Chapter 13

Question 1

Scheduling Definition

Answer 1

Scheduling is the planning involved in the sequencing and timing of work

Question 2

Scheduling Objective:

Answer 2

Efficiently and effectively utilize available capacity to achieve the organization’s objectives

Question 3

Scheduling Characteristics

Answer 3

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

Question 4

Common Scheduling Objectives

Answer 4

Meet customer due dates
Minimize completion time
Minimize work-in-process inventory
Maximize machine and/or labor utilization

Question 5

Bottleneck Resources

Answer 5

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.

Question 6

Batch Flow Scheduling

Answer 6

Very complex scheduling environment

Customers/jobs spend most of their time waiting

Question 7

Very complex scheduling environment

Answer 7

Variety of customers/jobs at any given time

Distinctive routings

Competing for common resources

Question 8

Customers/jobs spend most of their time waiting

Answer 8

Often 20% or less of total time in the system is spent being actively processed

Question 9

Input-Output Control

Answer 9

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

Question 10

Gantt Charts

Answer 10

• 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.

Question 11

Expediting Definition

Answer 11

Taking action to complete a job in less time than the normal schedule dictates.

Question 12

Expediting

Answer 12

• 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.

Question 13

The more expediting is used:

Answer 13

The less effective it becomes AND

The less efficient the system becomes

Question 14

What are dispatching rules?

Answer 14

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.

Question 15

dispatching rules

Answer 15

Dispatching rules, by themselves, can’t really be used to build a schedule

Question 16

Static Rules:

Answer 16

FISFS (First In System, First Served)

FCFS (First Come, First Served)

SPT (Shortest Processing Time)

MINDD (MINimum Due Date)

Question 17

FISFS (First In System, First Served)

Answer 17

Priority Value = Time of entry into the entire system

Question 18

FCFS (First Come, First Served)

Answer 18

Priority Value = Time of arrival at this work center

Question 19

SPT (Shortest Processing Time)

Answer 19

Priority Value = Estimated processing time at this work center

Question 20

MINDD (MINimum Due Date)

Answer 20

Priority Value = Due date of the job

Question 21

Dynamic Rules:

Answer 21

MINSOP (MINimum Slack time per # of OPerations remaining)

CR (Critical Ratio)

Question 22

MINSOP (MINimum Slack time per # of OPerations remaining)

Answer 22

Priority Value = Slack # operations remaining

Where: Slack = Due date – Current date – Total estimated remaining processing time

Question 23

Conclusions aboutBatch Flow Scheduling

Answer 23

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.

Question 24

Line Flow Environments

Answer 24

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

Question 25

Run-Out Method

Answer 25

1. For each product, obtain an inventory count, forecast the demand, and determine the line’s capacity.
2. Determine the appropriate lot size (run length) for each product.
3. Compute the production run times for one lot of each product.
4. Compute the run-out values for each product and schedule the highest priority job.
5. Identify when the production scheduled in Step #4 will be completed, compute anticipated inventory levels at that time, and compute new run-out values

Question 26

Theory of Constraints (TOC) characteristics

Answer 26

Proposed by Goldratt in The Goal (1983)
Focus on bottlenecks
Goal is to make money
Production does not count until it is sold!

Question 27

Key Elements of Theory of Constraints (TOC)

Answer 27

Throughput (sales - raw material costs)
Inventory (at cost of raw materials only)
Operating expenses (all other expenses)

Question 28

Theory of Constraints (TOC)

Answer 28

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.

Question 29

Ways of reducing the bottleneck or constraint:

Answer 29

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