Circuits Assembly - June 2008 - (Page 26)

Cover Story Improving Production Line Performance By Dr. Sabry Shaaban and Dr. Sarah Hudson Simulations make the case for uneven buffering, placed appropriately between workstations. esigning production lines with no form of mechanical pacing is not an easy affair. For instance, where to place operators who work at different rates, and where to keep unfinished items along the production line, are just some of the problems facing the line manager. One important decision is determining the size of the storage space in between workstations where partly finished products are kept, awaiting the next step of the process. This storage space, or buffer, has been the subject of numerous studies, such as buffer capacity allocation and placement. The underlying problem with buffer capacity decisions arises from two scenarios: In one, an operator temporarily works faster than their predecessor, so buffer stocks dwindle. In this case, the succeeding station will suffer from “starving” delays; the other possibility is if an operator temporarily works faster than their successor. In this case, the buffer soon fills up completely, and the preceding station will suffer from “blocking” delays. Figure 1 shows a five-station serial line with four buffers, where squares depict stations and diamonds represent buffers. So why not just put in place buffers with equal capacity between each workstation D (i.e., a balanced-buffer line)? There are often technical considerations to overcome. In some cases, the total buffer capacity needed for efficient working of the line has to be spaced unevenly. This is called an “unbalanced-buffer” line; i.e., the buffers are not all the same size. And based on our research, it may be beneficial to deliberately unbalance the buffers. It often has been assumed a line with equal buffer sizes gives the best performance, and anything else is going to lead to losses in efficiency. The work we have done shows this is not necessarily so; buffers of unequal size placed appropriately between workstations can sometimes even lead to increases in effectiveness. One way to measure line efficiency is to calculate the average buffer level (ABL) for the whole line; evidently, the ideal is as few work-in-process pieces in storage as possible, so ABL needs to be kept low. Another way is to calculate the time the line is inactive (idle time, or IT) as a percentage of total working time. This, too, needs to be kept as low as possible, to reduce labor costs. Buffer Placement Decisions We ran computer simulations on lines with five and eight workstations, with total buffer capacities (TB) of eight and 24 units for the shorter line, and 14 and 42 units for the longer (eight-station) line, giving average buffer capacities of two and six units, respectively, for both line lengths. The buffer capacities were then assigned unevenly along the lines. Figure 2 illustrates the general idea for some of the simpler patterns, the fivestation line with a total buffer capacity of eight units. circuitsassembly.com "! ! #! ! $! ! %! ! %! Figure 1. An unpaced production line. 26 Circuits Assembly JUNE 2008 http://circuitsassembly.com

Table of Contents Feed for the Digital Edition of Circuits Assembly - June 2008

Circuits Assembly - June 2008 Contents Letters Caveat Lector Industry News Market Watch Talking Heads Screen Printing Better Manufacturing China Goes ‘Upmarket’ Out of the Garage Improving Production Line Performance Manufacturing Steps Onto the Enterprise IT Stage Measuring and Controlling Wave Height and Dwell Time Wave Soldering Tech Tips Test and Inspection Process Doctor Pb-Free Lessons Learned Getting Lean Eastern Advances Semicon West Product Preview Product Spotlight Ad Index Assembly Insider Technical Abstracts

Circuits Assembly - June 2008

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