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Automated tool management in flexible manufacturing
Automated Tool Management in FlexibleManufacan g Hoda A. EIMaraghy, McMaster University, Hamilton, Ontario, Canada
Abstract The need for automated tooling in flexible machining, assembly, and sheet fabrication systems is reviewed. The various methods of implementing these systems, their benefits and drawbacks are discussed. The basic modules of automated tool transfer, storage, l o a d i n g / unloading, and management are described together with the appropriate level of automation for each module. The advantages and prerequisites for unmanned machining systems, the current sensing methods and the tool replacement strategies are also reviewed. The importance of a tool database, its uses and structure are highlighted. Finally, the design and evaluation of automated tooling systems and operating strategies, with the aid of discrete events computer simulation are discussed. An existing computer package which is capable of simulating automated tooling systems for flexible manufacturing systems is presented.
Keywords." Flexible Manufacturing, Tool Handling and Management, Automated Tooling, Simulation. l-he greatest emphasis in designing and implementing flexible manufacturing systems is usually placed on the work flow, handling, and storage. Whether the FM S is utilized tor machining, assembly or fabrication, it uses essential tools to perform its function. Such tools wear out, break, a n d / o r require resetting and maintenance to ensure successful
operation of the system. For this reason a tool flow also exists within the system. The number of cutting tools in a medium size FMS can easily run into thousands. Therefore, automated tool flow, storage, retrieval, loading and unloading presents a real challenge for the FMS system designer. However, effective solutions to the problem offer handsome rewards in increased productivity, profit and competitiveness. Since flexible manufacturing systems are capital intensive, a good return on investment can only be achieved when productivity is maximized and idle time, including that resulting from tooling problems, is minimized. Automated tool handling, transfer and control strategies make it possible to extend the operation of FMSs into the third shift with a minimum of human attention, and can significantly increase the efficiency and cost effectiveness of the current generation of FMSs. Utilization of machine tools can be improved by ensuring the availability of the necessary cutting tools when needed, eliminating manual tool change and setup during machine cycle, and monitoring tool wear and potential breakage which improves accuracy and reduces scrap. This can be achieved by maintaining an adequate supply of useable cutting tools, automating tool handling, transfer and load/ unload, monitoring tool usage, wear and premature failure, and incorporating control algorithms for the automatic replacement of tools. These measures increase the actual machining time and maximize
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the utilization of manpower, which results in a substantial reduction of manufacturing costs.
Automated Tooling To achieve an uninterrupted machining process, tool setup, change, and tool magazine load/ unload should take place outside the machining cycle. Automation of tool flow requires some essential elements: (!) tool transfer system, (2) tool storage facilities, (3) tool repair or replacement facilities, (4) automated tool loading and unloading mechanisms, and (5) a control logic for managing tool replacement and transfer activities. Tool Transfer. Automating the tool flow can be accomplished using the workpiece transfer system or a separate tool transfer system. Although a separate tool transfer system means additional cost, it may be necessary because of differences between tools and workpieces regarding size, holding, loading and unloading methods, frequency of motion and nature of flow. Workpiece flow tends to be a sequence of moves between workstations with infrequent trips to central or distributed stores. Tool flow, however, is mostly between individual workstations and one or more tool storage or maintenance facilities. A number of alternatives, which represent combinations of the two approaches mentioned above, may be used depending on the manufacturing system under consideration. For example, a number of standard tool kits or "libraries" may be designed for the various families of parts which are produced by the system. Such tool kits would travel with the workpieces and both automatic tool and component loading/unloading would take place at the same time. This closely allied tool and component flow is based on group technology (GT) concepts and bears resemblance to the currently used manual tool kitting techniques. Tool kits reduce capital investments in tool transfer systems as well as tools. Tool Storage, Loading and Unloading. The choice of the tool storage and handling modules, their capacity and location has a direct effect on reducing the total number of tools required in a FMS and minimizes redundancy. If tools are changed manually, then using the largest possible tool magazine would be best. However, if automated tool transfer between buffers is available, then smaller tool magazines would be sufficient.
For CNC lathes and turning centers, the capacity of the tool holding and changing unit varies between 8 and 20 tools. Therefore, the use of an additional tool storage unit would be very useful. Sandvik has introduced a block tooling system where tools are mounted on an indexing coromant tool magazine installed on the machine itself or located nearby. Hundreds of preset block tools can be stored and automatically retrieved and loaded by a special purpose manipulator which also loads the tools onto the turret. This tool storage and quick change system can be interfaced with machine control for automating tool loading and unloading. There are many special-purpose tool storage systems which can be used to supplement the tool storage capacity of machining centers. Such systems may consist of racks or carousels which carry a large number of tools, and a special load/unload mechanism. Some supplementary tool storage systems allow tool loading only when the spindle is stationary. In fully automated tooling systems appropriate for flexible manufacturing, tool exchange between the tool magazine and the secondary tool buffer should take place without interrupting the cutting process to minimize machine idle time. Therefore, extreme care must be exercised when selecting secondary tool storage buffers in order not to nullify one of the greatest benefits of an automated tooling system. Tool ControiSystem. A computerized control system is used to ensure the availability of the right tools when needed. This system schedules the tool transporter moves, interacts with machine controllers and monitoring devices to effect tool changes and resetting, and updates tool inventory and tool storage locations. When a new job order is scheduled, the computer tool control program scans the tool storage data files to ensure the availability of the necessary tools or initiates the appropriate actions to make them available. An elaborate database which includes information on tool codes, interchangeable tools and probes, useful tool life for various cutting conditions, and capacity of tool magazines and intermediate tool buffers is utilized by the tool control algorithms. Automated tool management systems must deal with the following issues: i. Ensuring that appropriate tools, in useable condition, are available at the machine when needed.
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2. Transporting tools to and from the machines safely and reliably. 3. Keeping track of tool usage and effecting tool changes when necessary. 4. Maintaining dynamic inventory of all tools in the system, their type, size, number and condition. 5. Monitoring tool inventory levels and initiating orders to replenish tool supply when needed. 6. Keeping the time spent waiting for tools or changing tools to a minimum. More information on available automated tooling systems for flexible manufacturing may be obtained from References 1 and 2.
Rationalizing Tool Usage While automated tool handling and management systems can improve the efficiency of flexible manufacturing, there are many other factors which must not be ignored. There should always be a conscious effort to rationalize and standardize the use of tools in order to reduce their variety and number. The benefits from this rationalization apply to both manual and automated tool management. The use of group technology concepts to select the part families for FMSs is a prerequisite for the success of the system operation and implementation of random routing, germane to flexible manufacturing. Group technology not only reduces set-up time between batches, but also helps streamline the tools required to manufacture the workpieces. A company introducing a FMS into its operation should use this opportunity to investigate the use of tools planned into the work over the years, with a view to making use of new and improved cutting tools technology, reducing the tool variety, increasing the use of standard tools, rationalizing the use of nonstandard tools, and making better use of tools interchangeability. Rationalizing tool usage and minimizing tool inventory is not a small job considering the large number of tools likely to be used in any typical FMS. This task is an ideal candidate for computerized coding and classification. Effort should be made to minimize the chances of tool breakage by adjusting the cutting parameters to reduce the cutting forces at critical moments, such as cutter entry into the workpiece. Such strategies may be built into the NC cutting program,
or implemented in real-time using adaptive control methods. The trade-off between the economic benefits derived from operating at optimum cutting conditions and frequent machine idle time due to tool breakage is a crucial factor in flexible manufacturing, especially during unmanned shifts. There are also other factors which can simplify and speed up tool change and maintenance, such as the use of tools which can be preset and reset automatically, and the use of standard holders with throwaway inserts. Enforcing effective quality control of the workpiece material can help reduce incidents of tool breakage due to material flaws. Practicing the rules of design for ease of manufacturability also helps eliminate design features, such as very small long holes, which are likely to produce tool breakage.
Level of Automation Varying degrees of computer automation may be adopted regarding tool storage, handling and transfer in flexible manufacturing systems. The degree of automation depends on the desirability and duration of unmanned operation, and the availability of sensors, reliable tool monitoring systems, real-time computer control algorithms, and a computerized tool database. The application of the various levels of automation in the three basic modules of a tooling system, namely: ( i ) central tool room, (2) tool storage/handling at work stations, and (3) transfer between tool room and workstations is discussed below. Central Tool Storage. The main activities which take place in the tool room are: I. Receive tool requirements for different machines, jobs or batches. 2. Prepare necessary tools to support scheduled production including presetting, changing tool inserts, resharpening tool cutting edge(s), placing tools in appropriate holders, and grouping tools into kits according to production requirements. 3. Responding to unexpected tool requests due to sudden tool breakage. 4. Recording relevant tool characteristics and tool usage data for future reference, reporting, and inventory control. 5. Replenishing tool stocks in an orderly and timely manner according to predefined criteria.
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in a manual tool room, operators will perform the above functions without the help of computers or a u t o m a t e d equipment which precludes the possibility of unmanned manufacturing. As more and more of the activities mentioned above are computerized and appropriate measures to deal automatically with requests for tools are implemented, achieving unmanned machining comes closer to reality. I-or example, computer terminals may be used in the tool room to interact with the rest of the production system and personnel, and facilitate the exchange of information about tool requirements, production schedules and emergency situations. Indeed the tool room may be one node in a distributed numerical control (DNC) system, with access to various production databases and realtime electronic data and messages exchange. For item 2 above, there will always be tasks related to tool preparation which need human operators. Such tasks are usually performed during the manned production shifts. The ability to respond automatically to scheduled tool requirements as well as unexpected tool requests, necessitates the use of an automated storage and retrieval system (AS RS) for tools in the tool room. This A S R S must be computer controlled and integrated with the rest of the F M S to enable it to automatically receive, interpret and respond to requests for tools. Preset and prepared tools are stored in the A S R S according to production plans and estimates of broken tools due to random failures. Tools may be stored in A S R S individually or in groups according to the adopted tool transfer and control method. A computer controlled handling device is used to fetch the necessary tools from the AS RS and load them on the tool transporter. A computer controlled A S R S and handling device is a prerequisite for implementing unmanned machining for one or more shifts, irrespective of the level of automation of other activities in the tool room. The storage capacity of the A S R S can be determined using computer simulation or inventory control methods. Recording of relevant tool data in the tool room may be automated using bar codes and code readers, as well as other suitable data entry devices such as keyboards. The operator may interface with a computerized tool database by responding to prompts appearing on the screen of his terminal. This data input is then used to update the database,
which in turn is used for filling tool requests, initiating restocking orders, issuing warning messages and preparing management reports. Decisions to purchase tools, and invoke inventory control measures can be assisted or a u t o m a t e d using any of the commercially available computer programs. Distributed Tool Storage BuJfers. Distributed tool storage buffers are usually placed next to workstations to c o m p l e m e n t the tool storage capacity of the machine itself. These buffers come in the form of stationary shelves or racks or movable carousels. Some of these storage units are vertical, others are horizontal. They may be totally separate from the machine and some can be integrated with the machine and its controller. A pick-and-place manipulator, with limited degrees of freedom, may be provided to a u t o m a t e tool transfer between the buffer and the tool magazine on the machine itself. This manipulator is a simple device which can be attached to the side of the machine. Alternatively, the tool transporter, which brings the tools to the machine, can be fitted with a manipulator arm for loading and unloading tools. Although this solution offers some economic savings by avoiding having a loading arm for each machine, it limits the tool transfer between the workstation and the additional tool buffer to the time when the tool transporter is at the machine. The storage capacity of these intermediate tool buffers and the need for a dedicated l o a d / u n l o a d arm for each machine can be assessed using computer simulation studies. Tool Transfer System. A bidirectional flow exists between the tool room and the various workstations to ensure availability of tools when needed. If tool transfer is accomplished manually in a F M S , tool kits are prepared in the tool room for each j o b or batch. These kits are sent to the appropriate workstation on a trolley, or by other means, as frequently as necessary according to the production plan and schedule. Unscheduled tool requests due to sudden breakage are also filled in the same manner. Worn tools are sent back to the tool crib where an appropriate action is taken (e.g., resharpening, replacing inserts, etc.). Transfer of tools between the tool room and the workstations may be automated in various ways. The transfer system used for moving the workpieces may be used for transferring tools as well. F o r example, tools needed for a given work-
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piece may be loaded on the same AGV which carries the part to the machine. The use of the material handling system to transport dedicated tool kits often interferes with the r a n d o m movement of workpieces and complicates material flow planning. Using the material handling system also increases the demand for labor to prepare dedicated tool sets during the daytime and significantly increases the necessary tool inventory and tool storage space. A large number of tools would remain idle and tied up with specific batches or workpieces. Another approach for automating the cutting tools transport is adopted by the Japanese machine tools m a n u f a c t u r e r Y a m a z a k i . Their s o l u t i o n requires transporting the tool drums themselves between the machining centers and tool room. A special gantry type crane, called the tools robot, is used for that purpose. In Yamazaki's first implementation, two tool drums were mounted back-toback on columns behind the machine tools and the gantry crane transfers the complete d r u m column assembly to the tool r o o m when needed. This approach has many inherent drawbacks. It introduces redundancy of tool d r u m s / c o l u m n s assembly as well as tools, it requires a specially powerful and precise gantry crane, it requires a larger than usual tool r o o m to store the tool drum assemblies, its use is limited to the Yamazaki machining centers, and above all it is cumbersome, expensive, and slow. A variation on this system has been recently introduced by Yamazaki. In the modified system, the tool drums are detachable. There are two tool drums, and while one is being used on the machine, the second is stored horizontally on a slide beside the machine. The horizontal tool drum can be exchanged during the machining cycle by a new d r u m brought to the machine by a tool drum shuttle cart called a shuttle robot. A rail-guided AGV transports tool drums between the machines and tool room. This system is more flexible and less bulky than the previously described Yamazaki a u t o m a t e d tool transfer system, but it still shares some of its drawbacks. A third approach for automating tool transfer between the machining centers and tool room involves transporting small groups of individual tools using an overhead transporter. When the tools arrive at the machine, they are loaded into an intermediate tool storage buffer or directly into the machine tool magazine by a simple robot arm.
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This approach was implemented in several systems such as the Czechoslovakian FM S developed by the V U O S O Research Institute which uses a monorail carrier to shuttle tools between the tool room and machine. -~ Special plastic cartridges are used to protect the tools during transportation and storage. Each preset tool is handled individually and can be used anywhere in the system. However, most of the standard tools are kept in the machine magazine as part of their standard tooling. Other tools are allowed to migrate a m o n g machines. A robot at each machine swaps tools between the tool carrier and the machine magazine. The robot then fetches tools from ti~e magazine and places them, one at a time, in a cartridge removal unit at the machine. This unit moves to the machine "ready" position where the tool exchanger replaces a used tool with the new one. The used tool in the plastic cartridge is then picked up by the robot and automatically returned to the magazine. The usage, location and condition of all tools are monitored by the central c o m p u t e r using a bar code reader in the robot arm. This data is used to decide on tool replacement, assignment and movement. Since the machine tools in flexible manufacturing systems process parts in random sequence, continuous regrouping of tools in tool magazines and exchange of tools frequently take place between magazines, temporary tool buffers and the tool transport cart. The automatic handling and transportation of individual tools requires extra features. For example, a protective cartridge must be used to protect the tool during transportation. This cartridge or cell must be standardized in size and geometry to facilitate handling and storage. Also, tools must be coded for identification purposes. Code is used to match a tool with tool requests in the tool room or at the machine before loading into the tool magazine. Empty tool transportation cells should not be allowed in order to avoid the tool taper contamination problems. This tool transfer method is well suited for retrofitting existing C N C machines and integrating them into F M S s or unmanned systems.
Unmanned Machining There are two crucial problems which must be overcome before the full potential of unmanned manufacturing can be realized. The first problem is the availability of effective, economical and reliable
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sensors to monitor the machine operation and detect signs of malfunction. The second problem is the development of efficient adaptive control algorithms which ensure quick enough response by the machines to input from sensors. It is normally necessary to take quick action, such as retreat of cutting tool or emergency stop of feed motion or spindle rotation, upon predicting eminent tool breakage. Economically feasible solutions to these problems, which encourage the machine tools builders to incorporated them in machine controls, is a prerequisite to the success of unattended operation of machines for a reasonably long time. A positive step toward achieving unmanned operation of manufacturing systems is the development of computer expert systems which can assist in operating, diagnosing and maintaining flexible manufacturing. Computer expert systems utilize knowledge-based interpretations using rules and knowledge provided by experts in the field. The artificial intelligence (AI) concepts and methodologies are adapted to allow decision making by the "expert system" through inference, deduction and reasoning on the basis of available complete or incomplete data. Such systems are designed to utilize multisensor raw data to monitor the operation of the various components of the manufacturing system and react to events such as equipment breakdown or traffic jam. When expert systems are implemented at the individual machine level, they are used to monitor the operation of the machine including tool utilization and performance and initiating appropriate actions, such as tool change or emergency stop, when necessary. The performance of cutting tools is one of the most critical factors governing the productivity of today's machining systems. Sensors are needed for identifying tools, setting their offset, and monitoring tool wear and breakage to compensate for undesirable dimensional effects or initiate tool replacement. The effective use of appropriate sensors can improve the reliability of the system operation and the quality of the products. In spite of the active research going on in this field, detecting sudden tool breakage remains immature compared with sensing tool wear.
Tool Life It is useful at this point to define what is meant
by tool life and explain how it may be used to devise a tool replacement strategy in a flexible manufacturing environment. A tool is no longer useful when it loses its ability to cut to specifications including geometric tolerance, surface roughness and established limits on cutting forces. The useful tool life for the same tool and operations may be longer if the tool is used on a different machine with greater tool offset, rigidity or load bearing capacity. Such variations between machines must be considered when designing sensing systems, control strategies, computer expert tool monitoring, and replacement systems. If the extent of progressive tool wear governs the useful tool life, then monitoring the cutting forces, surface finish, and size of workpiece can provide indications of the successful operation within predetermined performance limits. Sensors data can be used in real-time to compensate for tool wear by changing tool offset. However, the workpiece size and surface finish indicators of tool wear do not provide an early enough warning when tool failure is caused by fracture. Although the effect of tool breakage on cutting forces is measurable in real time, damage avoidance depends on the ability of the machine to quickly reduce feed, stop spindle or withdraw the tool. In a production environment, tool life is the economical useful life of a cutting tool before reconditioning or replacement. The units in which tool life is measured vary according to the manner in which the tool is used. It may be measured in number of tasks completed (e.g., holes drilled) or integral pieces produced under specified cutting conditions (e.g., feed, speed, depth of cut, etc.). Tool life may also be measured in machining time units, volume of material removed or surface area machined. Since today's tool sensing technology does not provide a complete, reliable and economical solution to the tool monitoring and failure detection problem, a rather conservative and deterministic approach is used in the production environment to define useful tool life. Such safe and methodical strategies for tool replacement, well before any damage to the workpiece or machine is likely to happen, do not optimize tool or machine utilization. Therefore, enhancing the capabilities of the machine to enable it to detect and respond to all modes of tool failure
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increases productivity by utilizing a larger portion of the total tool life. Attempts to develop analytical methods for predicting tool failure have not been very successful. This is mainly because the tool failure phenomenon is probabilistic in nature and failure mechanisms are not yet fully understood. Therefore, intensive efforts are directed toward identifying suitable signals to use as symptoms of tool failure.
Sensing Cutting Tool Failure A number of surveys of tool sensing methods have been published in the last few years, most notably by Micheletti et al, 3 Birla, 4 and Moriwaki. 5 Only an overall classification and brief discussion of the most common tool sensing methods are listed here. Sensing tool wear and breakage may be accomplished directly or indirectly depending on the nature of the cutting process and the type of sensor to be used. Direct sensing takes place by directly measuring or observing the tool condition, for example: 1. Sensing the existence of the tool edge. 2. Sensing the tool edge position. Indirect sensing is used when a correlation between tool condition and a given parameter is clearly established. Many parameters may be measured to indicate tool failure, such as: 1. Cutting forces. 2. Cutting temperature. 3. Power of spindle or feed drive. 4. Sound, acoustic emission. 5. Machine vibration 6. Workpiece dimensions. 7. Surface finish of workpiece. 8. Change in electrical resistance between tool and workpiece. For detailed discussions of the advantages and disadvantages of the various sensing methods, the reader is referred to the above mentioned references (References 3-5).
Tool Databases for FMSs A comprehensive and reliable tool database is very useful to the FMS user for several reasons. Firstly, the tool database is used in the day-to-day operation of the FMS to control tool inventory,
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satisfy requests for tools required for production, and help the NC part programmers in their efforts to streamline and standardize the cutting tools used in manufacturing. All the above benefits can be realized whether a manual or computerized database is used. The advantage of using computer databases, however, are numerous including ease of storing and retrieving data, minimizing human errors and duplication, maintaining the integrity of data, better control over the usage of data, and more timely reporting to management. Secondly, the tool database is essential for conducting any computer simulation analyses, which are extremely useful for evaluating the perfoimance of proposed tool handling and transfer solutions for a FMS. Thirdly, if an automated tool handling system is already in place, then a computerized tool database becomes a must. Furthermore, this database must be dynamic, with real-time capabilities to support multisensor tool monitoring and feedback, and allow simultaneous multiple access to the data. The tool database would be used continuously to assist real-time decisions to change, move, replace or replenish cutting tools. Fourthly, an effective computerized tool database is an essential ingredient of any computer expert system for FMS diagnosis. The tool database may reside in the central computer where communication with various users and workstations take place via appropriate input/ output ports. On the other hand, a distributed database may be used where each machine controller would contain all relevant tool data needed for that machine as well as the necessary control algorithms to deal with sensors input and data requests. These distributed tool databases would also be designed to allow data access and sharing by other workstations a n d / o r users depending on the design of the hierarchical control system in use. The pieces of information to be included in a tool database must be sufficient to support the following purposes: 1. 2. 3. 4. 5. 6. 7.
Long and short-term scheduling. Inventory control. Part programming. Tool kitting. Tool maintenance. Real-time process control. Tool preparation and assembly in tool room. A list of information contained in a typical tool
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database is given below. However, the organization of data and their structure will depend on the database management system used to create and use the tool database. It must be noted that this is not an exhaustive list and some variations may be necessary according to the specific application. Tool Description Manufacturer. Tool identifier code, name or symbol. Tool type, type of tool holder, clamping devices or tool assembly. Total number of available tools of each type. Tool geometry--shape, cutting edges, angles, etc. Tool size--also standard or nonstandard. Characteristics of cutting edge(s). Machines and processes where tool may be used. Tool compensation data--diameter, length. Tool Status Tool wear data. Useful tool life. Accumulated tool usage expressed in appropriate tool life units. Measured and adjusted tool dimensions (e.g., length). Total number of remaining tools of a given type. Total number of broken tools. While the tool description data remains the same, the information indicating the current tool status is updated continuously. Other tool related variables may be added to the tool database depending on the sophistication of the tool handling and control system and the degree of detail required in reporting. The benefits of setting up and using a structured tool database can be summarized as follows: 1. Minimizes tool redundancy. 2. Increases modularity and promotes universality and standardization. 3. Increases the effectiveness of tool data monitoring and management systems. 4. Enables real-time interaction with the automatic tool handling system and monitoring sensors. 5. Improves tool inventory control and rationalizes tool utilization. 6. Provides valuable records of tool data for use in simulation studies and report generation.
7. Provides timely feedback to management, reduces costs and improves productivity.
Nonmetal Cutting FMSs The discussion so far has focused on the issues related to automating handling, transporting and storing of cutting tools in flexible machining systems. However, flexible manufacturing systems are not limited to machining. Indeed, several flexible sheet metal forming systems and flexible assembly systems are operational to date. It is easy to draw parallels between these flexible manufacturing systems and flexible machining systems regarding the utilization of tools, in a broader sense, and show that the previous discussion and conclusions are still valid. Let us consider, for example, flexible sheet metal fabrication and forming systems where presses represent an important module of the overall facility. If the system is truly flexible, it must be capable of efficiently and quickly switching from one batch of products to the next to allow random parts routing and processing. This means the ability to change dies manually or automatically while minimizing downtime. Dies are costly pieces of tooling because of the precision and surface finish required to produce quality products. Some dies are very specific to a given press a n d / o r product (i.e., nonstandard) and others are more universal and interchangeable, and can be used on more than one press. In a flexible system, a variety of dies are normally needed to produce the range of products for which the system is designed. Some die storage buffer adjacent to the press, in the form of stationary racks, carousels or tooling magazines would be needed. Tooling flow will exist to transport dies between storage room and presses, as well as between presses if a die sharing strategy is utilized. The die loading and unloading mechanism, available at the press itself, may be manual, semiautomatic or totally automated depending on the press being used and the overall level of automation in the system. A number of standard and optional features are now available for moving the tooling in and out of a press, including automatic hydraulic clamping systems, quick die change mechanisms, floating air bolsters, fully powered track-guided rolling bolsters and fully automated changeover systems. Flexible assembly systems, including those
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utilizing programmable robots, provide another example where tool flow exists between a central storage preparation and calibration room and the various workstations. Tooling in this case refers to special robot tools, instrumented grippers and special assembly base plates. Since these tools are very expensive, uncontrolled duplication may be economically prohibitive and their rational use and exchange is highly desirable. The above two examples clearly show the similarity between handling tooling in cutting, sheet forming and fabrication, and assembly flexible manufacturing systems. Therefore, all arguments, discussions and conclusions regarding storage, movement, loading/unloading, and tool databases are equally valid for all cases.
Design and Simulation A number of important questions arise when a company decides to explore the merits of automated tooling for their FMS, as well as during the initial system design and selection stage. The main issues to be decided are: 1. Type of material handling system: dedicated computerized tool handling, combined tool/ w o r k p i e c e h a n d l i n g or m a n u a l t o o l transport. 2. Need for additional distributed tool storage buffers and their capacity. 3. Speed, capacity, and number of tool transporter(s). 4. Storage capacity of the central tool room. 5. Necessary tool inventory and restocking rules. 6. Tool requirements for unmanned shift(s). 7. Topology and layout of the tool transport system: straight line, loop or combination. 8. Need for a dedicated tool manipulator per machine. 9. Appropriate tool replacement strategy: individual or group replacement, free or restricted tool flow among machines (tool sharing). 10. Effect of the above factors on production rate and equipment utilization. 11. Effect of equipment breakdown on production and utilization. The selection of design parameters for a FMS tooling system and evaluation of the appropriate tooling strategy and operating rules represent a formidable challenge for the design team. The
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various alternatives must be carefully evaluated to assess their merits and drawbacks before a final selection is made. Discrete event computer simulation techniques are particularly useful in designing and evaluating automated tooling systems, answering the what if type of questions, and avoiding many costly mistakes. The programming and / or simulation language used to model and evaluate any system introduces some parameters and features which are specific to that language. However, the major and most important part of the simulation model is application dependent and can be described in generic terms. The following sections will focus on those generic aspects of flexible manufacturing systems tooling simulation models, main events and input/output data. Topology of Tooling System. There are two basic configurations to be considered, namely: a straight line or a loop as shown in Figures I and 2, or a combination. The tool transporter track may be bidirectional or unidirectional. In a dedicated tool handling system, the traffic does not usually warrant bidirectional movement. The track configuration is described by the coordinates of a suitable number of track points which represent connection with machines, intermediate buffers, tool room and other important decision points (e.g., branching or change of direction). The distance and average transporter speed are used to calculate tool transporting time. The location of intermediate tool storage buffers as well as the tool room are also defined as part of the system layout. Part A rrivaIPattern. The production schedule governs the parts availability, mix and arrival pattern. Random or deterministic arrival patterns can be easily simulated using suitable probability distribution and interarrival time. The parts introduced to the system trigger a chain of events aimed at satisfying their processing requirements. Equipment Breakdown. A realistic simulation of an automated tooling system should account for random breakdown events. These events may be defined using a random distribution characterized by the mean time between breakdowns and the mean repair time. When breakdown occurs, certain scheduled events must be altered according to prespecified rules to meet the production requirements. In practice, reliable breakdown data are difficult to obtain and simulating failures may not
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define the probability of failure. Data on abrupt tool failure is not normally provided by the tool manufacturer, nor is it readily available from users. Only those users who have used a tool monitoring system and maintained an updated tool database, can provide reliable abrupt tool breakage data. Unless reliable data is available, it is not advisable to include this option, otherwise, the simulation results will not be meaningful. ToolSeleetion/AssignmentStrategy. The tool replacement strategy is the heart of any automated tooling system, and its simulation represents the most challenge to the system modeler. A random tool flow strategy is used here to demonstrate the various events taking place when searching for a tool replacement, selecting and assigning the tool, performing the tool exchange, and updating the tool database. The random tool flow concept allows sharing of s t a n d a r d and special tools a m o n g machines and attempts to fill tool requests from the machine tool magazine first, then from intermediate tool buffers of the machine itself or other machines, and finally from the central tool room. The ultimate goal in planning and executing this strategy is reducing the distance travelled by the tool transporter, minimizing machine idle time, maximizing equipment utilization, and reducing tool redundancy and duplication. The priorities for using the tool transporter are set according to the operating rules selected by the user. The tool search strategy is summarized in Figure 3.
-/ ^ - - R o b o t ic Tool
Chm$~
C -- H~h~nin~ Unit
B -- S . ~ J i . r Tool Sta~-o~
O -- Tool Trar,,,p~t~
Figure 1 Outside Loop for Automatic
*-
Tool Transfer
Work,iece Hand]an 9 S y l t R
-~
A -- Ro~t~= too1 B -- -S~.,~-='---y1oo1 St.ce,o~
D -- Tool T~m~p~t~-
Computer Simulation
Figure 2 Linear Track
for Automatic
Tool Transfer
A discrete event computer simulation package, TOLSIM, was developed by Ha and EIMaraghy": for designing and evaluating automated tooling systems. The package is written in F O R T R A N IV and GASP IV and implemented on a DEC P D P I 1/ 34 and VAX i 1/ 730. A macro flow chart which outlines T O L S I M executive programs is shown in Figure 4. The package is capable of modeling various configurations of automated tool handling systems. TOLSIM Data Files. The simulator database consists of several data files for storing and updating information on tool status, workpieces (sequence of operations, necessary tools, arrival patterns), tool life, workstations, input, output and video display data using a master database manager.
be feasible. In those cases, some judgement should be exercised when evaluating the simulation results. Tool Wear and Breakage. A tool replacement event is triggered if progressive tool wear exceeds acceptable limits or when the tool suddenly breaks. In the simulation model, the tool usage is accumulated until the specified useful tool life expires. Correction factors, provided by the user, may be applied to account for variations in cutting conditions and improve the accuracy of simulated failure predictions. Such factors are normally based on the user's past experience. Sudden tool breakage is a random event and can be simulated using random distributions which
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Journal E~/ Manulacturing S.v,wem.~ V o l u m e 4 No. 1
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Input. The input to the simulation package is divided into the following categories: 1. Workstations--type, number, queues size, location, capacity of tool magazine, existing tools, capacity of intermediate tool storage buffer, downtime statistics, automatic tool change time, and part arrival patterns. 2. Parts--process plans, alternative routes and required tools. 3. Tools--code, tool life, probability distribution for abrupt breakage (optional), size, and assigned workstation if any. 4. Tool Transporter--tool carrying capacity (e.g., number and size of tool storage slots), speed, and track configuration. Output. The package produces performance statistics for various system components in the form of printed reports and graphs. The package also produces an animated display of the tool flow and transporter movements between workstations and
the tool room. A DEC VS/11 color refresh display or VT-100 terminal with advanced video option are used for the animated display. The output data can include any of the following: 1. Number of parts processed in each workstation and overall production. 2. Tool preparation time statistics per machine (mean, minimum, maximum and standard deviation). 3. Utilization of workstations and tool transporter. 4. Average size of queues per machine. 5. Number of broken or wornout tools. Typical screen displays produced by the TOLSIM package are shown in Figure 5. Figures 6 and 7 show examples of sensitivity analysis plots which are used to fine tune the tooling system parameters and design. For example, Figure 6 shows the effect of the storage capacity of an overhead tool transporter on the total time spent
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Journal o/ Manufarturing Systems Volume4 No. I
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Figure 6 Sample Sensitivity Plots Produced by TOLSIM
preparing tools for the next process in a flexible machining cell which contains five machining centers. Inadequate tool transporter capacity can add further delays when tool transportation between machines and the tool room is involved. The slope of the plots, in this case, would indicate that the EX-CELL-O is the least sensitive and the M PS-70 is the most sensitive. Similarly, Figure 7 shows that the effect of tool change time on the utilization of the selected three machines is negligible, while the tool transporter (cart) utilization is significantly affected. The TOLS1M package can be used to: (1) evaluate the tooling system layout and equipment utilization, (2) define the system design parameters such as capacity of tool transporter, intermediate tool storage and machine tool magazines and tool transporter speed, (3) estimate tool consumption and necessary inventory, and (4) evaluate tooling strategies and the effect of tool and machine failure on the overall
production and utilization. The TOLSI M package proved to be an effective and useful design tool for evaluating automated tooling systems used in flexible manufacturing.
Concluding Remarks Flexible manufacturing systems are captial intensive and a good return on investment can only be achieved by maximizing productivity and utilization, and minimizing idle time, including that resulting from tooling problems. This paper explored the need and benefits of automating tool handling and management in FMSs and described the various modes of implementation. Both hardware and software related issues were highlighted, including the tool database and simulation techniques. Automated tool management and handling has significant benefits irrespective of the selected level of automation. The standardization and streamlining of products,
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Journal o/ .~tanu/acluring Sv.~'tem~ Volume 4 No. I
References
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I. H. Hammer, J. Schuster. "A New Approach to I-lexible Automation of Boring and Milling Operations", tz.lur Metallhearheitung, 77. No. 5, 1983. 2. J. Zeleny. "Manufacturing Cells with Automatic '1ool l-low for Unmanned Machining of Box-like Workpieces", Proceedings t~ICIRP Seminars on Mam4lacturing Systems. Volume 12. No. 1, 1982, pp. 72-82. 3. G.I-. Micheletti, W. Koenig, H.R. Victor. "In-Process Tool Wear Sensors for Cutting Operations", Annals ¢~lthe CIRP. Volume 25, No. 2, 1976, pp. 483--496. 4. S.K. Birla. "Sensors for Adaptive Control and Machine Diagnostics", Paper 7.12, Machine Fool Controls, Technolog.r o f Machine Tool,~, Volume 4, Lawrence Livermore Laboratories, Livermore. California, 198 I. 5. [ . Moriwaki. "'Sensing and Prediction of Cutting Tool I-allure", Bulletin. Japan Society t~/ Precision Engineering, Volume 18, No. 2, June 1984, pp. 90-96. 6. D.S.M. Ho. "Simulation of Automatic Fool Transfer in Flexible Manufacturing Systems", M.Eng. Thesis, McMaster University, Hamilton, Ontario, Canada, 1983. 7. H.A. EIMaraghy. D.S.M. Ho. "Modelling and Simulation of Automatic Fool Management and Delivery in l-lexible Machining Systems", Proceedings o/the 1984 A S M E International Computers in L~gineering Conlerence, Las Vegas, Nevada, Advanced Automation: 1984 and Beyond, Volume 11. 1984, pp. 845-846.
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Eversheim, W. et al.. "'Manufacturing Cells In Unmanned Production". Proceeding,~"o f the 14th CIRP Seminar on ManH/acturing Srstems, Volume 12, No. 1, 1982. Flusty, J. and Andrews, G.C., "A Critical Review of Sensors lor Unmanned Machining", Proceedings .~or Seminar on FMS, Anaheim, California, sponsored by CASA/SME, 1983. Warnecke, H.J. and Steinhilper, R.,"New Concept of a Manufacturing Cell lor Unmanned Production", Proceedings o[ the 14th CIRP Seminar on Manu/acturing Srstems, Volume 12, No. I, 1982.
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Figure 7 Sample Sensitivity Plots Produced by TOLSIM
tools, and process plans which are associated with the implementation of this concept would have farreaching effects on the overall operation of the manufacturing enterprise.
Author Biography Hoda A. EIMaraghy is an Associate Professor of Mechanical Engineering at McMaster University, Hamilton, Canada, where she is also the Director of the Centre for Flexible Manufacturing Research and Development. Dr. EIMaraghy received her Bachelor of Engineering degree from Cairo U niversity where she taught for four years. She received a Masters and a Ph.D. degree in Mechanical Engineering from McMaster University where she was appointed to the faculty in 1976. She teaches courses in C A D / C A M and conducts research in flexible manufacturing, including flexible assembly by robots, sensors, application of At to off-line robot programming and simulation of manufacturing systems. Dr. EIMaraghy is a registered Professional Engineer in Ontario and has done cousulting engineering work for several industrial companies in North America. She is also a member of SME, CASA, ASME, NAMRI, and CAM-I.
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Abstract The need for automated tooling in flexible machining, assembly, and sheet fabrication systems is reviewed. The various methods of implementing these systems, their benefits and drawbacks are discussed. The basic modules of automated tool transfer, storage, l o a d i n g / unloading, and management are described together with the appropriate level of automation for each module. The advantages and prerequisites for unmanned machining systems, the current sensing methods and the tool replacement strategies are also reviewed. The importance of a tool database, its uses and structure are highlighted. Finally, the design and evaluation of automated tooling systems and operating strategies, with the aid of discrete events computer simulation are discussed. An existing computer package which is capable of simulating automated tooling systems for flexible manufacturing systems is presented.
Keywords." Flexible Manufacturing, Tool Handling and Management, Automated Tooling, Simulation. l-he greatest emphasis in designing and implementing flexible manufacturing systems is usually placed on the work flow, handling, and storage. Whether the FM S is utilized tor machining, assembly or fabrication, it uses essential tools to perform its function. Such tools wear out, break, a n d / o r require resetting and maintenance to ensure successful
operation of the system. For this reason a tool flow also exists within the system. The number of cutting tools in a medium size FMS can easily run into thousands. Therefore, automated tool flow, storage, retrieval, loading and unloading presents a real challenge for the FMS system designer. However, effective solutions to the problem offer handsome rewards in increased productivity, profit and competitiveness. Since flexible manufacturing systems are capital intensive, a good return on investment can only be achieved when productivity is maximized and idle time, including that resulting from tooling problems, is minimized. Automated tool handling, transfer and control strategies make it possible to extend the operation of FMSs into the third shift with a minimum of human attention, and can significantly increase the efficiency and cost effectiveness of the current generation of FMSs. Utilization of machine tools can be improved by ensuring the availability of the necessary cutting tools when needed, eliminating manual tool change and setup during machine cycle, and monitoring tool wear and potential breakage which improves accuracy and reduces scrap. This can be achieved by maintaining an adequate supply of useable cutting tools, automating tool handling, transfer and load/ unload, monitoring tool usage, wear and premature failure, and incorporating control algorithms for the automatic replacement of tools. These measures increase the actual machining time and maximize
Journal O~ Manu/acturing Systems V o l u m e 4 No.
the utilization of manpower, which results in a substantial reduction of manufacturing costs.
Automated Tooling To achieve an uninterrupted machining process, tool setup, change, and tool magazine load/ unload should take place outside the machining cycle. Automation of tool flow requires some essential elements: (!) tool transfer system, (2) tool storage facilities, (3) tool repair or replacement facilities, (4) automated tool loading and unloading mechanisms, and (5) a control logic for managing tool replacement and transfer activities. Tool Transfer. Automating the tool flow can be accomplished using the workpiece transfer system or a separate tool transfer system. Although a separate tool transfer system means additional cost, it may be necessary because of differences between tools and workpieces regarding size, holding, loading and unloading methods, frequency of motion and nature of flow. Workpiece flow tends to be a sequence of moves between workstations with infrequent trips to central or distributed stores. Tool flow, however, is mostly between individual workstations and one or more tool storage or maintenance facilities. A number of alternatives, which represent combinations of the two approaches mentioned above, may be used depending on the manufacturing system under consideration. For example, a number of standard tool kits or "libraries" may be designed for the various families of parts which are produced by the system. Such tool kits would travel with the workpieces and both automatic tool and component loading/unloading would take place at the same time. This closely allied tool and component flow is based on group technology (GT) concepts and bears resemblance to the currently used manual tool kitting techniques. Tool kits reduce capital investments in tool transfer systems as well as tools. Tool Storage, Loading and Unloading. The choice of the tool storage and handling modules, their capacity and location has a direct effect on reducing the total number of tools required in a FMS and minimizes redundancy. If tools are changed manually, then using the largest possible tool magazine would be best. However, if automated tool transfer between buffers is available, then smaller tool magazines would be sufficient.
For CNC lathes and turning centers, the capacity of the tool holding and changing unit varies between 8 and 20 tools. Therefore, the use of an additional tool storage unit would be very useful. Sandvik has introduced a block tooling system where tools are mounted on an indexing coromant tool magazine installed on the machine itself or located nearby. Hundreds of preset block tools can be stored and automatically retrieved and loaded by a special purpose manipulator which also loads the tools onto the turret. This tool storage and quick change system can be interfaced with machine control for automating tool loading and unloading. There are many special-purpose tool storage systems which can be used to supplement the tool storage capacity of machining centers. Such systems may consist of racks or carousels which carry a large number of tools, and a special load/unload mechanism. Some supplementary tool storage systems allow tool loading only when the spindle is stationary. In fully automated tooling systems appropriate for flexible manufacturing, tool exchange between the tool magazine and the secondary tool buffer should take place without interrupting the cutting process to minimize machine idle time. Therefore, extreme care must be exercised when selecting secondary tool storage buffers in order not to nullify one of the greatest benefits of an automated tooling system. Tool ControiSystem. A computerized control system is used to ensure the availability of the right tools when needed. This system schedules the tool transporter moves, interacts with machine controllers and monitoring devices to effect tool changes and resetting, and updates tool inventory and tool storage locations. When a new job order is scheduled, the computer tool control program scans the tool storage data files to ensure the availability of the necessary tools or initiates the appropriate actions to make them available. An elaborate database which includes information on tool codes, interchangeable tools and probes, useful tool life for various cutting conditions, and capacity of tool magazines and intermediate tool buffers is utilized by the tool control algorithms. Automated tool management systems must deal with the following issues: i. Ensuring that appropriate tools, in useable condition, are available at the machine when needed.
Journal ol Manula¢'turing Systems Volume 4 No, I
2. Transporting tools to and from the machines safely and reliably. 3. Keeping track of tool usage and effecting tool changes when necessary. 4. Maintaining dynamic inventory of all tools in the system, their type, size, number and condition. 5. Monitoring tool inventory levels and initiating orders to replenish tool supply when needed. 6. Keeping the time spent waiting for tools or changing tools to a minimum. More information on available automated tooling systems for flexible manufacturing may be obtained from References 1 and 2.
Rationalizing Tool Usage While automated tool handling and management systems can improve the efficiency of flexible manufacturing, there are many other factors which must not be ignored. There should always be a conscious effort to rationalize and standardize the use of tools in order to reduce their variety and number. The benefits from this rationalization apply to both manual and automated tool management. The use of group technology concepts to select the part families for FMSs is a prerequisite for the success of the system operation and implementation of random routing, germane to flexible manufacturing. Group technology not only reduces set-up time between batches, but also helps streamline the tools required to manufacture the workpieces. A company introducing a FMS into its operation should use this opportunity to investigate the use of tools planned into the work over the years, with a view to making use of new and improved cutting tools technology, reducing the tool variety, increasing the use of standard tools, rationalizing the use of nonstandard tools, and making better use of tools interchangeability. Rationalizing tool usage and minimizing tool inventory is not a small job considering the large number of tools likely to be used in any typical FMS. This task is an ideal candidate for computerized coding and classification. Effort should be made to minimize the chances of tool breakage by adjusting the cutting parameters to reduce the cutting forces at critical moments, such as cutter entry into the workpiece. Such strategies may be built into the NC cutting program,
or implemented in real-time using adaptive control methods. The trade-off between the economic benefits derived from operating at optimum cutting conditions and frequent machine idle time due to tool breakage is a crucial factor in flexible manufacturing, especially during unmanned shifts. There are also other factors which can simplify and speed up tool change and maintenance, such as the use of tools which can be preset and reset automatically, and the use of standard holders with throwaway inserts. Enforcing effective quality control of the workpiece material can help reduce incidents of tool breakage due to material flaws. Practicing the rules of design for ease of manufacturability also helps eliminate design features, such as very small long holes, which are likely to produce tool breakage.
Level of Automation Varying degrees of computer automation may be adopted regarding tool storage, handling and transfer in flexible manufacturing systems. The degree of automation depends on the desirability and duration of unmanned operation, and the availability of sensors, reliable tool monitoring systems, real-time computer control algorithms, and a computerized tool database. The application of the various levels of automation in the three basic modules of a tooling system, namely: ( i ) central tool room, (2) tool storage/handling at work stations, and (3) transfer between tool room and workstations is discussed below. Central Tool Storage. The main activities which take place in the tool room are: I. Receive tool requirements for different machines, jobs or batches. 2. Prepare necessary tools to support scheduled production including presetting, changing tool inserts, resharpening tool cutting edge(s), placing tools in appropriate holders, and grouping tools into kits according to production requirements. 3. Responding to unexpected tool requests due to sudden tool breakage. 4. Recording relevant tool characteristics and tool usage data for future reference, reporting, and inventory control. 5. Replenishing tool stocks in an orderly and timely manner according to predefined criteria.
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in a manual tool room, operators will perform the above functions without the help of computers or a u t o m a t e d equipment which precludes the possibility of unmanned manufacturing. As more and more of the activities mentioned above are computerized and appropriate measures to deal automatically with requests for tools are implemented, achieving unmanned machining comes closer to reality. I-or example, computer terminals may be used in the tool room to interact with the rest of the production system and personnel, and facilitate the exchange of information about tool requirements, production schedules and emergency situations. Indeed the tool room may be one node in a distributed numerical control (DNC) system, with access to various production databases and realtime electronic data and messages exchange. For item 2 above, there will always be tasks related to tool preparation which need human operators. Such tasks are usually performed during the manned production shifts. The ability to respond automatically to scheduled tool requirements as well as unexpected tool requests, necessitates the use of an automated storage and retrieval system (AS RS) for tools in the tool room. This A S R S must be computer controlled and integrated with the rest of the F M S to enable it to automatically receive, interpret and respond to requests for tools. Preset and prepared tools are stored in the A S R S according to production plans and estimates of broken tools due to random failures. Tools may be stored in A S R S individually or in groups according to the adopted tool transfer and control method. A computer controlled handling device is used to fetch the necessary tools from the AS RS and load them on the tool transporter. A computer controlled A S R S and handling device is a prerequisite for implementing unmanned machining for one or more shifts, irrespective of the level of automation of other activities in the tool room. The storage capacity of the A S R S can be determined using computer simulation or inventory control methods. Recording of relevant tool data in the tool room may be automated using bar codes and code readers, as well as other suitable data entry devices such as keyboards. The operator may interface with a computerized tool database by responding to prompts appearing on the screen of his terminal. This data input is then used to update the database,
which in turn is used for filling tool requests, initiating restocking orders, issuing warning messages and preparing management reports. Decisions to purchase tools, and invoke inventory control measures can be assisted or a u t o m a t e d using any of the commercially available computer programs. Distributed Tool Storage BuJfers. Distributed tool storage buffers are usually placed next to workstations to c o m p l e m e n t the tool storage capacity of the machine itself. These buffers come in the form of stationary shelves or racks or movable carousels. Some of these storage units are vertical, others are horizontal. They may be totally separate from the machine and some can be integrated with the machine and its controller. A pick-and-place manipulator, with limited degrees of freedom, may be provided to a u t o m a t e tool transfer between the buffer and the tool magazine on the machine itself. This manipulator is a simple device which can be attached to the side of the machine. Alternatively, the tool transporter, which brings the tools to the machine, can be fitted with a manipulator arm for loading and unloading tools. Although this solution offers some economic savings by avoiding having a loading arm for each machine, it limits the tool transfer between the workstation and the additional tool buffer to the time when the tool transporter is at the machine. The storage capacity of these intermediate tool buffers and the need for a dedicated l o a d / u n l o a d arm for each machine can be assessed using computer simulation studies. Tool Transfer System. A bidirectional flow exists between the tool room and the various workstations to ensure availability of tools when needed. If tool transfer is accomplished manually in a F M S , tool kits are prepared in the tool room for each j o b or batch. These kits are sent to the appropriate workstation on a trolley, or by other means, as frequently as necessary according to the production plan and schedule. Unscheduled tool requests due to sudden breakage are also filled in the same manner. Worn tools are sent back to the tool crib where an appropriate action is taken (e.g., resharpening, replacing inserts, etc.). Transfer of tools between the tool room and the workstations may be automated in various ways. The transfer system used for moving the workpieces may be used for transferring tools as well. F o r example, tools needed for a given work-
Journal ~1 Manula('turing Svstem~ Volume4
piece may be loaded on the same AGV which carries the part to the machine. The use of the material handling system to transport dedicated tool kits often interferes with the r a n d o m movement of workpieces and complicates material flow planning. Using the material handling system also increases the demand for labor to prepare dedicated tool sets during the daytime and significantly increases the necessary tool inventory and tool storage space. A large number of tools would remain idle and tied up with specific batches or workpieces. Another approach for automating the cutting tools transport is adopted by the Japanese machine tools m a n u f a c t u r e r Y a m a z a k i . Their s o l u t i o n requires transporting the tool drums themselves between the machining centers and tool room. A special gantry type crane, called the tools robot, is used for that purpose. In Yamazaki's first implementation, two tool drums were mounted back-toback on columns behind the machine tools and the gantry crane transfers the complete d r u m column assembly to the tool r o o m when needed. This approach has many inherent drawbacks. It introduces redundancy of tool d r u m s / c o l u m n s assembly as well as tools, it requires a specially powerful and precise gantry crane, it requires a larger than usual tool r o o m to store the tool drum assemblies, its use is limited to the Yamazaki machining centers, and above all it is cumbersome, expensive, and slow. A variation on this system has been recently introduced by Yamazaki. In the modified system, the tool drums are detachable. There are two tool drums, and while one is being used on the machine, the second is stored horizontally on a slide beside the machine. The horizontal tool drum can be exchanged during the machining cycle by a new d r u m brought to the machine by a tool drum shuttle cart called a shuttle robot. A rail-guided AGV transports tool drums between the machines and tool room. This system is more flexible and less bulky than the previously described Yamazaki a u t o m a t e d tool transfer system, but it still shares some of its drawbacks. A third approach for automating tool transfer between the machining centers and tool room involves transporting small groups of individual tools using an overhead transporter. When the tools arrive at the machine, they are loaded into an intermediate tool storage buffer or directly into the machine tool magazine by a simple robot arm.
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This approach was implemented in several systems such as the Czechoslovakian FM S developed by the V U O S O Research Institute which uses a monorail carrier to shuttle tools between the tool room and machine. -~ Special plastic cartridges are used to protect the tools during transportation and storage. Each preset tool is handled individually and can be used anywhere in the system. However, most of the standard tools are kept in the machine magazine as part of their standard tooling. Other tools are allowed to migrate a m o n g machines. A robot at each machine swaps tools between the tool carrier and the machine magazine. The robot then fetches tools from ti~e magazine and places them, one at a time, in a cartridge removal unit at the machine. This unit moves to the machine "ready" position where the tool exchanger replaces a used tool with the new one. The used tool in the plastic cartridge is then picked up by the robot and automatically returned to the magazine. The usage, location and condition of all tools are monitored by the central c o m p u t e r using a bar code reader in the robot arm. This data is used to decide on tool replacement, assignment and movement. Since the machine tools in flexible manufacturing systems process parts in random sequence, continuous regrouping of tools in tool magazines and exchange of tools frequently take place between magazines, temporary tool buffers and the tool transport cart. The automatic handling and transportation of individual tools requires extra features. For example, a protective cartridge must be used to protect the tool during transportation. This cartridge or cell must be standardized in size and geometry to facilitate handling and storage. Also, tools must be coded for identification purposes. Code is used to match a tool with tool requests in the tool room or at the machine before loading into the tool magazine. Empty tool transportation cells should not be allowed in order to avoid the tool taper contamination problems. This tool transfer method is well suited for retrofitting existing C N C machines and integrating them into F M S s or unmanned systems.
Unmanned Machining There are two crucial problems which must be overcome before the full potential of unmanned manufacturing can be realized. The first problem is the availability of effective, economical and reliable
Journal ~I ,~lanula~'turing St'.~tems Volume4
No. I
sensors to monitor the machine operation and detect signs of malfunction. The second problem is the development of efficient adaptive control algorithms which ensure quick enough response by the machines to input from sensors. It is normally necessary to take quick action, such as retreat of cutting tool or emergency stop of feed motion or spindle rotation, upon predicting eminent tool breakage. Economically feasible solutions to these problems, which encourage the machine tools builders to incorporated them in machine controls, is a prerequisite to the success of unattended operation of machines for a reasonably long time. A positive step toward achieving unmanned operation of manufacturing systems is the development of computer expert systems which can assist in operating, diagnosing and maintaining flexible manufacturing. Computer expert systems utilize knowledge-based interpretations using rules and knowledge provided by experts in the field. The artificial intelligence (AI) concepts and methodologies are adapted to allow decision making by the "expert system" through inference, deduction and reasoning on the basis of available complete or incomplete data. Such systems are designed to utilize multisensor raw data to monitor the operation of the various components of the manufacturing system and react to events such as equipment breakdown or traffic jam. When expert systems are implemented at the individual machine level, they are used to monitor the operation of the machine including tool utilization and performance and initiating appropriate actions, such as tool change or emergency stop, when necessary. The performance of cutting tools is one of the most critical factors governing the productivity of today's machining systems. Sensors are needed for identifying tools, setting their offset, and monitoring tool wear and breakage to compensate for undesirable dimensional effects or initiate tool replacement. The effective use of appropriate sensors can improve the reliability of the system operation and the quality of the products. In spite of the active research going on in this field, detecting sudden tool breakage remains immature compared with sensing tool wear.
Tool Life It is useful at this point to define what is meant
by tool life and explain how it may be used to devise a tool replacement strategy in a flexible manufacturing environment. A tool is no longer useful when it loses its ability to cut to specifications including geometric tolerance, surface roughness and established limits on cutting forces. The useful tool life for the same tool and operations may be longer if the tool is used on a different machine with greater tool offset, rigidity or load bearing capacity. Such variations between machines must be considered when designing sensing systems, control strategies, computer expert tool monitoring, and replacement systems. If the extent of progressive tool wear governs the useful tool life, then monitoring the cutting forces, surface finish, and size of workpiece can provide indications of the successful operation within predetermined performance limits. Sensors data can be used in real-time to compensate for tool wear by changing tool offset. However, the workpiece size and surface finish indicators of tool wear do not provide an early enough warning when tool failure is caused by fracture. Although the effect of tool breakage on cutting forces is measurable in real time, damage avoidance depends on the ability of the machine to quickly reduce feed, stop spindle or withdraw the tool. In a production environment, tool life is the economical useful life of a cutting tool before reconditioning or replacement. The units in which tool life is measured vary according to the manner in which the tool is used. It may be measured in number of tasks completed (e.g., holes drilled) or integral pieces produced under specified cutting conditions (e.g., feed, speed, depth of cut, etc.). Tool life may also be measured in machining time units, volume of material removed or surface area machined. Since today's tool sensing technology does not provide a complete, reliable and economical solution to the tool monitoring and failure detection problem, a rather conservative and deterministic approach is used in the production environment to define useful tool life. Such safe and methodical strategies for tool replacement, well before any damage to the workpiece or machine is likely to happen, do not optimize tool or machine utilization. Therefore, enhancing the capabilities of the machine to enable it to detect and respond to all modes of tool failure
Journal ~I ,~latlu/at'luring Sl'stem.~ Volume4
increases productivity by utilizing a larger portion of the total tool life. Attempts to develop analytical methods for predicting tool failure have not been very successful. This is mainly because the tool failure phenomenon is probabilistic in nature and failure mechanisms are not yet fully understood. Therefore, intensive efforts are directed toward identifying suitable signals to use as symptoms of tool failure.
Sensing Cutting Tool Failure A number of surveys of tool sensing methods have been published in the last few years, most notably by Micheletti et al, 3 Birla, 4 and Moriwaki. 5 Only an overall classification and brief discussion of the most common tool sensing methods are listed here. Sensing tool wear and breakage may be accomplished directly or indirectly depending on the nature of the cutting process and the type of sensor to be used. Direct sensing takes place by directly measuring or observing the tool condition, for example: 1. Sensing the existence of the tool edge. 2. Sensing the tool edge position. Indirect sensing is used when a correlation between tool condition and a given parameter is clearly established. Many parameters may be measured to indicate tool failure, such as: 1. Cutting forces. 2. Cutting temperature. 3. Power of spindle or feed drive. 4. Sound, acoustic emission. 5. Machine vibration 6. Workpiece dimensions. 7. Surface finish of workpiece. 8. Change in electrical resistance between tool and workpiece. For detailed discussions of the advantages and disadvantages of the various sensing methods, the reader is referred to the above mentioned references (References 3-5).
Tool Databases for FMSs A comprehensive and reliable tool database is very useful to the FMS user for several reasons. Firstly, the tool database is used in the day-to-day operation of the FMS to control tool inventory,
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satisfy requests for tools required for production, and help the NC part programmers in their efforts to streamline and standardize the cutting tools used in manufacturing. All the above benefits can be realized whether a manual or computerized database is used. The advantage of using computer databases, however, are numerous including ease of storing and retrieving data, minimizing human errors and duplication, maintaining the integrity of data, better control over the usage of data, and more timely reporting to management. Secondly, the tool database is essential for conducting any computer simulation analyses, which are extremely useful for evaluating the perfoimance of proposed tool handling and transfer solutions for a FMS. Thirdly, if an automated tool handling system is already in place, then a computerized tool database becomes a must. Furthermore, this database must be dynamic, with real-time capabilities to support multisensor tool monitoring and feedback, and allow simultaneous multiple access to the data. The tool database would be used continuously to assist real-time decisions to change, move, replace or replenish cutting tools. Fourthly, an effective computerized tool database is an essential ingredient of any computer expert system for FMS diagnosis. The tool database may reside in the central computer where communication with various users and workstations take place via appropriate input/ output ports. On the other hand, a distributed database may be used where each machine controller would contain all relevant tool data needed for that machine as well as the necessary control algorithms to deal with sensors input and data requests. These distributed tool databases would also be designed to allow data access and sharing by other workstations a n d / o r users depending on the design of the hierarchical control system in use. The pieces of information to be included in a tool database must be sufficient to support the following purposes: 1. 2. 3. 4. 5. 6. 7.
Long and short-term scheduling. Inventory control. Part programming. Tool kitting. Tool maintenance. Real-time process control. Tool preparation and assembly in tool room. A list of information contained in a typical tool
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database is given below. However, the organization of data and their structure will depend on the database management system used to create and use the tool database. It must be noted that this is not an exhaustive list and some variations may be necessary according to the specific application. Tool Description Manufacturer. Tool identifier code, name or symbol. Tool type, type of tool holder, clamping devices or tool assembly. Total number of available tools of each type. Tool geometry--shape, cutting edges, angles, etc. Tool size--also standard or nonstandard. Characteristics of cutting edge(s). Machines and processes where tool may be used. Tool compensation data--diameter, length. Tool Status Tool wear data. Useful tool life. Accumulated tool usage expressed in appropriate tool life units. Measured and adjusted tool dimensions (e.g., length). Total number of remaining tools of a given type. Total number of broken tools. While the tool description data remains the same, the information indicating the current tool status is updated continuously. Other tool related variables may be added to the tool database depending on the sophistication of the tool handling and control system and the degree of detail required in reporting. The benefits of setting up and using a structured tool database can be summarized as follows: 1. Minimizes tool redundancy. 2. Increases modularity and promotes universality and standardization. 3. Increases the effectiveness of tool data monitoring and management systems. 4. Enables real-time interaction with the automatic tool handling system and monitoring sensors. 5. Improves tool inventory control and rationalizes tool utilization. 6. Provides valuable records of tool data for use in simulation studies and report generation.
7. Provides timely feedback to management, reduces costs and improves productivity.
Nonmetal Cutting FMSs The discussion so far has focused on the issues related to automating handling, transporting and storing of cutting tools in flexible machining systems. However, flexible manufacturing systems are not limited to machining. Indeed, several flexible sheet metal forming systems and flexible assembly systems are operational to date. It is easy to draw parallels between these flexible manufacturing systems and flexible machining systems regarding the utilization of tools, in a broader sense, and show that the previous discussion and conclusions are still valid. Let us consider, for example, flexible sheet metal fabrication and forming systems where presses represent an important module of the overall facility. If the system is truly flexible, it must be capable of efficiently and quickly switching from one batch of products to the next to allow random parts routing and processing. This means the ability to change dies manually or automatically while minimizing downtime. Dies are costly pieces of tooling because of the precision and surface finish required to produce quality products. Some dies are very specific to a given press a n d / o r product (i.e., nonstandard) and others are more universal and interchangeable, and can be used on more than one press. In a flexible system, a variety of dies are normally needed to produce the range of products for which the system is designed. Some die storage buffer adjacent to the press, in the form of stationary racks, carousels or tooling magazines would be needed. Tooling flow will exist to transport dies between storage room and presses, as well as between presses if a die sharing strategy is utilized. The die loading and unloading mechanism, available at the press itself, may be manual, semiautomatic or totally automated depending on the press being used and the overall level of automation in the system. A number of standard and optional features are now available for moving the tooling in and out of a press, including automatic hydraulic clamping systems, quick die change mechanisms, floating air bolsters, fully powered track-guided rolling bolsters and fully automated changeover systems. Flexible assembly systems, including those
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utilizing programmable robots, provide another example where tool flow exists between a central storage preparation and calibration room and the various workstations. Tooling in this case refers to special robot tools, instrumented grippers and special assembly base plates. Since these tools are very expensive, uncontrolled duplication may be economically prohibitive and their rational use and exchange is highly desirable. The above two examples clearly show the similarity between handling tooling in cutting, sheet forming and fabrication, and assembly flexible manufacturing systems. Therefore, all arguments, discussions and conclusions regarding storage, movement, loading/unloading, and tool databases are equally valid for all cases.
Design and Simulation A number of important questions arise when a company decides to explore the merits of automated tooling for their FMS, as well as during the initial system design and selection stage. The main issues to be decided are: 1. Type of material handling system: dedicated computerized tool handling, combined tool/ w o r k p i e c e h a n d l i n g or m a n u a l t o o l transport. 2. Need for additional distributed tool storage buffers and their capacity. 3. Speed, capacity, and number of tool transporter(s). 4. Storage capacity of the central tool room. 5. Necessary tool inventory and restocking rules. 6. Tool requirements for unmanned shift(s). 7. Topology and layout of the tool transport system: straight line, loop or combination. 8. Need for a dedicated tool manipulator per machine. 9. Appropriate tool replacement strategy: individual or group replacement, free or restricted tool flow among machines (tool sharing). 10. Effect of the above factors on production rate and equipment utilization. 11. Effect of equipment breakdown on production and utilization. The selection of design parameters for a FMS tooling system and evaluation of the appropriate tooling strategy and operating rules represent a formidable challenge for the design team. The
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various alternatives must be carefully evaluated to assess their merits and drawbacks before a final selection is made. Discrete event computer simulation techniques are particularly useful in designing and evaluating automated tooling systems, answering the what if type of questions, and avoiding many costly mistakes. The programming and / or simulation language used to model and evaluate any system introduces some parameters and features which are specific to that language. However, the major and most important part of the simulation model is application dependent and can be described in generic terms. The following sections will focus on those generic aspects of flexible manufacturing systems tooling simulation models, main events and input/output data. Topology of Tooling System. There are two basic configurations to be considered, namely: a straight line or a loop as shown in Figures I and 2, or a combination. The tool transporter track may be bidirectional or unidirectional. In a dedicated tool handling system, the traffic does not usually warrant bidirectional movement. The track configuration is described by the coordinates of a suitable number of track points which represent connection with machines, intermediate buffers, tool room and other important decision points (e.g., branching or change of direction). The distance and average transporter speed are used to calculate tool transporting time. The location of intermediate tool storage buffers as well as the tool room are also defined as part of the system layout. Part A rrivaIPattern. The production schedule governs the parts availability, mix and arrival pattern. Random or deterministic arrival patterns can be easily simulated using suitable probability distribution and interarrival time. The parts introduced to the system trigger a chain of events aimed at satisfying their processing requirements. Equipment Breakdown. A realistic simulation of an automated tooling system should account for random breakdown events. These events may be defined using a random distribution characterized by the mean time between breakdowns and the mean repair time. When breakdown occurs, certain scheduled events must be altered according to prespecified rules to meet the production requirements. In practice, reliable breakdown data are difficult to obtain and simulating failures may not
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define the probability of failure. Data on abrupt tool failure is not normally provided by the tool manufacturer, nor is it readily available from users. Only those users who have used a tool monitoring system and maintained an updated tool database, can provide reliable abrupt tool breakage data. Unless reliable data is available, it is not advisable to include this option, otherwise, the simulation results will not be meaningful. ToolSeleetion/AssignmentStrategy. The tool replacement strategy is the heart of any automated tooling system, and its simulation represents the most challenge to the system modeler. A random tool flow strategy is used here to demonstrate the various events taking place when searching for a tool replacement, selecting and assigning the tool, performing the tool exchange, and updating the tool database. The random tool flow concept allows sharing of s t a n d a r d and special tools a m o n g machines and attempts to fill tool requests from the machine tool magazine first, then from intermediate tool buffers of the machine itself or other machines, and finally from the central tool room. The ultimate goal in planning and executing this strategy is reducing the distance travelled by the tool transporter, minimizing machine idle time, maximizing equipment utilization, and reducing tool redundancy and duplication. The priorities for using the tool transporter are set according to the operating rules selected by the user. The tool search strategy is summarized in Figure 3.
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A discrete event computer simulation package, TOLSIM, was developed by Ha and EIMaraghy": for designing and evaluating automated tooling systems. The package is written in F O R T R A N IV and GASP IV and implemented on a DEC P D P I 1/ 34 and VAX i 1/ 730. A macro flow chart which outlines T O L S I M executive programs is shown in Figure 4. The package is capable of modeling various configurations of automated tool handling systems. TOLSIM Data Files. The simulator database consists of several data files for storing and updating information on tool status, workpieces (sequence of operations, necessary tools, arrival patterns), tool life, workstations, input, output and video display data using a master database manager.
be feasible. In those cases, some judgement should be exercised when evaluating the simulation results. Tool Wear and Breakage. A tool replacement event is triggered if progressive tool wear exceeds acceptable limits or when the tool suddenly breaks. In the simulation model, the tool usage is accumulated until the specified useful tool life expires. Correction factors, provided by the user, may be applied to account for variations in cutting conditions and improve the accuracy of simulated failure predictions. Such factors are normally based on the user's past experience. Sudden tool breakage is a random event and can be simulated using random distributions which
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Journal E~/ Manulacturing S.v,wem.~ V o l u m e 4 No. 1
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Input. The input to the simulation package is divided into the following categories: 1. Workstations--type, number, queues size, location, capacity of tool magazine, existing tools, capacity of intermediate tool storage buffer, downtime statistics, automatic tool change time, and part arrival patterns. 2. Parts--process plans, alternative routes and required tools. 3. Tools--code, tool life, probability distribution for abrupt breakage (optional), size, and assigned workstation if any. 4. Tool Transporter--tool carrying capacity (e.g., number and size of tool storage slots), speed, and track configuration. Output. The package produces performance statistics for various system components in the form of printed reports and graphs. The package also produces an animated display of the tool flow and transporter movements between workstations and
the tool room. A DEC VS/11 color refresh display or VT-100 terminal with advanced video option are used for the animated display. The output data can include any of the following: 1. Number of parts processed in each workstation and overall production. 2. Tool preparation time statistics per machine (mean, minimum, maximum and standard deviation). 3. Utilization of workstations and tool transporter. 4. Average size of queues per machine. 5. Number of broken or wornout tools. Typical screen displays produced by the TOLSIM package are shown in Figure 5. Figures 6 and 7 show examples of sensitivity analysis plots which are used to fine tune the tooling system parameters and design. For example, Figure 6 shows the effect of the storage capacity of an overhead tool transporter on the total time spent
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Journal o/ Manufarturing Systems Volume4 No. I
FMS t o o I s - t P a n s ~ e l
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Figure 6 Sample Sensitivity Plots Produced by TOLSIM
preparing tools for the next process in a flexible machining cell which contains five machining centers. Inadequate tool transporter capacity can add further delays when tool transportation between machines and the tool room is involved. The slope of the plots, in this case, would indicate that the EX-CELL-O is the least sensitive and the M PS-70 is the most sensitive. Similarly, Figure 7 shows that the effect of tool change time on the utilization of the selected three machines is negligible, while the tool transporter (cart) utilization is significantly affected. The TOLS1M package can be used to: (1) evaluate the tooling system layout and equipment utilization, (2) define the system design parameters such as capacity of tool transporter, intermediate tool storage and machine tool magazines and tool transporter speed, (3) estimate tool consumption and necessary inventory, and (4) evaluate tooling strategies and the effect of tool and machine failure on the overall
production and utilization. The TOLSI M package proved to be an effective and useful design tool for evaluating automated tooling systems used in flexible manufacturing.
Concluding Remarks Flexible manufacturing systems are captial intensive and a good return on investment can only be achieved by maximizing productivity and utilization, and minimizing idle time, including that resulting from tooling problems. This paper explored the need and benefits of automating tool handling and management in FMSs and described the various modes of implementation. Both hardware and software related issues were highlighted, including the tool database and simulation techniques. Automated tool management and handling has significant benefits irrespective of the selected level of automation. The standardization and streamlining of products,
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Journal o/ .~tanu/acluring Sv.~'tem~ Volume 4 No. I
References
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I. H. Hammer, J. Schuster. "A New Approach to I-lexible Automation of Boring and Milling Operations", tz.lur Metallhearheitung, 77. No. 5, 1983. 2. J. Zeleny. "Manufacturing Cells with Automatic '1ool l-low for Unmanned Machining of Box-like Workpieces", Proceedings t~ICIRP Seminars on Mam4lacturing Systems. Volume 12. No. 1, 1982, pp. 72-82. 3. G.I-. Micheletti, W. Koenig, H.R. Victor. "In-Process Tool Wear Sensors for Cutting Operations", Annals ¢~lthe CIRP. Volume 25, No. 2, 1976, pp. 483--496. 4. S.K. Birla. "Sensors for Adaptive Control and Machine Diagnostics", Paper 7.12, Machine Fool Controls, Technolog.r o f Machine Tool,~, Volume 4, Lawrence Livermore Laboratories, Livermore. California, 198 I. 5. [ . Moriwaki. "'Sensing and Prediction of Cutting Tool I-allure", Bulletin. Japan Society t~/ Precision Engineering, Volume 18, No. 2, June 1984, pp. 90-96. 6. D.S.M. Ho. "Simulation of Automatic Fool Transfer in Flexible Manufacturing Systems", M.Eng. Thesis, McMaster University, Hamilton, Ontario, Canada, 1983. 7. H.A. EIMaraghy. D.S.M. Ho. "Modelling and Simulation of Automatic Fool Management and Delivery in l-lexible Machining Systems", Proceedings o/the 1984 A S M E International Computers in L~gineering Conlerence, Las Vegas, Nevada, Advanced Automation: 1984 and Beyond, Volume 11. 1984, pp. 845-846.
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Bibliography 0
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5
Eversheim, W. et al.. "'Manufacturing Cells In Unmanned Production". Proceeding,~"o f the 14th CIRP Seminar on ManH/acturing Srstems, Volume 12, No. 1, 1982. Flusty, J. and Andrews, G.C., "A Critical Review of Sensors lor Unmanned Machining", Proceedings .~or Seminar on FMS, Anaheim, California, sponsored by CASA/SME, 1983. Warnecke, H.J. and Steinhilper, R.,"New Concept of a Manufacturing Cell lor Unmanned Production", Proceedings o[ the 14th CIRP Seminar on Manu/acturing Srstems, Volume 12, No. I, 1982.
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15 20 Tool Change Time (see)
Figure 7 Sample Sensitivity Plots Produced by TOLSIM
tools, and process plans which are associated with the implementation of this concept would have farreaching effects on the overall operation of the manufacturing enterprise.
Author Biography Hoda A. EIMaraghy is an Associate Professor of Mechanical Engineering at McMaster University, Hamilton, Canada, where she is also the Director of the Centre for Flexible Manufacturing Research and Development. Dr. EIMaraghy received her Bachelor of Engineering degree from Cairo U niversity where she taught for four years. She received a Masters and a Ph.D. degree in Mechanical Engineering from McMaster University where she was appointed to the faculty in 1976. She teaches courses in C A D / C A M and conducts research in flexible manufacturing, including flexible assembly by robots, sensors, application of At to off-line robot programming and simulation of manufacturing systems. Dr. EIMaraghy is a registered Professional Engineer in Ontario and has done cousulting engineering work for several industrial companies in North America. She is also a member of SME, CASA, ASME, NAMRI, and CAM-I.
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