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机械专业英语董建国第20章


Unit 20

Unit 20
TEXT

READING MATERIAL

Unit 20

TEXT The EDM Process
It is known to every machinist that electrical energy is
convert

ed into motion by an electric motor on standard machine tools. Recently, we have discovered that electrical energy can be directly employed in metal removal. The main advantage is that metal of any hardness can be

machined by electrical machining processes with good
surface finish.

Unit 20
Electrical discharge machining is a form of metal removal in which pulsating direct current is applied to a shaped tool (electrode) and a work-piece, both of which are capable of conducting electricity. The two are held close to each other with

a nonconducting fluid serving as an insulator between them.
When a voltage high enough to break down the insulator is reached, a spark jumps the gap between the tool and the workpiece. As a result, this spark removes a small portion of material.

Unit 20
The surface finish of a part produced on an EDM machine
depends on the size of the sparks generated between it and the electrode. Big, powerful sparks produce a rough surface and small, less powerful sparks produce a smooth surface. It is reported that metal removal rates and surface finish are controlled by the frequency and intensity of the spark. High frequency, low-intensity sparks result in a low metal removal

rate and produce a smooth finish. Low frequency, high-intensity
sparks result in rapid metal erosion and a coarse finish.

Unit 20

LEARN TO SPEAK
Mr.Zhang:Hello, Mr. Cheng.

Mr.Cheng:Hi, Mr. Zhang.
Mr.Zhang:What are you doing? Mr.Cheng:I am machining a work-piece. Mr.Zhang:But there is a flame on the work-piece. Is this a kind of method of welding? Mr.Cheng:It is EDM process. Mr.Zhang:What does the EDM mean? Mr.Cheng:It means electrical discharge machining, and it uses the electrical energy remove directly the metal of work-piece.

Unit 20
Mr.Zhang:How does it work? Mr.Cheng:When machining pulsating direct current is applied

to a shaped tool(electrode) and a work-piece, both of which are
capable of conducting electricity. When the two are close to each other, a powerful sparks will remove a small portion of material. Mr.Zhang:What is its advantage? Mr.Cheng:The main advantage is that it can machine metal of

any hardness and form a good surface finish.
Mr.Zhang:Oh, how nice it is. Thanks a lot. Mr.Cheng:Not at all.

Unit 20

NEW WORDS AND EXPRESSIONS
electrical [I5lektrIk(E)l] adj. 电的,电力的

electrically
discharge discover pulsate current electrode close closely

[I?lektrIkElI] adv. 用电,电学上
[5dIstFB:dV] [dI5skQvE(r)] [5kQrEnt] [I5lektrEJd] [klEJs] [5klEJzlI] vt. 放电 v. 发现 vi. 脉动,波动 n. 电流 n. 电极 v. 关闭 adv. 接近地,紧密地

[pQl5seIt; (?@) 5pQlseIt]

nonconducting [nBnkEn?dQktIN]

adj. 不导电的

Unit 20
fluid insulator voltage reach spark jump gap result portion powerful report frequency erosion [5flu:Id] [5InsjJleItE(r)] [5vEJltIdV] [ri:tF] [spB:k] [dVQmp] [^Ap] [rI5zQlt] [5pC:F(E)n] [5paJEfJl] [rI5pC:t] [5fri:kwEnsI] [I5rEJV(E)n] n. 流体,液体 n. 绝缘体 n. 电压 vt. 到达,达到 n. 火花,电火花 n.&v. 跳 n. 间隙 n.&v. 结果 n. 部分 adj. 强有力 v.&n. 报告,通报 n. 频率,频繁 n. 腐蚀,侵蚀

Unit 20
electrical discharge machining(EDM)
direct current=D.C.

电火花加工
直流电

break down
close to each other as a result as result of result from result in a small portion of

破坏,毁坏,击穿
相互靠近 结果,因此 ……结果,由于 起因,由来 导致,结果形成 小部分

Unit 20

NOTES
1.It is known to every machinist that… 每一个机械师都知道…… ? ?It is…that…句型中,it是形式主语,that…引出的从 句是本句真正的主语。 2.…that metal of any hardness can be machined by electrical machining processes with good surface finish. 用电加工法能够加工任何硬度的金属,而且表面粗糙度

小。
? ?That引出的是表语从句,with good surface finish是 从句的状语。

Unit 20
3.Electrical discharge machining is a form of metal removal in which pulsating direct current is applied to a shaped tool (electrode) and a work-piece, both of which are capable of conducting electricity. 电火花加工是一种去除金属的方式。在此过程中,脉冲直 流电通到能导电的成形刀具(电极)和工件上。 ? 其中in which pulsating direct current is applied to a shaped tool (electrode) and a work-piece, both of which are capable of

conducting electricity是主句的定语从句,修饰表语a form of
metal removal,而both of which are capable of conducting electricity则是该从句中的从句,修饰a shaped tool (electrode)

and a work-piece。

Unit 20

EXERCISES
1.Discuss the following questions in English: (1) ?What is the electrical discharge machining(EDM)? (2) ?Which work can the EDM do? Give some examples. (3) ?Can the EDM machine the plastic work-piece? Why? 2.Use English replacing Chinese or use Chinese replacing English: (1) 用电火花加工的方法可以加工很硬的金属工件。

Unit 20
(2) 电火花加工所用的刀具就是电极。 (3) 电火花加工可以得到高质量的表面。 (4) ?The EDM use direct current to machine work-piece. (5) ?The work-piece and electrode are held close to each other with a non-conducting fluid serving as an insulator between them.

3.Place a“T”after sentences that are true and an“F”after those
that are false: (1) ?On standard machine tools, electrical energy is directly employed in metal removal.( ) (2) ?Metal of any hardness can be machined by electrical

machining processes with coarse surface finish.( )

Unit 20
(3) ?The surface finish of a part produced on an EDM machine depends on the size of part.( ) (4) ?Big, powerful sparks produce a smooth surface and small, less powerful sparks produce a rough surface.( ) (5) ?Metal material removal rates and surface finish can be controlled by the frequency and intensity of the spark.( ) (6) ?High frequency, low-intensity sparks result in a low metal removal rate and produce a smooth surface finish.( )

Unit 20 READING MATERIAL

Electrical Machining
Most electrical machining processes have come about due to the development of space-age metal alloys that are tough and hard

to machine. Grinding cutting, and shaping with cutting tools are
exacting, time-consuming, and expensive processes on such alloys. In some cases, machining with conventional equipment is almost impossible. A material like tungsten carbide, for example, is very difficult to machine even with diamond cutting tools.

Unit 20
On standard machine tools, electrical energy is converted by an electric motor into motion that is transmitted through a series of shafts, pulleys, belts, and gears to a cutting tool. In contrast, the electrical machining processes make direct use of electrical energy. This results in certain abilities that are unique to these processes. These include machining metals of any hardness, machining without cutting forces, machining without tool rotation or without touching the workpiece, machining odd shapes, and machining

dissimilar metals without loading the tool.
Three major electrical machining processes are EDM (electrical discharge machining), ECM (electro chemical machining), and

ECG (electrochemical grinding).

Unit 20 PROFESSIONAL WORDS AND PHRASES
unique [jJ5ni:k] odd [Bd] dissimilar [dI5sImIlE(r)] electrochemical [I?lektrEJ?kemIkEl] come about due to space-age time-consuming in contrast unique to electrochemical machining electrochemical grinding adj. 惟一的,独特的 adj. 奇数的,单数的 adj. 不同的,相异的 adj. 电气化学的 发生,出现 由于 空间时代 费时的 相比之下,相反,可是 ……所独有的 电化学加工 电化学磨削


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