ch15_ISM[1] - CHAPTER Short Term Scheduling 15 1. 2. 3. 4....

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247 15 CHAPTER Short Term Scheduling D ISCUSSION Q UESTIONS 1. Scheduling’s objective is to optimize the use of resources so that production objectives are met. 2. Four criteria for scheduling are: Minimizing completion time, maximizing utilization, mini- mizing work-in-process in inventory, and minimizing customer waiting time. There is a one-to-one correspondence between minimizing completion time and minimizing flowtime. 3. Loading is the assignment of jobs to work processing centers. Work centers can be loaded by capacity or by assigning specific jobs to specific work centers. Gantt charts and assignment method are loading techniques. 4. Five priority sequencing rules are: ¡# First come, first served (FCFS); or First in, first out (FIFO): Jobs are sequenced in the order in which they arrive at the workstation. ¡# Earliest due date (EDD): Jobs are sequenced in the order in which they are due for delivery to the customer. ¡# Shortest processing time (SPT): Jobs are sequenced in order of the processing time required at the workstation, with the job requiring the least processing time at the workstation scheduled first. ¡# Longest processing time (LPT): Jobs are sequenced in order of the processing time required at the workstation, with the job requiring the longest processing time at the workstation scheduled first. ¡# Critical ratio (CR): Jobs are sequenced in order of increasing critical ratio (the ratio of time required by work left to be done to time left to do the work ). 5. SPT minimizes the average flow time, average lateness, and average number of jobs in the system. It maximizes the number of jobs completed at any point. The disadvantage is that long jobs are pushed back in the schedule. 6. A due date may range from a (meaningless) promise to a contractual obligation. It is a target with or without penalties. 7. Flow time is the length of time a job is in the system; lateness is completion time minus due date. 8. Most students will go for EDD, to gain minimum lateness. Others will go for SPT, on the grounds that the team can’t afford to tackle a job with an early due date and a long processing time. Interesting to see student assumption about sequence, damage, etc. 9. Johnson’s rule is used to sequence several jobs through two or three work centers. 10. Four effectiveness measures for dispatching rules: Average completion time, average number of jobs in the sys- tem, average job lateness, and utilization. 11. The assignment method involves adding and subtracting appropriate numbers in the problem’s table in order to find the lowest opportunity cost for each assignment. The four steps are detailed in the chapter. 12. Theory of constraints steps: 1. identify the constraints. 2. develop a plan for overcoming the identified constraints.
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ch15_ISM[1] - CHAPTER Short Term Scheduling 15 1. 2. 3. 4....

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