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PART III Manufacturing Systems chapter 13 Introduction to Manufacturing Systems CHAPTER CONTENTS 13.1 Components Production Material 13.1.3 Computer 13.1.4 13.2 of a Manufacturing 13.1.1 13.1.2 Hurnen System Control System sescuroes Classification of Manufacturing 13.2.1 Types of Operations 13.2.2 Number 13.2.3 System Machines Handling Systems Performed of Workstations Automation Level 13.2.4 Part or Product Variety 13.3 13.4 Overview of the Classification 13.4.1 Type I Manufacturing 13.4.2 Type II Manufacturing 13.4.3 Type III Manufacturing Manufacturing Progress Scheme Systems: Systems: Systems: Functions Single Stations Multi-Station Production (Learning Cells lines Curvesl In this part of the book, we consider how automation and material handling technologies are synthesized to create manufacturing systems. We define a manufacturing system to be a collection of integrated equipment and human resources, whose function is to perform one or more processing and/or assembly operations on a starting raw material, part, or set of parts. The integrated equipment includes production machines and tools, material handling and work positioning devices, and computer systems. Human resources are required either full time or periodically to keep the system running. The manufacturing system is "5 Chap. "6 : Enterpri •• level i i Introduction to Manufacturing I s~;;~:~~~~~';;> Systems Manufacturing ~upporlsY5tem.~ Qualilyconlrol sy,telt1'l FII<"I<'ry Producllon system 13 level Facilities AUI<,>rnaU,m and comrcl tecb.nnlugies Malerial handling !e~hnolngies Figure 13.1 The position of the manufacturing production system system in the larger where the value-added work is accomplished on the part or product. The position of the manufacturing system in the larger production system is seen in Figure 13.1. Examples of manufacturing systems include: • one worker tending one machine, which operates on semi-automatic • a duster of semi-automatic machines, attended by one worker • a fully automated assembly machine, periodically • a group of automated similar parIs attended machines working on automatic • a team of workers performing assembly operations cycle by a human worker cycles to produce a family of on a production line. In the present chapter, we classify these manufacturing systems and examine their features and performance. In other chapters in this part of the book, we discuss the various manufacturing systems of greatest commercial and technological importance. 13.1 COMPONENTS OF A MANUFACTURING SYSTEM A manufacturing system consists of several components. nents usually include: In a given system, these compo- • production machines plus tools, fixtures, and other related hardware • material handling system • computer systems to coordinate and/or control the above components • human workers In this section, we discuss each of these components and the variety of types within each category. In the following section, we consider how these components are combined and organized in different ways to achieve various objectives in production. Sec. 13.1 I Components of a Manufacturing 13.1.1 System 377 Production Machines In virtually all modern manufacturing systems, most of the actual processing or assembly work is accomplished by machines or with the aid of tools. The machines can be classified as (1) manually operated, (2) semi-automated.or (3) fully automated. Manually operated machines are directed or supervised by a human worker. The machine provides the power for the operation and the worker provides the control. Conventional machine tools (e.g., lathes, milling machines, drill presses) fit into this category. The worker must be at the machine continuously. A semi-automated machine performs a portion of the work cycle under some fonn of program control, and a human worker tends to the machine for the remainder of the cycle, by loading and unloading it or performing some other task each cycle. An example of this category i~a CNC lathe controlled for most of the work cycle by the part program, but rcquiring a worker to unload the finished part and load the next workpiece at the end of the part program. In these cases, the worker must attend to the machine every cycle, but continuous presence during the cycle is not always required. If the automatic machine cycle takes,say,lO min, w hiie the part unloading and loading portion of the work cycle only takes 1 min, then there may be an opportunity for one worker to tend more than one machine. We analyze this possibility in Chapter 14 (Section 14.4.2). What distinguishes a fully automated machine from its semi-automated cousin is its capacity to operate for extended periods of time with no human attention. By extended periods of time, we generally mean longer than one work cycle. A worker is not required to be present during each cycle. Instead, the worker may need to tend the machine every tenth. cycle or every h.undredth cycle. An example of this type of operation is found in many injection molding plants, where the molding machines run on automatic cycle, but periodically the collection bin full of molded parts at the machine must be taken away and replaced by an empty bin. In manufacturing systems, we use the term workstation 10 refer 10 a location in the factory where some well-defined task or operation is accomplished by an automated machine, a worker-and-machine combination, or a worker using hand tools and/or portable powered tools. In this last case, there is no definable production machine at the location. Many assembly tasks are in this category. A given manufacturing system may consist of one or more workstations. A system with multiple stations is called a production line, or assembly line, or machine cell, or other name, depending on its configuration and function. 13.1.2 Material Handling System In most processing and assembly operations performed on discrete parts and products. the following ancillary functions must be provided: (J) loading and unloading work units and (2) positioning the work units at each station. In manufacturing systems composed of multiple workstations, a means of (3) transporting work units between stations is also required. These functions are accomplished by the material handling system. In many cases, the units are moved by the workers themselves, but more often some form of mechanized or automated material transport system (Chapter 10) is used to reduce human effort. Most material handling systems used in production also provide (4) a temporary storage function. The purpose of storage in these systems is usually to make sure that work is always pr~sent for the stations, that is, that the stations are not starved (meaning thai they have nothing to work on). 378 Chap. 13 I Introduction to Manufacturing Systems Some of the issues related to the material handling system are often unique to the particular type of manufacturing system. and so it makes sense to discuss the details of each handling system when we discuss the manufacturing system itself in later chapters. OUT discussion here is concerned with general issues relating to the material handling system. LUliding. PUI:iWuning, (1f1(J Unloading. These matenal haudliug functions occur at each workstation. Loading involves moving the work units into the production machine or processing equipment from a source inside the station. For example, starting parts in batch processing operations are often stored in containers (pallets, tote bins.etc.] in the immediately vicinity of the station. For most processing operations, especially those requiring accuracy and precision. the work unit must be positioned in the production machine. Positioning provides for the part to be in a known location and orientation relative to the wurkhead or tooling that performs the operation. Positioning in the production equipment is often accomplished using a workholder.A workholder is a device that accurately locates, orients, and clamps the part for the operation and resists any forces that may occur during processing. Common workhotders include jigs, fixtures, and chucks. When the production operation bas been completed, the work unit must be unloaded, that is, removed from the production machine and either placed in a container at the workstation or prepared for transport to the next workstation in the processing sequence. "Prepared for transport" may consist of simply loading the part onto a conveyor leading to the next station. When the production machine is manually operated or semi-automatic, loading, positioning, and unloading are performed by the worker either by hand or with the aid of a hoist. In fully automated stations, a mechanized device such as an industrial robot, parts feeder. coil feeder (in sheet metal stamping), or automatic pallet changer is used to accomplish these material handling functions. Work Transport Between Stations. In the context of manufacturing systems, work transport means moving parts between workstations in a multi-station system. The transport function can be accomplished manually or by the most appropriate material transport equipment. In some manufacturing systems, work units are passed from station to station by hand. Manual work transport can be accomplished by moving the units one at a lime or in batches, Moving parts in batches is generally more efficient, according to the Unit Load Principle (Section 9.3). Manual work transport is limited to cases in which the parts are small and light, so that the manual labor is ergonomically acceptable. When the load to be moved exceeds certain weight standards, powered hoists (Section 10.5) and similar lift equipment are used. Manufacturing systems that utilize manual work transport include manual assembly lines and group technology machine cells. Various types of mechanized and automated material handling equipment are widely used to transport work units in manufacturing systems, We distinguish two general categories of work transport, according to the type of routing between stations; (1) variable r?uting and (2) fixed routing. In V{lfiab~e routing, work units are transported through a vanety of different station sequences. ThIS means that the manufacturing system is processing or assembling different work units. Variable routing transport is associated with job shop production and many batch production operations. Manufacturing systems that use variable routing include group technology machine cells (Chapter 15) and flexible manufacturing systems (Chapter 16). In fixed routing, the work units always flow through the same sequence of stations. This means that the work units are identical or similar enough Sec. 13.1 I Components of a Manufacturing aza System Wurkstations eee Cumpleted work units Starting workunns (0) ;,~;:~~~ Slarting wurkunits (b) Figure 13.2 'Types of routing in multiple station manufacturing tems: (a) variable routing and (b) fixed routing. TABLE 13.1 sys- Common Material Transport Equipment Used for Variable and Fixed Routing in Multiple Station Manufacturing Systems Type of Part Routing Material Handling Equipment" Variable routing Automated guided vehicle system Power-and-free overhead conveyor Monorail system Fixed routing Powered roller conveyor Belt conveyor Drag chain conveyor Overhead trolley conveyor Rotary indexing mechanisms Walking beam transfer equipment Cert-on-treck •Described in Chapte,s 10and convsvor 18 that the processing sequence is identical. Fixed routing transport is used on production lines (Chapters 17 and 18). The difference between variable and fixed routing is portrayed in Figure 13.2. Table 13.1 lists some of the typical material transport equipment used for the two types of part routing. Pallet Fixtures and Work Carriers in Transport Systems. Depending on the geometry of the work units and the nature of the processing and/or assembly operations Chap. 3•• 13 / Introduction to Manufacturing Sv~ems to be performed, the transport system may be designed to accommodate some form of pallet fixture. A pailel future is a workholder that is designed to be transported by the material handling system. The part is accurately attached to the fixture on the upper face of the pallet, and the under portion of the pallet is designed to be moved, located, and damped in position at each workstation in the system. Since the part is accurately located in the fixture, and the pallet is accurately clamped at the station, the part is therefore accurately located at each station for processing or assembly. Use of pallet fixtures is common in automated manufacturing systems, such as single machine cells with automatic pallet changers, transfer lines, and automated assembly systems. The fixtures can be designed with modular features that allow them to be used for different workpart geometries. By changing components and making adjustments in the fixture, variations in part sizes and shapes can be accommodated. These modular pallet fIXtures are ideal for use in flexible manufacturing systems. Alternative methods of workpart transport avoid the use of pallet fixtures. Instead, parts are moved by the handling system either with or without work carriers. A work carrier is a container (e.g., tote pan, flat pallet, wire basket) that holds one or more parts and can be moved in the system. Work carriers do not fixture the part(s) in an exact position. Their role is simply to contain parts during transport. When the parts arrive at the desired destination, any locating requirements for the next operation must be satisfied at that station. (This is usually done manually.) An alternative to using pallet fixtures or work carriers is direct transport, in which the transport system is designed to move the work unit itself. The obvious benefit of this arrangement is that it avoids the expense of pallet fixtures or work carriers as well as the added cost of providing for their return to the starting point in the system for reuse. In manually operated manufacturing systems, direct transport is quite feasible, since any positioning required at workstations can be accomplished by the worker. In automated manufacturing systems, in particular systems that require accurate positioning at workstations, the feasibility of direct transport depends on the part's geometry and whether an automated handling method can be devised that is capable of moving, locating, and clamping the part with sufficient precision and accuracy, Not all part shapes allow for direct handling by a mechanized or automated system. 13.1.3 Computer Control Syatem In roday's automated manufacturing systems. a computer is required to control the automated and semi-automated equipment and to participate in the overall coordination and management of the manufacturing system. Even in manually driven manufacturing systerns, such as a completely manual assembly tine, a computer system is useful to support production.Typical computer system functions include the following: • Communicate instructions to workers. In manually operated workstations that perform different tasks on different work units, processing or assembly instructions for the specific work unit must be communicated to the operator, • Download part programs to computer-controlled machines (e.g., CNC machine tools). • Material handling system control. This function is concerned with controlling the matenal handling system and coordinating its activities with those of the workstations. • Schedule production. Certain production site of the manufacturing system. scheduling functions are accomplished at the Sec, 13.2 ! Classification of Manufacturing 38' Systems • Safety Moniroring. This function ensures that the system does not operate in an unsate condition. The goal of safety monitoring is to protect hoth the human workers manning the vvsrcrn and the equipment comprising the system • Qualif v Control, The purpme ~,r this ([>ntJUI fuuctiou is to detect ,md possibly reject defective work units produced by the system. 13.1.4 Human Resources In many manufacturing systems, humans perform some or all ofthe value-added work that is accomplished on the parts or products, In these cases, the human workers are referred to as directlabor. Through their physicallabor, they directly add to the value of the work unit by performing manual work on it ur by controlling the machines that perform the work. In manufacturing systems that are full',' aumrnared.dlrect labor is still needed 10 perform such aetivilies ns loading and unloading paris to and from the 3ystcm, changing tools, resharpening toots. and similar functions. Human workers are also needed for automated manufacturing systems to manage or support the system as computer programmers.compurer operators, part programmers for CNC machine tools (Chapter 6),maintenanee and repair personnel. and similar indirect labor tasks. In automated systems, the distinction between direct and indirect labor is not always precise. 13.2 CLASSIFICATION OF MANUFACTURING SYSTEMS In this scctior, we explore the variety of manufacturing system types and develop a classificauon scheme based on the factors that define and distinguish the different types. The factors arc: (I) types of operations performed.(2) number of workstations and system layout. (3) level of auromauon.and (4) part product variety. The four factors in our man ufacturing systems classification scheme are defined in Table 13.2 and discussed below. (}T TABLE 13.2 Factors in Manufacturing Factor Part or product variety Scheme Alternatives Types of operations performed Number of workstations system layout level of automation Systems Classification and Processing operations versus assembly operations Type of processing or assembly operation One station versus more than one station For more than one station, variable routing versus fixed routing Manual or semi-automated workstations that requirefull-time operator attention versus fully automated that require only periodic worker attention All work units identicat versus variations in work units that require differences in processing 382 Chap. 13 I Introduction 13.2.1 Types of Operations to Manufacturing Systems Performed First of all. manufacturing systems are distinguished by the types of operations they perform.At the highest level. the distinction is between (1 J processing operations on individual work units and (2) ussemhty operations to combine individual parts into assembled entities, Beyond this distinction. there are the technologies of the individual processing and assembly operations (Section 2.2.1) Additional parameters of the product that playa role in determining the design of the manufacturing system include: type of material processed, size and weight of the part or product, and part geometry. For example. machined parts can be classified according to part geometry as rotational or nonrotanonal. Rotational parts arc cylindrical or disk-shaped and require turning and related rotational operations. Non rotational (also called pnnneric) parts are rectangular or cube-like and require milling and related machining operations to shape them. Manufacturing systems that perform machining operations must be distinguished according to whether they make rotational or nonrotational parts. The distinction is important not only because of differences in the machining processes and machine tools required. but also because the material handling system must be engineered differently for the two cases, 13.2.2 Number of Workstations and System layout The number of workstations is a key factor in our classification scheme. It exerts a strong influence on the performance of the manufacturing system in terms of production capacity, productivity, cost per unit, and maintainability. Let us denote the number of workstations in the system hy the symbol n, The individual stations in a manufacturing system can be identified by the subscripts, where i '" 1,2, ... ,n. This might be useful in identifying parameters of the individual workstations, such as operation time or number of workers at a station The numher of workstations in the manufacturing system is a convenient measure of its size. As the number of stations is increased, the amount of work that can be accomplished by the system increases. This translates into a higher production rate, certainly as compared with a single workstation's output, but also compared with the same number of single stations working independently. There must be a synergistic benefit obtained from multiple stations working in a coordinated manner rather than independently; otherwise, it makes more sense for the stations to work as independent entities. The synergistic henefit might be denved from the fact that the totality of work performed on the part or product is too complex to engineer at a single workstation. There are too many individual tasks to perform at one workstation. By assigning separate tasks to individual stations, the task performed at each station is simplified. More stations also mean that the system is more complex and therefore more difficult to manage and maintain. The system consists of more workers, more machines, and more parts being handled. The logistics and coordination of the system becomes more involved. Maintenance problems occur more frequently. Closely related to number of workstations is the arrangement of the workstations, that is, the wily the stations are laid out-This, of course.applies mainly to systems with multiple stations. Are the stations arranged for variable routing or fixed routing? Workstation layouts organized for variable routing can have a variety of possible configurations, while lay- Sec. 13.2 I Classification of Manufacturing Systems 383 outs organized for fixed routing are usually arranged linearly, as in a production line. The layout of stations i, an important factor in determining the most appropriate material handling systern Our classification scheme is applicable to manufacturing systems that perform either processing or assembly operations. Although these operations are different, the manufacturing sy<;terns to perform them possess similar configurations. According to number of stations and the layout of the stations, our classification scheme has three levels: TYPe I Single station. This is the simplest case, consisting of one workstation usually including a production machine that can be manually operated, tomated, or fully automated. (n '" I), semi-au- Type H Multiple stations with variable routing. This manufacturing system consists of two or more stations (n > 1) that are designed and arranged 10 accommodate the processing or assembly of different part or product styles. Type HI Multiple stations with fixed routing. This system has two or more workstations (n > 1), which are laid out as a production line. 13.2.3 level of Automation The level of automation is another factor that characterizes the manufacturing system. As defined above, the workstations (machines) in a manufacturing system can be manually operated, semi-automated, or automated. Manning Level. Closely correlated with the level of automation is the proportion of time that direct labor must be in attendance at each station. The manning level of a workstation, symbolized M" is the proportion of time that a worker is in attendance at the station. If M, '" 1 for station i, it means that one worker must be at the station continuously. If one worker tends four automatic machines. then M, = 0.25 for each of the four machines, assuming each machine requires the same amount of attention. On portions of an automobile final assembly line, there are stations where multiple workers perform assembly tasks on the car, in which case M, = 2 or 3 or more. In general, high values of M, (M, 2: 1) indicate manual operations at the worketation, while low values (M, < 1) denote some form of automation. The average manning level of a multi-station manufacturing system is a useful indicator of the direct labor content of the system. Let us define it as follows: (13.1) where M "" average manning level for the system; Wu = number of utility workers assigned to the system; w, = number of workers assigned specifically to station i; for i = 1,2, ... ,11; and w = total number of workers assigned to the system. Utility workers are workers who are not specifically assigned to individual processing or assembly stations; instead they perform functions such as: (1) relieving workers at stations for personal breaks, (2) maintenance and repair of the system, (3) 1001 changing, and (4) loading andior unloading work units to and from the system. Even a fully automated multi-station manufacturing system is likely to have one or more workers who are responsible for keeping it running. .. , Chap, 13 l Introduction to Manufacturing Systems Automation in the Classification Scheme. Including automation in our classification scheme, we have two possible automation levels for single stations and three possible levels for multi-station systems. The two levels for single stations (type I) are: M = manned station and A = fully automated. The manned station is identified by the fact that one or more workers must be at the station every cycle. This means that any machine at the station is manually operated or semi-automatic and that manning is equal to or greater than one (M 2:: 1). However, in some cases, one worker may be able to attend more than one machine, if the semi-automatic cycle is long relative to the service required each cycle of the worker (thus, M < I). We address this issue in Section 14.4.2.A fully automated station requires less than full-time attention of a worker (M < 1). For multi-station systems (types II and III), the levels M and A are applicable, and a third level is possible: H = hybrid, in which some stations are manned and others are fully automated. Listing the alternatives, we have the following Type I M Single-station manned cell. The basic case is one machine and one worker (n '" 1,w = l).The machine is manually operated or semi-automated, and the worker must be in continuous attendance at the machine. Type I A Single station automated cell. This is a fully automated machine capable of unattended operation (M < 1) for extended periods of time (longer than one machine cycle). A worker must periodically load and unload the machine or otherwise service it Type II M Multi-station manual system with variable routing. This has multiple stations that are manually operated or semi-automated.The layout and work transport system allow for various routes to be followed by the parts or products made by the system. Work transport between stations is either manual or mechanized. Type II A Multi-station automated system with variable routing. This is the same as the previous system, except the stations are fully automated (n > 1, w, = 0, M < 1). Work transport is also fully automated. Type II H Murti-sration hybrid system with variable routing. This manufacturing system contains both manned and automated stations. Work transport is manual, automated, or a mixture (hybrid). Type III M Multi-station manual system with fized routing. This manufacturing system consists of two or more stations (n > 1), with one or more workers at each station (U', ~ 1). The operations are sequential, thus necessitating a fixed routing, usually laid out as a production line. Work transport between stations is either manual or mechanized. Type A Multi-station automated system with fixed routing. This system consists of two or more automated stations (n > 1, U', == 0, M < 1) arranged as a production line or similar configuration. Work transport is fully automated. 'Iype III H Multi-station hybrid system withftxed routing. This system includes both manned andautomatedstations(n > 1,w, ~ 1 for some stations,w, == o for other stations, M > 0). Work transport is manual, automated, or a mixture (hybrid). III The eight types of manufacturing system are depicted in Figure 13.3.