Manufacturing Operations Scheduling

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1ãã 2002 South-Western/Thomson Learning2002 South-Western/Thomson Learning TMTM

Slides preparedSlides prepared

by John Loucksby John Loucks

Chapter 16

Manufacturing Operations Scheduling

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Overview

Scheduling Process-Focused Manufacturing Scheduling Product-Focused Manufacturing

Computerized Scheduling Systems

Wrap-Up: What World-Class Companies Do

Scheduling

Process-Focused

Manufacturing

Process-Focused Manufacturing

Process-focused factories are often called job shops. A job shop’s work centers are organized around

similar types of equipment or operations.

Workers and machines are flexible and can be

assigned to and reassigned to many different orders.

Job shops are complex to schedule.

Scheduling and Shop-Floor Decisions

MaterialRequirementsPlan (MRP)

CapacityRequirementsPlan (CRP)

Order-Processing orRouting Plans

Planned

Order ReleasesReport

Work Center

Loading andOvertime Plan

Assignment of

Orders toWork Centers

MasterProductionSchedule (MPS)

Product DesignandProcess Plans

Day-to-Day Scheduling and Shop-Floor Decisions

Pre-production Planning

Design the product in customer order Plan the operations the product must pass through .....

this is the routing plan

Work moves between operations on a move ticket

Common Shop Floor Control Activities

The production control department controls andmonitors order progress through the shop.

Assigns priority to orders

Issues dispatching lists

Tracks WIP and keeps systems updated

Controls input-output between work centers

Measures efficiency, utilization, and productivity

of shop

Shop Floor Planning and Control

Input-Output Control Gantt Chart

Finite and Infinite Loading

Forward and Backward Scheduling

Gantt Charts

Gantt charts are useful tools to coordinate jobsthrough shop; graphical summary of job status and

loading of operations

Gantt Charts

Machining

Fabrication

Assembly

Tue. Wed. Thu. Fri. Sat.Work Centers Mon.

Scheduled Progress Setup, Maint.

Assigning Jobs to Work Centers:

How Many Jobs/Day/Work Center

Infinite loading Assigns jobs to work centers without regard to

capacity

Unless excessive capacity exists, long queues occur

Finite loading

Uses work center capacity to schedule orders

Popular scheduling approach

Integral part of CRP

Assigning Jobs to Work Centers:

Which Job Gets Built First?

Forward scheduling Jobs are given earliest available time slot in

operation

excessive WIP usually results

Backward scheduling

Start with promise date and work backwardthrough operations reviewing lead times todetermine when a job has to pass through eachoperation

Less WIP but must have accurate lead times

Order-Sequencing Problems

Sequencing Rules Criteria for Evaluating Sequencing Rules

Comparison of Sequencing Rules

Controlling Changeover Costs Minimizing Total Production Time

Order-Sequencing Problems

We want to determine the sequence in which we will process a group of waiting orders at a work center.

Many different sequencing rules can be followed in

setting the priorities among orders.

There are numerous criteria for evaluating the

effectiveness of the sequencing rules.

Order-Sequencing Rules

First-Come First-Served (FCFS) Next job to process is the one that arrived firstamong the waiting jobs

Shortest Processing Time (SPT)

Next job to process is the one with the shortest processing time among the waiting jobs

Earliest Due Date (EDD)

Next job to process is the one with the earliestdue (promised finished) date among the waiting jobs

Order-Sequencing Rules

Least Slack (LS) Next job to process is the one with the least [timeto due date minus total remaining processingtime] among the waiting jobs

Critical Ratio (CR) Next job to process is the one with the least [timeto due date divided by total remaining processingtime] among the waiting jobs

Least Changeover Cost (LCC)

Sequence the waiting jobs such that total machinechangeover cost is minimized

Evaluating the Effectiveness

of Sequencing Rules

Average flow time - average amount of time jobsspend in shop

Average number of jobs in system -

Average job lateness - average amount of time job’s

completion date exceeds its promised delivery date

Changeover cost - total cost of making machine

changeovers for group of jobs

Experience Says:

First-come-first-served Performs poorly on most evaluation criteria

Does give customers a sense of fair play

Shortest processing time

Performs well on most evaluation criteria But have to watch out for long-processing-time

orders getting continuously pushed back

Critical ratio

Works well on average job lateness criterion

May focus too much on jobs that cannot becompleted on time, causing others to be late too.

Example: Sequencing Rules

Use the FCFS, SPT, and Critical Ratio rules tosequence the five jobs below. Evaluate the rules on

the bases of average flow time, average number of

jobs in the system, and average job lateness.

Job Processing Time Time to Promised Completion

A 6 hours 10 hours

B 12 16

C 9 8D 14 14

FCFS Rule A > B > C > D > E Processing Promised Flow

Job Time Completion Time Lateness

A 6 10 6 0

B 12 16 18 2C 9 8 27 19

D 14 14 41 27

E 8 7 49 42

49 141 90

FCFS Rule Performance

Average flow time:

141/5 = 28.2 hours

Average number of jobs in the system:141/49 = 2.88 jobs

Average job lateness:

90/5 = 18.0 hours

SPT Rule A > E > C > B > D Processing Promised Flow

A 6 10 6 0

B 8 7 14 7

C 9 8 23 15

D 12 16 35 19

E 14 14 49 35

49 127 76

SPT Rule Performance

Average flow time:

127/5 = 25.4 hours

76/5 = 15.2 hours

Critical Ratio Rule E > C > D > B > A

Processing Promised Flow

E (.875) 8 7 8 1C (.889) 9 8 17 9

D (1.00) 14 14 31 17

B (1.33) 12 16 43 27

A (1.67) 6 10 49 39

49 148 93

Critical Ratio Rule Performance

Average flow time:

148/5 = 29.6 hours

93/5 = 18.6 hours

Comparison of Rule PerformanceAverage Average Average

Flow Number of Jobs Job

Rule Time in System Lateness

FCFS 28.2 2.88 18.0

SPT 25.4 2.59 15.2

CR 29.6 3.02 18.6

SPT rule was superior for all 3 performance criteria.

Controlling Changeover Costs

Changeover costs - costs of changing a processingstep in a production system over from one job to

another

Changing machine settings

Getting job instructions

Changing material

Changing tools

Usually, jobs should be processed in a sequence thatminimizes changeover costs

Controlling Changeover Costs

Job Sequencing Heuristic First, select the lowest changeover cost among all

changeovers (this establishes the first two jobs in

the sequence)

The next job to be selected will have the lowest

changeover cost among the remaining jobs that

follow the previously selected job

Example: Minimizing Changeover Costs

Hardtimes Heat Treating Service has 5 jobswaiting to be processed at work center #11. The job-

to-job changeover costs are listed below. What

should the job sequence be?

Jobs That PrecedeA B C D E

A -- 65 80 50 62

B 95 -- 69 67 65

C 92 71 -- 67 75D 85 105 65 -- 95

E 125 75 95 105 --

Follow

Example: Minimizing Changeover Costs

Develop a job sequence:A follows D ($50 is the least c.o. cost)

C follows A ($92 is the least following c.o. cost)

B follows C ($69 is the least following c.o. cost)

E follows B (E is the only remaining job)

Job sequence is D – A – C – B – E

Total changeover cost = $50 + 92 + 69 + 75 = $286

Minimizing Total Production Time

Sequencing n Jobs through Two Work Centers When several jobs must be sequenced through two

work centers, we may want to select a sequence

that must hold for both work centers

Johnson’s rule can be used to find the sequence

that minimizes the total production time through

both work centers

Johnson’s Rule

1. Select the shortest processing time in either workcenter

2. If the shortest time is at the first work center, put the

job in the first unassigned slot in the schedule. If the

shortest time is at the second work center, put the jobin the last unassigned slot in the schedule.

3. Eliminate the job assigned in step 2.

4. Repeat steps 1-3, filling the schedule from the frontand back, until all jobs have been assigned a slot.

Example: Minimizing Total Production Time

It is early Saturday morning and The FinestDetail has five automobiles waiting for detailingservice. Each vehicle goes through a thoroughexterior wash/wax process and then an interiorvacuum/shampoo/polish process.

The entire detailing crew must stay until the lastvehicle is completed. If the five vehicles aresequenced so that the total processing time isminimized, when can the crew go home. They willstart the first vehicle at 7:30 a.m.

Time estimates are shown on the next slide.

Exterior Interior

Job Time (hrs.) Time (hrs.)

Cadillac 2.0 2.5Bentley 2.1 2.4

Lexus 1.9 2.2

Porsche 1.8 1.6

Infiniti 1.5 1.4

Exterior

Interior

0 1.9 3.9 6.0 7.8 9.3 12.0

0 1.9 4.1 6.6 9.0 10.6 12.0

Idle C B P I

It will take from 7:30 a.m. until 7:30 p.m. (notallowing for breaks) to complete the five vehicles.

Scheduling

Product-Focused

Manufacturing

Batch Scheduling

EOQ for Production Lot Size How many units of a single product should be

included in each production lot to minimize annual

inventory carrying cost and annual machine

changeover cost?

Example: EOQ for Production Lots

CPC, Inc. produces four standard electronicassemblies on a produce-to-stock basis. The annualdemand, setup cost, carrying cost, demand rate, and

production rate for each assembly are shown on thenext slide.

a) What is the economic production lot size for eachassembly?

b) What percentage of the production lot of power

units is being used during its production run?c) For the power unit, how much time will pass

between production setups?

Annual Setup Carry Demand Prod.

Demand Cost Cost Rate Rate

Power Unit 5,000 $1,200 $6 20 200

Converter 10,000 600 4 40 300

Equalizer 12,000 1,500 10 48 100

Transformer 6,000 400 2 24 50

% of Power Units Used During Productiond/p = 20/200 = .10 or 10%

Time Between Setups for Power Units

EOQ/d = 1,490.7/20 = 74.535 days

Batch Scheduling

Run-Out Method Attempts to use the total production capacity

available to produce just enough of each product

so that if all production stops, inventory of each

product runs out at the same time

Example: Run-Out Method

QuadCycle, Inc. assembles, in batches, four bicycle models on the same assembly line. The

production manager must develop an assembly

schedule for March.

There are 1,000 hours available per month for bicycle assembly work. Using the run-out method

and the pertinent data shown on the next slide,

develop an assembly schedule for March.

Assembly March AprilInventory Time Forec. Forec.

On-Hand Required Demand Demand

Bicycle (Units) (Hr/Unit) (Units) (Units)

Razer 100 .3 400 400Splicer 600 .2 900 900

Tracker 500 .6 1,500 1,500

HiLander 200 .1 500 500

Convert inventory and forecast into assembly hours

Assemb. March MarchInvent. Time Forec. Invent. Forec.

On-Hand Req’d. Dem. On-Hand Dem.

Bicycle (Units) (Hr/Unit) (Units) (Hours) (Hours)

Razer 100 .3 400 30 120

Splicer 600 .2 900 120 180

Tracker 500 .6 1,500 300 900

HiLander 200 .1 500 20 50

Total 470 1,250

(1) (2) (3) (4) (5)

(1) x (2) (2) x (4)

E l R O M h d

Compute aggregate run-out time in monthsAggregate Run-out Time =

= [(Total Inventory On-Hand in Hours)

+ (Total Assembly Hours Available per Month)- (March’s Forecasted Demand in Hours)]

/ (April’s Forecasted Demand in Hours)

= (470 + 1,000-

1,250)/1,250 = .176 months

C t i d S h d li

Computerized Scheduling

Develops detailed schedules for each work centerindicating starting and ending times

Develops departmental schedules

Generates modified schedules as orders move

Many packages available.... select one most

appropriate for your business

E d f Ch t 16

End of Chapter 16

Manufacturing Operations Scheduling

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