The Russell Auto Repair Shop uses a shop floor control system based on forward s

Question

The Russell Auto Repair Shop uses a shop floor control system based on forward scheduling to determine auto completion dates for their customers, and to determine when to release a car to the shop for repairing. The control system generates a Gantt chart on a daily basis, along with a number of statistics. The shop is open for business eight hours per day, and there are three repair centers in the shop. Activity for the most recent few days is shown below. The jobs are listed in order of customer arrival, and are scheduled for processing on a first-come-first-served basis. Determine what the promise times should be, along with the makespan, the average flowtime, and the average queue time.

Job Workcenter Est. Hours

1 A/1, B/3, C/2

2 A/2, C/3

3 B/1, A/2, C/1

4 A/3, C/2, B/2

5 C/1, B/4, A/2

Explanation / Answer

There are five jobs and three centers in the shop. The jobs are listed in order of customer arrival and are scheduled for processing on a first-come-first-served basis. The given data is as follows:

From the above table (scheduling on the basis of first-come-first-served basis) only job1 is completed on the first day within 8 hours whereas the job5 will be completed on the third day. Accordingly Promise times, makespan, average flowtime and average queue times can be calculated.

Makespan time = Time of completion of last job - Starting time of first job

Average flowtime = Sum of flowtimes of all jobs / Number of jobs

Average queue time = Sum of queue times / number of jobs

In case the workstations take up the jobs as they form the queue, the revised schedule may be as follows:

Now the makespan time is 13 hours as compared to earlier 20 hours.

Job1 and job2 may be given the promise times of 8 hours that is delivery at the end of same day. Job3 next day morning and job4 and job5 after 13 and 10 hours (Job5 can be delivered before job4)

Flowtime for Job1 is 8 hours with waiting of 2 hours

Flowtime for Job2 is 6 hours with waiting time of 1 hour

Flowtime for Job3 is 9 hours with waiting time of 5 hours

Flowtime for Job4 is 13 hours with waiting time of 6 hours

Flowtime of Job5 is 10 hours with waiting time of 3 hours

Average flowtime = (8+6+9+13+10)/5

Average queue time = (2+1+5+6+3)/5

Estimated Hours Job/   Workcenter A B C 1 1 3 2 2 2 -- 3 3 2 1 1 4 3 2 2 5 2 4 1 Total 10 7 9

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