Chapter 13 Flashcards
Scheduling Definition
Scheduling is the planning involved in the sequencing and timing of work
Scheduling Objective:
Efficiently and effectively utilize available capacity to achieve the organization’s objectives
Scheduling Characteristics
Short range planning horizon (a year or less)
Time units of weeks, days, hours, minutes, seconds, etc.
Allocation only, no acquisition
Trade-offs between conflicting objectives
Common Scheduling Objectives
Meet customer due dates
Minimize completion time
Minimize work-in-process inventory
Maximize machine and/or labor utilization
Bottleneck Resources
A bottleneck is a work center whose capacity is less than the demand placed on it during a given time period.
Bottleneck resources constrain the capacity of the entire operation.
Batch Flow Scheduling
Very complex scheduling environment
Customers/jobs spend most of their time waiting
Very complex scheduling environment
Variety of customers/jobs at any given time
Distinctive routings
Competing for common resources
Customers/jobs spend most of their time waiting
Often 20% or less of total time in the system is spent being actively processed
Input-Output Control
Usually applied at shop level
Input rate and capacity dictate output rate
Seek desired (equilibrium) level then match input rate to output rate
Does not result in the creation of a schedule
Gantt Charts
- Developed by Henry Gantt in 1917
- Timelines for each resource
- Customers/jobs are first prioritized
- Resource time is then allocated to them in priority sequence
- Make-span time, idle time, queue time, and resource utilization measures are used to evaluate the quality of the schedule.
- Lot splitting/transfer batches/operation overlapping may be used to expedite.
Expediting Definition
Taking action to complete a job in less time than the normal schedule dictates.
Expediting
- The simplest form of expediting would, at a given work center, process the expedited job as soon as the job currently being processed there is completed.
- A more aggressive form would be to stop processing the current job to begin processing the expedited job immediately
- Lot-splitting/operation overlapping is a very aggressive form of expediting.
The more expediting is used:
The less effective it becomes AND
The less efficient the system becomes
What are dispatching rules?
If you have more than one customer waiting at a service area (or jobs at a work station), how do you select which one to serve/process next? The criterion you use for selecting the next job is your dispatching rule.
dispatching rules
Dispatching rules, by themselves, can’t really be used to build a schedule
Static Rules:
FISFS (First In System, First Served)
FCFS (First Come, First Served)
SPT (Shortest Processing Time)
MINDD (MINimum Due Date)
FISFS (First In System, First Served)
Priority Value = Time of entry into the entire system
FCFS (First Come, First Served)
Priority Value = Time of arrival at this work center
SPT (Shortest Processing Time)
Priority Value = Estimated processing time at this work center
MINDD (MINimum Due Date)
Priority Value = Due date of the job
Dynamic Rules:
MINSOP (MINimum Slack time per # of OPerations remaining)
CR (Critical Ratio)
MINSOP (MINimum Slack time per # of OPerations remaining)
Priority Value = Slack # operations remaining
Where: Slack = Due date – Current date – Total estimated remaining processing time
Conclusions aboutBatch Flow Scheduling
Performance is highly sequence dependent.
Waiting time depends upon job interference in the schedule and available capacity.
Finding optimal schedules is not practical, but good heuristics are available.
Line Flow Environments
One Product: The schedule is largely determined by the process (it is “built into the line”).
Multiple Products (produced on the same line): Must determine sequence and lengths of runs
Run-Out Method
- For each product, obtain an inventory count, forecast the demand, and determine the line’s capacity.
- Determine the appropriate lot size (run length) for each product.
- Compute the production run times for one lot of each product.
- Compute the run-out values for each product and schedule the highest priority job.
- Identify when the production scheduled in Step #4 will be completed, compute anticipated inventory levels at that time, and compute new run-out values
Theory of Constraints (TOC) characteristics
Proposed by Goldratt in The Goal (1983)
Focus on bottlenecks
Goal is to make money
Production does not count until it is sold!
Key Elements of Theory of Constraints (TOC)
Throughput (sales - raw material costs)
Inventory (at cost of raw materials only)
Operating expenses (all other expenses)
Theory of Constraints (TOC)
A constraint is anything that is slowing down production—a bottleneck.
The bottleneck determines the capacity of the system.
Implication: The operations manager should focus on the bottleneck to increase capacity and throughput (and make more money).
Bottleneck resources should be scheduled to achieve maximum throughput.
Non-bottlenecks should be scheduled to keep the bottleneck busy.
Bottleneck resources should have a work-in-process queue.
It is acceptable for non-bottleneck resources to be idle.
Find ways to relieve or reduce the bottleneck.
Ways of reducing the bottleneck or constraint:
Increase capacity.
Divert work that doesn’t need to go through the bottleneck.
Catch rejects before they get to the bottleneck.
Increase output at the bottleneck.
- Larger batches
- Reduced set-up times
