機械專業(yè)英語董建國第19章

1、Unit 19TEXT READING MATERIAL Unit 19Unit 19TEXTTool Life The higher the cutting speed, the shorter the tool life. By tool life is meant the time of cutting until the tool becomes too worn to do its work effectively and must be sharpened.Although it cannot be universally applied, many cutting operati

2、ons on most materials obey the empirical relationship：VTn=C Where V is the cutting speed in surface meters per second, T is the tool life in cutting seconds, and C is a constant, subject to the variables of work material, cutting tool material, tool shape, and other factors. The constant C is equal

3、to the cutting velocity for a tool life of l min. Figure19-1 shows a typical tool life curve that follows this mathematical relationship. Unit 19Fig. 19-1 Tool life curves for turning steel barUnit 19 If such a machining formula is available, the production planner can use it to work out the lowest

4、cost of machining. Obviously the fastest possible machining speed will give the lowest cost per cubic millimeter of chips, if no other factors are involved. However, this ideal condition may mean frequent tool changes and sharpenings. In the time required to change tool bits perhaps 25 cubic millime

5、ter or more of chip making may be lost. The production planner must then select a compromise speed. He may select the speed that will give a tool life extending over a full shift of production or over the whole production run, if it is reasonably short. Or he may balance the cost of tool change and

6、tool sharpening against the speed of machining to provide a minimum overall cost.Unit 19LEARN TO SPEAKMr. Zhang: Hello, Mr. Cheng. Nice to see you again!Mr. Cheng: Im glad to meet you.Mr. Zhang: What are you doing?Mr. Cheng: I am preparing an experiment of tool life.Mr. Zhang: Oh, tool life? What do

7、es it mean?Mr. Cheng: Tool life is the time of cutting until the tool becomes too worn to do its work efficiently and must be sharpened.Mr. Zhang: How can we decide that the tool must be sharpened?Unit 19Mr. Cheng: If the tool is broken or the cutting operation is very difficult and the machining qu

8、ality is not satisfactory, the tool must be sharpened.Mr. Zhang: It seems difficult to judge it.Mr. Cheng: Not the case. It is very easy for a skilled worker.Mr. Zhang: Oh, I see. I think I have a long way to go.Mr. Cheng: Do not worry. You can do your best if you work hard.Mr. Zhang: Youre very kin

9、d.Mr. Cheng: Thats OK.Unit 19NEW WORDS AND EXPRESSIONSobey E5beI v. 服從，順從empirical em5pIrIk(E)l adj. 經(jīng)驗的，實驗的subject 5sQbdVIkt n. 題目，主題，科目 adj. 從屬的，易受的 sEb5dVekt vt. 使受到，使服從typical 5tIpIk(E)l adj. 典型的，標準的curve k:v n. 曲線 v. 繪曲線planner n. 計劃者Unit 19cubic 5kju:bIk adj. 立方體的，立方的involve In5vBlv vt. 包含，涉及i

10、deal aI5dIEl adj. 理想的compromise 5kBmprEmaIz n. 折衷 v. 折衷minimum 5mInImEm n. 最小值 adj. 最小(值)overall EJvEr5C:l adj. 全部的，全面的，總的carbide 5kB:baId n. 碳化物Unit 19too.to v 太以致于不能subject to 受支配的，以為條件 的，易受的work out 解決，算出，加工出tool bit 刀頭production run 生產(chǎn)(運行)周期overall cost 總成本sintered carbide 硬質(zhì)合金Unit 19NOTES 1By t

11、ool life is meant the time of cutting until the tool becomes too worn to do its work effectively and must be sharpened. 刀具耐用度是指刀具磨損過大以致無法有效地切削而必須刃磨之前的切削時間。 by+名詞A+is meant+名詞B “A是指B”或“A就是B” 這是科技英語中對某一術(shù)語下定義時常用的句型。主語是名詞B，放在謂語動詞is meant 的后面，是一個倒裝句。例如： Unit 19 By cutting speed is meant the rate at which

12、 the work passes the cutter. 切削速度是指工件通過刀具的速率。 too+形容詞或副詞+to V “太(以致)不” 這種句型中，后面的to v 有否定的意義。如： This problem is too difficult to understand. 這個問題太難，理解不了。 Modern high-strength alloys are often too hard to machine. 現(xiàn)代高強度合金常常硬得難以機械加工。 Unit 19 2Although it cannot be universally applied, many cutting oper

13、ations on most materials obey the empirical relationship： 雖然下述經(jīng)驗公式不能普遍適用，但對大多數(shù)材料的許多加工是成立的： 代詞it是指后面主句中的the empirical relationship。英語中有時人稱代詞先出現(xiàn)，而所代的名詞在后，譯成漢語時，要作適當(dāng)處理。 3Where V is the cutting speed in surface meters per second, T is the tool life in cutting seconds, and C is a constant, subject to the

14、 variables of work material, cutting tool material, tool shape, and other factors. Unit 19 式中V是切削速度，以米/秒表示；T是刀具耐用度，以切削秒數(shù)表示；C是一個常數(shù)，隨工件材料、刀具材料、刀具形狀和其它因素而變化。 where引出的從句，說明上面的公式。這種場合where 一般譯成“式中”、“其中”。 本句中subject to是形容詞短語，說明前面的名詞a constant; subject to 表示“以為條件”，“受制于”。 4Obviously the fastest possible mac

15、hining speed will give the lowest cost per cubic millimeter of chips, if no other factors are involved.Unit 19 顯然，若不涉及其它因素，以可能達到的最高切削速度來加工，就能使切削每立方毫米切屑的成本最低。 possible用在形容詞的最高級后面，用來加重語氣，可譯成：“盡可能”、“能時”。如： the best means possible 盡可能好的方法Unit 19EXERCISES 1Discuss the following questions in English: (1)

16、How to define the metal cutting operation called“turning”? (2) How to define the term“cutting speed”? (3) How to define the word“machinability(切削性)”? (4) Can you say the two classes of metallic materials that are used to make metal-cutting tools? What are they?Unit 19 2Use English for replacing Chin

17、ese or use Chinese for replacing English: (1) 刀具的耐用度與很多因素有關(guān)。 (2) 加工時的切削速度對刀具的壽命有很大的影響。 (3) 制造刀具的材料有很多種，如高速鋼、硬質(zhì)合金、陶瓷等。 (4) The lower the cutting speed, the longer the tool life is. (5) Tool life curve follows mathematical relationship. (6) The cutting velocity is very important for metal to be cut. U

18、nit 19 3Place a“T”after sentences that are true and an“F”after those that are false: (1) Rake is the slope of the top of the tool away from the cutting edge.( ) (2) The chip movement and pressure causes the dome to become soft, so that particles of it break away, and become welded to the chip and to

19、 the body of the work-piece.( ) (3) Continuous chip formation is associated with brittle work-piece material, and cutting tools with a small rake angle.( ) (4) In the formation of the chip by tear, there is a general movement of the chip over the tool face.( )Unit 19 (5) The purpose of the first ,or

20、 roughing cut, is to remove the surface of the work-piece as rapidly as possible.( )(6) The purpose of the last or finishing cut is to produce a work-piece of the required size that has the quality of surface finish desired.( ) Unit 194Fill the blanks with the words given below:being, inside, tear,

21、outside, rake, mechanical, physical, clearance, having, shear. (1) The force F acting upon the tool can be reduced by_ a large rake angle, and the tendency of welding to occur can be eliminated by making the cutting tool from a material that is non-metallic. (2) In practice, the_ of the chip display

22、 the “step” produced by the intermittent slip, but the_ of the chip is burnished smooth by the action of the chip rubbing upon the tool face. Unit 19 (3) _ involves compression of the work-piece material adjacent to the tool face, so that a crack runs ahead of the cutting tool, and towards the body

23、of the work-piece. (4) The force F acting upon the tool is reduced by increasing the _ angle. (5) The rake angle controls the chip formation, and is in turn governed by the_ properties of the material being cut.Unit 19READING MATERIALThe Shape of the Cutting Tool It is obvious that the narrower the

24、blade of a cutting tool, the less force or power will be required to “wedge” it through the material to be cut. Therefore, when cutting soft materials with a cutting tool made of a much harder and stronger substance, the blade can be sharpened to a long thin edge. As the hardness of the material to

25、be cut increases, however, the strength of the cutting edge must also be increased. Fig.19-2 shows forces action on a lathe tool. Cutting a soft iron bar, for example, Unit 19will cause a fine steel blade to dull immediately, even though the steel in the blade is harder than the metal being cut. If

26、you were to examine the edge of this blade under a glass, you would see that it is either bent or broken. The shape of a cutting tool, then, helps to determine the strength of a cutting edge. This explains why a knife edge will dull almost upon contact with metal while the more obtuse cutting edge of a cold chisel will stand up to continuous pounding into metal. Unit 19Fig. 19-2 Forces action on a lathe toolUnit 19 The cutting-edge of any metal-cutting tool must