《机械专业英语》课件第20章.ppt

1、Unit 20TEXT LEARN TO SPEAK NEW WORDS AND PHRASES NOTES EXERCISES READING MATERIAL PROFESSIONAL WORDS AND PHRASES Wire-cut EDM(1)Principle of EDMElectrical discharge machining，commonly known as EDM，is a controlled metal removal process whereby an electric spark is used to cut(erode)the workpiece，whic

2、h then takes the shape opposite to that of the cutting tool or electrode.The electrode and the workpiece are both submerged in a dielectric fluid，which is generally a light lubricating oilThis dielectric fluid should be a nonconductor of electricity.A servomechanism maintains a gap of about 0.0005 t

3、o 0.001 in.(0.01 to 0.02 mm)between the electrode and the work,preventing them from coming into contact with each other.A direct current of low voltage and high amperage is delivered to the electrode at the rate of approximately 20,000 hertz(Hz)These electrical energy impulses become sparks which ju

4、mp the gap between the electrode and the workpiece through the dielectric fluidIntense heat is created in the localized area of the spark impact；the metal melts and a small particle of molten metal is expelled from the workpiece.The dielectric fluid，which is constantly being circulated，carries away

5、the eroded particles of metal and also helps in dissipating the heat caused by the spark There are two types of EDM machines used in industry：the vertical EDM machine and the wire-cut EDM machine(as shown in Fig.20-l).Since the wire-cut EDM is generally used for machining complex forms which require

6、 NC programming，only this type will be discussed in detail Wire-cut EDMThe wire-cut EDM is a discharge machine which uses NC movement to produce the desired contour or shape on a partIt does not require a special-shaped electrode；instead，it uses a continuous traveling wire under tension as the elect

7、rodeThe electrode or cutting wire can be made of brass，copper，or any other electrically conductive material ranging in diameter from 0.002 to 0.012 in.(0.05 to 0.30 mm).The path that the wire follow is controlled along a two-axis(XY)contour，eroding(cutting)a narrow slot through the workpieceThis con

8、trolled movement is continuous and simultaneous in increments of 0.000,05 in(0.001 mm).Any contour may be cut to a high degree of accuracy and is repeatable for any number of successive parts.A dielectric fluid，usually demonized water which is constantly being circulated,carries away the eroded part

9、icles of metalThe dielectric fluid maintains the proper conductivity between the wire and the workpiece and assists in reducing the heat caused by the spark Fig.20-1 Electrical discharge machines(a)Vertical EDM；(b)Wire-cut EDM Parts of the Wire-cut EDMThe main parts of the wire-cut EDM are the bed，s

10、addle，table，column，arm，UV axis head，wire feed and dielectric systems，and machine control unit(MCU)(as shown in Fig.20-2)Fig.20-2 The main parts of a wire-cut EDM Bed：The bed is a heavy，rugged casting used to support the working parts of the wire-cut EDMGuide rails(“ways”)are machined on the top sect

11、ion，and these guide and align major parts of the machine.Saddle：The saddle is fitted on top of the guide rail and may be moved in the XY direction by feed servomotors and ball lead screwsTable：The table，mounted on top of the saddle，is U-shaped and contains a series of drilled and tapped holes equall

12、y spaced around the top surfaceThese are used for the workpiece holding and clamping devices.Column：The column，which is attached to the bed，supports the wire feed system，the UV axes，and the capacitor switchesWire feed system：The wire feed system is used to provide a continual feed of new wire(electr

13、ode)for the cutting operationThe wire is fed from a supply spool through a series of guides and guide rollers which apply tension to the wire.The wire travels in a continuous path past the workpiece，and the used wire is rewound on a take-up spoolDielectric system：The dielectric system contains filte

14、rs，an ion exchanger，and a cooler.This system provides a continuous flow of clean deionized water at a constant temperatureThe deionized water stabilizes the cutting operation，flushes away particles of electrode and workpiece material that have been eroded，and cools the workpiece MCU：The MCU can be s

15、eparated as three individual panels(1)The control panel for setting the cutting condition(2)The control panel for machine setup(3)The control panel for manual data input(MDI)and cathode-ray tube(CRT)character display(as showin in Fig.20-3)Fig.20-3 Control panel LEARN TO SPEAKMr.Zhang:Hi,Miss.Li.Mr.L

16、i:Hello,Mr.Cheng.Mr.Zhang:Would you mind asking you some questions?Miss.Li:No problem.What do you want to know?Mr.Zhang:How many types of EDM machines are there used in industry?Miss Li:There are two.Mr.Zhang:Can you describe it in detail?Miss Li:Yes.There are vertical EDM machine and the wire-cut E

17、DM machine.And the wire-cut EDM is generally used for machining complex forms which require NC programming.Mr.Zhang:What is the main parts of the wire-cut EDM?Miss Li:The main parts of the wire-cut EDM are the bed，saddle，table，column，arm，UV axis head，wire feed and dielectric systems，and machine cont

18、rol unit(MCU).Mr.Zhang:Is the MCU can be separated as three individual panels?Miss Li:Yes.Mr.Zhang:Can you describe it in detail?Miss Li:The control panel for setting the cutting condition,the control panel for machine setupAnd the control panel for manual data input(MDI)and cathode-ray tube(CRT)cha

19、racter displayMr.Zhang:Youre so kind for telling me about these.Miss Li:For my pleasure.Mr.Zhang:Bye-bye.Miss Li:Bye.NEW WORDS AND PHRASES spark n.火花；火星 vi.发出火星；发出闪光 n火花 erode vt.&vi.侵蚀，腐蚀，电蚀 submerge vt.&vi.(使)潜入水中，淹没，浸入 vt.完全掩盖，遮掩 nonconductor n.不传导体，绝缘体 amperage n.安培数，电流强度 impulse n.脉搏，脉冲 dischar

20、ge vt.卸船；偿还;放电 dissipate vt.&vi.驱散;消失;耗散 vt.浪费 dielectric n.电介质,绝缘体 adj.非传导性的 deionize v.除去离子 rugged adj.结实的，坚固的 align v.调整，校准 clamp v.夹住 ion n.离子 capacitor n.电容器 CRT 阴极射线管 Wire-Cut EDM 线切割电火花加工 guide roller 导轮 NOTES1.Electrical discharge machining，commonly known as EDM，is a controlled metal removal

21、 process whereby an electric spark is used to cut(erode)the workpiece，which then takes the shape opposite to that of the cutting tool or electrode电火花加工通常简称EDM，它是采用电火花放电腐蚀工件来控制金属切削的加工方法。这样加工出的形状与切割刀具即电极的形状相反。句中关系副词“whereby”相当于by which、by what，意思是“借以”。“which”引出的非限定性定语从句修饰“the workpiece”。2.These electr

22、ical energy impulses become sparks which jump the gap between the electrode and the work-piece through the dielectric fluid脉冲能量在电极与工件间的工作液中火花放电。句中“which”引导的定语从句修饰sparks。“dielectric fluid”可译为“工作液”。3.The path that the wire follows is controlled along a two-axis(XY)contour，eroding(cutting)a narrow slot

23、 through the workpiece切割线被控制在X-Y平面内沿工件轮廓曲线移动，从而在工件上电蚀出一条狭缝。句中“that the wire follows”为定语从句，修饰“The path”。句子的主干是The path is controlled along。“eroding”是现在分词短语作伴随状语。4.A dielectric fluid，usually deionized water which is constantly being circulated,carries away the eroded particles of metal工作液通常是除去离子的水，它不断

24、循环流动，冲走腐蚀的金属微粒。deionized water：除去离子的水。“which is constantly being circulated”为定语从句，修饰“deionized water”。句子主干是A dielectric fluid carries away the eroded particles of metal.5.The wire is fed from a supply spool through a series of guides and guide rollers which apply tension to the wire新线从贮丝筒的供线端绕过一系列导轮

25、与导柱，这些导轮和导柱给切割线提供一定张紧力。“which”引导的定语从句修饰“guides and guide rollers”，说明它们所起的作用。supply spool：贮丝筒。applyto：把施加于。EXERCISES1.Discuss the following questions in English:(1)How can EDM remove metal from a workpiece?(2)Why must an appropriate gap be maintained between the electrode and the work?(3)What is the

26、dielectric fluid used for?(4)What kind of metal can be used as an electrode?(5)How many main parts does a wire-cut EDM have?2.Use English replace Chinese or use Chinese replace English.(1)电容器(2)线切割电火花加工(3)工作液(4)EDM(5)CRT(6)MCU 3.Place a“T”after sentences that are true and an“F”after those that are f

27、alse.(1)EDM uses an electric spark to cut the workpiece into the same shape as the electrode has.()(2)The dielectric fluid is used only to carry away the eroded particles of metal and assist in reducing the heat caused by the spark.()(3)A wire-cut EDM machine cant cut a contour to a high degree of a

28、ccuracy.()(4)The dielectric system of a wire-cut EDM provides a continuous flow of clean deionized water at a constant temperature in order to keep the operation done successfully.()READING MATERIALWire-cut EDM(2)Machine Operating Systems Various systems on wire-cut EDM machines play an important pa

29、rt in the efficient operation of the machine tool.Servo systems，dielectric fluid，electrode，and the MCU are the main operating components of wire-cut EDM machines The ServomechanismThe EDM power supply controls the cutting current levels and the feed rate of the drive motors.It also controls the trav

30、eling speed of the wireEDM machines are equipped with a servo control mechanism that automatically maintains a constant gap of approximately 0.001 to 0.002 in.(0.02 to 0.05 mm)between the wire and the workpieceIt is important that there be no physical contact between the wire(electrode)and the workp

31、iece；otherwise arcing could damage the workpiece and break the wire.The servomechanism also advances the wire into the workpiece as the operation progresses and senses the work-wire spacing and slows or speeds up the drive motors as required to maintain the proper arc gap.Precise control of the gap

32、is essential to a successful machining operation.If the gap is too large，ionization of the dielectric fluid does not occur and machining can not take place.If the gap is too small,the wire will touch the workpiece，causing it to melt and break.Precise gap control is accomplished by a circuit in the p

33、ower supply comparing the average gap voltage to a preselected reference voltage.The difference between the two voltages is the input signal,which tells the servomechanism how far and how fast to feed the wire and when to retract from the work-piece.This is usually indicated by a reverse servo light

34、When chips in the spark gap reduce the voltage below a critical level,the servomechanism causes the wire to retract until the chips are flushed out by the dielectric fluidThe servo system should not be too sensitive to“short-lived”voltages caused by chips being flushed out；otherwise the wire would b

35、e constantly retracting,thereby seriously affecting machining rates.The Dielectric Fluid One of the most important factors in a successful EDM operation is the removal of the particles(chips)from the working gap.Flushing these particles out of the gap with the dielectric fluid will produce good cutt

36、ing conditions，while poor flushing will cause erratic cutting and poor machining conditions.The dielectric fluid in the wire-cut EDM process is usually deionized water.This is tap water that is circulated through an ion-exchange resin.The deionized water makes a good insulator,while untreated water

37、is a conductor and is not suitable for the EDM process.The ion exchanger is a compound of positive ion exchange resin(cation)and negative ion exchange resin(anion).When water is applied to this compound,the ion exchange reaction starts and is continuously repeated until all the impurities are comple

38、tely removed from the water，thus producing pure water.The amount of deionization is measured by its specific resistance.For most operations，the lower the resistance，the faster the cutting speed.However，the resistance of the dielectric fluid should be much higher when carbides and high-density graphi

39、te are cut(The dielectric fluid system as show in Fig.20-4).Fig.20-4 The dielectric fluid system The dielectric fluid in the wire-cut EDM process serves several functions:(1)It helps to initiate the spark between the wire(electrode)and the workpiece.(2)It serves as an insulator between the wire and

40、the workpiece.(3)It flushes away the particles of disintegrated wire and workpiece to prevent shorting.(4)It acts as a coolant for both the wire and the workpiece.The dielectric fluid must be circulated under constant pressure if it is to flush away the particles and assist in the machining process.

41、The flow of water is governed by two control valves:one controlling the water flowing above the workpiece and one controlling the flow under the workpiece.When starting to supply the water for the cutting process,apply a steady amount“down-stream”and then slowly apply the water from underneath until

42、 there is a flaring effect at the top surface of the workpiece(as shown in Fig.20-5).Fig.20-5 A steady stream of dielectric must be supplied to the workpiece If red sparks occur during the cutting operation，the water supply is inadequateTo overcome this problem，increase the flow of water until blue

43、sparks appearNote:Excessive water flow can cause the wire to be deflected，causing erratic cutting and loss machining accuracy.The ElectrodeThe electrode in wire-cut EDM is a spool of wire ranging from 0.002 to 0.012 in.(0.05 to 0.30 mm)in diameter and from 2 to 100 1b(0.90 to 45.36 kg)in weightThe l

44、ength of wire on a spool can provide over 500h of unattended machining timeThe electrode of the wire-cut machine continuously travels from a supply spool to a take-up spool so that it is constantly renewed.When this type of electrode is used，the wear on the wire does not affect the accuracy of the c

45、ut because new wire is being fed past the workpiece continuously at rates from a fraction of an inch to several inches per minuteBoth the electrode wire and the material removal rate from the workpiece depend on such things as the materials thermal conductivity，its melting point，and the duration and

46、 intensity of the electrical pulsesAs in conventional machining，some EDM materials have better cutting and wearing qualities than others；therefore，electrode materials must have the following characteristics:(1)Be a good conductor of electricity;(2)Have a high melting point;(3)Have a high tensile str

47、ength;(4)Have good thermal conductivity;(5)Produce efficient metal removal from die workpiece.Research and experimentation are continually increasing metal removal rates and finding good，economical materials for the manufacture of EDM wire electrodes.Brass，copper，tungsten，molybdenum，and zinc are som

48、e of the materials which have found certain applications as electrode materialsThe most widely used material is brass wire 0.008 in.(0.20 mm)in diameterA normal overcut of about 0.001 in.(0.02 mm)per side will produce an internal corner with a radius of about 0.005 in.(0.12 mm).Smaller diameters of

49、brass wire are also used for many applicationsTungsten and molybdenum wire，which have a very high melting point and high tensile strength，permit small-diameter wire of 0.002 in(0.05 mm)in diameter to be used for cutting fine radial and intricate shapes.Stratified wire，which consists of a copper core

50、 with a thin surface layer of zinc，gives the wire the high conductivity of copper combined with the cooling effect of zincThis allows the use of higher current，increasing the energy level of the EDM spark and，as a result，the metal removal rate.PROFESSIONAL WORDS AND PHRASES servo n伺服系统 ionization n离