Capacity Planning, Aggregate Scheduling, Master Schedule, and
Short-Term Scheduling Capacity Planning 1. Facility size 2.
Equipment procurement Aggregate Scheduling 1. Facility utilization
2. Personnel needs 3. Subcontracting Master Schedule 1. MRP 2.
Dis-aggregation of master plan Long-term Intermediate-term
Short-term Intermediate-term Short-term Scheduling 1. Work center
loading 2. Job sequencing
Slide 2
Scheduling Low-Volume Systems Loading - assignment of jobs to
process centers Sequencing - determining the order in which jobs
will be processed Job-shop scheduling Scheduling for low-volume
systems with many variations in requirements
Slide 3
Assigning jobs to work centers Considerations Job priority
(e.g., due date) Capacity Work center hours available Hours needed
for job Approaches Gantt charts (load & scheduling) - capacity
Assignment method - job to specific work center Loading Jobs in
Work Centers
Slide 4
Loading Two Approaches to loading: - Finite Loading limit the
amount of work that is released to a given work center on the basis
of capacity consideration. Input/output control is a method for
keeping track of assigned resource (input) and the work completed
by a resources (output). This helps identify a backlog (input rate
in greater that output rate). The remove a backlog to slow down the
input.
Slide 5
Loading - Infinite loading assign job to center regardless of
available capacity. This requires that proper planning has been
done to make sure enough capacity is available to take care of all
work to be done.
Slide 6
8-6 Scheduling Definition: Determination of when something is
to be done and the tasks and activities required to do it.
Establishing the timing of the use of equipment, facilities and
human activities in an organization Scheduling aids in on-time
completion Direct link to value perceived by customers Scheduling
improves the utilization of the firms resources Direct link to
productivity
Slide 7
Strategic Implications of Short-Term Scheduling By scheduling
effectively, companies use assets more effectively and create
greater capacity per dollar invested, which, in turn, lowers cost
This added capacity and related flexibility provides faster
delivery and therefore better customer service Good scheduling is a
competitive advantage which contributes to dependable delivery
Slide 8
The Goals of Short-Term Scheduling Minimize completion time
Maximize utilization (make effective use of personnel and
equipment) Minimize WIP inventory (keep inventory levels low)
Minimize customer wait time
Slide 9
Scheduling Services Considerations Scheduling services
different from manufacturing Inability to store or inventory
services Random nature of customer requests for service Point of
customer contact Back-office operations Front-office
operations
Slide 10
Qualitative factors Number and variety of jobs Complexity of
jobs Nature of operations Quantitative criteria Average completion
time Utilization (% of time facility is used) WIP inventory
(average # jobs in system) Customer waiting time (average lateness)
Choosing a Scheduling Method
Slide 11
Forward and Backward Scheduling Forward scheduling: begins the
schedule as soon as the requirements are known jobs performed to
customer order schedule can be accomplished even if due date is
missed often causes buildup of WIP Backward scheduling: begins with
the due date of the final operation; schedules jobs in reverse
order used in many manufacturing environments, catering, scheduling
surgery
Slide 12
Scheduling Difficulties Variability in Setup times Processing
times Interruptions Changes in the set of jobs No method for
identifying optimal schedule Scheduling is not an exact science
Ongoing task for a manager
Slide 13
Minimizing Scheduling Difficulties Set realistic due dates
Focus on bottleneck operations Consider lot splitting of large
jobs
Slide 14
Sequencing Sequencing: Determine the order in which jobs at a
work center will be processed. Priority rules: Simple heuristics
used to select the order in which jobs will be processed. Job time:
Time needed for setup and processing of a job.
Slide 15
Priority Rules for Dispatching Jobs First come, first served
The first job to arrive at a work center is processed first
Earliest due date The job with the earliest due date is processed
first Shortest processing time The job with the shortest processing
time is processed first Longest processing time The job with the
longest processing time is processed first Critical ratio The ratio
of time remaining to required work time remaining is calculated,
and jobs are scheduled in order of increasing ratio. FCFS EDD SPT
LPT CRCR
Slide 16
Assumptions of Priority Rules The setup of jobs is known Setup
time is independent of processing sequence Setup time is
deterministic There will be no interruptions in processing such as:
Machine breakdowns Accidents Worker illness
Slide 17
Process first job to arrive at a work center first Average
performance on most scheduling criteria Appears fair &
reasonable to customers Important for service organizations
Example: Restaurants First Come, First Served Rule
Slide 18
SPT and LPT Rule SPT Schedule a job with shortest processing
time first LPT Schedule a job with longest processing time
first
Slide 19
Process job with earliest due date first Widely used by many
companies If due dates important If MRP used Due dates updated by
each MRP run Performs poorly on many scheduling criteria Earliest
Due Date Rule
Slide 20
Advantages of the Critical Ratio Scheduling Rule Use of the
critical ratio can help to: determine the status of a specific job
establish a relative priority among jobs on a common basis relate
both stock and make-to-order jobs on a common basis adjust
priorities and revise schedules automatically for changes in both
demand and job progress dynamically track job progress and
location
Slide 21
Two Work Center Sequencing Johnsons Rule: technique for
minimizing completion time for a group of jobs to be processed on
two machines or at two work centers. Minimizes total idle time
Several conditions must be satisfied
Slide 22
Johnsons Rule Conditions Job time must be known and constant
Job times must be independent of sequence Jobs must follow same
two-step sequence Job priorities cannot be used All units must be
completed at the first work center before moving to second
Slide 23
Johnson's Rule - Scheduling N Jobs on Two Machines 1. List all
jobs and their processing times each machine. 2. Select first the
job with the shortest processing time. If the shortest processing
time lies with the first machine, the job is scheduled first; if
with the second machine, the job is scheduled last. 3. Once a job
is scheduled, eliminate it. Apply steps 2-3 to the remaining jobs
until all jobs are assigned. The process will move toward the
center of the sequence.
Slide 24
Limitations of Rule-Based Dispatching Systems Scheduling is
dynamic; therefore, rules need to be revised to adjust to changes
in process, equipment, product mix, etc. Rules do not look upstream
or downstream; idle resources and bottleneck resources in other
departments may not be recognized Rules do not look beyond due
dates
Slide 25
Number of Servers Single Server Multiple Servers Examples:
Multiple Single Servers Examples:
Slide 26
Queue Discipline First come -- first served (FCFS): Multiple
Priorities: Examples:
Slide 27
8-27 Project Management A project is a set of activities aimed
at meeting a goal, with a defined beginning and end.
Slide 28
8-28 Project Management with Certain Time Estimates Summary of
steps: Determine activities that need to be accomplished Determine
precedence relationships and completion times Construct network
diagram Determine the critical path Determine early start and late
start schedules
Slide 29
8-29 Project Scheduling When the Times of Activities are
Uncertain Summary of steps: Determine the activities that need to
be accomplished Determine the precedence relationships and
completion times Construct the network diagram Determine the
critical path Determine the early start and late start schedules
Calculate the variances for the activity times Calculate the
probability of completing by the desired due date
Slide 30
8-30 Project Scheduling with Time Uncertainty The Heuristic
approach to dealing with timing uncertainty Based on understanding
of individual activities as conforming to a beta distribution Take
three time estimates Optimistic - What is the (realistic) fastest
we can get an activity done? Pessimistic - What is the (realistic)
worst case scenario for delay? Most likely - What is our most
likely estimate?
Slide 31
Process job with earliest due date first Widely used by many
companies If due dates important If MRP used Due dates updated by
each MRP run Performs poorly on many scheduling criteria Earliest
Due Date Rule
Slide 32
Best Methods among Priority Rule SPT and EDD are superior in
all measure. SPT is generally the best for minimizing flow and
average number of jobs.
Slide 33
Ratio of time remaining to work time remaining Process job with
smallest CR first Performs well on average lateness CR Time
remaining Work daysremaining Due date- Today's date Work (lead)
time remaining = = Critical Ratio (CR)
Slide 34
Advantages of the Critical Ratio Scheduling Rule Use of the
critical ratio can help to: determine the status of a specific job
establish a relative priority among jobs on a common basis relate
both stock and make-to-order jobs on a common basis adjust
priorities and revise schedules automatically for changes in both
demand and job progress dynamically track job progress and
location
Slide 35
Two Work Center Sequencing Johnsons Rule: technique for
minimizing completion time for a group of jobs to be processed on
two machines or at two work centers. Minimizes total idle time
Several conditions must be satisfied
Slide 36
Johnsons Rule Conditions Job time must be known and constant
Job times must be independent of sequence Jobs must follow same
two-step sequence Job priorities cannot be used All units must be
completed at the first work center before moving to second
Slide 37
Limitations of Rule-Based Dispatching Systems Scheduling is
dynamic; therefore, rules need to be revised to adjust to changes
in process, equipment, product mix, etc. Rules do not look upstream
or downstream; idle resources and bottleneck resources in other
departments may not be recognized Rules do not look beyond due
dates
Slide 38
Queuing Theory Where is waiting? Service facility Fast-food
restaurants post office grocery store bank Manufacturing Equipment
awaiting repair Phone or computer network Product orders
Slide 39
Measures of System Performance Average number of customers
waiting Average time customers wait System utilization
Slide 40
Application of Queuing Theory We can use the results from
queuing theory to make the following types of decisions: How many
servers to employ Whether to use a single fast server or a number
of slower servers Whether to have general purpose or faster
specific servers Goal: Minimize total cost = cost of servers + cost
of waiting
Slide 41
System Characteristics Number of servers Arrival and service
pattern Infinite Population The arrival is not affected by waiting
line (public places, e.g. gas station) Finite Population The
arrival is affect by waiting line. Queue discipline