.0
12.0
I
10.6 12.0
It will take from 7:30 a.m. until 7:30 p.m. (not
allowing for breaks) to complete the five vehicles.
7
Scheduling
Product-Focused
Manufacturing
8
Product-Focused Scheduling

Two general types of product-focused production:
Batch - large batches of several standardized
products produced
Continuous - few products produced
continuously.... minimal changeovers


9
Scheduling Decisions

If products are produced in batches on the same
production lines:
How large should production lot size be for each
product?
When should machine changeovers be scheduled?
If products are produced to a delivery schedule:
At any point in time, how many products should
have passed each operation if time deliveries are to
be on schedule?

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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 electronic
assemblies on a produce-to-stock basis. The annual
demand, setup cost, carrying cost, demand rate, and
production rate for each assembly are shown on the
next slide.
a) What is the economic production lot size for each
assembly?
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
Converter
Equalizer
Transformer
5,000 $1,200
10,000
600
12,000 1,500
6,000
400
$6
4
10
2
20
40
48
24
200
300
100
50
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Example: EOQ for Production Lots

Economic Production Lot Sizes
EOQ = (2DS/C[p/(p-d)]
EOQ1 = (2(5,000)(1,200)/6[200/(200-20)]  1, 490.7
EOQ2 = (2(10,000)(600)/4[300/(300-40)]  1,860.5
EOQ3 = (2(12,000)(1,500)/10[100/(100-48)]  2,631.2
EOQ4 = (2(6,000)(400)/2[50/(50-24)]  2,148.3
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Example: EOQ for Production Lots

% of Power Units Used During Production
d/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

Limitations of EOQ Production Lot Size
Uses annual “ballpark” estimates of demand and
production rates, not the most current estimates
Not a comprehensive scheduling technique – only
considers a single product at a time
Multiple products usually share the same scarce
production capacity
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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
Bicycle
Razer
Splicer
Tracker
HiLander
Assembly March
April
Inventory
Time
Forec.
Forec.
On-Hand Required Demand Demand
(Units) (Hr/Unit) (Units) (Units)
100
600
500
200
.3
.2
.6
.1
400
900
1,500
500
400
900
1,500
500
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Example: Run-Out Method

Convert inventory and forecast into assembly hours
(1)
(2)
(3)
(4)
(5)
Assemb.
Invent.
Time
On-Hand Req’d.
Bicycle
(Units) (Hr/Unit)
March
March
Forec. Invent. Forec.
Dem. On-Hand Dem.
(Units) (Hours) (Hours)
Razer
Splicer
Tracker
HiLander
.3
.2
.6
.1
400
900
1,500
500
30
120
300
20
120
180
900
50
(2) x (4)
Total
470
1,250
100
600
500
200
(1) x (2)
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Example: Run-Out Method

Compute aggregate run-out time in months
Aggregate 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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Example: Run-Out Method

Develop March’s Production Schedule
(6)
Bicycle
Razer
Splicer
Tracker
HiLander
(3) x .176
(7)
(8)
(9)
March’s March’s
Desired Desired
Assembly
Ending End.Inv. Required
Time
Inventory & Forec. Production Allocated
(Units)
(Units)
(Units)
(Hours)
70
158
264
88
(3) + (6)
470
1,058
1,764
588
(7) - (1)
370
458
1,264
388
(8) x (2)
111.0
91.6
758.4
38.8
999.8
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Computerized Scheduling




Develops detailed schedules for each work center
indicating 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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Wrap-Up: World-Class Practice

In process-focused factories:
MRP II refined.... promises are met, shop loading
is near optimal, costs are low, quality is high
In product-focused factories:
EOQ for standard parts containers, this sets S, lot
sizes are lower, inventories slashed, customer
service improved
Scheduling is integral part of a computer information
system
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End of Chapter 16
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