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Shop-Floor Planning and Control in Manufacturing

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Operations Scheduling
Scheduling in a
Process-Focused Environment
Terminology
пЃ† Job
shop - a functional organization whose departments
or work centers are organized around particular types of
operations
пЃ† Order
- quantity of a unique part number
пЃ† Routing - network of processes or work centers through
which the order must pass before it is completed
пЃ† Schedule
- allocate resources to be used to complete
each job, when, and for what period of time. Often
performed in two phases - loading of jobs and sequencing
of jobs
MTSU
3
Possible Objectives of
Job-Shop Scheduling
пЃ† Meet
promise or due dates
пЃ† Minimize lead time or make span
пЃ† Minimize setup time or cost
пЃ† Minimize work-in-process inventory
пЃ† Maximize resource utilization
MTSU
4
Elements of the Job-Shop
Scheduling Problem
пЃ† Job
arrival patterns
пЃ† Number and variety of machines in the shop
пЃ† Ratio of workers to machines in the shop
пЃ† Flow pattern of jobs through the shop
пЃ† Priority rules or methods for allocating jobs
to workers or machines
пЃ† Schedule evaluation criteria
MTSU
5
Shop-Floor Control Activities
пЃ† Assigning
a priority to each order
пЃ† Issuing dispatching lists to each work center
– Orders to be produced
– Order priorities
– Order due date
пЃ† Keeping
the work-in-process inventory updated
пЃ† Providing input-output control
пЃ† Measuring the efficiency, utilization and
productivity
MTSU
6
Scheduling and
Shop-Floor Decisions
Design and
processing plans
MPS
MRP
CRP
Routing plans
Planned
order release
report
Work center
loading and
overtime plan
Assignment of
orders to
work centers
1. Set priorities of orders
2. Assign resources to orders
3. Shop-floor control-dispatching
MTSU
7
Some Approaches to Developing
Job Shop Schedules
пЃ† Priority
rules
пЃ† Forward scheduling
пЃ† Backward scheduling
пЃ† Finite and infinite scheduling
MTSU
8
Examples of
Sequencing (Priority) Rules
пЃ† First-come,
first served (FCFS)
– sequence is based on the order jobs arrive
пЃ† Shortest
processing time (SPT)
– job with the shortest processing time among
waiting jobs is next
пЃ† Earliest
due date (EDD)
– job with the earliest due date among waiting time
is next
пѓІ
more
MTSU
9
Rules (continued)
пЃ† Start
date
– Due date minus normal lead time
пЃ† Slack
time remaining (STR)
– due date minus processing time remaining
пЃ† Slack
time per operation
пЃ† Critical ratio (CR)
– job with the least critical ratio (time to due date
divided by total remaining production time)
among the waiting jobs is next
MTSU
10
Forward Scheduling
пЃ† Jobs
are assigned to the earliest unassigned
time slots in work centers
пЃ† Excessive in-process inventories usually
result because jobs tend to sit and wait for
their next work-center assignments
MTSU
11
Backward Scheduling
пЃ† Starting
with an order’s promised delivery
date, schedulers work backward through
each stage of production using lead times to
determine when the order should pass
through each stage
пЃ† An order is assigned to the latest possible
time slot in work centers to allow the
promised delivery date to be met
MTSU
12
Finite and Infinite Loading
пЃ† Infinite
loading approach
– jobs are assigned to work centers without
considering the capacities of the work centers
пЃ† Finite
loading approach
– capacities of work centers are allocated among
a list of jobs
– no more work is scheduled to a work center
during any hour than the capacity of the work
center
MTSU
13
Possible Criteria for
Evaluating a Sequence
пЃ† Average
flow time
– average amount of time jobs spend in the
system
пЃ† Average
number of jobs in the system
пЃ† Average job lateness
– average amount of time that a job’s completion
date exceeds its promised delivery date
MTSU
14
Shop-Floor Control
Gantt Charts
пЃ† A plan
of resource commitments by each
work center to orders is plotted on the chart
пЃ† Actual performance is then plotted on the
same chart to identify deviations from the
plan
пЃ† This type of chart assists the manger in
coordinating the orderly flow of jobs
between work centers
MTSU
16
Input-Output Control
пЃ† The
amount of work flowing to a work center
is compared to the amount flowing out
пЃ† Identifies such problems as insufficient
capacity, excessive capacity, and production
difficulties
пЃ† For example if too much is flowing into the
work center compared to its capacity, then
excessive WIP inventory precedes the work
center
MTSU
17
Minimizing Total Production
Time For a Special Case
пЃ† Sequencing
n jobs through two work
centers
1 Select the shortest processing time in either
work center
2 If this time is on the first work center, place that
job in the first open slot
3 If this time is on the last work center, place that
job in the last open slot
4 Eliminate the job assigned and go back to Step
1
MTSU
18
Synchronous Manufacturing
Synchronous Manufacturing and the Theory of
Constraints
пЃ† The
Goal of the firm
пЃ† The
hockey-stick phenomenon
пЃ† Performance
пЃ† Capacity
measurement
and flow issues
MTSU
20
The Hockey Stick Phenomenon
Output
($)
1
пЃ† The
2
Period
3
4
end-of-period rush!
MTSU
21
Goldratt Proclaims
The goal of a firm is to make money.
MTSU
22
Performance Measurement
Financial
пЃ† Net
–
profit
an absolute measurement in dollars
пЃ† Return
–
a relative measure based on investment
пЃ† Cash
–
on investment
flow
a survival measurement
MTSU
23
Performance Measurement
Operational
пЃ† 1.
–
пЃ† 2.
–
пЃ† 3.
–
Throughput
the rate at which money is generated by the
system through sales
Inventory
all the money that the system has invested in
purchasing things it intends to sell
Operating expenses
all the money that the system spends to turn
inventory into throughput
MTSU
24
Productivity
пЃ† Does
not guarantee profitability
– Has throughput increased?
–
Has inventory decreased?
–
Have operational expenses decreased?
пЃ† PRODUCTIVITY IS ALL THE ACTIONS
THAT BRING A COMPANY CLOSER TO
ITS GOALS
MTSU
25
Unbalanced Capacity
пЃ† Earlier,
we discussed balancing assembly
lines.
–
The goal was constant cycle time across all
stations
пЃ† Synchronous
manufacturing views constant
workstation capacity as a bad decision.
пЃ† Why
is this the case?
MTSU
26
The Statistics of Dependent Events
Process Time (A)
Process Time (B)
пЃ† Rather
than balancing capacities, the flow
of product through the system should be
balanced
MTSU
27
Some Capacity Related
Terminology
пЃ† Capacity
–
Available time for production
пЃ† Bottleneck
–
Capacity is less than demand placed on resource
пЃ† Nonbottleneck
–
Capacity is greater than demand placed on resource
пЃ† Capacity-constrained
–
resource (CCR)
Capacity is close to demand placed on resource
MTSU
28
What’s Going to Happen?
Case A
X
D em a n d /m on th
P r ocess tim e/u n it
A va il. tim e/m on th
Y
Market
X
B ottlen eck
2 0 0 u n its
1 h ou r
2 0 0 h ou r s
MTSU
Y
N on bottlen eck
2 0 0 u n its
4 5 m in s
2 0 0 h ou r s
29
What’s Going to Happen?
Case
Case BB
Y
Y
D em a n d /m on th
P r ocess tim e/u n it
A va il. tim e/m on th
X
X
Market
Market
X
B ottlen eck
2 0 0 u n its
1 h ou r
2 0 0 h ou r s
MTSU
Y
N on bottlen eck
2 0 0 u n its
4 5 m in s
2 0 0 h ou r s
30
What’s Going to Happen?
Case C
Market
Assembly
X
D e m a n d /m o n th
P r o c e s s tim e /u n it
A v a il. tim e /m o n th
Y
X
B o ttle n e c k
2 0 0 u n its
1 h our
200 h ours
MTSU
Y
N o n b o ttle n e c k
2 0 0 u n its
4 5 m in s
200 h ours
31
What’s Going to Happen?
Case D
Market
X
D em a n d /m on th
P r ocess tim e/u n it
A va il. tim e/m on th
Market
Y
X
B ottlen eck
2 0 0 u n its
1 h ou r
2 0 0 h ou r s
MTSU
Y
N on bottlen eck
2 0 0 u n its
4 5 m in s
2 0 0 h ou r s
32
Saving Time
Bottleneck
Nonbottleneck
What are the consequences of saving
time at each process?
MTSU
33
Drum, Buffer, Rope
Bottleneck (drum)
A
B
Communication
(rope)
C
D
E
F
Market
Inventory
buffer
(time buffer)
MTSU
34
Quality Implications
пЃ† More
–
tolerant than JIT systems
Excess capacity throughout system
пЃ† Except
–
for the bottleneck
Quality control needed before bottleneck
MTSU
35
Inventory Cost Measurement
Dollar Days
пЃ† A measurement
of the value of inventory
and the time it stays within an area
Example
Dollar Days = (value of inventory)(number of days within a department)
MTSU
36
Benefits from Dollar Day
Measurement
пЃ† Marketing
–
пЃ†
Purchasing
–
пЃ†
discourages holding large amounts of finished goods
inventory
discourages placing large purchase orders that on the
surface appear to take advantage of quantity
discounts
Manufacturing
–
discourage large work in process and producing
earlier than needed MTSU
37
Comparing Synchronous
Manufacturing to MRP
пЃ† MRP uses
backward scheduling
пЃ† Synchronous
manufacturing uses forward
scheduling
MTSU
38
Comparing Synchronous
Manufacturing to JIT
пЃ† JIT
is limited to repetitive manufacturing
пЃ† JIT
requires a stable production level
пЃ† JIT
does not allow very much flexibility in
the products produced
MTSU
39
Comparing Synchronous
Manufacturing to JIT
пЃ† JIT
still requires work in process when used
with kanban so that there is "something to
pull."
пЃ† Vendors
need to be located nearby because
the system depends on smaller, more
frequent deliveries
MTSU
40
MTSU
41
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