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McGraw-Hill/Irwin
2009 McGraw-Hill Companies, All Rights Reserved
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Chapter 19
Operations Scheduling
McGraw-Hill/Irwin
2009 The McGraw-Hill Companies, All Rights Reserved
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OBJECTIVES
Work Center Defined Typical Scheduling and Control Functions Job-shop Scheduling Examples of Scheduling Rules Shop-floor Control Principles of Work Center Scheduling Issues in Scheduling Service Personnel
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Work Center
A work center is an area in a business in which productive resources are organized and work is completed Can be a single machine, a group of machines, or an area where a particular type of work is done
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Capacity and Scheduling
Infinite loading (Example: MRP) Finite loading Forward scheduling Backward scheduling (Example: MRP)
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Types of Manufacturing Scheduling Processes and Scheduling Approaches
Type of Process Continuous process High-volume manufacturing Med-volume manufacturing Low-volume manufacturing
Typical Scheduling Approach Finite forward of process, machine limited Finite forward of line, machined limited Infinite forward of process, labor and machined limited Infinite forward of jobs, labor and some machine limited
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Typical Scheduling and Control Functions
Allocating orders, equipment, and personnel Determining the sequence of order performance Initiating performance of the scheduled work Shop-floor control
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Work-Center Scheduling Objectives
Meet due dates Minimize lead time Minimize setup time or cost Minimize work-in-process inventory Maximize machine utilization
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Priority Rules for Job Sequencing
1. First-come, first-served (FCFS) 2. Shortest operating time (SOT) 3. Earliest due date first (DDate) 4. Slack time remaining (STR) first 5. Slack time remaining per
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Priority Rules for Job Sequencing (Continued)
6. Critical ratio (CR)
(Due date - Current date) CR = Number of days remaining
7. Last come, first served (LCFS) 8. Random order or whim
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Example of Job Sequencing: First-Come First-Served
Jobs (in order of arrival) A B C D Processing Due Date Time (days) (days hence) 4 5 7 10 3 6 1 4
Suppose you have the four jobs to the right arrive for processing on one machine
What is the FCFS schedule?
Do all the jobs get done on time? No, Jobs B, C, and D are going to be late
Answer: FCFS Schedule
Jobs (in order of arrival) A B C D Processing Due Date Flow Time Time (days) (days hence) (days) 4 5 4 7 10 11 3 6 14 1 4 15
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Example of Job Sequencing: Shortest Operating Time
Suppose you have the four jobs to the right arrive for processing on one machine
Jobs (in order of arrival) A B C D
Processing Due Date Time (days) (days hence) 4 5 7 10 3 6 1 4
What is the SOT schedule?
Do all the jobs get done on time?
Answer: Shortest Operating Time Schedule
Jobs (in order of arrival) D C A B Processing Due Date Flow Time Time (days) (days hence) (days) 1 4 1 3 6 4 4 5 8 7 10 15 No, Jobs A and B are going to be late
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Example of Job Sequencing: Earliest Due Date First
Suppose you have the four jobs to the right arrive for processing on one machine
Jobs (in order of arrival) A B C D
Processing Due Date Time (days) (days hence) 4 5 7 10 3 6 1 4
What is the earliest due date first schedule?
Do all the jobs get done on time?
Answer: Earliest Due Date First
Jobs (in order of arrival) D A C B Processing Due Date Flow Time Time (days) (days hence) (days) 1 4 1 4 5 5 3 6 8 7 10 15
No, Jobs C and B are going to be late
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Example of Job Sequencing: Critical Ratio Method
Suppose you have the four jobs to the right arrive for processing on one machine
What is the CR schedule?
Jobs (in order of arrival) A B C D
Processing Due Date Time (days) (days hence) 4 5 7 10 3 6 1 4
Do all the jobs get done on time?
In order to do this schedule the CRs have be calculated for each job. If we let today be Day 1 and allow a total of 15 days to do the work. The resulting CRs and order schedule are: CR(A)=(5-4)/15=0.06 (Do this job last) CR(B)=(10-7)/15=0.20 (Do this job first, tied with C and D) CR(C)=(6-3)/15=0.20 (Do this job first, tied with B and D) CR(D)=(4-1)/15=0.20 (Do this job first, tied with B and C)
No, but since there is threeway tie, only the first job or two will be on time
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Example of Job Sequencing: Last-Come First-Served
Jobs (in order of arrival) A B C D Processing Due Date Time (days) (days hence) 4 5 7 10 3 6 1 4
Suppose you have the four jobs to the right arrive for processing on one machine
What is the LCFS schedule?
Do all the jobs get done on time?
Answer: Last-Come First-Served Schedule
Jobs (in order of arrival) D C B A Processing Due Date Flow Time Time (days) (days hence) (days) 1 4 1 3 6 4 7 10 11 4 5 15
No, Jobs B and A are going to be late
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Johnsons Rule
The primary objective of Johnson's Rule is to find an optimal sequence of jobs to reduce makespan (the total amount of time it takes to complete all jobs). It also reduces the number of idle time between the two work centers. List the jobs and their times at each work center. Select the job with the shortest time. If the job is for the first work center, then schedule the job first. If that job is for the second work center then schedule the job last. Break ties arbitrarily. Eliminate the job selected from further consideration. Repeat steps 2 and 3, working towards the center of the job schedule until all jobs have been scheduled.
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Example of Job Sequencing: Johnsons Rule (Part 1)
Suppose you have the following five jobs with time requirements in two stages of production. What is job the sequence using Johnsons Rule?
Time in Hours Stage 1 Stage 2 1.50 1.25 2.00 3.00 2.50 2.00 1.00 2.00
Jobs A B C D
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Example of Job Sequencing: Johnsons Rule (Part 2) First, select the job with the smallest time in either stage. That is Job D with the smallest time in the first stage. Place that job as early as possible in the unfilled job sequence below. Time in Hours Stage 1 Stage 2 1.50 1.25 2.00 3.00 2.50 2.00 1.00 2.00
Jobs A B C D
Drop D out, select the next smallest time (Job A), and place it 4th in the job sequence.
Drop A out, select the next smallest time. There is a tie in two stages for two different jobs. In this case, place the job with the smallest time in the first stage as early as possible in the unfilled job sequence. Then place the job with the smallest time in the second stage as late as possible in the unfilled sequence. Job Sequence 1 Job Assigned D 2 B 3 C 4 A
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Shop-Floor Control: Major Functions
1. Assigning priority of each shop order 2. Maintaining work-in-process quantity information 3. Conveying shop-order status information to the office
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Shop-Floor Control: Major Functions (Continued)
4. Providing actual output data for capacity control purposes 5. Providing quantity by location by shop order for WIP inventory and accounting purposes 6. Providing measurement of efficiency, utilization, and productivity of manpower and machines
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Input/Output Control
Input
Work Center
Output
Planned input should never exceed planned output
Focuses attention on bottleneck work centers
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Principles of Work Center Scheduling
1. There is a direct equivalence between work flow and cash flow 2. The effectiveness of any job shop should be measured by speed of flow through the shop 3. Schedule jobs as a string, with process steps back-to-back 4. A job once started should not be interrupted
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Principles of Job Shop Scheduling (Continued)
5. Speed of flow is most efficiently achieved by focusing on bottleneck work centers and jobs 6. Reschedule every day 7. Obtain feedback each day on jobs that are not completed at each work center 8. Match work center input information to what the worker can actually do
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Principles of Job Shop Scheduling (Continued)
9. When seeking improvement in output, look for incompatibility between engineering design and process execution 10. Certainty of standards, routings, and so forth is not possible in a job shop, but always work towards achieving it
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Personnel Scheduling in Services
Scheduling consecutive days off Scheduling daily work times Scheduling hourly work times
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Question Bowl
A Work Center may be which of the following? a. A single machine b. A group of machines c. An area where a particular type of work is performed d. All of the above e. None of the above Answer: d. All of the above
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Question Bowl
When work is assigned to a work center simply based on what is needed over time, we would refer to this as which of the following scheduling systems? a. A finite loading of work b. An infinite loading of work c. Forward scheduling d. All of the above e. None of the above
Answer: b. An infinite loading of work
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Question Bowl
Typical scheduling and controlling of operations include which of the following functions? a. Allocating orders at work centers b. Allocating equipment at work centers c. Allocating personnel at work centers d. All of the above e. None of the above Answer: d. All of the above
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Question Bowl
Typical scheduling and controlling of operations include which of the following functions? a. Determining the job sequences b. Dispatching c. Expediting late and critical orders d. All of the above e. None of the above
Answer: d. All of the above
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Question Bowl
Which of the following are standard measures of schedule performance used to evaluate priority rules? a. Meeting due dates b. Maximizing job flow time c. Maximizing work-in-process inventory d. All of the above e. None of the above Answer: a. Meeting due dates (Correct answer can also include minimizing WIP inventory, idle time, and job flow time.)
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Question Bowl
Which priority rule uses the calculation of the difference between the due date and the current date divided by the number of work days remaining? a. STR b. SOT c. DDate d. FCFS e. None of the above Answer: e. None of the above (Correct answer can is CR or critical ratio.)
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Question Bowl
The major functions of a shopfloor control are which of the following? a. Conveying shop-order status b. Measuring efficiency c. Assigning priorities d. Maintaining WIP quantity information e. All of the above
Answer: e. All of the above (Correct answer can also include providing quantity by location and actual output data.)
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Question Bowl
Which of the following are Tools of Shop-Floor Control? a. Daily dispatch lists b. Scrap reports c. Rework reports d. All of the above e. None of the above
Answer: d. All of the above (Correct answer can also include all status and exception reports and input/output control reports.)
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Question Bowl
Which of the following is a Principle of Work-Center Scheduling? a. There is a direct equivalence between work flow and cash flow b. Certainty of routings are very possible in a shop c. Reschedule only once a week d. All of the above e. None of the above
Answer: a. There is a direct equivalence between work flow and cash flow (There are nine other principles.)
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End of Chapter 19
McGraw-Hill/Irwin
2009 The McGraw-Hill Companies, All Rights Reserved
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