外文翻譯--電火花加工技術(shù)-11頁[中英word]【中英文文獻(xiàn)譯文】,中英word,中英文文獻(xiàn)譯文,外文,翻譯,電火花,加工,技術(shù),11,十一,word,中英文,文獻(xiàn),譯文 Electrical-Discharge Machining 1. Electrical-discharge Machining Electrical-discharge machining (EDM),or spark machining, as it is also called, removes material with repetitive spark discharges from a pulsating DC power supply, with a dielectric flowing between the work piece and the tool. The principle of the EDM process is illustrated by the simplified diagram. The tool is mounted on the chuck attached to the machine spindle whose motion is controlled by a servo-controlled feed drive. The workpiece is placed in a tank filled with a dielectric fluid; a depth of at least 50mm over the work surface is maintained to eliminate the risk of fire. The tool and workpiece are connected to a pulsating DC power supply. Dielectric fluid is circulated under pressure by a pump, usually through a hole or holes in the tool electrode. A spark gap of about 0.025 to 0.05mm is maintained by the servomotor. In power supplies for EDM the input power is first converted into continuous DC power by conventional solid-state rectifiers. The flow of this DC power is then controlled by a bank of power transistors which are switched by a digital multivibrator oscillator circuit. The high-power pluses output is then applied to the tools and work piece to produce the sparks responsible for material removal. Each spark generates a localized high temperature on the order of 12000℃ in its immediate vicinity. This heat caused part of the surrounding dielectric fluid to evaporate; it also melts and vaporizes the metal to form a small crater on the work surface. Since the spark always occurs between the points of the tool and work piece that are closest together, the high spots of the work are gradually eroded, and the form of the tool is reproduced on the work .The condensed metal globules, formed during the process, are carried away by the flowing dielectric fluid. As the metal is eroded, the tool is fed toward the work piece by a servo-controlled feed mechanism. Each pulse in the EDM cycle lasts for only a few microseconds. Repeated pulses, at rates up to 100000 per second, result in uniform erosion of material from the work piece and from the electrode. As the process progressed, the electrode is advanced by the servo drive toward the work piece to maintain a constant gap distance until the final cavity is produced. Application Electrical-discharge machining can be used for all electrically conducting materials regardless of hardness. The process is most suited to the sinking of irregularly shaped holes, slots, and cavities. Fragile work pieces can be machined without breakage. Holes can be of various shapes and can be produced at shallow angles in curved surfaces without problems of tool wander. The EDM process finds greatest application at present in toolmarking, particularly in the manufacture of press tools, extrusion dies, forging dies, and molds. Graphite electrodes produced by copy milling from patterns are often used. A great advantage of EDM is that the tool or die can be machined after it is hardened and hence great accuracy can be achieved. Tools of cemented carbide can be machined after final sintering, which eliminates the need for an intermediate partial sintering stage, thus eliminating the inaccuracies resulting from final sintering after holes, slots, and so on, are machined. Electrical-discharge machining can be used effectively to drill small high-aspect-ratio holes. Diameters as small as 0.3mm in material 20mm or more in thickness can be readily achieved. With efficient flushing, holes with aspect ratios as high as 100:1 have been produced. The process has been used successfully to produce very-small-diameter holes in hardened fuel-injector nozzles. Varying numbers of holes in a precise patten can be drilled around the injector tip. 2．Numerical Control Numerical control (NC) is a method of controlling the movements of machine components by directly inserting coded instructions in the form of numerical data (numbers and data) into the system. The system automatically interprets these data and converts it to output signals. These signals, in turn control various machine components, such as turning spindles on and off, changing tools, moving the workpiece or the tools along specific paths, and turning cutting fluids on and off. In order to appreciate the importance of numerical control of machines, let’s briefly review how a process such as machining has been carried out traditionally. After studying the working drawing of a part, the operator sets up the appropriate process parameters(such as cutting speed, feed, depth of cut, cutting fluid, and so on), determines the sequence of operations to be performed, clamps the workpiece in a workholding device such as a chuck or collet, and proceeds to make the part. Depending on part shape and the dimensional accuracy specified, this approach usually requires skilled operators. Furthermore, the machining procedure followed may depend on the particular operator, and because of the possibilities of human error, the parts produced by the same operator may not all be identical. Part quality may thus depend on the particular operator or even the same operator on different days or different hours of the day. Because of our increased concern with product quality and reducing manufacturing costs, such variability and its effects on product quality are no longer acceptable. This situation can be eliminated by numerical control of the machining operation. We can illustrate the importance of numerical control by the following example. Assume that holes have to be drilled on a part in the positions shown in the picture. In the traditional manual method of machining this part, the operator positions the drill with respect to the workpiece, using as reference points any of the three method shown. The operator then proceeds to drill these holes. Let’s assume that 100 parts, having exactly the same shape and dimensional accuracy, have to be drilled. Obviously, this operation is going to be tedious because the operator has to go through the same motions again and again. Moreover, the probability is high that, for various reasons, some of the paths machined will be different from others. Let’s further assume that during this production run, the order for these paths is changed, so that 10 of the paths now require holes in different positions. The machinist now has to reset the machine, which will be time consuming and subject to error. Such operations can be performed easily by numerical control machines that are capable of producing parts repeatedly and accurately and of handling different parts by simply loading different part programs. In numerical control, data concerning all aspects of the machining operation, such as locations, speeds, feeds, and cutting fluid, are stored on magnetic tape ,cassetts, floppy or hard disks, or paper or plastic (Mylar, which is a thermoplastic polyester) tape. Data are stored on punched 25mm wide paper or plastic tape, as originally developed and still used. The concept of NC control is that holes in the tape represent specific information in the form of alphanumeric codes. The presence (on) or absence (off) of these holes is read by sensing devices in the control panel, which then actuate relays and other devices (called hard-wired controls). These devices control various mechanical and electrical systems in the machine. This method eliminated manual setting of machine positions and tool paths or the use of templates and other mechanical guides and devices. Complex operations, such as turning a part having various contours and die sinking in a milling machine, can be carried out. Numerical control has had a major impact on all aspects of manufacturing operations. It is a widely applied technology, particularly in the following areas: a) Machining centers. b) Milling, turning, boring, drilling, and grinding. c) Electrical-discharge, laser-beam, and electron-beam machining. d) Water-jet cutting. e) Punching and nibbling. f) Pipe bending and metal spinning. g) Spot welding and other welding and cutting operation. h) Assembly operations. Numerical control machines are now used extensively in small-and-medium-quantity (typically 500 parts or less) of a wide variety of parts in small shops and large manufacture facilities. Older machines can be retrofitted with numerical control. Advantages and Limitations Numerical control has the following advantages over conventional method of machine control: 1. Flexibility of operation and ability to produce complex shapes with good dimensional accuracy, repeatability, reduced scrap loss, and high production rates, productivity, and product quality. 2. Tooling costs are reduced, since templates and other fixtures are not required. 3. Machine adjustments are easy to make with minicomputer and digital readout. 4. More operations can be performed with each setup, and less lead time for setup and machining is required compared to conventional methods. Design changes are facilitated, and inventory is reduced. 5. Programs can be prepared rapidly and can be recalled at any time utilizing microprocessors. Less paperwork is involved. 5. Faster prototype production is possible 6. Required operator skill is less, and the operator has more time to attend to other tasks in the work area. The major limitations of NC are the relatively high cost of the equipment and the need for programming and special maintenance, requiring trained personal. Because NC machines are complex systems, breakdowns can be very costly, so preventive maintenance is essential. However, these limitations are often easily outweighed by the overall economic advantages of NC. 3．Scope of CAD/CAM Computer-aided design is the use of computer systems to facilitate the creation, modification, analysis, and optimization of a design. In this context the term computer system means a combination of hardware and software. Computer-aided manufacturing is the use of a computer system to plan, manage, and control the operation of a manufacturing plant. An appreciation of the scope of CAD/CAM can be obtained by considering the stages that must be completed in the design and manufacture of a product, as illustrate by the product cycle shown. The inner loop of this figure includes the various steps in the product cycle and outer loop show some of the functions of CAD/CAM superimposed the product cycle. Based on market and customer requirements, a product is conceived, which may well be a modification of previous products. This product is then designed in detail, including any required design analysis, and drawings and parts lists are prepared. Subsequently, the various components and assemblies are planned for production, which involves the selection of sequences of processes and machine tools and the estimation of cycle times, together with the determination of process parameters, such as feeds and speeds. When the product is in production, scheduling and control of manufacture take place, and the order and timing of each manufacturing step for each component and assembly is determined to meet an overall manufacturing schedule. The actual manufacturing and control of product quality then takes place according to the schedule and the final products are delivered to the customers. Computer-based procedures have been or are being developed to facilitate each of these stages in the product cycle. Computer-aided design and drafting techniques have been developed. These allow a geometric model of the product and its components to be created in the computer. This model can then be analyzed using specialized software packages, such as those for finite element stress analysis, mechanisms design, and so on. Subsequently, drawings and parts lists can be produced with computer-aided drafting software and plotters. Computer-aided process-planning systems, including the preparation of NC programs, are available that produce work plans, estimates, and manufacturing instruction automatically from geometric descriptions of the components and assemblies. For scheduling and production control, large amounts of data and numerous relatively simple calculations must be carried out. One example is the determination of order quantities by subtracting stock levels from forecasts of the number of items required during a particular manufacturing period. Many commercial software packages are available for scheduling, inventory control, and shop floor control, including materials requirements planning (MRP) system. At the shop floor levels computers are used extensively for the control and monitoring of individual machines. There is a difference in the time scale required for processing data and the issuing of instruction for these various applications of computers in the product cycle. For example, design and process-planning functions are carried out once for each new product and the time scale required is on the order of weeks to years for the completion of the whole task. Scheduling and production controls tasks will be repeated once every production period (usually one week) throughout the year. A t the machine-control level instructions must be issued continually with a time scale of micro-or nanoseconds in many cases. One of the major objectives of CAM is the integration of the various activities in the product cycle into one unified system, in which data is transferred from one function to another automatically. This leads to the concept of computer-integrated manufacture (CIM), with the final objective being the “paperless” factory. Several developments have taken place, but no totally integrated CIM systems have yet been achieved. Since the design and process-planning function are carried out once in the product cycle, these are the most suitable functions for integration. This integration is particular desirable because the geometric data generated during the design process is one of the basic inputs used by process planning when determining appropriate manufacturing sequences and work plans. Consequently, various activities in design and process planning can share a common design and manufacturing data base. With such a system ,geometric models of the products and components are created during the design process. This data is then accessed by various downstream activities, including NC programming, process planning, and robot programming. The programs and work plans generated by these activities are also added to the data base. Production control and inventory control programs can then access the work plans, time estimate, and parts lists (bill of material file), in preparing the manufacturing schedules, for example. 電火花加工技術(shù) 1．電火花加工 電火花加工（EDM），顧名思義，它是通過脈沖直流電源不斷產(chǎn)生火花放電來去除工件材料的，且在工件與工具之間有絕緣液體介質(zhì)。 電火花加工的工作原理如簡圖所示。工具夾在卡盤上，卡盤與由伺服進(jìn)給系統(tǒng)控制的主軸相連。工件放在充滿絕緣液體介質(zhì)的工作槽中。在工作表面至少要維持50mm的距離，是為了消除火災(zāi)的隱患。工具和工件與脈沖電源的兩輸出端相連。絕緣液體介質(zhì)通過工具電極的小孔，經(jīng)油泵加壓，強迫循環(huán)的。伺服系統(tǒng)控制電火花間隙為0.025-0.05mm。 電火花加工的電源首先是將輸入的電源通過晶體整流管轉(zhuǎn)化為直流電源，直流電源又受到通過數(shù)字多諧振蕩電路轉(zhuǎn)換來的晶體管的控制。輸出的高頻脈沖作用到工具和工件上，產(chǎn)生電火花來去除工件材料。 每個電火花瞬間產(chǎn)生高達(dá)12000℃的局部高溫，這些熱量使部分絕緣液體介質(zhì)蒸發(fā)，也使工件表面蝕除一小部分金屬，在工件表面形成一個小凹坑。由于在極間距離相對最近擊穿放電，工件表面逐漸被蝕除掉，工具的形狀復(fù)制到工件上了。在此過程中形成的一些濃縮的金屬小屑被流動的絕緣液體介質(zhì)排除出去。隨著金屬被蝕除掉，工具電極通過飼服進(jìn)給系統(tǒng)控制向工件進(jìn)給。 電火花加工中每個脈沖延續(xù)的時間只有幾個微秒，經(jīng)過不斷的重復(fù)放電，工件和工具電極有一樣的腐蝕形狀。隨著電火花加工的進(jìn)行，工具電極不斷向工件進(jìn)給，直到加工完成，一直保持一定的放電間隙。 應(yīng)用 電火花加工能加工任何硬度的導(dǎo)電材料，且大部分用于加工不規(guī)則的孔，槽和型腔。那些剛度低的工件也可以加工。電火花加工還可加工出各種形狀的孔以及曲面上角度很小的孔，且不存在工具漂移的問題。 目前，電火花加工極廣泛地用于模具制造，特別是壓力機模具，擠壓模，鍛模和鑄模等。通過模型復(fù)制制造出來的石墨電極也經(jīng)常使用。 電火花加工的優(yōu)點就是工具在硬化處理后仍能加工出來，因此能達(dá)到很高的精度。硬質(zhì)合金的工具在燒結(jié)后也能加工出來。 電火花加工能有效的加工出又小又深的孔。已經(jīng)在直徑只有0.3mm的材料上鉆出深20mm甚至更深的孔。經(jīng)過有效的吹氮脫氣，可以加工出寬徑比為100：1的孔。電火花加工已成功地用于已淬硬噴油嘴的極小孔的加工，能在噴油嘴周圍精確的鉆出大量的孔。 2．?dāng)?shù)字控制 數(shù)控是一種用數(shù)字控制機床各部件運動的方法，通過直接向系統(tǒng)輸入指令代碼（數(shù)字和字母）來完成的。系統(tǒng)自動將這些指令代碼轉(zhuǎn)化成信號輸出。這些信號依次控制機床各種部件的運動，比如主軸的啟動和停止，刀具的轉(zhuǎn)換，沿指定路徑移動刀具和工件，控制切削液的通斷等等。 為了說明數(shù)控機床的重要性，我們來簡單回顧一下傳統(tǒng)機床的加工過程。操作者研究零件工作圖后，調(diào)整合適的加工參數(shù)（如切削速度，進(jìn)給量，切削深度，切削液等等），安排加工順序，然后將工件夾緊在夾具（如卡盤或夾頭）上，再開始加工。根據(jù)所規(guī)定的工件形狀和尺寸精度，這種加工通常需要熟練的操作工。而且，其后續(xù)加工是由各個操作者完成的。由于存在不可避免的人為誤差，即便由同一個人加工出來的零件也不可能完全相同。因此，零件的質(zhì)量就可能取決于操作者的操作水平，甚至取決于該工人在不同時期或不同時間的狀態(tài)。由于我們越來越關(guān)注加工質(zhì)量和降低加工成本，所以我們不再允許存在零件偏差和產(chǎn)品的質(zhì)量影響，而通過數(shù)控加工就可以消除以上這些情況。 我們可以通過以下的例子來說明數(shù)控加工的重要性。假如要在圖示位置的零件上鉆這幾個孔，當(dāng)傳統(tǒng)的手工操作機床加工此零件時，操作者可選圖示三種方法中的任一種，使鉆頭與工件上的點相對應(yīng)著，然后鉆這些孔。假如要加工100個同樣形狀，同樣尺寸，同樣精度的零件，很明顯，操作者會覺得很枯燥，因為操作者要一遍又一遍重復(fù)同樣的動作，而且，由于各種原因。有些零件加工出來的不一樣的可能性是很高的。我們進(jìn)一步假設(shè)，在操作過程中，零件的加工要求要改變，現(xiàn)在要在不同的位置加工出10個孔，機械師必須馬上調(diào)整機床，這樣既浪費時間又增加了加工誤差。而數(shù)控機床能夠重復(fù)而準(zhǔn)確地加工工件，而且可通過簡單地輸入不同程序來加工不同的零件。因此，使用數(shù)控機床就可以輕而易舉地完成此類加工。 在數(shù)控系統(tǒng)中，與加工過程各種相關(guān)的數(shù)據(jù)如工件的定位，切削速度，進(jìn)給量和切削液，儲存在磁盤，盒式錄音帶，軟盤，硬盤，紙帶或塑料紙（熱塑性樹脂）上。將數(shù)據(jù)存儲在25mm寬的穿孔紙帶或塑脂帶上，這種數(shù)據(jù)存儲方法使用最早并沿用至今。數(shù)控的概念就是紙帶上的孔表示以字母代碼表達(dá)的特定信息。這些孔的打開和關(guān)閉由控制面板的感應(yīng)元件控制，然后驅(qū)動繼電器和其他機械導(dǎo)向裝置。一些復(fù)雜操作如切削具有不同輪廓，外形的零件或在鉆床上刻模也可以實現(xiàn)了。 數(shù)控加工在制造各方面有著深遠(yuǎn)的影響，特別在以下的加工領(lǐng)域中廣泛使用： a） 加工中心。 b） 銑，車，鏜，鉆，磨。 c） 放電加工，激光加工和電子束加工。 d） 水射流切削。 e） 沖孔和分段沖模。 f） 彎管和金屬旋壓 g） 點焊，其他焊接和切削加工。 h） 裝配。 數(shù)控機床廣泛使用在小型或大型機械制造中，加工出品種繁多， 少量或中等批量（小于或等于500）的零件?，F(xiàn)在也可以用數(shù)控改裝舊的機床了。 優(yōu)點和局限性 數(shù)控加工與傳統(tǒng)的機加工相比，具有以下的優(yōu)點： 1． 操作簡便，能加工出尺寸精度高的復(fù)雜形狀的工件，重復(fù)性好，能降低材料的浪費，生產(chǎn)速度快，生產(chǎn)效率高，加工質(zhì)量高。 2． 降低工具的成本，因為一些模型和工件夾具都不需要了。 3． 通過微機和數(shù)字輸出，很容易調(diào)整機床。 4． 每一步工序可以同時加工所多個零件，與傳統(tǒng)的機加工相比，裝夾和加工的時間減少了。圖樣的轉(zhuǎn)換也更容易了。 5． 能很快準(zhǔn)備好加工工序，微處理器在任何時候都能存儲這些工序，手工計算已經(jīng)不需要了。 6． 可以快速加工出原來的模型。 7． 不需要更多的操作者技術(shù)了，操作者在車間有更多的時間從事其他工作了。 數(shù)控加工的最大局限性就是設(shè)備的成本相對比較高，需要程序控制，特別的維護和經(jīng)過專門培訓(xùn)的操作者。因為數(shù)控機床系統(tǒng)復(fù)雜，且每個零部件都比較昂貴，因此這種維護是必不可少的。然而，這些局限性在經(jīng)濟上經(jīng)常比數(shù)控加工的優(yōu)點更為突出。 3．計算機輔助設(shè)計/計算機輔助制造的范圍 計算機輔助設(shè)計是利用計算機系統(tǒng)更方便對設(shè)計進(jìn)行創(chuàng)造，修改，分析，優(yōu)化設(shè)計等等。在這里，計算機系統(tǒng)包括硬件和軟件。計算機輔助制造是利用計算機系統(tǒng)對制造車間進(jìn)行設(shè)計，管理，操作控制。通過考慮產(chǎn)品設(shè)計和制造完成的整個過程，我們可以對計算機輔助設(shè)計/計算機輔助制造的范圍做一個評估。圖中內(nèi)圈為產(chǎn)品生產(chǎn)過程中的各個環(huán)節(jié)，外圈則是在產(chǎn)品基本生產(chǎn)環(huán)節(jié)上所增加的計算機輔助設(shè)計/計算機輔助制造的功能。 基于市場和顧客的需求，生產(chǎn)商家必須構(gòu)思產(chǎn)品，這樣可以對原產(chǎn)品進(jìn)行改進(jìn)。然后這產(chǎn)品進(jìn)行詳細(xì)設(shè)計，通過各種需要的設(shè)計分析，準(zhǔn)備好圖紙和零件明細(xì)表。其次，要對各個零部件的生產(chǎn)作出規(guī)劃，其中包括安排加工順序，選擇機床，估算生產(chǎn)周期，確定工藝參數(shù)（如進(jìn)給量和切削速度）。當(dāng)產(chǎn)品進(jìn)行生產(chǎn)時，按照整個制造的安排來確定每個零部件每個步驟制造的時間。根據(jù)安排表來保證產(chǎn)品制造和控制的質(zhì)量，然后把成品賣給顧客。 計算機程序已經(jīng)或者正在開發(fā)，這樣方便了生產(chǎn)循環(huán)的每個環(huán)節(jié)。計算機輔助設(shè)計和繪圖技術(shù)也得到開發(fā)，這就要求產(chǎn)品的幾何模型和組成在計算機里生成。這模型可以用特定的軟件包，比如有限元法受力分析，機械設(shè)計等等來分析。接下來，通過計算機輔助繪圖軟件和繪圖儀可以畫出圖紙和零件明細(xì)表。包含有編制數(shù)控程序功能的計算機輔助工藝過程系統(tǒng)設(shè)計，可以根據(jù)零件的幾何參數(shù)和裝配要求自動地編出作業(yè)計劃，進(jìn)行計算，生成加工指令。 為了達(dá)到生產(chǎn)管理的目標(biāo)，必須需要大量數(shù)據(jù)和進(jìn)行眾多相對簡單的計算。例如，將某一生產(chǎn)周期所需物料的預(yù)測量減去庫存量，便可確定該物料的定貨量。許多商業(yè)軟件包可以提供時序安排，庫存管理，車間管理包括物質(zhì)需求計劃體系。在車間里，計算機更廣泛地用于對每臺機器的監(jiān)視和控制。 在產(chǎn)品生產(chǎn)環(huán)節(jié)中，時間標(biāo)度所需的程序數(shù)據(jù)與各種計算機應(yīng)用的指令是不相同的。例如，對每一種新產(chǎn)品及其工藝過程進(jìn)行設(shè)計，整個工作所需的時間及數(shù)周乃至幾年。時間安排和生產(chǎn)控制工作在一年的每個生產(chǎn)時期（通常為一星期）都要重復(fù)。在機器控制條件下，在許多情況下，那些指令出現(xiàn)的時間只會持續(xù)微秒甚至納秒。 計算機輔助制造最大的目標(biāo)就是在生產(chǎn)環(huán)節(jié)中各種活動成為一元化系統(tǒng)。數(shù)據(jù)能自動從一種功能轉(zhuǎn)換為另一種功能。這就有了計算機集成制造這個概念。最后的目標(biāo)就是無紙傳送信息。在每個生產(chǎn)周期對產(chǎn)品及工藝過程設(shè)計，有對集成最適合的功能。這種集成是很有必要的，因為設(shè)計過程中生成的幾何參數(shù)是在制定合適的制造過程和作業(yè)計劃時確定工藝過程所需的基本輸入數(shù)據(jù)之一。因此，在工藝過程設(shè)計中的各個活動可以共享同一個設(shè)計和制造數(shù)據(jù)庫。有了這樣一個系統(tǒng)，產(chǎn)品和零部件的幾何模型在設(shè)計過程中就可以設(shè)計出來。這些數(shù)據(jù)可以通過以下途徑存取，包括數(shù)控程序，工藝過程設(shè)計，自動控制程序。通過這些活動得到的技術(shù)和工作計劃也存到數(shù)據(jù)庫中。產(chǎn)品控制和庫存控制程序也可存取工作計劃，預(yù)算時間，零件列表等等。 Page 12 of 12