PRODUCTION SCHEDULING

P.VIDYUT CHANDRAProfessor

Mechanical Engineering Dept

Scheduling key issues

The two key problems in production scheduling are, according to Wight(1984), "priorities" and "capacity.“

In other words, "What should be done first?" and "Who should do it?"

INTRODUCTION

• Production scheduling activities are common but complex.

• Each perspective has a particular scope and its own set of assumptions.

• Different perspectives lead naturally to different approaches to improving production scheduling.

Decision Prospectives

The following paragraphs will briefly cover three important perspectives:

• problem-solving,• decision-making, and • organizational.

Decision-making is a critical function for any organization, especially manufacturing firms that operate in a hostile, dynamic, and complex environment

Key Decisions

The following are among the key decisions in a production scheduling system:

• releasing jobs for production,• assigning resources (people, equipment, or production

lines) to tasks,• reassigning resources from one task to another,• prioritizing tasks that require the same resources,• sequencing production tasks,• determining when tasks should begin and end, and• interrupting tasks that should be halted

Schedule Real Time

Many manufacturing facilities generate and update production schedules, which are plans that state when certain controllable activities (e.g., processing of jobs by resources) should take place.

When under implementation stageOne also react quickly to unexpected events and

revise schedules in a cost-effective manner.

Example

Rescheduling is then practically mandatory in order to minimize the effect of such disturbances in the performance of the system. There are many types of disturbances that can upset the plan, including machine failures, processing time delays, rush orders, quality problems, and unavailable material.

Rescheduling is the process of updating an existing production schedulein response to disruptions or other changes. The following is a partial list ofpossible disruptions:• Arrival of a new, urgent job,• Cancellation of a job, change to a job's due date, or other changein job specification,• Machine failure, repair, or other change in status,• Delay in the arrival of required material or other problem withmaterial delivery,• Poor quality parts that require rework or making new parts,• Incorrect predictions of setup time, processing time, or otheractions, and• Absentee workers or changes to worker assignments.Possible responses include changing when activities will occur, using asubstitute machine or person, authorizing overtime, subcontracting work toanother firm, and changing the manufacturing process.

Scheduling

• Scheduling is the allocation of starts and finish time to each particular order. Therefore scheduling

• can bring productivity in shop floor by providing a calendar for processing a set of jobs.

• The single machine-scheduling problem consists of n jobs with the same single operation on each of the jobs, while the flow shop-scheduling problem consists of n jobs with m operations on each of the jobs. In this problem, all the jobs will have the same process sequences. The job shop scheduling problem contains n jobs with m operations on each of the jobs; but, in this case, the process sequences of the jobs will be different from each other.

Single machine scheduling problem

The basic single machine scheduling problem is characterized by the following conditions:

• 1. A set of independent, single-operation jobs is available for processing at time zero.

• 2. Set-up time of each job is independent of its position in jobs sequence. So, the set-up time of

• each job can be included in its processing time.• 3. Job descriptors are known in advance.• 4. One machine is continuously available and is never kept

idle when work is waiting.• 5. Each job is processed till its completion without break.

In flow shop scheduling problem,

There are n jobs; each require processing on m different machines.

The order in which the machines are required to process a job is called process sequence of that job.

The process sequences of all the jobs are the same. But the processing times for various jobs on a

machine may differ. If an operation is absent in a job, then the processing time of the operation of

that job is assumed as zero.

CHARATERISTIC

The flow-shop scheduling problem can be characterized as given below:

• A set of multiple-operation jobs is available for processing at time zero (Each job requires m operations and each operation requires a different machine).

• Set-up times for the operations are sequence independent, and are included in processing times.

• Job descriptors are known in advance.• m different machines are continuously available.• Each individual operation of jobs is processed till its

completion without break.

Major Difference

• The main difference of the flow shop scheduling from the basic single machine scheduling is that the inserted idle time may be advantageous in flow shop scheduling. Though the current machine is free, if the job from the previous machine is not released to the current machine, we cannot start processing on that job. So, the current machine has to be idle for sometime. Hence, inserted idle time on some machines would lead to optimality.