Sequencing and Scheduling

Scheduling
Scheduling
Scheduling: The allocation of resources over
time to accomplish specific tasks.
Demand scheduling: A type of scheduling
whereby customers are assigned to a definite
time for order fulfillment.
Workforce scheduling: A type of scheduling
that determines when employees work.
Operations scheduling: A type of scheduling in
which jobs are assigned to workstations or
employees are assigned to jobs for specified
time periods.
Performance Measures
Job flow time: The amount of time a job spends in the
service or manufacturing system. Also referred to as
throughput time or time spent in the system, including
service.
Makespan: The total amount of time required to complete a
group of jobs.
Past due (Tardiness): The amount of time by which a job
missed its due date or the percentage of total jobs
processed over some period of time that missed their due
dates.
Work-in-process (WIP) inventory: Any job that is waiting in
line, moving from one operation to the next, being delayed,
being processed, or residing in a semi-finished state.
Total inventory: The sum of scheduled receipts and on-
hand inventories.
Utilization: The percentage of work time that is productively
spent by an employee or machine.
Gantt Charts
Gantt chart: Used as a tool to monitor the progress of
work and to view the load on workstations.
The chart takes two basic forms: (1) the job or activity
progress chart, and (2) the workstation chart.
The Gantt progress chart graphically displays the
current status of each job or activity relative to its
scheduled completion date.
The Gantt workstation chart shows the load on the
workstations and the nonproductive time.
Gantt Progress Chart
Gantt Progress Chart for an Auto Parts Company
Start activity
Scheduled activity time
Finish activity
Current Actual progress
Nonproductive time date
Job 4/17 4/18 4/19 4/20 4/21 4/22 4/23 4/24 4/25 4/26
Ford
Plymouth
Pontiac
Gantt Workstation Chart
Gantt Workstation Chart for Hospital Operating Rooms

Operations Scheduling
Operations schedules are short-term plans designed to
implement the master production schedule.
Operations scheduling focuses on how best to use
existing capacity.
Often, several jobs must be processed at one or more
workstations. Typically, a variety of tasks can be
performed at each workstation.
Job shop: A firm that specializes in low- to medium-
volume production and utilizes job or batch processes.
Flow shop: A firm that specializes in medium- to high-
volume production and utilizes line or continuous
processes.
Manufacturing Process
Shipping Department
Raw Materials
Legend:
Batch of parts
Workstation
Job Shop Dispatching
Dispatching: A method of generating schedules in job

shops whereby the decision about which job to process
next is made using simple priority rules whenever the
workstation becomes available for further processing.
Priority sequencing rules: The rules that specify the
job processing sequence when several jobs are waiting
in line at a workstation.
Job Shop Dispatching
Earliest due date (EDD): A priority sequencing rule that specifies
that the job with the earliest due date is the next job to be processed.
First-come, first-served (FCFS): A priority sequencing rule that
specifies that the job arriving at the workstation first has the highest
priority.
Shortest processing time (SPT): A priority sequencing rule that
specifies that the job requiring the shortest processing time is the
next job to be processed.
Critical ratio (CR): A ratio that is calculated by dividing the time
remaining until a jobs due date by the total shop time remaining for
the job. CR = (Due date Todays date)/Total shop time remaining
Total Shop Time = Setup, processing, move, and expected
waiting times of all remaining operations, including the operation
being scheduled.
Scheduling Jobs for
Multiple Workstations
Priority sequencing rules can be used to schedule more than
one operation. Each operation is treated independently.
Identifying the best priority rule to use at a particular
operation in a process is a complex problem because the
output from one process becomes the input for another.
Computer simulation models are effective tools to determine
which priority rules work best in a given situation.
When a workstation becomes idle, the priority rule is applied
to the jobs waiting for that operation, and the job with the
highest priority is selected.
When that operation is finished, the job is moved to the next
operation in its routing, where it waits until it again has the
highest priority.
Scheduling Problems
One machine, many jobs
Two machines, many jobs
Three machines, many jobs
Many machines, many jobs
One machine, many jobs
Total time is independent of sequence
SPT minimizes average flow time
Examples- Mumbai airport
Two machines, many jobs
All Jobs follow same sequence
Johnsons Rule
Jobs have different sequence Jacksons
Rule
Johnsons 3 machine rule
Johnsons Rule
Johnsons rule: A procedure that minimizes makespan
when scheduling a group of jobs on two workstations.
Step 1. Find the shortest processing time among the
jobs not yet scheduled. If two or more jobs are tied,
choose one job arbitrarily.
Step 2. If the shortest processing time is on workstation
1, schedule the corresponding job as early as possible.
If the shortest processing time is on workstation 2,
schedule the corresponding job as late as possible.
Step 3. Eliminate the last job scheduled from further
consideration. Repeat steps 1 and 2 until all jobs have
been scheduled.
Example 16.5
Johnsons Rule at the Morris Machine Co.
Time (hr)
Motor Workstation 1 Workstation 2
M1 12 22
M2 4 5
M3 5 3
M4 15 16
M5 10 8
Eliminate
Eliminate
Eliminate
Eliminate M5
M2 M3
M1 from
and
from
from consideration.
the only job
consideration.
consideration. The
The
The next
remaining
next
next to shortest
be
shortest
shortest time
time
time is
is
Shortest time is 3 hours at workstation 2, so
M5 isat
M2
scheduled
M1 at at Workstation
is M4.
workstation
workstation #1,
#2, 1,schedule
so
so so schedule
schedule M1
M5 M2 first.
next.
next to last.
schedule job M3 last.
Sequence = M2 M1 M4 M5 M3
Example 16.5
Johnsons Rule at the Morris Machine Co.
The schedule minimizes the idle time of workstation 2

and gives the fastest repair time for all five motors.
No other sequence will produce a lower makespan.
Gantt Chart for the Morris Machine Company Repair Schedule
Workstation
1 M2 M1 M4 M5 M3 Idleavailable
(4) (12) (15) (10) (5) for further work
2 Idle M2
(5) Idle M1
(22)
M4
(16)
M5
(8)
M3
(3)
0 5 10 15 20 25 30 35 40 45 50 55 60 65
Day
Classwork
Job 1 2 3 4 5 6 7 8
A 4 5 8 10 6 5 10 6
B 8 10 2 4 5 7 6 6
Johnsons 3 Machine Rule
Domination Largest on B is not greater
than smallest on A or C. Domination
guarantees optimality
Conversion Convert into a 2 M/C
problem as M1 = A + B, M2 = B + C
Find optimal sequence for this 2 machine
problem
Schedule on 3 machines based on the
sequence above
Classwork
Job 1 2 3 4 5 6
A 8 6 10 7 9 7
B 7 4 6 5 5 4
C 13 12 11 12 13 10
M1 15 10 16 12 14 11
M2 20 16 17 17 18 14

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Scheduling

Gantt Workstation Chart for Hospital Operating Rooms

Dispatching: A method of generating schedules in job

The schedule minimizes the idle time of workstation 2

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