Assembly Line Balancing with Processing Alternatives: An Application

The conventional approach to the assembly line balancing problem assumes that the manufacturing methods to be used have been predetermined. However, in practice the design engineer has several alternatives available in the choice of processing, typically involving a trade-off between labor or capital intensive options. The choice of manufacturing method is frequently viewed as an investment or capital budgeting decision, contrasting projected savings in labor cost with the additional fixed cost for the more capital intensive alternatives. The manufacturing tasks (based on the selected processing alternatives) are then assigned to work stations so as to minimize the number of work stations (i.e., labor costs) necessary to achieve a desired production rate. This paper describes a method of simultaneously considering both the choice of manufacturing alternatives and the assignment of tasks to stations so as to minimize total costs (labor and fixed) over the expected life of the production line. The importance of considering these decisions jointly results from the fact that the benefits obtained from specific manufacturing alternatives should not be limited to anticipated labor savings alone. The true measure of achievable labor savings can only be determined after an assignment of tasks to stations has been chosen. For instance, although a processing alternative may reduce the total work content of a set of tasks, if the resulting line balancing assignment does not reduce the number of stations required to achieve a desired production rate, the assumed savings will not be achievable and will serve only to increase the idle time of the line. On the other hand, an apparently trivial reduction in the time to complete a certain task may lead to a more efficiently balanced line, producing a much greater real savings in labor cost than had been anticipated, due to a reduction in both work content and idle time for the line. The combined processing alternative line balancing problem can be formulated as an integer programming problem. Two alternate formulations are provided which differ in the degree of flexibility in selecting a cycle time. A branch and bound procedure is described for the fixed cycle time situation which takes advantage of the special structure of the problem to provide an efficient method capable of solving problems of practical interest. The effectiveness of the proposed procedure is demonstrated by application to an actual redesign of an assembly line for a major auto-industry supplier.