Operations Management
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Operations Management. Chapter 15 – Short-Term Scheduling. PowerPoint presentation to accompany Heizer/Render Principles of Operations Management, 7e Operations Management, 9e . Outline. Global Company Profile: Delta Air Lines The Strategic Importance of Short-Term Scheduling

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Operations management

Operations Management

Chapter 15 –

Short-Term Scheduling

PowerPoint presentation to accompany

Heizer/Render

Principles of Operations Management, 7e

Operations Management, 9e


Outline

Outline

  • Global Company Profile: Delta Air Lines

  • The Strategic Importance of Short-Term Scheduling

  • Scheduling Issues

    • Forward and Backward Scheduling

    • Scheduling Criteria


Outline continued

Outline – Continued

  • Scheduling Process-Focused Facilities

  • Loading Jobs

    • Input-Output Control

    • Gantt Charts

    • Assignment Method


Outline continued1

Outline – Continued

  • Sequencing Jobs

    • Priority Rules for Dispatching Jobs

    • Critical Ratio

    • Sequencing N Jobs on Two Machines: Johnson’s Rule

    • Limitations of Rule-Based Dispatching Systems

  • Finite Capacity Scheduling (FCS)


Outline continued2

Outline – Continued

  • Theory of Constraints

    • Bottlenecks

    • Drum, Buffer, Rope

  • Scheduling Repetitive Facilities

  • Scheduling Services

    • Scheduling Service Employees with Cyclical Scheduling


Learning objectives

Learning Objectives

When you complete this chapter you should be able to:

Explain the relationship between short-term scheduling, capacity planning, aggregate planning, and a master schedule

Draw Gantt loading and scheduling charts

Apply the assignment method for loading jobs


Learning objectives1

Learning Objectives

When you complete this chapter you should be able to:

Name and describe each of the priority sequencing rules

Use Johnson’s rule

Define finite capacity scheduling

List the steps in the theory of constraints

Use the cyclical scheduling technique


Delta airlines

Delta Airlines

  • About 10% of Delta’s flights are disrupted per year, half because of weather

  • Cost is $440 million in lost revenue, overtime pay, food and lodging vouchers

  • The $33 million Operations Control Center adjusts to changes and keeps flights flowing

  • Saves Delta $35 million per year


Strategic importance of short term scheduling

Strategic Importance of Short-Term Scheduling

  • Effective and efficient scheduling can be a competitive advantage

    • Faster movement of goods through a facility means better use of assets and lower costs

    • Additional capacity resulting from faster throughput improves customer service through faster delivery

    • Good schedules result in more dependable deliveries


Scheduling issues

Scheduling Issues

  • Scheduling deals with the timing of operations

  • The task is the allocation and prioritization of demand

  • Significant issues are

    • The type of scheduling, forward or backward

    • The criteria for priorities


Scheduling decisions

Scheduling Decisions

Table 15.1


Scheduling flow

Scheduling Flow

Figure 15.1


Forward and backward scheduling

Due Date

Now

Forward and Backward Scheduling

  • Forward scheduling starts as soon as the requirements are known

  • Produces a feasible schedule though it may not meet due dates

  • Frequently results in buildup of work-in-process inventory


Forward and backward scheduling1

Due Date

Now

Forward and Backward Scheduling

  • Backward scheduling begins with the due date and schedules the final operation first

  • Schedule is produced by working backwards though the processes

  • Resources may not be available to accomplish the schedule


Forward and backward scheduling2

Due Date

Now

Forward and Backward Scheduling

  • Backward scheduling begins with the due date and schedules the final operation first

  • Schedule is produced by working backwards though the processes

  • Resources may not be available to accomplish the schedule

Often these approaches are combined to develop a trade-off between a feasible schedule and customer due dates


Different processes different approaches

Different Processes/ Different Approaches

Table 15.2


Scheduling criteria

Scheduling Criteria

Minimize completion time

Maximize utilization of facilities

Minimize work-in-process (WIP) inventory

Minimize customer waiting time

Optimize the use of resources so that production objectives are met


Scheduling process focused facilities

Scheduling Process-Focused Facilities

Schedule incoming orders without violating capacity constraints

Check availability of tools and materials before releasing an order

Establish due dates for each job and check progress

Check work in progress

Provide feedback

Provide work efficiency statistics and monitor times


Planning and control files

Planning Files

An item master file contains information about each component

A routing file indicates each component’s flow through the shop

A work-center master file contains information about the work center

Control Files

Track the actual progress made against the plan

Planning and Control Files


Loading jobs

Loading Jobs

  • Assign jobs so that costs, idle time, or completion time are minimized

  • Two forms of loading

    • Capacity oriented

    • Assigning specific jobs to work centers


Input output control

Input-Output Control

  • Identifies overloading and underloading conditions

  • Prompts managerial action to resolve scheduling problems

  • Can be maintained using ConWIP cards that control the scheduling of batches


Input output control example

Work Center DNC Milling (in standard hours)

Input-Output Control Example

Figure 15.2


Input output control example1

Work Center DNC Milling (in standard hours)

Explanation:

250 input,

270 output implies

–20 change

Explanation:

270 input,

270 output implies

0 change

Input-Output Control Example

Figure 15.2


Input output control example2

Input-Output Control Example

Options available to operations personnel include:

Correcting performances

Increasing capacity

Increasing or reducing input to the work center


Gantt charts

Gantt Charts

  • Load chart shows the loading and idle times of departments, machines, or facilities

  • Displays relative workloads over time

  • Schedule chart monitors jobs in process

  • All Gantt charts need to be updated frequently to account for changes


Gantt load chart example

Day

Work Center

Monday

Tuesday

Wednesday

Thursday

Friday

Metalworks

Job 349

Job 350

Mechanical

Job 349

Job 408

Electronics

Job 408

Job 349

Painting

Job 295

Job 408

Job 349

Processing

Unscheduled

Center not available

Gantt Load Chart Example

Figure 15.3


Gantt schedule chart example

Start of an activity

End of an activity

Scheduled activity time allowed

Maintenance

Actual work progress

Nonproduction time

Point in time when chart is reviewed

Now

Gantt Schedule Chart Example

Figure 15.4


Assignment method

Assignment Method

  • A special class of linear programming models that assign tasks or jobs to resources

  • Objective is to minimize cost or time

  • Only one job (or worker) is assigned to one machine (or project)


Assignment method1

Typesetter

JobABC

R-34$11$14$ 6

S-66$ 8$10$11

T-50$ 9$12$ 7

Assignment Method

  • Build a table of costs or time associated with particular assignments


Assignment method2

Assignment Method

Create zero opportunity costs by repeatedly subtracting the lowest costs from each row and column

Draw the minimum number of vertical and horizontal lines necessary to cover all the zeros in the table. If the number of lines equals either the number of rows or the number of columns, proceed to step 4. Otherwise proceed to step 3.


Assignment method3

Assignment Method

Subtract the smallest number not covered by a line from all other uncovered numbers. Add the same number to any number at the intersection of two lines. Return to step 2.

Optimal assignments are at zero locations in the table. Select one, draw lines through the row and column involved, and continue to the next assignment.


Assignment example

Step 1a - Rows

Step 1b - Columns

Typesetter

Typesetter

Typesetter

ABC

Job

R-34$11$14$ 6

S-66$ 8$10$11

T-50$ 9$12$ 7

ABC

Job

R-34$ 5$ 8$ 0

S-66$ 0$ 2$ 3

T-50$ 2$ 5$ 0

ABC

Job

R-34$ 5$ 6$ 0

S-66$ 0$ 0$ 3

T-50$ 2$ 3$ 0

Assignment Example


Assignment example1

Step 2 - Lines

Step 3 - Subtraction

Typesetter

Typesetter

ABC

Job

R-34$ 5$ 6$ 0

S-66$ 0$ 0$ 3

T-50$ 2$ 3$ 0

ABC

Job

R-34$ 3$ 4$ 0

S-66$ 0$ 0$ 5

T-50$ 0$ 1$ 0

Assignment Example

The smallest uncovered number is 2 so this is subtracted from all other uncovered numbers and added to numbers at the intersection of lines

Because only two lines are needed to cover all the zeros, the solution is not optimal


Assignment example2

Step 2 - Lines

Step 4 - Assignments

Typesetter

Typesetter

ABC

Job

R-34$ 3$ 4$ 0

S-66$ 0$ 0$ 5

T-50$ 0$ 1$ 0

ABC

Job

R-34$ 3$ 4$ 0

S-66$ 0$ 0$ 5

T-50$ 0$ 1$ 0

Assignment Example

Start by assigning R-34 to worker C as this is the only possible assignment for worker C.

Job T-50 must go to worker A as worker C is already assigned. This leaves S-66 for worker B.

Because three lines are needed, the solution is optimal and assignments can be made


Assignment example3

From the original cost table

Minimum cost = $6 + $10 + $9 = $25

Step 4 - Assignments

Typesetter

Typesetter

ABC

Job

R-34$ 3$ 4$ 0

S-66$ 0$ 0$ 5

T-50$ 0$ 1$ 0

ABC

Job

R-34$11$14$ 6

S-66$ 8$10$11

T-50$ 9$12$ 7

Assignment Example


Sequencing jobs

Sequencing Jobs

  • Specifies the order in which jobs should be performed at work centers

  • Priority rules are used to dispatch or sequence jobs

    • FCFS: First come, first served

    • SPT: Shortest processing time

    • EDD: Earliest due date

    • LPT: Longest processing time


Sequencing example

Sequencing Example

Apply the four popular sequencing rules to these five jobs


Sequencing example1

Sequencing Example

FCFS: Sequence A-B-C-D-E


Sequencing example2

Sum of total flow time

Number of jobs

Average completion time = = 77/5 = 15.4 days

Total job work time

Sum of total flow time

Utilization = = 28/77 = 36.4%

Sum of total flow time

Total job work time

Average number of jobs in the system

= = 77/28 = 2.75 jobs

Total late days

Number of jobs

Average job lateness = = 11/5 = 2.2 days

Sequencing Example

FCFS: Sequence A-B-C-D-E


Sequencing example3

Sequencing Example

SPT: Sequence B-D-A-C-E


Sequencing example4

Sum of total flow time

Number of jobs

Average completion time = = 65/5 = 13 days

Total job work time

Sum of total flow time

Utilization = = 28/65 = 43.1%

Sum of total flow time

Total job work time

Average number of jobs in the system

= = 65/28 = 2.32 jobs

Total late days

Number of jobs

Average job lateness = = 9/5 = 1.8 days

Sequencing Example

SPT: Sequence B-D-A-C-E


Sequencing example5

Sequencing Example

EDD: Sequence B-A-D-C-E


Sequencing example6

Sum of total flow time

Number of jobs

Average completion time = = 68/5 = 13.6 days

Total job work time

Sum of total flow time

Utilization = = 28/68 = 41.2%

Sum of total flow time

Total job work time

Average number of jobs in the system

= = 68/28 = 2.43 jobs

Total late days

Number of jobs

Average job lateness = = 6/5 = 1.2 days

Sequencing Example

EDD: Sequence B-A-D-C-E


Sequencing example7

Sequencing Example

LPT: Sequence E-C-A-D-B


Sequencing example8

Sum of total flow time

Number of jobs

Average completion time = = 103/5 = 20.6 days

Total job work time

Sum of total flow time

Utilization = = 28/103 = 27.2%

Sum of total flow time

Total job work time

Average number of jobs in the system

= = 103/28 = 3.68 jobs

Total late days

Number of jobs

Average job lateness = = 48/5 = 9.6 days

Sequencing Example

LPT: Sequence E-C-A-D-B


Sequencing example9

Sequencing Example

Summary of Rules


Comparison of sequencing rules

Comparison of Sequencing Rules

  • No one sequencing rule excels on all criteria

  • SPT does well on minimizing flow time and number of jobs in the system

  • But SPT moves long jobs to the end which may result in dissatisfied customers

  • FCFS does not do especially well (or poorly) on any criteria but is perceived as fair by customers

  • EDD minimizes lateness


Critical ratio cr

Time remaining

Workdays remaining

Due date - Today’s date

Work (lead) time remaining

CR = =

Critical Ratio (CR)

  • An index number found by dividing the time remaining until the due date by the work time remaining on the job

  • Jobs with low critical ratios are scheduled ahead of jobs with higher critical ratios

  • Performs well on average job lateness criteria


Critical ratio example

Critical Ratio Example

Currently Day 25

With CR < 1, Job B is late. Job C is just on schedule and Job A has some slack time.


Critical ratio technique

Critical Ratio Technique

Helps determine the status of specific jobs

Establishes relative priorities among jobs on a common basis

Relates both stock and make-to-order jobs on a common basis

Adjusts priorities automatically for changes in both demand and job progress

Dynamically tracks job progress


Sequencing n jobs on two machines johnson s rule

Sequencing N Jobs on Two Machines: Johnson’s Rule

  • Works with two or more jobs that pass through the same two machines or work centers

  • Minimizes total production time and idle time


Johnson s rule

Johnson’s Rule

List all jobs and times for each work center

Choose the job with the shortest activity time. If that time is in the first work center, schedule the job first. If it is in the second work center, schedule the job last.

Once a job is scheduled, it is eliminated from the list

Repeat steps 2 and 3 working toward the center of the sequence


Johnson s rule example

Johnson’s Rule Example


Johnson s rule example1

B

E

D

C

A

Johnson’s Rule Example


Johnson s rule example2

B

E

D

C

A

WC 1

B

E

D

C

A

WC 2

Johnson’s Rule Example

Time0310202833


Johnson s rule example3

B

E

D

C

A

WC 1

B

E

D

C

A

WC 2

B

E

D

C

A

A

B

E

D

C

Johnson’s Rule Example

Time0310202833

Time 01357910111213171921 22 23252729313335


Limitations of rule based dispatching systems

Limitations of Rule-Based Dispatching Systems

Scheduling is dynamic and rules need to be revised to adjust to changes

Rules do not look upstream or downstream

Rules do not look beyond due dates


Finite capacity scheduling

Finite Capacity Scheduling

  • Overcomes disadvantages of rule-based systems by providing an interactive, computer-based graphical system

  • May include rules and expert systems or simulation to allow real-time response to system changes

  • Initial data often from an MRP system

  • FCS allows the balancing of delivery needs and efficiency


Finite capacity scheduling1

  • MRP Data

  • Master schedule

  • BOM

  • Inventory

  • Routing files

  • Work center information

Tooling and other resources

Priority rules

  • Expert systems

  • Simulation models

Setups and run time

Finite Capacity Scheduling

Interactive Finite Capacity Scheduling

Figure 15.5


Finite capacity scheduling2

Finite Capacity Scheduling


Theory of constraints

Theory of Constraints

  • Throughput is the number of units processed through the facility and sold

  • TOC deals with the limits an organization faces in achieving its goals

Identify the constraints

Develop a plan for overcoming the constraints

Focus resources on accomplishing the plan

Reduce the effects of constraints by off-loading work or increasing capacity

Once successful, return to step 1 and identify new constraints


Bottlenecks

Bottlenecks

  • Bottleneck work centers are constraints that limit output

    • Common occurrence due to frequent changes

  • Management techniques include:

    • Increasing the capacity of the constraint

    • Cross-trained employees and maintenance

    • Alternative routings, procedures, or subcontractors

    • Moving inspection and test

    • Scheduling throughput to match bottleneck capacity


Drum buffer rope

Drum, Buffer, Rope

  • The drum is the beat of the system and provides the schedule or pace of production

  • The buffer is the inventory necessary to keep constraints operating at capacity

  • The rope provides the synchronization necessary to pull units through the system


Scheduling repetitive facilities

Scheduling Repetitive Facilities

  • Level material use can help repetitive facilities

    • Better satisfy customer demand

    • Lower inventory investment

    • Reduce batch size

    • Better utilize equipment and facilities


Scheduling repetitive facilities1

Scheduling Repetitive Facilities

  • Advantages include:

    • Lower inventory levels

    • Faster product throughput

    • Improved component quality

    • Reduced floor-space requirements

    • Improved communications

    • Smoother production process


Scheduling services

Scheduling Services

Service systems differ from manufacturing


Scheduling services1

Scheduling Services

  • Hospitals have complex scheduling system to handle complex processes and material requirements

  • Banks use a cross-trained and flexible workforce and part-time workers

  • Retail stores use scheduling optimization systems that track sales, transactions, and customer traffic to create work schedules in less time and with improved customer satisfaction


Scheduling services2

Scheduling Services

  • Airlines must meet complex FAA and union regulations and often use linear programming to develop optimal schedules

  • 24/7 operations like police/fire departments, emergency hot lines, and mail order businesses use flexible workers and variable schedules, often created using computerized systems


Demand management

Demand Management

  • Appointment or reservation systems

  • FCFS sequencing rules

  • Discounts or other promotional schemes

  • When demand management is not feasible, managing capacity through staffing flexibility may be used


Scheduling service employees with cyclical scheduling

Scheduling Service Employees With Cyclical Scheduling

  • Objective is to meet staffing requirements with the minimum number of workers

  • Schedules need to be smooth and keep personnel happy

  • Many techniques exist from simple algorithms to complex linear programming solutions


Cyclical scheduling example

Cyclical Scheduling Example

Determine the staffing requirements

Identify two consecutive days with the lowest total requirements and assign these as days off

Make a new set of requirements subtracting the days worked by the first employee

Apply step 2 to the new row

Repeat steps 3 and 4 until all requirements have been met


Cyclical scheduling example1

Cyclical Scheduling Example


Cyclical scheduling example2

Cyclical Scheduling Example


Cyclical scheduling example3

Cyclical Scheduling Example


Cyclical scheduling example4

Cyclical Scheduling Example


Cyclical scheduling example5

Cyclical Scheduling Example


Cyclical scheduling example6

Cyclical Scheduling Example


Cyclical scheduling example7

Cyclical Scheduling Example


Cyclical scheduling example8

Cyclical Scheduling Example


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