mrp scheduling criteria - kmutnb gantt loading and scheduling chartsdraw gantt loading and...

Manufacturing Resource Planning

MRPMRP

OutlineOutline

�� The The Strategic Importance of ShortStrategic Importance of Short--Term SchedulingTerm SchedulingTerm SchedulingTerm Scheduling

�� Scheduling IssuesScheduling Issues

��Forward and Backward SchedulingForward and Backward Scheduling

��Scheduling CriteriaScheduling Criteria

Outline Outline –– ContinuedContinued

�� Scheduling ProcessScheduling Process--Focused Focused �� Scheduling ProcessScheduling Process--Focused Focused FacilitiesFacilities

�� Loading JobsLoading Jobs

�� InputInput--Output ControlOutput Control

��Gantt ChartsGantt Charts

��Assignment MethodAssignment Method


�� Sequencing JobsSequencing Jobs

��Priority Rules for Dispatching JobsPriority Rules for Dispatching Jobs

��Critical RatioCritical Ratio

��Sequencing N Jobs on Two Machines: Sequencing N Jobs on Two Machines: Johnson’s RuleJohnson’s Rule

�� Limitations of RuleLimitations of Rule--Based Dispatching Based Dispatching �� Limitations of RuleLimitations of Rule--Based Dispatching Based Dispatching SystemsSystems

�� Finite Capacity Scheduling (FCS)Finite Capacity Scheduling (FCS)


�� Theory of ConstraintsTheory of Constraints

��BottlenecksBottlenecks

��Drum, Buffer, RopeDrum, Buffer, Rope

�� Scheduling Repetitive Scheduling Repetitive FacilitiesFacilities

Learning ObjectivesLearning Objectives

When you complete this chapter you should be able to:

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

2.2. Draw Gantt loading and scheduling chartsDraw Gantt loading and scheduling charts

3.3. Apply the assignment method for loading Apply the assignment method for loading 3.3. Apply the assignment method for loading Apply the assignment method for loading jobsjobs

Learning ObjectivesLearning Objectives

When you complete this chapter you should be able to:

4.4. Name and describe each of the priority Name and describe each of the priority sequencing rulessequencing rules

5.5. Use Johnson’s ruleUse Johnson’s rule

6.6. Define finite capacity schedulingDefine finite capacity scheduling6.6. Define finite capacity schedulingDefine finite capacity scheduling

7.7. List the steps in the theory of List the steps in the theory of constraintsconstraints

MRPII: General conceptsMRPII: General concepts

�� MRP MRP allows for the input of sales forecasts from allows for the input of sales forecasts from sales and marketing. These forecasts determine sales and marketing. These forecasts determine sales and marketing. These forecasts determine sales and marketing. These forecasts determine the raw materials demand. the raw materials demand.

�� MRP MRP and MRPII systems draw on a master and MRPII systems draw on a master production schedule, the break down of specific production schedule, the break down of specific plans for each product on a line. plans for each product on a line.

�� MRPII MRPII facilitates the development of a detailed facilitates the development of a detailed production schedule that accounts for machine production schedule that accounts for machine production schedule that accounts for machine production schedule that accounts for machine and labor capacity, scheduling the production runs and labor capacity, scheduling the production runs according to the arrival of materials. according to the arrival of materials.

�� An An MRPII output is a final labor and machine MRPII output is a final labor and machine schedule. schedule. 8

BenefitsBenefits

��MRP II systems can provide:MRP II systems can provide:��Better Better control of inventoriescontrol of inventories��Better Better control of inventoriescontrol of inventories

��Improved Improved schedulingscheduling

��Productive Productive relationships with suppliersrelationships with suppliers

��For design / engineering:For design / engineering:��Improved Improved design controldesign control

��Better Better quality and quality controlquality and quality control

��For financial and costing:For financial and costing:��Reduced Reduced working capital for inventoryworking capital for inventory

��Improved Improved cash flow through quicker deliveriescash flow through quicker deliveries

��Accurate Accurate inventory recordsinventory records

9

Strategic Importance of ShortStrategic Importance of Short--Term Term SchedulingScheduling

�� Effective and efficient scheduling can Effective and efficient scheduling can �� Effective and efficient scheduling can Effective and efficient scheduling can be a competitive advantagebe a competitive advantage

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

�� Additional capacity resulting from faster Additional capacity resulting from faster �� Additional capacity resulting from faster Additional capacity resulting from faster throughput improves customer service throughput improves customer service through faster deliverythrough faster delivery

�� Good schedules result in more Good schedules result in more dependable deliveriesdependable deliveries

Scheduling IssuesScheduling Issues

�� Scheduling deals with the timing of Scheduling deals with the timing of operationsoperationsoperationsoperations

�� The task is the allocation and The task is the allocation and prioritization of demandprioritization of demand

�� Significant issues areSignificant issues are

��The type of scheduling, forward or The type of scheduling, forward or backwardbackward

��The criteria for prioritiesThe criteria for priorities

Scheduling DecisionsScheduling Decisions

OrganizationOrganization Managers Must Schedule the FollowingManagers Must Schedule the Following

Arnold Palmer Arnold Palmer HospitalHospital

Operating room useOperating room usePatient admissionsPatient admissionsHospitalHospital Patient admissionsPatient admissionsNursing, security, maintenance staffsNursing, security, maintenance staffsOutpatient treatmentsOutpatient treatments

University of University of MissouriMissouri

Classrooms and audiovisual equipmentClassrooms and audiovisual equipmentStudent and instructor schedulesStudent and instructor schedulesGraduate and undergraduate coursesGraduate and undergraduate courses

Lockheed Martin Lockheed Martin factoryfactory

Production of goodsProduction of goodsPurchases of materialsPurchases of materialsWorkersWorkers

Hard Rock CafeHard Rock Cafe Chef, waiters, bartendersChef, waiters, bartendersDelivery of fresh foodsDelivery of fresh foodsEntertainersEntertainersOpening of dining areasOpening of dining areas

Delta Air LinesDelta Air Lines Maintenance of aircraftMaintenance of aircraftDeparture timetablesDeparture timetablesFlight crews, catering, gate, ticketing personnelFlight crews, catering, gate, ticketing personnelTable 15.1Table 15.1

Forward and Backward Forward and Backward SchedulingScheduling

�� Forward scheduling starts as soon as Forward scheduling starts as soon as �� Forward scheduling starts as soon as Forward scheduling starts as soon as the requirements are knownthe requirements are known

�� Produces a feasible schedule though it Produces a feasible schedule though it may not meet due datesmay not meet due dates

�� Frequently results in Frequently results in �� Frequently results in Frequently results in buildup of workbuildup of work--inin--process inventoryprocess inventory

Due Due DateDateNowNow


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

�� Schedule is produced by working Schedule is produced by working backwards though the processesbackwards though the processesbackwards though the processesbackwards though the processes

�� Resources may not Resources may not be available to be available to accomplish the accomplish the scheduleschedule



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

�� Schedule is produced by working Schedule is produced by working backwards though the processesbackwards though the processesbackwards though the processesbackwards though the processes

�� Resources may not Resources may not be available to be available to accomplish the accomplish the scheduleschedule


Different Processes/ Different Different Processes/ Different ApproachesApproaches

ProcessProcess--focused focused facilitiesfacilities

ForwardForward--looking scheduleslooking schedulesMRP due datesMRP due datesfacilitiesfacilities MRP due datesMRP due datesFinite capacity schedulingFinite capacity scheduling

Work cellsWork cells ForwardForward--looking scheduleslooking schedulesMRP due datesMRP due datesDetailed schedule done using work cell Detailed schedule done using work cell priority rulespriority rules

Repetitive facilitiesRepetitive facilities ForwardForward--looking schedule with a looking schedule with a balanced linebalanced linebalanced linebalanced linePull techniques for schedulingPull techniques for scheduling

ProductProduct--focused focused facilitiesfacilities

ForwardForward--looking schedule with stable looking schedule with stable demand and fixed capacitydemand and fixed capacityCapacity, setCapacity, set--up, and run times knownup, and run times knownCapacity limited by longCapacity limited by long--term capital term capital investmentinvestmentTable 15.2Table 15.2

Scheduling CriteriaScheduling Criteria

1.1. Minimize completion timeMinimize completion time1.1. Minimize completion timeMinimize completion time

2.2. Maximize utilization of facilitiesMaximize utilization of facilities

3.3. Minimize workMinimize work--inin--process (WIP) process (WIP) inventoryinventory

4.4. Minimize customer waiting timeMinimize customer waiting time4.4. Minimize customer waiting timeMinimize customer waiting time

Optimize the use of resources so that Optimize the use of resources so that production objectives are metproduction objectives are met

Scheduling ProcessScheduling Process--Focused FacilitiesFocused Facilities

1.1. Schedule incoming orders without violating Schedule incoming orders without violating 1.1. Schedule incoming orders without violating Schedule incoming orders without violating capacity constraintscapacity constraints

2.2. Check availability of tools and materials Check availability of tools and materials before releasing an orderbefore releasing an order

3.3. Establish due dates for each job and check Establish due dates for each job and check progressprogress

4.4. Check work in progressCheck work in progress

5.5. Provide feedbackProvide feedback

6.6. Provide work efficiency statistics and Provide work efficiency statistics and monitor timesmonitor times

Planning and Control FilesPlanning and Control Files

Planning FilesPlanning Files

1.1. An item master file contains information about each componentAn item master file contains information about each component

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

3.3. A workA work--center master file contains information about the work centercenter master file contains information about the work center

Control FilesControl Files

Track the actual progress made against the planTrack the actual progress made against the plan

Loading JobsLoading Jobs

�� Assign jobs so that costs, idle Assign jobs so that costs, idle �� Assign jobs so that costs, idle Assign jobs so that costs, idle time, or completion time are time, or completion time are minimizedminimized

�� Two forms of loadingTwo forms of loading

��Capacity orientedCapacity oriented

��Assigning specific jobs to work Assigning specific jobs to work centerscenters

InputInput--Output ControlOutput Control

�� Identifies overloading and Identifies overloading and �� Identifies overloading and Identifies overloading and underloading conditionsunderloading conditions

�� Prompts managerial action to resolve Prompts managerial action to resolve scheduling problemsscheduling problems

�� Can be maintained using ConWIP Can be maintained using ConWIP cards that control the scheduling of cards that control the scheduling of cards that control the scheduling of cards that control the scheduling of batchesbatches

InputInput--Output Control ExampleOutput Control Example

Work Center DNC Milling (in standard hours)

Week Ending 6/6 6/13 6/20 6/27 7/4 7/11

Planned Input 280 280 280 280 280

Actual Input 270 250 280 285 280

Cumulative Deviation –10 –40 –40 –35

Planned Output 320 320 320 320

Actual Output 270 270 270 270

Work Center DNC Milling (in standard hours)


Cumulative Change in Backlog

0 –20 –10 +5

Figure 15.2Figure 15.2


Work Center DNC Milling (in standard hours)Work Center DNC Milling (in standard hours)

Week Ending 6/6 6/13 6/20 6/27 7/4 7/11

Planned Input 280 280 280 280 280

Actual Input 270 250 280 285 280


Planned Output 320 320 320 320

Actual Output 270 270 270 270Explanation:270 input,

Explanation: 250 input,270 output implies


Cumulative Change in Backlog

0 –20 –10 +5

270 input, 270 output implies 0 change

270 output implies –20 change



Options available to operations Options available to operations Options available to operations Options available to operations personnel include:personnel include:

1.1. Correcting performancesCorrecting performances

2.2. Increasing capacityIncreasing capacity

3.3. Increasing or reducing input to Increasing or reducing input to the work centerthe work center

Gantt ChartsGantt Charts

�� Load chart shows the loading and idle Load chart shows the loading and idle times of departments, machines, or times of departments, machines, or times of departments, machines, or times of departments, machines, or facilitiesfacilities

�� Displays relative workloads over timeDisplays relative workloads over time

�� Schedule chart monitors jobs in Schedule chart monitors jobs in processprocessprocessprocess

�� All Gantt charts need to be updated All Gantt charts need to be updated frequently to account for changesfrequently to account for changes

Gantt Load Chart ExampleGantt Load Chart Example

DayMonday Tuesday Wednesday Thursday FridayWork DayMonday Tuesday Wednesday Thursday FridayWork

Center

Metalworks

Mechanical

Electronics

Painting

Job 349

Job 349

Job 349

Job 408

Job 408

Job 408

Job 350

Job 349

Job 295


Painting Job 349Job 408

ProcessingProcessing UnscheduledUnscheduled Center not availableCenter not available

Job 295

Gantt Schedule Chart Gantt Schedule Chart ExampleExample

Day Day Day Day Day Day Day Day Start of an Start of an activityactivity

JobDay

1Day

2Day

3Day

4Day

5Day

6Day

7Day

8

A

BMaintenance

activityactivity

End of an End of an activityactivity

Scheduled Scheduled activity time activity time allowedallowed

Actual work Actual work progressprogress


C

NowNow

Nonproduction Nonproduction timetime

Point in time Point in time when chart is when chart is reviewedreviewed

Assignment MethodAssignment Method

�� A special class of linear programming A special class of linear programming �� A special class of linear programming A special class of linear programming models that assign tasks or jobs to models that assign tasks or jobs to resourcesresources

�� Objective is to minimize cost or timeObjective is to minimize cost or time

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


�� Build a table of costs or time Build a table of costs or time �� Build a table of costs or time Build a table of costs or time associated with particular associated with particular assignmentsassignments

TypesetterTypesetter

JobJob AA BB CC

RR--3434 $11$11 $14$14 $ 6$ 6RR--3434 $11$11 $14$14 $ 6$ 6

SS--6666 $ 8$ 8 $10$10 $11$11

TT--5050 $ 9$ 9 $12$12 $ 7$ 7


1.1. Create zero opportunity costs by Create zero opportunity costs by repeatedly subtracting the lowest costs repeatedly subtracting the lowest costs

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

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


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

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

Assignment ExampleAssignment Example

AA BB CCJobJob


JobJob

RR--3434 $11$11 $14$14 $ 6$ 6

SS--6666 $ 8$ 8 $10$10 $11$11

TT--5050 $ 9$ 9 $12$12 $ 7$ 7


Step 1a Step 1a -- RowsRows


Step 1b Step 1b -- ColumnsColumns

AA BB CCJobJob

RR--3434 $ 5$ 5 $ 8$ 8 $ 0$ 0

SS--6666 $ 0$ 0 $ 2$ 2 $ 3$ 3

TT--5050 $ 2$ 2 $ 5$ 5 $ 0$ 0

TypesetterTypesetterAA BB CC

JobJob

RR--3434 $ 5$ 5 $ 6$ 6 $ 0$ 0

SS--6666 $ 0$ 0 $ 0$ 0 $ 3$ 3

TT--5050 $ 2$ 2 $ 3$ 3 $ 0$ 0



Step 2 Step 2 -- LinesLinesThe smallest uncovered number is 2 The smallest uncovered number is 2 so this is subtracted from all other so this is subtracted from all other uncovered numbers and added to uncovered numbers and added to

AA BB CCJobJob

RR--3434 $ 5$ 5 $ 6$ 6 $ 0$ 0

SS--6666 $ 0$ 0 $ 0$ 0 $ 3$ 3

TT--5050 $ 2$ 2 $ 3$ 3 $ 0$ 0


Step 3 Step 3 -- SubtractionSubtraction


uncovered numbers and added to uncovered numbers and added to numbers at the intersection of linesnumbers at the intersection of lines

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

AA BB CCJobJob

RR--3434 $ 3$ 3 $ 4$ 4 $ 0$ 0

SS--6666 $ 0$ 0 $ 0$ 0 $ 5$ 5

TT--5050 $ 0$ 0 $ 1$ 1 $ 0$ 0



Step 2 Step 2 -- LinesLines


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

AA BB CCJobJob

RR--3434 $ 3$ 3 $ 4$ 4 $ 0$ 0

SS--6666 $ 0$ 0 $ 0$ 0 $ 5$ 5

TT--5050 $ 0$ 0 $ 1$ 1 $ 0$ 0

TypesetterTypesetterworker C.worker C.

Step 4 Step 4 -- AssignmentsAssignments


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

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

AA BB CCJobJob

R-34 $ 3$ 3 $ 4$ 4 $ 0

S-66 $ 0$ 0 $ 0 $ 5$ 5

T-50 $ 0 $ 1$ 1 $ 0$ 0



Step 4 Step 4 -- AssignmentsAssignments

TypesetterTypesetterTypesetterTypesetterAA BB CC

JobJob

R-34 $ 3$ 3 $ 4$ 4 $ 0

S-66 $ 0$ 0 $ 0 $ 5$ 5

T-50 $ 0 $ 1$ 1 $ 0$ 0

TypesetterTypesetterAA BB CC

JobJob

RR--3434 $11$11 $14$14 $ 6$ 6

SS--6666 $ 8$ 8 $10$10 $11$11

TT--5050 $ 9$ 9 $12$12 $ 7$ 7


From the original cost table

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

Sequencing JobsSequencing Jobs

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

�� Priority rules are used to dispatch or Priority rules are used to dispatch or sequence jobssequence jobs

��FCFS: First come, first servedFCFS: First come, first served

��SPT: Shortest processing timeSPT: Shortest processing time��SPT: Shortest processing timeSPT: Shortest processing time

��EDD: Earliest due dateEDD: Earliest due date

�� LPT: Longest processing timeLPT: Longest processing time

Sequencing ExampleSequencing Example

Apply the four popular sequencing rules to Apply the four popular sequencing rules to these five jobsthese five jobs

JobJob

Job Work Job Work (Processing) Time(Processing) Time

(Days)(Days)

Job Due Job Due DateDate

(Days)(Days)

AA 66 88

BB 22 66

these five jobsthese five jobs

BB 22 66

CC 88 1818

DD 33 1515

EE 99 2323


FCFS: Sequence AFCFS: Sequence A--BB--CC--DD--EE

Job Job SequenceSequence

Job Work Job Work (Processing) (Processing)

TimeTimeFlow Flow TimeTime


Job Job LatenessLateness

AA 66 66 88 00

BB 22 88 66 22

CC 88 1616 1818 00CC 88 1616 1818 00

DD 33 1919 1515 44

EE 99 2828 2323 55

2828 7777 1111


FCFS: Sequence AFCFS: Sequence A--BB--CC--DD--EE






AA 66 66 88 00

BB 22 88 66 22

CC 88 1616 1818 00

Average completion time = = 77/5 = 15.4 daysSum of total flow time

Number of jobs

Utilization = = 28/77 = 36.4%Total job work time

Sum of total flow time

Average number of jobs in the system = = 77/28 = 2.75 jobs

Sum of total flow timeCC 88 1616 1818 00

DD 33 1919 1515 44

EE 99 2828 2323 55

2828 7777 1111


Total job work time

Average job lateness = = 11/5 = 2.2 daysTotal late days

Number of jobs


SPT: Sequence BSPT: Sequence B--DD--AA--CC--EE






BB 22 22 66 00

DD 33 55 1515 00

AA 66 1111 88 33AA 66 1111 88 33

CC 88 1919 1818 11

EE 99 2828 2323 55

2828 6565 99


SPT: Sequence BSPT: Sequence B--DD--AA--CC--EE






BB 22 22 66 00

DD 33 55 1515 00

AA 66 1111 88 33

Average completion time = = 65/5 = 13 daysSum of total flow time

Number of jobs




Sum of total flow timeAA 66 1111 88 33

CC 88 1919 1818 11

EE 99 2828 2323 55

2828 6565 99


Total job work time


Number of jobs


EDD: Sequence BEDD: Sequence B--AA--DD--CC--EE






BB 22 22 66 00

AA 66 88 88 00

DD 33 1111 1515 00DD 33 1111 1515 00

CC 88 1919 1818 11

EE 99 2828 2323 55

2828 6868 66


EDD: Sequence BEDD: Sequence B--AA--DD--CC--EE






BB 22 22 66 00

AA 66 88 88 00

DD 33 1111 1515 00


Number of jobs




Sum of total flow timeDD 33 1111 1515 00

CC 88 1919 1818 11

EE 99 2828 2323 55

2828 6868 66


Total job work time


Number of jobs


LPT: Sequence ELPT: Sequence E--CC--AA--DD--BB






EE 99 99 2323 00

CC 88 1717 1818 00

AA 66 2323 88 1515AA 66 2323 88 1515

DD 33 2626 1515 1111

BB 22 2828 66 2222

2828 103103 4848


LPT: Sequence ELPT: Sequence E--CC--AA--DD--BB






EE 99 99 2323 00

CC 88 1717 1818 00

AA 66 2323 88 1515


Number of jobs




Sum of total flow timeAA 66 2323 88 1515

DD 33 2626 1515 1111

BB 22 2828 66 2222

2828 103103 4848


Total job work time


Number of jobs


Summary of RulesSummary of Rules

RuleRule

Average Average Completion Completion Time (Days)Time (Days)

Utilization Utilization (%)(%)

Average Number Average Number of Jobs in of Jobs in

SystemSystem

Average Average Lateness Lateness

(Days)(Days)

FCFSFCFS 15.415.4 36.436.4 2.752.75 2.22.2

SPTSPT 13.013.0 43.143.1 2.322.32 1.81.8

EDDEDD 13.613.6 41.241.2 2.432.43 1.21.2

LPTLPT 20.620.6 27.227.2 3.683.68 9.69.6

Comparison of Comparison of Sequencing RulesSequencing Rules

�� No one sequencing rule excels on all criteriaNo one sequencing rule excels on all criteria

�� SPT does well on minimizing flow time and number of jobs in the SPT does well on minimizing flow time and number of jobs in the systemsystem

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

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

�� EDD minimizes latenessEDD minimizes lateness

Critical Ratio (CR)Critical Ratio (CR)

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

�� Jobs with low critical ratios are scheduled Jobs with low critical ratios are scheduled ahead of jobs with higher critical ratiosahead of jobs with higher critical ratios

�� Performs well on average job lateness Performs well on average job lateness criteriacriteriacriteriacriteria

CR = =CR = =Due date Due date -- Today’s dateToday’s date

Work (lead) time remainingWork (lead) time remaining

Time remainingTime remaining

Workdays remainingWorkdays remaining

Critical Ratio ExampleCritical Ratio Example

Currently Day Currently Day 2525

JobJobDue Due DateDate

Workdays Workdays RemainingRemaining Critical RatioCritical Ratio

Priority Priority OrderOrder

AA 3030 44 (30 (30 -- 25)/4 = 1.2525)/4 = 1.25 33

BB 2828 55 (28 (28 -- 25)/5 = .6025)/5 = .60 11

CC 2727 22 (27 (27 -- 25)/2 = 1.0025)/2 = 1.00 22

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

Critical Ratio TechniqueCritical Ratio Technique

1.1. Helps determine the status of specific jobsHelps determine the status of specific jobs

2.2. Establishes relative priorities among jobs Establishes relative priorities among jobs on a common basison a common basis

3.3. Relates both stock and makeRelates both stock and make--toto--order jobs order jobs on a common basison a common basis

4.4. Adjusts priorities automatically for changes Adjusts priorities automatically for changes 4.4. Adjusts priorities automatically for changes Adjusts priorities automatically for changes in both demand and job progressin both demand and job progress

5.5. Dynamically tracks job progressDynamically tracks job progress

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

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

�� Minimizes total production time and Minimizes total production time and idle timeidle timeidle timeidle time

Johnson’s RuleJohnson’s Rule

1.1. List all jobs and times for each work centerList all jobs and times for each work center

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

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

4.4. Repeat steps 2 and 3 working toward the Repeat steps 2 and 3 working toward the center of the sequencecenter of the sequence

Johnson’s Rule ExampleJohnson’s Rule Example

JobJobWork Center 1 Work Center 1

(Drill Press)(Drill Press)Work Center 2 Work Center 2

(Lathe)(Lathe)

AA 55 22AA 55 22

BB 33 66

CC 88 44

DD 1010 77

EE 77 1212




(Lathe)(Lathe)

AA 55 22

BB EE DD CC AA

AA 55 22

BB 33 66

CC 88 44

DD 1010 77

EE 77 1212




(Lathe)(Lathe)

AA 55 22AA 55 22

BB 33 66

CC 88 44

DD 1010 77

EE 77 1212

TimeTime 00 33 1010 2020 2828 3333

BB AACCDDEE

B ACDEWC

1

WC 2




(Lathe)(Lathe)

AA 55 22AA 55 22

BB 33 66

CC 88 44

DD 1010 77

EE 77 1212

TimeTime 00 33 1010 2020 2828 3333

BB AACCDDEE

TimeTime�� 00 11 33 55 77 99 1010 1111 1212 1313 1717 1919 21 22 2321 22 232525 2727 2929 3131 3333 3535

B ACDE

B ACDE

WC 1

WC 2

BB EE DD CC AA

Limitations of RuleLimitations of Rule--Based Based Dispatching SystemsDispatching Systems

1.1. Scheduling is dynamic and rules Scheduling is dynamic and rules 1.1. Scheduling is dynamic and rules Scheduling is dynamic and rules need to be revised to adjust to need to be revised to adjust to changeschanges

2.2. Rules do not look upstream or Rules do not look upstream or downstreamdownstreamdownstreamdownstream

3.3. Rules do not look beyond due datesRules do not look beyond due dates

Finite Capacity SchedulingFinite Capacity Scheduling

�� Overcomes disadvantages of ruleOvercomes disadvantages of rule--based based systems by providing an interactive, systems by providing an interactive, systems by providing an interactive, systems by providing an interactive, computercomputer--based graphical systembased graphical system

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

�� Initial data often from an MRP systemInitial data often from an MRP system�� Initial data often from an MRP systemInitial data often from an MRP system

�� FCS allows the balancing of delivery needs FCS allows the balancing of delivery needs and efficiencyand efficiency


MRP Data

• Master • Routing files

• Work center

Interactive Finite Capacity SchedulingInteractive Finite Capacity Scheduling

• Master schedule

• BOM

• Inventory

Priority rules

• Expert

• Work center information

Tooling and other

resources

Setups and • Expert

systems

• Simulation models

Setups and run time



Theory of ConstraintsTheory of Constraints

�� Throughput is the number of units Throughput is the number of units processed through the facility and soldprocessed through the facility and soldprocessed through the facility and soldprocessed through the facility and sold

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

1.1. Identify the constraintsIdentify the constraints

2.2. Develop a plan for overcoming the constraintsDevelop a plan for overcoming the constraints

3.3. Focus resources on accomplishing the planFocus resources on accomplishing the plan3.3. Focus resources on accomplishing the planFocus resources on accomplishing the plan

4.4. Reduce the effects of constraints by offReduce the effects of constraints by off--loading work or increasing loading work or increasing capacitycapacity

5.5. Once successful, return to step 1 and identify new constraintsOnce successful, return to step 1 and identify new constraints

BottlenecksBottlenecks

�� Bottleneck work centers are constraints that Bottleneck work centers are constraints that limit outputlimit outputlimit outputlimit output

�� Common occurrence due to frequent changesCommon occurrence due to frequent changes

�� Management techniques include:Management techniques include:�� Increasing the capacity of the constraintIncreasing the capacity of the constraint

�� CrossCross--trained employees and maintenancetrained employees and maintenance

�� Alternative routings, procedures, or subcontractorsAlternative routings, procedures, or subcontractors�� Alternative routings, procedures, or subcontractorsAlternative routings, procedures, or subcontractors

�� Moving inspection and testMoving inspection and test

�� Scheduling throughput to match bottleneck capacityScheduling throughput to match bottleneck capacity

Drum, Buffer, RopeDrum, Buffer, Rope

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

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

�� The rope provides the synchronization The rope provides the synchronization necessary to pull units through the systemnecessary to pull units through the system

Scheduling Repetitive Scheduling Repetitive FacilitiesFacilities

�� Level material use can help Level material use can help �� Level material use can help Level material use can help repetitive facilities repetitive facilities

��Better satisfy customer Better satisfy customer demanddemand

�� Lower inventory Lower inventory investmentinvestmentinvestmentinvestment

��Reduce batch sizeReduce batch size

��Better utilize equipment Better utilize equipment and facilitiesand facilities

Scheduling Repetitive Scheduling Repetitive FacilitiesFacilities

�� Advantages include:Advantages include:�� Advantages include:Advantages include:

1.1. Lower inventory levelsLower inventory levels

2.2. Faster product throughputFaster product throughput

3.3. Improved component qualityImproved component quality

4.4. Reduced floorReduced floor--space requirementsspace requirements4.4. Reduced floorReduced floor--space requirementsspace requirements

5.5. Improved communicationsImproved communications

6.6. Smoother production processSmoother production process

mrp scheduling criteria - kmutnb gantt loading and scheduling chartsdraw gantt loading and...

Documents