Sequencing & Scheduling Problems

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Job Shop Scheduling

K. Aswathappa, K. Shridhara Bhat

8/14/2013 1A.J. INSTITUTE OF MANAGEMENT


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Scheduling:

Scheduling pertains to establishing the time of the use of specific

resources within an organization. Scheduling means organizing a

production line to produce products in time efficiently with leastuse of time & maximum utilisation of resources. It relates to the

use of equipment, machines, facilities & human activities

(especially men & machines ). Scheduling involves developing

& assigning specific dates for the start & completion of the

necessary tasks or operation in production shop floor.



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Objectives of Scheduling:

- To increase the efficiency of production or productivity;

- To fix up delivery dates for various manufacturing activities &

for the finished products;

- To reduce idle time of labour & machines, which might be

caused due to waiting for materials, waiting for movement,

waiting for inspection etc;

- To utilize machines & labour in such a way that the output is

produced within the established lead time so as to deliver the

products/service in time & complete production in the shortestcycle time possible at minimum total cost of production.



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Problems in Scheduling:

- Lack of correct & up to date information concerning lead time,

operation or processing time, lot size & prevailing load on

machines & work centres;

- Resource constraints capacity shortfall & delay in supply ofmaterials;

- Absenteeism, lack of skill & experience in labour & inefficiency

of labour;

- Type of production i.e. Job Production, Batch Production, MassProduction & Continuous Production;

- Problems in machine loading & line balancing.



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Scheduling Techniques in Job Shops:

- Forward Scheduling:In this approach, each task is scheduled to occur at the earliest

time that the necessary material will be on hand & capacity will

be available. It assumes that procurement of material &

operations start as soon as the customers requirements are known.

The customers place their orders on a needed-as-soon possible

basis. The earliest completion date, assuming that everything

goes as planned, could be quoted to the potential customer. Some

buffer time may be added to determine a date that is more likely

to be achievable, if it is acceptable to the customer.( Steel mills & machine tool manufactures where jobs are

manufactured to customer orders & delivery is requested to as

early as possible basis).



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- Backward Scheduling:

The scheduling technique is often used in assembly-type

industries that commit in advance to specific delivery dates.

After determining required schedule dates for major sub-

assemblies, the schedule uses these required dates for each

component & works backward to determine the proper release

date for each component manufacturing order. The jobs start

date is determined by setting back from the finish date the

processing time for the job. Backward scheduling is also knownas reverse scheduling.



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Sequencing:

Sequencing is the process of determining the sequence of

processing of all jobs at each work centre or machine. It

establishes the priorities for processing the jobs which are waiting

in the queue at each work centre or machine. When the machine

or work centre becomes free, the job with the highest priority isassigned.

Some of the criteria used are:

- Work-in-progress inventory cost;

- Idle time;

- Average flow time;

- Average time to complete a job etc.



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Principal Assumptions:

- No machine can process more than one operation at a time;

- Each operation once started must be performed till completion;- Time intervals for processing are independent of the order in

which operations are performed;

- A job is processed as soon as possible subject to the ordering

requirement.

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Assignment:

The objective of assignment problem is to assign a number ofjobs to the equal number of destinations (persons) at a minimum

cost or maximum profit.

Steps:- Prepare a cost matrix. If the cost matrix is not a square matrix

then add a dummy row (column) with zero cost element.

- Subtract the minimum element in each row from all the

elements of the respective rows.

- Modify the resulting matrix by subtracting the minimumelement of each column from all the elements of the columns.

- Draw minimum number of horizontal & vertical lines to cover

all zeros in the resulting matrix.



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* Four different jobs can be done on four different machines &

take down time costs are prohibitively high for change over's.The matrix below gives the cost in rupees of producing job I on

machine J.



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Sequencing & Scheduling Problems