Sequencing and Rules of sequencing

The sequencing aims to find the order in which different jobs are to be processed on different machines, so that ideal time is minimized and utilization is optimized. It is essential for a smooth flow of material and effective utilization of manpower and machine.

Rules of sequencing:

  • If nothing is mentioned the processing order for machine remain constant
  • One job on one machine at a time
  • Once a job is initiated, it must be fully completed
  • Time taken by the job from one machine to another is negligible
  • Irrespective of order the processing time for the job remains constant

Terms used in sequencing:

  • Job flow time: Job flow time is the time from some starting point until that particular job is completed
  • Makespan time: It is the time from when processing begins on the first job on the set, until the last job is completed.
  • Tardiness: Tardiness is also known as lateness. It is the amount of the time by which a job is delayed beyond its due date.
  • Average no. of jobs in system: It is the term used to represent on an average no of jobs present all the time within the system, until one set of job is completed. It is the ratio of total job flow time and the over make span time

Sequencing rule for N jobs one machine:

  • Shortest Processing Time rule:

In this job rules, jobs are in sequence to increase their processing time.

  • Earliest Due Date Rules:

In this rules jobs are sequenced in increasing order, of their due date.

  • Critical ratio:

Critical ratio is defined as the ration between the due date and the processing time.

  • Slack-Time remaining:

Slack time is known as the difference between the due date and the processing time

N jobs on 2 machines:

If the process jobs are sequenced by using Johnson’s rules or Johnson algorithms, the steps involved in the process are.

  • Find out the minimum of A_i \:\: and \:\:B_i
  • If the minimum is for a particular job on machine A then perform that job at the start
  • If the minimum is for a particular job on machine B then perform the job in the final stage.
  • Strike of the jobs which is assigned. So that it cannot be taken into consideration again
  • Continue in the similar manner until all the jobs are assigned.

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