Operations Management Short-Term Scheduling 短期批程 Chapter 15 Week #16-2

Outline GLOBAL COMPANY PROFILE: DELTA AIRLINES THE STRATEGIC IMPORTANCE OF SHORT-TERM SCHEDULING (短期批程在策略上的重要性) SCHEDULING ISSUES (批程問題) Forward and Backward Scheduling Scheduling Criteria SCHEDULING PROCESS-FOCUSED WORK CENTERS

Outline - Continued LOADING JOBS IN WORK CENTERS Input-Output Control Gantt Charts Assignment Method SEQUENCING JOBS IN WORK CENTERS Priority Rules for Dispatching Jobs Critical Ratio Sequencing N Jobs on Two Machines: Johnson’s Rule

Outline - Continued LIMITATIONS OF RULE-BASED DISPATCHING SYSTEMS FINITE SCHEDULING THEORY OF CONSTRAINTS BOTTLENECK WORK CENTERS REPETITIVE MANUFACTURING SCHEDULING FOR SERVICES Scheduling Service Employees with Cyclical Scheduling

Learning Objectives When you complete this chapter, you should be able to : Identify or Define: Gantt charts Assignment method Sequencing rules Johnson’s rules Bottlenecks

Learning Objectives - continued When you complete this chapter, you should be able to : Describe or Explain: Scheduling Sequencing Shop loading Theory of constraints

Delta Airlines 10% of Delta’s flights disrupted per year - half of those by weather Cost: $440-million in: lost revenue overtime pay food and lodging vouchers $33-million hi-tech nerve center 18 staff $35-million savings One approach to take in discussing Delta Airlines is to ask students to describe Delta’s short term problem - and to be sure to include as many factors as possible. Very likely, the first attempts will be fairly short. Students will probably need prodding - not to identify the factors, but to include them. Once the list has been made, ask the students to explain how they would like to account for these factors in their scheduling model. Another question for students: Is the problem for Delta, as discussed in the case, one of developing a schedule under a complex set of constraints, or, rather, one of devising a minimal set of modifications to an already developed schedule?

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 You might stress to students that scheduling is what the end-customer actually sees.

Short-Term Scheduling Deals with timing of operations Short run focus: Hourly, daily, weekly Types Today Due Date B E Forward Scheduling Backward Scheduling

Short-Term Scheduling Examples © 1984-1994 T/Maker Co. Hospital Outpatient treatments Operating rooms University Instructors Classrooms Factory Production Purchases Students should be asked to provide additional examples. They should also be asked to identify any “unusual circumstances” which might pertain to their examples.

Scheduling Decisions Organization Managers Must Schedule Operating room use Patient admission Nursing, security, maintenance staffs Outpatient treatments Classrooms and audiovisual equipment Student and instructor schedules Graduate and undergraduate courses Production of goods Purchase of materials Workers Mount Sinai Hospital Indiana University Lockheed-Martin Factory Here you might ask students to identify the scheduling decisions for a business with which they are familiar. McDonalds, grocery store, and doctor’s office are useful examples.

Scheduling Decisions Organization Managers Must Schedule Chefs, waiters,bartenders Delivery of fresh foods Entertainers Opening of dining areas Maintenance of aircraft Departure timetables Flight crews, catering, gate, and ticketing personnel Hard Rock Cafe Delta Airlines

Capacity Planning, Aggregate Scheduling, Master Schedule, and Short-Term Scheduling 1. Facility size 2. Equipment procurement Long-term Aggregate Scheduling 1. Facility utilization 2. Personnel needs 3. Subcontracting Intermediate-term Master Schedule 1. MRP 2. Disaggregation of master plan Intermediate-term Students should be asked to identify three decisions at each level. Short-term Scheduling 1. Work center loading 2. Job sequencing Short-term

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 Students should be asked to consider how, using either process, they would modify the schedule when (a) an order is cancelled, (b) a priority, rush order is accepted, or (c) a delay is encountered.

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 While we would like to minimize customer wait time, an alternate goal might be to make customer wait time consistent - we could always ask them to come later.

Choosing a Scheduling Method 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) Ask students to identify at least one example of a scheduling based on qualitative factors, and one one quantitative factors.

Scheduling Methods Differ by Process Level Use Methods Repetitive-Focused Product-Focused Process-Focused Variety of Methods

Process-Focused Work Centers High variety, low volume systems Products made to order Products need different materials and processing Complex production planning and control Production planning aspects Shop loading Job sequencing

Requirements for Scheduling Process-Focused Work Centers Schedule incoming orders without violating capacity constraints of individual work centers Check availability of tools and materials before releasing an order to a department Establish due dates for each job and check progress against need dates and order lead times Check work-in-progress as jobs move through the shop Provide feedback on plant and production activities Provide work-efficiency statistics and monitor operator times for payroll and labor distribution analyses

Types of Planning Files Item master file - contains information about each component the firm produces or purchases Routing file - indicates each component’s flow through the shop Work-center master file - contains information about the work center such as capacity and efficiency It is helpful to discuss the purpose of each of the planning files. What is the data used for?

Process-Focused Planning System Aggregate Forecast & Resource Production Firm Orders Availability Planning Material Master Requirements Production Planning Scheduling No, modify CRP, MRP, or MPS Capacity Shop It is helpful to actually walk the students through the steps illustrated in this slide. Requirements Realistic? Floor Yes Planning Schedules

Loading Jobs in Work Centers 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

Input-Output Control Week Ending 6/6 6/13 6/20 6/27 7/4 7/11 Planned input 280 Actual input 270 250 285 Cumulative Deviation -10 -40 -35 Planned Output 320 Actual Output -50 -100 -150 -200 Cumulative Change in Backlog -20 -+5 Explanation: 270 input, 270 output, implies 0 change Explanation: 250 input, 270 output implies –20 change (20 standard hours less in the work center)

Order Release Begins Shop Loading No Release Order Hold Release Priority & Capacity OK? Gross Requirements Scheduled Receipts Available Net Requirements Planned Order Receipt Planned Order Release Yes 1 2 100 3

Options for Managing Facility Work Flow Correcting performance Increasing capacity Increasing or reducing input to the work center by: routing work to or from other work centers increasing or decreasing subcontracting producing less (or more) Students should be asked to consider which of these options allow the most flexibility.

Gantt Load Chart Shows relative workload in facility Disadvantages Does not account for unexpected events Must be updated regularly Work Center M T W Th F Metal Works Job 349 Job 350 Mechanical Job D Job G Electronics Job B Job H Painting Job C Job E Job I A characteristic of most of our project management and schedule management tools is that they require management attention for updating on a regular basis.

Gantt Load Chart for Week of March 6 Shows relative workload in facility Disadvantages Does not account for unexpected events Must be updated regularly Day Work Center Monday Tuesday Wednesday Thursday Friday Metalworks Job 349 Job 350 Mechanical Job 406 Electronics Job 408 Painting Job 295 Center not available (for example, maintenance time, repairs, shortages) Processing Unscheduled

Gantt Scheduling Chart Start of an activity Scheduled activity time allowed Point in time when chart is reviewed S T W F Day Job Job A Job B Job C Maintenance Now End of an activity Actual work progress Non-production time Day 1 Day 2 Day 3 Day 4 Day 5 Day 6 Day 7

Assignment Method Assigns tasks or jobs to resources Type of linear programming model Objective Minimize total cost, time etc. Constraints 1 job per resource (e.g., machine) 1 resource (e.g., machine) per job Students should be asked to consider what other assumptions must be made when using the assignment model for job scheduling.

Assignment Method - Four Steps Subtract the smallest number in each row from every number in that row; then subtract the smallest number in every column from every number in that column Draw the minimum number of vertical and horizontal straight lines necessary to cover all zeros in the table If the number of lines equals either the number of rows or the number of columns, then you can make an optimal assignment (Step 4) Otherwise: Subtract the smallest number not covered by a line from every other uncovered number. Add the same number to any number(s) lying at the intersection of any two lines. Return to Step 2 Optimal assignments will always be at the zero locations of the table It should be noted to students that there are other methodologies available for solving the assignment problem (branch-and-bound, etc.)

Assignment Method – Type Setter Example Job A B C R-34 $11 $14 $ 6 S-66 $ 8 $10 T-50 $9 $12 $7 Initial set-up

Step 1a & 1b Step 1a Step 1b Typesetter Job A B C R-34 5 8 S-66 2 3 S-66 2 3 T-50 Typesetter Job A B C R-34 5 6 S-66 3 T-50 2 Step 1a Step 1b

Step 2 Smallest uncovered number Typesetter Job A B C R-34 5 6 S-66 3 S-66 3 T-50 2 Smallest uncovered number

Step 3 Make assignments R-34 →C, S-66S → B, T-50 →A Typesetter Job A B S-66 5 T-50 1 Make assignments R-34 →C, S-66S → B, T-50 →A

Sequencing Challenge Order release Job Packet Job XYZ Which job do I run next? Dispatch List Order Part Due Qty XYZ 6014 123 100 ABC 6020 124 50 Production Control Production

Sequencing Specifies order jobs will be worked Sequencing rules First come, first served (FCFS) Shortest processing time (SPT) Earliest due date (EDD) Longest processing time (LPT) Critical ratio (CR) Johnson’s rule The next seven slides provide additional discussion of these rules.

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 CR

First Come, First Served Rule 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

Shortest Processing Time Rule Process job with shortest processing time first. Usually best at minimizing job flow and minimizing the number of jobs in the system Major disadvantage is that long jobs may be continuously pushed back in the queue.

Longest Processing Time Rule Process job with longest processing time first. Usually the least effective method of sequencing.

Earliest Due Date Rule 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

Critical Ratio (CR) Ratio of time remaining to work time remaining = Work days remaining Due date - Today' s date = Work (lead ) time remaining Process job with smallest CR first Performs well on average lateness

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

Criteria to Evaluate Priority Rules You might point out (even though you have probably noted it before) that ratings or rankings of the different scheduling rules will differ depending upon the criteria used.

Job Sequencing Example Job Work Processing time in days Job Due Date (day) A 6 8 B 2 C 18 D 3 15 E 9 23

FCFS Parameter Value Average completion time 15.4 days Utilization 36.4% Average number of jobs in the system 2.75 jobs Average job lateness 2.2 days Sequence A B C D E

SPT Parameter Value Average completion time 13 days Utilization 43.1% Average number of jobs in the system 2.32 jobs Average job lateness 1.8 days Sequence B D A C E

EDD Parameter Value Average completion time 13.6 days Utilization 41.2% Average number of jobs in the system 2.43 jobs Average job lateness 1.2 days Sequence B A D C E

LPT Parameter Value Average completion time 20.6 days Utilization 27.2% Average number of jobs in the system 3.68 jobs Average job lateness 9.6 days Sequence E C A D B

Summary Rule Average Completion Time (days) Utilization (%) Average Number of Jobs in the System Average Lateness (Days) FCFS 15.4 36.4 2.75 2.2 SPT 13.0 43.1 2.32 1.8 EDD 13.6 41.2 2.43 1.2 LPT 20.6 27.2 3.68 9.6

Processing time in days Critical Ratio (CR) Job Job Work Processing time in days Job Due Date (day) Critical Ratio A 6 8 0.75 B 2 0.33 C 18 0.44 D 3 15 0.20 E 9 23 0.39 Sequence A C E B D

Johnson’s Rule Used to sequence N jobs through 2 machines in the same order © 1995 Corel Corp. Saw Drill Job A Job B Job C Jobs (N = 3)

Johnson's Rule - Scheduling N Jobs on Two Machines All jobs are to be listed, and the time each requires on a machine shown. Select the job with the shortest activity time. If the shortest time lies with the first machine, the job is scheduled first; if with the second machine, the job is scheduled last. Once a job is scheduled, eliminate it. Apply steps 2-3 to the remaining jobs, working toward the center of the sequence.

Johnson’s Rule Steps 2 1 List jobs & activity times Select job with shortest time Machine? Schedule FIRST LAST Eliminate job from list Jobs left? Break arbitrarily Ties? Yes 1 2 Stop No

Johnson’s Rule - Example Job Work Center 1 (Drill Press) Work Center 2 (Lathe) A 5 2 B 3 6 C 8 4 D 10 7 E 12

Johnson’s Rule - Example Step 1 A Step 2 B A Step 3 B C A Step 4 B D C A Step 5 B E D C A

Graphical Depiction of Job Flow Work center 1 Work center 2 0 3 10 20 28 33 0 3 9 10 20 22 28 29 33 35 Time => B E D C A B E D C A = Idle = Job completed

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 You might point out to students that this is another case of models which are insufficiently robust to handle all factors. Ask students to consider how these limitations might be overcome. (Finite scheduling?) What is the role of information technology?

Finite Scheduling System

Theory of Constraints Deals with factors limiting company’s ability to achieve goals Types of constraints Physical Example: Machines, raw materials Non-physical Example: Morale, training Limits throughput in operations

Theory of Constraints A Five Step Process Identify the constraints Develop a plan for overcoming the identified constraints Focus resources on accomplishing the constraints identified in step 2 Reduce the effects of the constraints by off-loading work or by expanding capability Once one set of constraints is overcome, return to the first step and identify new constraints

Bottleneck Work Centers Bottleneck work centers have less capacity than prior or following work centers They limit production output © 1995 Corel Corp.

Techniques for Dealing With Bottlenecks Increase the capacity of the constraint Ensure well-trained and cross-trained employees are available to operate and maintain the work center causing the constraint Develop alternate routings, processing procedures, or subcontractors Move inspections and tests to a position just before the constraint Schedule throughput to match the capacity of the bottleneck

The 10 Commandments for Correct Scheduling Utilization of a non-bottleneck resource is determined not by its own capacity but by some other constraint in the system Activating a resource is not synonymous with utilizing a resource An hour lost at a bottleneck is an hour lost of the whole system An hour saved at a non-bottleneck is a mirage The transfer batch may not, and many times should not, be equal to the process batch Discussion should include not only a definition of the commandment but an explanation of how it impacts the production system.

The 10 Commandments for Correct Scheduling The amount processed should be verifiable and not fixed Capacity and priority need to be considered simultaneously, not sequentially Damage from unforeseen problems can be isolated and minimized Plant capacity should not be balanced The sum of the local optimums is not equal to the global optimum

Repetitive Manufacturing - Advantages of Level Material Use Lower inventory levels, releasing capital for other uses Faster product throughput Improved component quality and hence improved product quality Reduced floor space requirements Improved communication among employees because they are closer together Smoother production process because large lots have not “hidden” the problems

Scheduling for Services Appointment systems - doctor’s office Reservations systems - restaurant, car rental First come, first served - deli Most critical first - hospital trauma room Students should be asked to consider what special problems arise in scheduling services.

Cyclical Scheduling Plan a schedule equal in weeks to the number of people being scheduled Determine how many of each of the least desirable off-shifts must be covered each week Begin the schedule for one worker by scheduling the days off during the planning cycle (at a rate of 2 days per week on average)

Cyclical Scheduling - Continued Assign off-shifts for the first worker Repeat this pattern for each other worker, but offset by one week from the previous Allow each worker to pick his/her “slot” or “line” in order of seniority Mandate that any changes from the chosen schedule are strictly between the personnel wanting to switch