Design of a Naval Tender Job Shop425The project described in this chapter deals with the determination of the optimalman/machine configuration of a naval tender job shop. The approach illustrateshow to complement the strengths of two important modeling techniques: mathema-tical programming and simulation. The problem can be characterized as one ofcapacity planning, where a great deal of uncertainty exists as to the demands on thesystem.The approach taken is hierarchical in the sense that an aggregate planningmodel first suggests a man/machine configuration for the job shop and then a detailedmodel evaluates the performance of this configuration in a simulated environment.The aggregate model extends over a six -month planning horizon and has a mixed-integer programming structure. Once a proposed configuration for the job shop isgenerated by the aggregate planning model, the detailed model addresses the un-certainties and the precedence relations that affect the job-shop environment on anhour-by-hour basis. If the detailed evaluation of the configuration is unacceptable,constraints are modified in the aggregate model and the procedure is repeated.This hierarchical approach, which combines optimization and simulation,provides a viable way of eliminating the weaknesses inherent in the two modelingapproaches. The mixed-integer programming model cannot incorporate the detailedprecedence relationships among jobs or include uncertainties explicitly, withoutbecoming so large that it would be impossible to solve. On the other hand, thesimulation model does not generate alternative man/machine configurations, butmerely evaluates those presented to it. By using the two approaches jointly , it ispossible both to generate a reasonable set of alternative configurations and toevaluate each one against a set of scheduling environments.The hierarchical approach also facilitates the decision-maker's interaction withthe models, and allows for comprehensive testing of a wide variety of options thatcan result in robust and efficient solutions.10.1426Design of a Naval Tender Job Shop10.1 THE PROBLEM DESCRIPTIONIn order to support its fleet of ships, the U.S. Navy maintains a number of special-purpose ships, called naval tenders, which are dedicated to performing maintenancefunctions for the fleet. The purpose of this project is to develop an analytic approachfor determining the machine configuration and manpower allocation for a naval-tender machine shop. Although this objective might be regarded as quite specific,the naval-tender machine shop can be considered a typical example of an intermit-tent-production, open job shop wherein general-purpose equipment and trainedmechanics are held ready to meet a widely fluctuating demand for repair and manu-facturing work. In this specific case, the work is generated by the fleet of ships forwhich the tender is responsible. The suggested design approach can be extendedeasily to other job-shop configurations.The principal functions of the naval-tender machine shop are to repair pumps,valves, and similar mechanical equipment; to manufacture machinery replacementitems; to perform grinding and engraving work; and to assist other tender shops.The typical modern naval-tender machine shop contains milling machines, drillpresses, grinders, engine lathes, a furnace, a dip tank, bandsaws, shapers, turretlathes, boring mills, a disintegrator, an arbor press, and various other equipment.The shop normally is supervised by three chief petty officers, and the operationspersonnel include several petty officers (first-, second-, and third-class) as well as alarge number of "non-rated" machinery repairmen.In order to reduce the scope of the study to a more manageable size, we haveexcluded from our analysis the engraving and grinding sections, since there is virtuallyno cross-training between these sections and the remaining part of the tender, andtheir use does not overlap with the remaining activities of the naval tender. It wouldbe straightforward to extend our suggested approach to cover grinding and engravingoperations if proper data were available.The Use of Numerically-Controlled MachinesA primary concern of our study is to examine the applicability of numerically-controlled machine technology to naval tenders. Numerical control provides forthe automatic operation of machinery, using as input discrete numerical data andinstructions stored on an appropriate medium such as punched or magnetized tape.The motions and operations of numerically-controlled machine tools are controlledprimarily , not by an operator, but by an electronic director, which interprets codedinstructions and directs a corresponding series of motions on the machine. Numeri-cally-controlled machine tools combine the operations of several conventionalmachines, such as those used in milling, drilling , boring, and cutting operations.To evaluate the decision-making problem properly, it is important to examinesome of the advantages that numerically-controlled machines offer over conventionalmachine tools.First, the combination of many machining activities into one machine maydecrease setup losses, transportation times between machine groups, and waitingtimes in queues. Jobs then tend to spend less time in the shop, and so generally therewill be less work-in-process and less need for finished-good inventories.Second, the programmed instructions provided to the numerically-controlledmachines can be transmitted, by conventional data lines or via a satellite system,to tenders in any of the seas and oceans. This creates an opportunity to developcentralized design, engineering, parts-programming, and quality-control organi-zation, which can offer many economical, tactical, and manufacturing advantages.Third, because numerically-controlled machines can be programmed to performrepetitive tasks very effectively, the jobs that they complete may require less reworkand can be expected to result in less scrap. Also, superior quality control can begained without relying on an operator to obtain close tolerances, and significantsa vings in inspection time can be realized.Fourth, numerical control can have major impacts on tooling considerations.Tool wear can be accounted for, at given speed and feed rates, by automaticallymodifying the tooling operation to compensate for the changes in tool shape. Withthis compensation, numerous