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ISQA 459/559 Mellie Pullman

Scheduling Shop Floor

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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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Load Chart

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Progress Chart

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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

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Group In-Class Problem

Hall Activity 1

Mopping

Activity 2

Buffing

A 4 3

B 2 7

C 6 5

D 4 5

E 3 4

F 5 1