Lead-time reduction through flexible routing: Application to shape deposition manufacturing

Manufacturing process plans are usually defined by the sequence of operations a job has to go through to transform raw materials to a finished product. Restricting process plans to be a sequence of operations often overconstraints the process plan beyond what processing technology would require. Relaxing the strict ordering in which operations are prescribed in conventional process plans can result in substantial reductions of lead-time in manufacturing. Machine scheduling can take advantage of this fact by optimizing simultaneously the job schedule and order of operations in each job. This approach has been proposed by other researchers. However, static optimization has well-known problems dealing with process variability, schedule ‘nervousness’ when the program conditions are perturbed (e.g. addition of a new job, machine downtime) and the effect of a planning horizon. This paper presents results for Shape Deposition Manufacturing that show that reductions of queuing time of up to 80% can be achieved by delaying ordering decisions until the job is already in the shop even in the absence of global optimization.