7.1: Introduction

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A workstation performs one, or one set of, operations on an item. Multiple workstations are required to perform all operations necessary to produce a finished product or perform a required service. Suppose one, or at most a few, types of items need to be processed and that this can be accomplished by processing all items in the same sequence. In this case, a set of single workstations organized into a serial line is appropriate, possibly with a material handling device moving items between the workstations.

In Figure 7-1, a material handling device such as a conveyor lies between seven workstations that comprise a serial line. Items enter at the far left and depart at the far right. Each workstation in left to right sequence performs particular operations on the item. A finished product leaves the system after completing the operation at the right most workstation.

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An assembly line is one kind of serial system that typically employ human workers. A transfer line is another type of serial system whose workstations consist of automatic machines with a common control system.

In a lean system, the work to be done in processing an item would be balanced between the workstations. The the average operating time at each workstation should be as close to the same as possible. However, it is possible that these average times vary or due to random variation the processing time for an individual part varies between the workstations and the operation time for different parts vary at any particular workstation.

Thus when a workstation completes its operation on an item, the following workstation may still be processing another item. By placing the completed item in a buffer between the stations, the preceding workstation may begin processing another item. If the buffer is full, the preceding station has no where to place the completed item and cannot begin working on another. In this case, the workstation is in the BLOCKED state. Time in the BLOCKED state is unproductive and may increase lead time. Preventing blocking requires buffer space that may be scarce (or a reduction in variability). Thus, minimizing lead time and minimizing buffer between workstations space trade off against each other.