[Psychology] Mechanical Assemblies Phần 9 pptx

444 16 ASSEMBLY SYSTEM DESIGN pulling work from upstream stations based on customer orders rather than pushing work downstream based on a pre- planned schedule. The aim is to produce what is wanted, when it is wanted, and where it is wanted. To accomplish this, the system is run by passing orders upstream in the form of "kanbans." Kanban is the Japanese word for ticket, but the kanbans act like money in the sense that they are used by downstream stations to buy parts from upstream stations. For this reason, if the orders are entered at the very end of the line, a signal rep- resenting what was just made will propagate upstream, causing the same things to be made over and over. In order to guarantee that the actual mix of incoming orders is reflected upstream, and to combat the variations caused by model mix, Toyota employs production smoothing or load leveling, which are discussed next. Furthermore, as discussed in Section 16.1.3, the order stream may be inserted in the middle of the line instead of the very end. 16.I.2.C. Production Smoothing or Load Leveling Orders from customers do not arrive in the best sequence for production. Suppose the plant makes car A and car B, among others. Assume car A takes much less than the average time to make, while car B takes much longer. If 10 orders each for car A and car B arrive, it may disrupt the line to schedule them each in a solid batch. If the factory operates at a standard pace, operators working on a solid batch of 10 A's will have time left over and nothing to do. On the other hand, operators working on a solid batch of 10 B's will fall behind. It is better to interleave these orders as ABAB so that over these 20 cars the operators will take about the average time. Another kind of smoothing is also pursued. Suppose the plant receives orders for sedans, hardtops, and wagons in the following proportions: 50% sedans, 25% wagons, and 25% hardtops. If these different cars use some different parts, then demand for the parts will vary. As in other respects, a goal of TPS is to reduce variation and thus reduce the need for buffer stocks that absorb that variation. On this basis, one should not make all the day's sedans first, then all the wagons, and then all the hardtops. Instead, one should interleave them in a pattern like SSWHSSWHSSWH ([Monden], pp. 68-69). Naturally, these two formulae for sequencing the cars cannot both be obeyed, although one can approach both goals. Toyota actually favors the second kind of smoothing and gives it priority when solving its sequencing problem each day ([Monden], p. 254). If time for W is longer than for S and H is shorter, one might then make the above cars in the sequence SHSWSHSWSHSW if that smoothed the different station times better. 16.l.2.d. Short Setup Times Since the TPS involves mixing the different orders rather thoroughly in order to keep variation in demand down, some upstream processes, particularly machining and stamping operations, have to change over frequently. This will never be economical unless changeovers can be done quickly. This is a topic of its own, exemplified by the single minute exchange of dies process (SMED) ([Shingo]). 16.l.2.e. Single Piece Flow In the TPS, individual orders are treated individually, so that large batches of parts and assemblies are not made. This is sometimes called single piece flow. Among the advantages are short waiting times for parts of a particular type, low work in process inventories, and quick discov- ery of mistakes. If 5,000 of part A are made before any of part B are made, products that need part B will wait while all 5,000 As are made, or else a large (wasteful) supply of B's parts must be held in inventory. If a mistake is found in the 500th A, all 5,000 may contain the mistake and have to be reworked or scrapped. Single piece flow supports another element of the TPS called the visible control system, in which it is easy to see what is happening to every part. [Linck] reports that automobile component plants that use single piece flow have lower mistake rates and can make more units with fewer employees in less floorspace than batch process plants making the same components. Single piece flow is accomplished in machining operations by creating a cell architecture. A few operators walk individual parts from machine to machine. The parts follow their required machining sequence but the operators visit the machines in the sequence in which they finish and need a new part. The operators make the parts called for by the kanbans. If demand falls, fewer operators are assigned to the cell and fewer kanbans arrive. The alternative to single piece flow is batch processing. Batch size is governed by the economic lot size formula, which balances cost and time for changeovers with cost of holding the batch as work in process inventory. According to this formula, shorter changeovers make smaller batches economical, although this forces transport events to hap- pen more often and may require more resources to carry out these events. 16.1. THE TOYOTA PRODUCTION SYSTEM 445 In industries like aircraft, where the products are large, there is no alternative to single piece flow. In addition to the advantages of single piece flow discussed above, batch processing requires investments in transport equipment that can carry a whole batch or a large fraction of it. This can create problems of its own in the form of a transport department with its own procedures and costly equipment. 19 16.l.2.f. Quality Control and Troubleshooting In order for a low work in process inventory system to operate successfully, there must be very few assembly mistakes. The TPS emphasizes mistake reduction by several means, including foolproofing operations and empower- ing operators to inspect their own work. Reduction in inventories also makes problems appear rapidly because workers are affected quickly when their buffers run out. Ohno called this "lowering the water so you can see the rocks." It is the reverse of the strategy of using buffers as protection against unforeseen events. 16.1.2.g. Extension to the Supply Chain It took Toyota a number of years to discover that the TPS had to be extended to its suppliers in order to gain full advantage. The basic issue is the need to reduce costs all down the supply chain. The TPS recognizes waste in the form of idle labor and idle parts or assemblies. The cost of production at any stage in the supply chain is mostly the cost of parts and assemblies purchased from the stage below. Labor (and equipment depreciation) is a small proportion of the cost. But, summed over the entire chain, labor is the largest proportion, as discussed in Chapter 18. Thus, if a company looks only at its own operations, it will focus more on the materials and less on the labor. But if it looks at the whole chain, it will focus on labor. Since Toyota knew how to make efficient use of both labor and materials in its own plants, it undertook to teach its suppliers to do the same. It also taught its suppliers how to get along with less fixed equipment and to be able to cut costs when demand fell. 19 A car engine plant visited by the author consisted of separate machining lines linked by transport vehicles that brought several parts at once. When a line lacked parts, its operators blamed the transport department. The transport department blamed the upstream line for not notifying it when parts were ready to ship. The problem was solved by directing the downstream operators to get the parts themselves. 16.1.3. Layout of Toyota Georgetown Plant Toyota's design for the Georgetown, Kentucky, plant shows a sophisticated mix of pull- and push-type production (Figure 16-17). As described in [Mishina], final orders are smoothed as described above and sent to the beginning (not the end) of the line just after the press shop. The line runs as a conventional push-type conveyor from that point forward. However, the subassembly feeder lines and supplier lines operate on a pull basis and supply parts according to what is consumed by the main line. Since the main line is sequenced to represent the average flow of orders, the supplier and subassembly lines produce versions according to that average or use the concept of delayed commitment to modify their output at or near the end of their sub-lines in order to satisfy each individual order. A small amount of inventory in the form of a "convenience store" is held at the ends of these lines as well. 16.1.4. Volvo's 21-Day Car Volvo has built a factory in Ghent, Belgium, that delivers a car to a customer twenty-one days after it is ordered. Typical delivery intervals are six to eight weeks in most countries. A variety of techniques, many of them similar to Toyota's, contribute to Volvo's ability to deliver this quickly. Unlike the Denso panel meter, where product design and assembly process design were crucial en- ablers, Volvo's process uses largely standard part design and fabrication processes and depends instead on carefully managed logistics. Volvo has decided carefully where and when to make each subassembly (make ahead and keep in stock, make only when the customer orders, make at lineside, make at supplier, etc.). The elements of the approach are illustrated in Figure 16-18. Like Denso, Volvo presents customers with a limited amount of variety from which to choose, although the range is still generous. Three body styles and twenty colors are available. The customer can choose seat coverings, interior colors, and any or none of the following: roof rails, air conditioning, cruise control, electric win- dows, and electric mirror. Several engine options are also available, as are transmission options. The strategy includes partitioning these items according to their value and the time it takes to make them. High-value long-lead items like engines, transmissions, seats, and instrument panel assemblies are made at nearby 446 16 ASSEMBLY SYSTEM DESIGN FIGURE 16-17. Layout of Toyota Georgetown Plant as of 1992. This figure shows an in-house supplier for engines, a first-tier supplier of seats, and a second-tier supplier of seat covers. One or two hours of parts from suppliers not shown are arrayed along the assembly line in what Mishina calls "stores." Press shop, engine shop, seat supplier, and seat-cover supplier operate pull systems. Final assembly starting in the body shop is a push system. According to this layout, finished engines are drawn from a store rather than being built to match a particular car. At an auto plant in Germany, the engine assembly line is notified 4.5 hours before an engine is needed by the adjacent assembly plant. Since it takes 3 hours to assemble an engine from finished parts, there is no need for a store at the end of the engine line. However, blocks are machined in large batches, and it takes three weeks to generate all the necessary varieties. (Observed by the author in 1996.) In the Volvo 21-day car system described in the next section, orders enter at the output of the paint shop buffer. This, too, permits engines to be assembled to suit each car. (Adapted from [Mishina]. Copyright © 1999 Ashgate Publishing Ltd. Used by permission.) FIGURE 16-18. Volvo's 21-Day Car. The customer orders the car and many parts are marshaled in the time leading up to assembly day. A fixed variety of body styles and colors is made almost regardless of orders. Due to the possible unreliability of paint processes, cars are not painted to order. Instead, painted cars are stored in a buffer and a specific order begins to be built when one of these bodies is assigned to a customer. Many items, such as seats, are built in nearby plants to match the order and are ready at the time they are needed on the final assembly line. (Information provided by M. Etienne DeJaeger of Volvo.) 16.J. DISCRETE EVENT SIMULATION 447 plants. Basic engines are standard and made in Sweden, but accessories can be added quickly in the final assembly plant to meet a customer's needs. Seats are similar, with power motors and fabric coverings being matters of customer choice. Medium value items with short process times like steering columns are built in the final assembly plant from standard parts that are small and not too valuable. There are big stocks at lineside of low cost small parts. A big ballet of signals, conveyor lines, and trucks mesh these items together during an eighteen-hour period that begins with welding together stamped body parts and painting them. (Eighteen hours is typical for this overall process at most car plants.) Three body types and twenty colors makes sixty customer choices, and a buffer of three hundred vehicles ahead of final assembly thus contains five of each possible type, ready to pick when a customer's order becomes active. Seat and engine plants are notified after welding but before painting, giving them between four and nine hours notice that a particular item will be needed. A finished car rolls off the line every 1.5 minutes, two shifts a day. 16J. DISCRETE EVENT SIMULATION 20 An important step in the design of many manufacturing systems is the simulation of system operation. Simulation may be incorporated in the design process for specifying system characteristics or it may be used to verify the performance of a proposed system after the specification process is complete. Simulation of the type described here, called discrete event simulation, is a very powerful tool in operations research and is widely used for such problems as route and equipment scheduling for transportation systems. Consequently, numerous software tools and lan- guages exist for system simulation. It is beyond the scope of this text to cover any particular simulation software package in depth or even to list all the available pack- ages. Rather, the purpose of this section is to describe, in a general sense, how and when simulation may be effectively applied to the design of manufacturing systems. For a more detailed description of simulation and the available tools, the reader is referred to the references ([Pooch and Wall], [Fishman]). Simulation is the operation of computer models of systems for the purpose of studying deterministic and stochas- tic phenomena expected to occur in those systems. Sim- ulation is instrumental in the design process because it allows the engineer or analyst to: 1. Study the performance of systems without building them. 2. Study the impact of different operational strategies without implementing them. 20 This section is based in part on Chapter 15 of [Nevins and Whitney]. 3. Study the impact of major external uncontrollable events such as component failures without requiring them to occur. 4. Expand or compress time to study phenomena otherwise too fast or too slow to observe. 5. Realistically represent random events and non- linear effects like finite buffer sizes that are difficult to capture mathematically. The key to any simulation effort is the formulation of a model of the system under study. The results obtained through simulation can be only as accurate as the under- lying model. The model is an abstract representation of a system or part of a system. The model describes, in some convenient way, how the system will behave under all conditions that it is likely to experience. All discrete event simulation tools share a common modeling viewpoint—that of entities, activities, and queues. The model is a network of activities and queues through which the entities flow. The essence of construct- ing the model is to specify the network and the logic that governs that flow. Entities are objects that flow through the system or resources that reside in the system. Examples of entities are workers, robots, machine tools, and production parts. Activities are the productive elements of system behavior and require the participation of one or more entities in order to occur. Examples of activities are the machining of a part or the replacement of a machine's cutting tool. Finally, queues are places where entities collect when not participating in any activity. Queues may represent real aspects of the system such as inventories of materials, or they may represent fictitious quantities such as raw materials that have not yet entered the system or machines 448 16 ASSEMBLY SYSTEM DESIGN in the idle state ready to be assigned work. In some cases, the behavior of queues may be of specific interest because the size of an inventory queue or time that machines are idle are important aspects of system performance. Each activity has a duration, which can be a random number. The simulation starts by finding all the activities that can start because they have all the entities they need. The simulator then advances the clock until the next event, which is caused by completion of the ongoing activity that has the shortest time-to-go. The simulator dis- tributes its entities to different queues according to the model and then looks to see if any other activities can start or finish. The simulation continues in this way until a time limit is reached or for some reason no activities can start. The concepts of entities, activities, and queues are illustrated by a simplified model shown in Figure 16-19. This figure, called an activity cycle diagram, depicts the various activities as rectangles, the queues as circles, and the "flow" of entities as connecting lines. The flow of entities along the connecting lines is instantaneous; at all times, every entity must be either involved in an activity or waiting in a queue. The connecting lines represent the possible state changes for each class of entity. Two classes of entities are included: pallets and a cutting tool. The pallets can move between the activities and queues defined by the network paths shown by solid lines. The cutting tool is constrained to the network paths shown in dashed lines. The process that this model simulates can be described as follows: • A part is loaded onto an empty pallet. The part is machined using the cutting tool. The finished part is removed from the pallet, which returns to the beginning of the system. Provision has been made for the cutting tool to be replaced when worn or broken. While the tool is being replaced, no machining can occur. Similarly, if there are no empty pallets, parts cannot be fed into machining. Two features illustrated in the figure are especially important to discrete event simulation: cooperation and branching. Machining cannot occur without the cooperation of a pallet and the cutting tool. The cutting tool may branch from queue "sharp tools" to either activity "machining" or activity "replace tool." The model must specify some logic for determining which branch to follow. This model could be used to study how in-process storage requirements change when activity durations and tool replacement strategies are varied. Commonly, simulation is used to do the following: 1. Determine resource utilizations to identify bottle- necks in system performance and to fine-tune the line balance. In the above example, simulation would have shown that machine utilization was less than expected because of the idle time caused by waiting for a sharp tool. FIGURE 16-19. Example Activity Cycle Diagram. 16.K. HEURISTIC MANUAL DESIGN TECHNIQUE FOR ASSEMBLY SYSTEMS 449 2. Investigate scheduling strategies. System performance is often affected by changing the scheduling and priority of activities. For example, simulation could show that a system's throughput could be im- proved by giving highest priority to the repair of the machines with the highest utilization. 3. Determine inventory levels. These may be inventory levels or buffer sizes that result from operation of the system in a prescribed manner, or the inventory or buffer sizes required to achieve system performance unconstrained by the effects of finite buffer size. 4. Investigate the impact of different batching strategies for batch-process systems. The usefulness of the simulation to the system designer relies on the use of other tools such as economic analysis. Without proper interpretation of its results, simulation would be merely a trial and error process. Simulation will yield the characteristics of a single point in design space: It is the responsibility of the designer, using other methods such as those described elsewhere in this chapter, to optimize the system within the design space. Discrete event simulation is a valuable tool in the design and specification of manufacturing systems. It is not, however, a substitute for analytical methods. It is useful when a system is complex or subject to random behavior and as a means of verifying results obtained by an analysis based on unproven assumptions. A rough analysis is always a prerequisite for formulating a simulation model. 16.K. HEURISTIC MANUAL DESIGN TECHNIQUE FOR ASSEMBLY SYSTEMS This section and the next one deal with specific steps in designing an assembly system for the base case where one or a few versions of a product are to be assembled. This section describes a manual design method while the next shows how to use a computer algorithm to help with part of the process. Some of the steps in this process are illustrated with the staple gun 21 whose DFA is considered in Chapter 15. Five hundred thousand of these items are made each year. 16.K.1. Choose Basic Assembly Technology In this manual method, it will be assumed that one dom- inant assembly method will be used: manual, fixed automation, or flexible automation. The computer algorithm described in the next section chooses the most appropriate technology for each operation or group of operations and generates mixed-technology designs. 16.K.2. Choose an Assembly Sequence We learned in Chapter 7 how to generate and select assembly sequences. Different sequences may favor different assembly technologies. For example, if the assembly 2 'The staple gun example is based on work by MIT students Benjamin Arellano, Dawn Robison, Kris Seluga, Thomas Speller, and Hai Truong, and Technical University of Munich student Stefan von Praun. sequence requires turning the product over many times, manual assembly (or manual operation of the turnover steps) may be the best choice. A product whose different versions require different part counts or different sequences may be feasible via a fixed automation machine that allows stations to be skipped if their part is not needed by that version. More often, such products are assembled by robots or people. 16.K.3. Make a Process Flowchart A process flowchart is a diagram that follows the pattern of the assembly sequence, indicating separately each subassembly that is built and introduced to the line. The flowchart also includes all nonassembly operations that require attention, time, or equipment, such as inspections, lubrication, or record-keeping. Figure 16-20 is the process flowchart for the staple gun. 16.K.4. Make a Process Gantt Chart Gantt charts are commonly used in scheduling any kind of work sequence. An example appears in Figure 16-21. Time runs along the horizontal axis, while the tasks from the process flowchart are arrayed down the vertical axis in sequence from first to last. Times for tasks that occur in series must be placed end to end in the chart. Operations on subassemblies that can be done in parallel are shown going on at the same time as other tasks. An estimate of the time required for each task should be calculated using 450 16 ASSEMBLY SYSTEM DESIGN FIGURE 16-20. Process Flowchart for the Staple Gun. G1 and G2 are greasing operations. FIGURE 16-21. Assembly Gantt Chart for the Staple Gun with Station Assignments. Times for individual steps are shown for stations 1 and 3, while aggregate times are shown for the others. Two seconds transport time between stations is not shown. Also not represented is any downtime loss, which the designers of this system assumed would be 15%. The makespan without these effects is 163 seconds. Next Page 16.K. HEURISTIC MANUAL DESIGN TECHNIQUE FOR ASSEMBLY SYSTEMS 451 Equation (16-4) or some other suitable method. A time appropriate to the resource being used must be chosen. The total time (makespan) needed to assemble one unit can then be read off the chart. 16.K.5. Determine the Cycle Time Assuming that the number of assemblies needed per year is known, the required cycle time can be computed using Equation (16-7). This cycle time reflects an assumption about how many shifts will be needed. It is easiest to start by assuming one shift operation. 16.K.6. Assign Chunks of Operations to Resources Equation (16-6) tells us how many equal-sized time chunks are needed to do all the operations. The longest time chunk (called t in Equation (16-4)) should not be longer than the cycle time (T in Equation (16-7)). Our goal is to assign chunks of operations to resources so that all the work gets done and each resource has about the same amount of work to do. In Figure 16-21, the number of chunks is eight. In this case, several time chunks are longer than the operation times in those chunks, so one manual or flexible resource can do several tasks. In general, the operation time may exceed the cycle time, may be about the same, or may be much less. Each case is handled differently. First, see if some operations take much longer than others. If so, consider providing additional stations in parallel to do those operations, as shown in Figure 16-10a. Keep doing this until those operations can be done in approximately one cycle. Next, look for operations that take much less time than the others and see if they can be clustered into one workstation so that their total time is approximately one cycle. An example is shown in Figure 16-1 Ob. This option is feasible only if the resource can do more than one task; this is inapplicable to fixed automation, whose operation times by definition are the same for each step in the assembly and consist of one step only. At this point, one may have a line of stations which, operating in series, can produce the assemblies at the required rate. Even after chunking the operations into approximately equal time clusters, there still may not be enough time to make all the needed assemblies unless a very large number of parallel stations is used. This would be unwieldy and take up a lot of space. Instead, consider adding a second or even a third shift of operation. Equivalently, consider simply building more than one identical system. Either approach effectively multiplies the required cycle time by two or three over that calculated at first and may enable the system to finish the needed assemblies in the available time. Naturally, adding shifts will affect the economics (discussed below) in different ways, depending on whether the system is manual or not. The reason is that adding a second shift doubles the labor cost while the same machines can be used on any number of shifts without buying them again. Only the people needed to tend the machines must be paid for a second (or third) time. Duplicating the system means buying additional machines as well as hiring additional people. The plan for the staple gun shown in Figure 16-21 can deliver the required 500,000 units per year if it is operated for two shifts per day. Its cycle time of 22 seconds plus 2 seconds station move time permits just over 1,000 units to be made per shift at 85% uptime. 16.K.7. Arrange Workstations for Flow and Parts Replenishment The above steps create a list of stations and identify the time sequence of their operation, or equivalently the sequence in which assemblies must visit the stations. The next step is to arrange these stations into a floor layout, perhaps using one of the layout types discussed in Sec- tion 16.F. In doing so, the designer must account for space for people to work and move about, space for the assembly equipment and work tables, and access paths and storage space for incoming parts and finished assemblies. Buffers between stations must also be considered, especially on either side of the slowest station. Areas for rework following test operations must also be provided. The floor area must be arranged so that paths of transport vehicles do not cross each other and present safety problems or traffic jams. If the system contains robots or fixed automation, good practice is to leave plenty of space between stations for people to stand in if a station is broken for an extended period. Figure 16-22 shows the assembly system for the staple gun. The station times shown here include an extra 2 seconds for passing the work from one station to the next, in addition to the process times shown in Figure 16-21. Previous Page 452 16 ASSEMBLY SYSTEM DESIGN FIGURE 16-22. Assembly System Design for the Staple Gun. This system is estimated to require an investment of $32,000 and yield a unit assembly cost of $0.90 counting only direct labor at $15/hr. Note that the operators are inside this loop while parts arrive from the outside. A door is provided to permit the operators to enter and leave. Table 16-4 shows the parts supply strategy for this system. Based on the size of the parts and the rate at which they are consumed, different delivery schedules are appropriate for the parts needed at each station. 16.K.8. Simulate System, Improve Design The above design process creates a system that is suf- ficient to meet average demand under average operating conditions. Many sources of variation will affect its operation, usually negatively. For this reason, it is necessary to make a discrete event simulation of the proposed design to see how it works. As discussed in Section 16.J, the result could be addition of buffers, enlargement of buffer space, improvement in anticipated machine downtimes, hiring of additional repair or part replenishment people, and so on. 16.K.9. Perform Economic Analysis and Compare Alternatives The above procedure creates an assembly system based on assuming a given assembly technology, together with its costs. These consist of investment in equipment plus the ongoing cost of labor. In some situations, floor space is assigned an overhead cost or even taxed as real estate 16.K. HEURISTIC MANUAL DESIGN TECHNIQUE FOR ASSEMBLY SYSTEMS 453 TABLE 16-4. Parts Supply Schedule for the Staple Gun Station 1 Station 2 Station 3 Station 4 Station 5 Station 6 Station 7 Station 8 Different Parts Supplied 4 7 5 3 1 4 2 1 Bulk Bins in Rack at Any Time 2 5 1 1 1 3 1 0 Size of Bin 17x7x2.5 17x7x2.5 17x7x2.5 17x7x2.5 17x7x2.5 17x7x2.5 17x7x2.5 17x7x2.5 Trays in Rack at Any Time 8 5 15 2 1 3 1 5 Size of Trays 17x7x5 17x7x5 17x7x5 17x7x5 17x7x5 17x7x5 17x7x5 17x7x5 Maximum Supply Interval Ihr Ihr 1 hr 1.15hr 4hr 1 hr 8hr Ihr Recommended Supply interval 0.5 hr 0.5 hr 0.5 hr 0.5 hr 3hr 0.5 hr 8hr 0.5 hr FIGURE 16-23. Robotic Assembly System Proposed for Staple Guns. This system can make 500,000 units per year operating one shift. Each station operates in 10 seconds, and 2 seconds are allowed for station-station move time. It is estimated to require an investment of $1.26 million. There are nine automated stations plus four manual stations (not shown) that prepare subassemblies S1 through S4. Each unit bears about $0.59 to repay this investment at prevailing interest rates. by the surrounding municipality. To see if the proposed system is the most economical, an economic analysis of it must be made. Following this, a different design must be created and subjected to all of the above steps so that its performance and cost may be compared to the first one. This process is repeated as many times as the designer has imagination or time, until a satisfactory design is obtained. Naturally, if design of the system is outsourced to a vendor, the vendor will do all this tedious work but will most likely choose the assembly methods it is most familiar with and prepared to deliver. In the case of the staple gun, an alternate design con- sisting of fixed automation and robots was designed and compared to the manual line described above. It is shown in Figure 16-23. Economic analysis, as explained in more detail in Chapter 18, shows that it would cost slightly more to assemble one staple gun on this system than on the manual system, even though it would make all the needed staple guns in one shift. It also faced considerable technical challenges in accomplishing the more difficult assembly tasks. [...]... strip; the parts may be made right on the strip, like the connector pins in Figure 17 -9; or the strip may be placed in an injection molding machine and the parts molded into the strip Reliability Changing Time Magazine feeder Short carrier strip feeder 99 .92 % 5 minutes 99 .93 % 7 minutes Tape feeder Screwdriver 99 .97 % 99 .96 % 2 minutes 4 minutes Comments Preparation takes a lot of time It takes relatively... Raton, FL: CRC Press, 199 3 [Scholl] Scholl, A., Balancing and Sequencing of Assembly Lines, Heidelberg: Physica Verlag, 199 5 [Shingo] Shingo, S., A Revolution in Manufacturing: The SMED System, Stamford, CT: Productivity Press, 198 5 [Spear and Bowen] Spear, S., and Bowen, H K., "Decoding the DNA of the Toyota Production System," Harvard Business Review, September-October, pp 96 -106, 199 9 [Taguchi] Taguchi,... Ltd., 199 9 [Monden] Monden, Y, Toyota Production System: An Integrated Approach to Just-in-Time, Norcross, GA: Engineering & Management Press, 199 8 [Nevins and Whitney] Nevins, J L., and Whitney, D E., editors, Concurrent Design of Products and Processes, New York: McGraw-Hill, 198 9 [Nof et al.] Nof, S Y, Wilhelm, W E., and Warnecke, H.-J., Industrial Assembly, New York: Chapman and Hall, 199 7 [Peschard... and Raff, D., editors, Aldershot, UK: Ashgate Publishing, Ltd., 199 9 [Fishman] Fishman, G S., Discrete Event Simulation, New York: Springer-Verlag, 2001 [Gershwin] Gershwin, S B., Manufacturing Systems Engineering, Englewood Cliffs, NJ: Prentice-Hall, 199 4 [Goldratt] Goldratt, E M., The Goal, Great Barrington, MA: North River Press, 199 2 [Graves and Holmes-Redfield] Graves, S C., and HolmesRedfield,... Publications, 198 6 [Whitney] Whitney, D E., "Nippondenso Co Ltd: A Case Study of Strategic Product Design," Research in Engineering Design, vol 5, pp 1-20, 199 3 [Womack, Jones, and Roos] Womack, J P., Jones, D T., and Roos, D., The Machine that Changed the World, New York: Rawson Associates, 199 0 ASSEMBLY WORKSTATION DESIGN ISSUES "If the work must be done in 60 seconds and your robot needs 59 seconds,... 4,5,6 2 3 4 5 6 7 8 9 SI S4, 20, 27 22,23 21 S3 S2 8 1-3,7 9 Should the buffers upstream (downstream) of a bottleneck be half full (empty) or totally full (empty)? 464 16 ASSEMBLY SYSTEM DESIGN 16.R FURTHER READING [Boothroyd] Boothroyd, G., Assembly Automation and Product Design, New York: Marcel Dekker, 199 2 [Chow] Chow, W M., Assembly Line Design, New York: Marcel Dekker, 199 0 [Cooprider] Cooprider,... numbers of parts or can even be of different sizes 16.N.2 Denso Alternator Line (~ 198 6) The Denso alternator assembly line comprises twenty robots, designed and built by Denso (Figure 16- 29) 16.N EXAMPLE LINES FROM INDUSTRY: DENSO 4 59 FIGURE 16- 29 Denso Robotic Assembly Line for Alternators This system is arranged in a loop Assemblies are carried on pallets which return to the start of the line to pick... Alternatives," S M thesis, MIT Mechanical Engineering Department, February 198 7 [Linck] Linck, J., "A Decomposition-Based Approach for Manufacturing System Design," Ph.D thesis, MIT Mechanical Engineering Department, June 2001 [Milner] Milner, J., "The Assembly Sequence Selection Problem: An Application of Simulated Annealing," S M thesis, MIT Sloan School of Management, May 199 1 [Mishina] Mishina, K.,... applications, one or a few stations will be used Assemblies can circulate inside each station, returning to the assembly robots several times as they change tools and add more parts Also, assemblies can circulate among several stations for the same purpose Parts are placed in the stocker at the rear of the station ([Hibi]) 16.N.4 Denso Roving Robot Line for Starters (~ 199 8) The roving robot line shown in Figure... author observed Denso employees helping each other during a visit in 197 4 An employee who was ahead ran downstream to help the adjacent employee who had fallen behind, then ran back to work off her own backlog In 198 1, Hitachi described a slightly different roving robot concept in which the robots carried the partially finished assemblies as well as the tools, and they obtained parts from the different . Line (~ 198 6) The Denso alternator assembly line comprises twenty robots, designed and built by Denso (Figure 16- 29) . 16.N. EXAMPLE LINES FROM INDUSTRY: DENSO 4 59 FIGURE 16- 29. Denso . permits engines to be assembled to suit each car. (Adapted from [Mishina]. Copyright © 199 9 Ashgate Publishing Ltd. Used by permission.) FIGURE 16-18. Volvo's 21-Day Car. The . Transport Betw Stations 9 10 When a resource can be used: | - Operation ; Tool time (s) i number ___£___ Tool cost RBS RBB TRN C 40000_ C_80000__ C rho 2.5_ rho 2.5_ rho e 90 e 90 e v 6.00