1 isqa 459/559 mellie pullman scheduling shop floor
Post on 22-Dec-2015
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TRANSCRIPT
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Learning Objectives
• Explain the different kinds of scheduling• Describe different shop loading methods• Describe priority rules• Describe scheduling performance
measures
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Definitions• Routing:
• The operations to be performed, their sequence, the work centers visited, & the time standards
• Bottleneck:• A resource whose capacity is less than the demand placed on it
• Due date:• When the job is supposed to be finished
• Slack:• The time that a job can be delayed & still finish by its due date
• Queue:• A waiting line
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High Volume Operations
• High volume flow operations generally have fixed routings
• Bottlenecks are easily identified• Commonly use line-balancing to
design the process around the required tasks• Examples?
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Low Volume Operations• Low volume job shop operations are
designed for flexibility. • Each product or service may have its own
routing (scheduling is much more difficult)• Bottlenecks move around depending upon
the products being produced at any given time• Examples?
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Gantt Charts• Graphical tools used to illustrate workloads &
help monitor job progress
• Load charts: • Illustrates the workload relative to the capacity of a
resource
• Progress charts: • Illustrates the planned schedule compared to actual
performance
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Loading Workstations• Infinite loading:
• Ignores capacity constraints, but helps identify bottlenecks in a proposed schedule – enabling proactive management
• Finite loading:• Allows only as much work to be assigned as a
station should be able to handle – but doesn’t prepare for inevitable slippage
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Example
• Infinite:• Schedule
in time period needed
• Finite:• Schedule
according to capacity limits
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Forward Vs. Backward Scheduling
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
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Operations Sequencing• A short-term plan of actual jobs based on available
capacity & priorities• Priority rules:
• Decision rules to allocate the relative priority of jobs at a work center
• Local priority rules: determines priority based only on jobs at that workstation
• Global priority rules: also considers the remaining workstations a job must pass through
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Common Priority Rules• First come, first served (FCFS)• Last come, first served (LCFS)• Earliest job due date (EDD)• Shortest processing time (SPT)• Longest processing time (LPT)• Min Critical ratio: (CR)
• (Time until due date)/(processing time remaining)
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How to use priority rules• Decide which rule to use.
• List all the jobs waiting to be processed at the work-center and their job time • Job time includes setup and processing time
• Using your priority rule, determine which job has the highest priority and should be worked on 1st , 2nd, 3rd ,etc.
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Machine Shop with 6 jobs waiting
• Use SPT
(shortest processing time)• Determine sequence of
jobs
Job Number Job Time
(setup & run)
A 3 days
B 7 days
C 6 days
D 4 days
E 2 day
F 5 days
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Performance Measurement• Job flow time:
• Time a job is completed minus the time the job was first available for processing
• Average jobs in system:• Measures amount of work-in-progress =Total job flow time/Make-span
• Make-span:• The entire time it takes to finish a batch of jobs
• Job lateness:• Whether the job is completed ahead of, on, or behind
schedule
• Job tardiness:• How long after the due date a job was completed
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More Data on the jobs
Job Number
Job Time
(setup & run)
Due Date
(days from now)
Remaining job time at other WC
Critical Ratio
A 3 days 15 6 15/(3+6)
B 7 days 20 8
C 6 days 30 5
D 4 days 20 3
E 2 day 22 7
F 5 days 20 5
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Completion Data
Job Number
Compet-ion Date
Due Date
(days from now)
Lateness (days)
Tardiness (days)
A 5 15 -10 0
B 27 20 +7 7
C 20 30 -10 0
D 9 20 -11 0
E 2 22 -20 0
F 14 20 -6 0
Negative lateness means job is finished ahead of due date. 0=on time
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Group In-Class Activity• Each team should use each of these rules
calculate all performance measures:• Critical Ratio• Longest Process Time• EDD
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Worksheet for your method:____________Job # Job
Time
(setup & run)
Due Date
-days from now
Remaining job time at other WC
Job Flow Job Lateness
Job Tardiness
CR
A 3 days 15 6
B 7 days 20 8
C 6 days 30 5
D 4 days 20 3
E 2 day 22 7
F 5 days 20 5
AVE
Make-span ____________Job flow-time ____________Average jobs in system ____________
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What are the company’s objectives?What priority rule works best?
• Fairness?
• Minimizing mean job tardiness?
• Minimizing mean job flow time, lateness, and average jobs in the system?
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Sequencing through 2 work centersJohnson’s rule• All jobs are processed through 2 work centers
sequentially1. List the jobs and processing times2. Find the shortest activity processing time among all
jobs (not yet schedule).• If it is the first activity, put job needing that activity in the
earliest available position in the sequence• If it is the second activity, put the job needing that activity in
the last avaiable position in the job sequence. • One you schedule a job, it’s eliminated from further
consideration
3. Repeat step 2 until every job is on the schedule
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Sample Problem
Metal Job Activity 1
Metal
Cutting
Activity 2
Deburr & Grind
A 1 2
B 3 5
C 2 4
D 5 4
E 4 2
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Sample Problem (days)
Metal Job
Activity 1
Metal
Cutting
Activity 2
Deburr & Grind
A 1
(first)
2
B 3
(fourth)
5
C 2
(second)
4
D 5 4
(fifth)
E 4 2
(third)
1 2 3 4 5 6 7 8 9 10
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12
13
14
15
16
17
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Act1
A C C B B B D D D D D E E E E
Act
2
A A C C C C B B B B B D D D D E E
Ties: pick either