POM 3430: Ch 15 Scheduling

priority rules steps

1. decide which priority rule to use 2. list all the jobs waiting to be processed at the work center and their job time 3. using your priority rule, determine which job has the highest priority and should be worked on first, second, third, and so on

OPT principles: bottlenecks determine throughput and system inventory

a bottleneck resource determines the throughput for the system; it also determines how much inventory is needed in the system to ensure the continuous operation of the bottleneck resource

input/output control

a capacity control technique for monitoring the flow of jobs between work centers

progress chart

a chart that visually shows the planned schedule compared to actual performance

load chart

a chart that visually shows the workload relative to the capacity at a resource

mean job flow time and the mean number of jobs in the system each measure ...

a company's responsiveness

when you are using backward scheduling and forward scheduling together ...

a difference between the start times of the first activity indicates slack in the schedule

theory of constraints (TOC)

a management philosophy that extends the concepts of OPT; a system's output is determined by 3 kinds of constraints (internal resource constraint, market constraint, and policy constraint)

internal resource constraint

a regular bottleneck

operation sequencing (job sequencing)

a short term plan of actual jobs to be run in each work center based on available capacity and priorities -we expect a work center to have several jobs waiting to be processed, so we decide on the sequence for processing the jobs

policy constraint

a specific policy dictates the rate of production

Johnson's rule

a technique for minimizing makespan in a two stage, unidirectional process; the job flow must be unidirectional (the first activity for every job is the same, and you must finish it before you can begin the second activity aka wash the clothes before you dry the clothes)

optimized production technology (OPT)

a technique used to schedule bottleneck systems; emerged in the late 1970s by Eli Goldratt

Bottleneck

a work stage that cannot meet the production quota even at its maximum throughput capacity, thereby delaying or stopping the flow of operations.

OPT principles:usage and activation of a resource are not the same

activation of a resource means the resource is used to process materials or products; usage means that the resource activated is contributing positively to the company's performance

new software packages tend to use real time finite scheduling; these programs ...

alert the scheduler when a problem arises and may recommend appropriate rescheduling

bottleneck

any resource whose capacity is less than the demand placed on it

OPT principles: nonbottleneck usage is determined by some other constraint within the system

at the floral shop, the order processing and the delivery service are non bottlenecks. Their level of usage is determined by the flower arrangers (the bottleneck resource)

capacity constrained resource

bottleneck caused by inefficient usage

routing: the workstation or processing point that needs the greatest amount of time is the system's ...

bottleneck, which determines how many products or services the system can complete -thus, the goal is to sequence the operations so they need the least control possible

when companies schedule a job shop...

bottlenecks are common

OPT classifies resources as either ...

bottlenecks or non bottlenecks, and makes bottlenecks the basis for scheduling and capacity planning

planned outputs use ...

capacity planning techniques

companies measure scheduling effectiveness according to their ...

competitive priorities -ex: if your company is concerned with customer response time, you measure scheduling effectiveness in terms of response time

schedules should be established by ...

considering all constraints simultaneously

makespan has no link to ...

customer due dates; you can have an efficient schedule in terms of finishing a batch of jobs but still have relatively poor customer service

priority rule

determines the priority of jobs at a work center -typically classified as local or global

priority rules steps: step #1 decide which priority rule to us e

different priority rules achieve different results

if your company competes on cost, it's concerned with ...

efficiency

a major concern with flow operations is...

employee boredom with repetitive tasks -companies use techniques like job enrichment, job enlargement, and job rotation to reduce boredom and maximize line output

companies benefit from both...

finite and infinite loading

objective of low volume operations

flexibility, both in product variation and product volume

a company's overall strategy provides the ...

framework for making decision in many operational areas

companies achieve cost efficiency in a high volume operations through ...

high levels of labor and equipment utilization -design of the work environment ensures a smooth flow of products or customers through the system

companies with low volume operations use ...

highly skilled employees, general purpose equipment, and a process layout

non bottleneck resources can be ...

idle and still not affect the output of the system because output is determined by the bottleneck resource, not by capacity at the non bottleneck resources

OPT principles: an hour lost at a bottleneck resource is an hour lost forever

if an organization's goal is to maximize throughput, the bottleneck must be fully used

two kinds of work scheduling or work loading

infinite load ing and finite loading

RFID technology can be...

integrated into scheduling software to track milestones in the production process

actual input

is compared with planned input to ensure that enough work enters the measured work center

the disadvantage to finishing a job early

it causes an inventory buildup if items are not delivered before the due date

when a job is finished ahead of schedule...

it has negative lateness -ex: if job x is due on day 15 and it's finished on day 12, it has a lateness of negative 3 days

disadvantage of finite loading

it tends to break down over the long term: problems arise and the schedule slips, causing jobs to be rescheduled -finite loading is why you may have to wait at the doctor's office

priority rules steps: step #2 list all the jobs waiting to be processed at the work center and their job time

job time includes setup and processing time

first come, first served (FCFS)

jobs are processed in the order in which they arrive at a machine or work center

with forward scheduling, it's not unusual for...

jobs to be finished before their due date

businesses make schedules to show how...

labor, materials, and equipment should be used

backward scheduling shows you how ...

late the job can be started and still be finished on time

in low volume operations, products are...

made to order -each product or service can have its own routing through a unique sequence of workstations, processes, materials, or setups; as a results, scheduling is COMPLEX

well defined schedules allow a company to...

make promised to customers concerning the completion time of the service provided

global priority

makes a priority decision based on information that includes the remaining working centers a job must pass through -similar to critical ratio or slack per remaining operations

local priority rule

makes a priority decision based on jobs currently at that work center -ex: the highest priority might be given to the job that arrives first or the job that can be done the fastest

job flow time

measurement of the time a job spends in the shop before it is finished; measures response time

job tardiness

measures how long after the due date the job is completed; positive job lateness values

average number of jobs in the system

measures the WIP inventory and also affects response time -the greater the number of jobs in the system, the longer the queues and subsequently the longer the job flow times

job lateness

measures whether the job is done ahead of, on, or behind schedule; a measure of customer service

a work center needs priority rules when ...

multiple jobs await processing (but not if only a single jobs needs processing)

1-800 flowers

nationwide network of over 1500 independent florists, allows customers to enter a website and private chat room to discuss their order with a customer service representative in real time

in low volume operations, equipment is ...

not dedicated to particular jobs but is available for all jobs

bottlenecks typically result when ...

one operation in a job takes longer than the other operations

input/output controls monitors the ..

planned inputs and outputs at a work center against the actual inputs and outputs

Gantt chart

planning and control chart designed to graphically show workloads or to minter job progress -named after Henry Gantt, who developed these charts in the early 1900s -a Gantt chart is a visual representation of a schedule over time

the finite loading schedule shows how a company ...

plans to use available capacity at each work center -in a manufacturing company, the schedule shows the jobs to be done at a particular work center if the work center uses a set number of production hours each day -in a service organizations, a doctor's office is a good example; to spend 10 minutes with each patient, the doctor can have 6 patients scheduled per hour

companies providing low volume, customized, higher margin products or services (such as customer furniture maker or an upscale restaurant) focus on ...

process -they need general purpose equipment, more highly skilled employees, and flexible process flows

flow operations

processes designed to handle high volume, standard products -can be repetitive operations for discrete products like automobiles, appliances, or bread, ors services like license renewals at the DMV -can also be continuous operations for goods produced in a continuous flow as in a product like gasoline or a service like waste treatment

a company providing a high volume, standardized, consistent quality, lower margin product or service (such as commercial bakery or fast food restaurant) focuses on...

product and layout -this type of operation needs dedicated equipment, less skilled employees, and a continuous or repetitive process flow

companies differentiate themselves based on ...

product volume and product variety; this differentiation affects how the company organizes its operations

operation sequencing sets...

projected start and finish times and expected queues -a job's priority is its position in the sequence

routing

provides information about the operations to be performed, their sequence, the work centers, and the time standards (used by flow operations)

infinite loading identifies ...

resource bottlenecks for a proposed schedule so that planners can find solutions proactively, such as changing the schedule and increasing the resource capacity

market constraint

results when market demand is less than production capacity; since companies don't want excess inventory buildup, the market determines the rate of production

at times, all jobs must be processed through the ...

same two work centers sequentially

lead times are the result of the ...

schedule and are predetermined

infinite loading

scheduling that calculates the capacity needed at work centers in the time period needed without regard to the capacity available to do the work -infinite loading schedules work without regard to capacity limits; they let you know how much capacity you need to meet at schedule

forward scheduling

scheduling that determines the earliest possible completion date for a job -processing starts immediately when a job is received, regardless if the due date -each job activity is scheduled for completion as soon as possible

finite loading

scheduling that loads work centers up to a predetermined amount of capacity

high volume standard items, either discrete or continuous, have...

smaller profit margins, so cost efficiency is important

backward scheduling

starts with the due date for an order and works backward to determine the start date for each activity -begin scheduling the job's last activity so that the job is finished right on the due date

to calculate makespan, we...

subtract the starting time of the first job from the completion time of the last job in the group

According to OPT, companies should schedule bottleneck resources to full capacity and schedule non bottlenecks to ...

support the bottleneck resources

slack

the amount of time a job can be delayed and still be finished by its due date -you can start job immediately but you do not have to do so; you can start it any time up to the start time in your backward schedule and still meet the due date

makespan

the amount of time intakes to finish a batch of jobs -measures efficiency; mean job lateness and mean job tardiness measure due date performance

finite loading develops the operational schedule that uses ...

the available capacity

earliest due date (EDD)

the job due the earliest has the highest priority

shortest processing time (SPT)

the job that requires the least processing time has the highest priority

longest processing time (LPT)

the job that requires the longest processing time has the highest priority

slack per remaining operations (S/RO)

the job with the least slack per remaining operations is given the highest priority -calculate by diving slack by remaining operations

critical ratio (CR)

the job with the smallest ratio of time remaining until due date to its processing time remaining has the highest priority

last come, first served (LCFS)

the last job into the work center or at the top of the stack is processed first

if quick customer responses is critical to your company,

the number of jobs waiting in the system should be relatively low

planned inputs are based on...

the operational schedule

throughput

the quantity of finished goods that can be sold

process batch

the quantity produced at a resource before the resource is switched over to produce another product

job lateness is the difference between...

the time a job is finished and the time it is supposed to be finished (its due date)

priority rules assume that...

there is no variability in either the setup line or the run time of the job

OPT principles: an hour lost at a non bottleneck resource is just a mirage

time lost at a non bottleneck resource is critical only if the lost time causes the resource to become a bottleneck

OPT principles: balance the process rather than the flow

traditionally, managers try to make the same amount of capacity available in each departments or work center; this means every resource is a bottleneck

OPT principles: the transfer batch does not have to equal the process batch

transfer batch = the quantity of items moved at the same time from one resource to the next

infinite loading identifies ...

uneven workloads and bottlenecks

low volume operations (job shop)

used for high quality, customized products such as customer stereo systems or custom automobile paint jobs, or for services such as personal fitness with higher profit margins

actual output

used to identify possible problems in the work center, such as equipment problem or unexpected absences

queue

waiting line

job flow time includes ...

waiting time, setup time, processing time, and possible delays

OPT principles: the process batch should be variable

we do not always have to produce the same quantity, but instead we should produce what is needed

if you do not know what your workload is...

you cannot tell a customer how long it will take to do a job