20.0 production scheduling

8/6/2019 20.0 Production Scheduling.

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Production Scheduling.


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Scheduling can be defined as prescribing of when and

where each operation necessary to manufacture theproduct is to be performed.

It is also defined as establishing of times at which to

begin and complete each event or operationcomprising a procedure.

The principle aim of scheduling is to plan the sequence

of work so that production can be systematically

arranged towards the end of completion of all products

by due date.


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Production Scheduling and the Hierarchy of Production Decisions.

First, the organization must forecast demand for aggregate

sales over some predetermined planning horizon.

These forecast provides the input for determining the

aggregate production and workforce level for the planning

horizon. The aggregate production plan must then be translated

into master production schedule (MPS).

The MPS results in specific production goals by product and

time period.

Material requirements planning (MRP) is one method for

meeting specific production goals of finished-goods

inventory generated by the MPS.

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Production Scheduling

The MRP system explodes the production levels

one obtains from the MPS analysis back in time toobtain production targets at each level of assembly

by time period.

The result of the MRP analysis is specific planned

order releases for final products, subassemblies andcomponents.

Finally, the planned order releases must be

translated into set of tasks and the due dates

associated with those tasks.

This level of detailed planning results in the shop

floor schedule


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Important Characteristics of Job Shop Scheduling Problems.

1. The job arrival pattern:

It is often viewed that job shop problems are static and

proceeded with solutions on the current state but most practical

shop scheduling problems are dynamic in nature.

2. Number and variety of machines in the shops:

A particular job shop may have unique feature that could make

implementing a solution obtained from scheduling algorithm

difficult.

3. Number of workers in the shop:

Both the number of workers and the variety of machines inthe shop determine shop capacity.

A breakdown of a single machine or loss of critical employee

could result in a bottleneck and reduction in the shop capacity.


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4. Particular flow patterns:

Because material handling issues are often treated separately

from scheduling issues, infeasible flow pattern may result.

5. Evaluation of alternative rules:

The choice of objective will determine the suitability and

effectiveness of a sequencing rule .

It is common for more than one objective to be important, so that

it may be impossible to determine a unique optimal rule.


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Objectives of Job shop Management.

One of the difficulties of scheduling is that many, often conflicting,objectives are present. Some of the most common objectives are

1. Meet due dates.

2. Minimum work-in-process(WIP) inventory.

3. Minimize the average flow time through the system.

4. Provide for high machine/worker time utilization. (Minimum

machine/worker idle time).

5. Provide for accurate job status information.6. Reduce setup times.

7. Minimize production and worker costs.


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Principles of Scheduling.

1. The principle of optimum task size:Scheduling tends to achieve maximum efficiency when

the task sizes are small, and all tasks of same order of

magnitude.

2. Principle of optimum production plan:

The planning should be such that it imposes an equal

load on all plants.

3. Principle of optimum sequence:

Scheduling tends to achieve the maximum efficiency

when the work is planned so that work hours are

normally used in the same sequence.


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Performance Measures.

The following performance measures are commonly

used for scheduling. Job Flow Time: The amount of time a job spends in the

service or manufacturing system. Minimizing job flow

times supports the competitive priorities of cost (lowerinventory) and time (delivery speed).

Job flow time = Time of completion - Time job was

available for first processing operation.

Job flow time is sometimes referred to as throughout

time or time spent in the system, including service.


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Tardiness and lateness: Tardiness is the positive

difference between the completion time (flow time)

and the due date of the job.

A tardy job is the one that is completed after the

due date.

Lateness refers to the difference between the jobcompletion time and its due date, and differs from

tardiness in that lateness can be either positive or

negative

Minimizing the tardiness supports the competitivepriorities of cost (penalties for missing due dates),

quality (perceptions of poor service) and time (on-

time delivery).


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Work-in-process Inventory:

Any job that is waiting in line, from one operation to

the next, being delayed for some reason, being

processed , or residing in the semi finished state.

Minimizing the WIP Inventory supports the

competitive priority of cost (inventory holding costs).


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Total Inventory: This performance measure is used to

measure how effective schedules for manufacturingprocess are.

The sum of scheduled receipt and on-hand inventories

is the total inventory.

Total inventory = Scheduled receipts for all items + On-

hand inventories of all items.

Minimizing total inventory supports the competitive

priority of cost(inventory holding costs). Essentially, total inventory is the sum of WIP and

finished goods inventories.


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Utilization:

The percentage of work time thatis productively spent by an employee or a

machine is called Utilization.

Maximizing the utilization of a process

supports the competitive priority of cost(slack

capacity).


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Inputs to Scheduling.

1. Performance standards: The information regarding theperformance standards (standard times for operations)

helps to know the capacity in order to assign required

machine hours to the facility.

2. Units in which loading and scheduling is to be expressed.

3. Effective capacity of the work center.

4. Demand pattern and extent of flexibility to be provided

for rush orders.5. Overlapping of operations.

6. Individual job schedules.


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Scheduling Strategies.

Scheduling strategies vary widely among firms andrange from no scheduling to very sophisticated

approaches.

These strategies are grouped into four classes:

1. Detailed scheduling:

Detailed scheduling for specific jobs that are

arrived from customers is impracticable in actual

manufacturing situation. Changes in orders, equipment breakdown, and

unforeseen events deviate the plans.


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2. Cumulative scheduling:

Cumulative scheduling of total work load is usefulespecially for long range planning of capacity needs.

This may load the current period excessively and

under load future periods.

It has some means to control the jobs.

3. Cumulative detailed:

Cumulative detailed combination is both feasible and

practical approach if master schedule has fixed and

flexible portions.


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4. Priority decision rules:

Priority decision rules are scheduling guides

that are used independently and in conjunction

with one of the above strategies, i.e., first come

first serve.

These are useful in reducing Work-In-Process

(WIP) inventory.


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Priority sequencing rules:

The rules that specify the job processing sequence when several jobs

are waiting in line at a workstation.

First-Come ,First-Served : The job arriving at the workstation first

has the highest priority under a first-come ,first-served (FCFS)

rule.

Earliest Due Date: The job with the earliest due date (EDD) is thenext job to be processed.

Shortest processing Time: The job requiring the shortest

processing time (SPT) at the workstation is processed next.

Critical Ratio: The critical ratio (CR) is calculated by dividing thetime remaining until a jobs due date by the total shop time

remaining for the job, which is defined as the setup, processing,

move and expected waiting times of all remaining operations,

including the operations being scheduled.


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The formula for Critical ratio is

CR= (Due date - Todays date) / (Total shop time remaining)

The difference between the due date and todays date must be

in the same units as the total shop time remaining.

A ratio less than 1.0 implies that the job is behind schedule.

A ratio greater than 1.0 implies that the job is ahead of

schedule.

The job with the lowest CR is scheduled next.


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Numerical Problem:


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Scheduling Models


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Two-Machine Flow Shop

Johnsons Rule Example

A 5 6

B 16 5

C 8 2D 9 17

E 4 6

Machine Machine

Job Center 1 Center 2

The minimum processing time, 2, is given by Job C on

Machine 2. So, Schedule Job C in the end.


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A 5 6

B 16 5

D 9 17

E 4 6

Machine Machine




C

After removing job C, the minimum processing time, 4, is

given by Job E on Machine 1. So, Schedule Job E in the

beginning.


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A 5 6

B 16 5

D 9 17

Machine Machine




CE

Job E is removed. Now, there is a tie. Minimum processing

time is given by Jobs A and B. Break ties arbitrarily.

Schedule one of Job A or Job B.


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B 16 5

D 9 17

Machine Machine




CAE

Job A is chosen arbitrarily. Job A is scheduled in the

beginning, because its minimum time is given on Machine 1.

Beginning means position 2 because position 1 is taken.


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D 9 17

Machine Machine




CBAE

Next, Job B is scheduled. Its scheduled in the end, because

its minimum processing time is given on Machine 2.


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Machine Machine




CBDAE

The sequencing is complete after assigning the remaining

Job D to the remaining position. Next, the makespan is

computed.


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E

A

DB

C

CBDAE

Each triplet above shows the starting, processing, and finishing times of an operation.

Johnsons rule guarantees that the above schedule gives the best value (44) of

makespan.



Machine Machine



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Johnsons Rule

example:2

JOBJOB PROCESS 1PROCESS 1 PROCESS 2PROCESS 2

AA 66 88BB 1111 66

CC 77 33

DD 99 77

EE 55 1010

CE A BD


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d h d l


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P d ti S h d li


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Waiting Line Model.

When job arrivals/order arrivals is random in nature and

we have limited facilities to process jobs or orders, the

jobs or orders pile up in the form of a queue or a waiting

line .

This is particularly true in service organizations or in jobproduction shops.

The production in charge has to use waiting line model

to reduce the waiting time of jobs and to see that

machines are optimally loaded.

P d i S h d li


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Thank You

20.0 production scheduling

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