履帶式液壓挖掘機(jī)液壓系統(tǒng)設(shè)計(jì)【含說(shuō)明書+CAD圖紙】
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COMPUTER AIDED MANUFACTURING
The term Computer Aided Manufacturing (CAM) covers many areas from information processing and decision making to manufacturing and machining, which makes giving a single definition for CAM extremely difficult. D. Kochan gave a very fitting definition for CAM, with its diversity and wide range of use, in his book, “CAM can be defined as computer-aided preparation of manufacturing including decision-making, process and operational planning, software design techniques, and artificial intelligence, and manufacturing with different types of automation (NC machine, NC machine centers, NC machining cells, NC flexible manufacturing systems ), and different types of realization (CNC single unit technology, DNC group technology ).”
Since CAM has such a wide range of uses, a better way to look at CAM is through CAM technologies. The CAM technologies covered are group technology, manufacturing database, automated and tolerancing.
The essential role of the computer in the production function is to capture and process the data relating to a large number of transactions which continuously take place in different departments of the company. The initial research activity for CAM was Numerical Control (NC) for machine tools at the Massachusetts Institute of Technology in 1953. The first programming language was Automatically Programming Tools (APT) created at MIT, and it was the pattern for many further developments. Currently, many manufacturing functions have been addressed by CAM including the following :
Numerical Control (NC)
Computer numerical control (CNC)
Direct numerical control (DNC)
Computer controlled conveyor systems
Computer controlled machining process
Computer aided process monitoring
Computer aided fixturing design
Computer aided tooling design
Computer aided tolerancing analysis
Computer aided cost estimating
Material requirement planning
Computer aided process planning
Computerized machinability data systems
Manufacturing resources planning
Computer aided decision support systems
Development of work standards
Computer aided line balancing
Production and inventory planning
Computer aided scheduling
Computer aided quality control
Computer aided inspection
Computer Numerical Control. Numerical control (NC) is a form of programmable automation in which the processing equipment is controlled by means of numbers, letters , and other symbols. The numbers, letters, and symbols are coded in an appropriate format to define a program is of instructions for the particular workpiece or job. When the job changes, the program is what makes NC suitable for low-volume and medium-volume production, and it is much easier to write new programs than to make major alterations to the processing equipment.
The principle of numerical control was first applied to the milling process, and then later to the turning process, flame cutting, drilling, and grinding. NC technology is now used more and more for other manufacturing processes, such as forming (fine forging, rolling, etc.) engraving, and laser cutting.
The current NC equipment is relatively more mature. Many machines possess multiple processing functions, such as milling centers which can perform vertical and horizontal milling, drilling, boring, reaming, slotting, shaping, and turning processes. Of course, with a high capacity automated tooling library, CNC machines’functions can be considerably more abundant.
Programmable Logic Controller . Programmable logic controller are widely used in computer aided manufacturing. Actually , PLCs are used in virtually every segment of industry where automation is required. PLCs represent one of the faster growing segments of the electronics industry. Since their inception, PLCs have proved to be the salvation of many manufacturing plans which previously relied on electro-mechanical control system. A PLC is a solid-state device designed to perform logic functions previously accomplished by electro-mechanical relays. The design of most PLCs is similar to that of a computer . Basically, the PLC is an assembly of solid-state digital logic elements designed to make logical decisions and provide outputs. Programmable logic controllers are used for the control and operation of manufacturing process equipment and machinery.
Computer sided material handling. Material handling is a very important factor in how efficiently a workshop or company can be operated. An efficient MH system will help reduce waiting time, and it may even help increase safety or the effectiveness of the entire manufacturing process.
Cabbert and Brown indicated that as much as 60% of the total production cost may be accounted for by material handling. It is also evidenced that most discrete manufacturing products spend 90% of their manufacturing lead time on the duration of material handling and storage. With MH accounting for such a large amount of the total production cost, it is obvious that reducing the amount of time a product is handled will dramatically reduce production costs. One way of helping reduce these costs is by using computers to do some material handling.
There is a great variety of material handling equipment available commercially and there are many types of MH approaches used today. One of these approaches is to use a computer database to store listings of MH equipment and the user’s input of factor values. The computer takes the user’s required level of, and preferred importance for, each criterion, and the feasible MH equipment for the task at hand, and produces a category of equipment from which the user can choose the proper type or piece of MH equipment.
Computer Monitoring and diagnostics for manufacturing process. In a computer monitoring and diagnostic system, the aim of monitoring is to detect failures, while the aim of diagnostics includes fault lacalization and indentification. Both monitoring and diagnostics should appear at all levels of the control-monitoring hierarchy.
There are some essential requirements that almost every monitoring and diagnostic system should possess. Some of the requirements for a monitoring system are: (1) the ability to measure and process relatively numerous analogue and digital signals; (2)the capability of profound preprocessing of measured signals, including statistical and frequency based analysis; (3) the ability for complex, multi-parameter decisions; (4)modular, extendable, reconfigurable structure; (5) programmability in all functions; and (6) standardized bi-directional software/hardware interface to the CNC/DNC controllers. Some of the requirements for a diagnostic system are: (1) the system should easily provide knowledge about the causal interrelationship when faults arise, to enable even workers who are not well acquainted with the process to lacalize faults; (2) the consequences of faults should be readily available in the system so that the severity of a given fault for the further production process can be estimated; (3) the user should have the possibility of repairing the fault alone, i.e.repair instructions should be available to the users in a suitable form;(4) the operation of the expert system should be possible by employees who have no previous experience with computers; and (5) after a short training period , the system should be maintained by the employees running the facility so that the presence of expert engineers is no longer necessary.
There are three major types of M/D systems that can be classified by their place and function in the manufacturing system. These M/D systems are : (1) autonomous subsystem monitoring, which gets only messages containing environment or condition descriptions from upper levels of control, and supplies all of the elements of the monitoring process with instructions, parameters, or settings needed for measuring, processing, classification, and intervention; (2) complementary subsystem monitoring, which undertakes only the task of measuring and processing and passes classification and intervention to the system level; and (3) semi-autonomous monitoring, which performs only simple, quick monitoring functions autonomously on its own level, and turns to upper levels in the case of sophisticated classification and intervention tasks.
The ideal computer monitoring and diagnostic system can be summed up as being a system that can be used during the absence of the human expert, for example, when the expert ins on vacation, during breaks, or if a company wants to have three shifts with few people on the third shift.
Selected from ＜＜ design of machine elements ＞＞edited by M.F. Spotts
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