機械加工外文翻譯---金屬加工性能

1、<p><b>　　金屬加工性能</b></p><p><b>　　簡介</b></p><p>　　加工條件和材料的物理性能對材料的切削加工有直接影響。被描述為”加工的物質(zhì)條件”的各種條件和特性，它們單獨或累加地直接影響和決定加工材料的加工性能。運轉(zhuǎn)條件、刀具材料、幾何尺寸和加工工件的需求作業(yè)間接影響加工性能，并且經(jīng)常用于克服加工材

2、料所呈現(xiàn)的復(fù)雜情況。另一方面如果忽視它們這些因素，這些因素可以導(dǎo)致出現(xiàn)增加加工難度的情況。對影響加工性能和加工工藝所有因素的完全理解有助于選擇材料和工件設(shè)計，已達(dá)到最佳加工方案和最大生產(chǎn)效率的目的。</p><p><b>　　2 加工材料的條件</b></p><p>　　以下八個因素決定加工材料的的條件，顯微結(jié)構(gòu)、晶粒尺寸、熱處理、化學(xué)成分、制造、硬度、屈服強度、

3、拉伸強度。</p><p>　　顯微結(jié)構(gòu)：一種金屬的顯微結(jié)構(gòu)是指通過蝕刻和拋光表面，在顯微鏡下檢測金屬的晶粒或晶體結(jié)構(gòu)。具有相似晶粒顯微結(jié)構(gòu)的金屬由相似的加工性能特性，但是在同一個加工工件上有多種影響加工性能的晶粒顯微結(jié)構(gòu)。</p><p>　　粒度：金屬的晶粒尺寸和機構(gòu)是金屬加工性能的一般衡量指標(biāo)。晶粒規(guī)則和細(xì)小的金屬更加容易切削和拋光。這種金屬不但延展性好，而且具有“粘性”晶粒尺寸中等

4、大小的金屬呈現(xiàn)折中的金屬切削和拋光加工特性。金屬硬度一定與晶粒尺寸相關(guān)，并且它是一般作為機械加工的指標(biāo)。</p><p>　　熱處理：提供所需的金屬特性，在固態(tài)狀態(tài)下，金屬經(jīng)過間或的一系列加熱和冷卻處理。金屬可以改善加工性能，如減少脆性、消除應(yīng)力、增強硬度或其他變化。</p><p>　　化學(xué)成分：金屬的化學(xué)成分是決定金屬加工性能的主要因素。化學(xué)成分的影響不總是很明確。因為多種元素構(gòu)成的金

5、屬合金，每種元素都對金屬加工性能有影響，而他們影響的疊加則不確定。關(guān)于鋼鐵的化學(xué)成分與加工性能聯(lián)系可以這樣歸納，但非鐵合金過于復(fù)雜多樣而不能這樣歸納。</p><p>　　制造：無論金屬是被熱軋、冷軋、冷拉、熔煉、鍛造都會影響它的晶粒尺寸、塑性、硬度、晶體結(jié)構(gòu)和其加工性能?！凹庸ぁ笔侵赣脗鹘y(tǒng)的加工工藝，捶打或材料成型為可以隨時改變的組件或構(gòu)件。加工金屬分為棒材、鋼坯、卷板、條料、板材或管材。</p>

6、<p>　　鑄造要將熔融金屬澆鑄到一個模具中以得到要求低的、相似的形狀，有些情況是不需要加工的。澆鑄需要的模具可以用沙子、石膏、金屬或其他材料制作而成。</p><p>　　硬度：書本上硬度的定義材料抵抗變形的趨勢。硬度的衡量經(jīng)常用布氏硬度或洛氏硬度方法。測量硬度的方法是采用預(yù)訂的負(fù)載或重力，用一個特定尺寸和形狀的壓頭壓緊測試材料的表面。布氏硬度或洛氏硬度讀數(shù)與壓痕到材料表面的距離有關(guān)。越大的壓痕，布

7、氏硬度或洛氏硬度讀數(shù)越??；大的布氏硬度或洛氏硬度讀數(shù)，則壓痕到材料表面的距離就很小。根據(jù)定義，是一種硬度極大的金屬。圖1展示了如何測量硬度。</p><p>　　布氏硬度測試要嵌入一個特定直徑的鋼球，用一公斤的負(fù)載在材料表面測試。布氏硬度值（BHN）是在測試工件表面留下的球形壓痕的每單位面積負(fù)載（平方毫米）。這種標(biāo)準(zhǔn)化的測量方法提供了統(tǒng)一的方法來測量比較不同加工材料的硬度或某種材料硬化過程。</p>

8、<p>　　洛氏硬度測試可以采用多種尺寸的壓頭和多種負(fù)載測試。硬度測試或洛氏硬度有幾種不同的硬度等級。就實際應(yīng)用來說有三種最受流行的等級。</p><p>　　這個測試的設(shè)計如下：</p><p>　　洛氏硬度 測試</p><p><b>　　等級 應(yīng)用</b></p><p>　　

9、A 鎢合金、其他薄的、硬的、條狀硬質(zhì)材料。</p><p>　　B 中等硬度、退火條件下的低碳鋼。</p><p>　　C 材料硬度大于洛氏硬度100HBS</p><p>　　在一般實際加工中，低硬度材料可以使生產(chǎn)效率提高，因為切削速度依據(jù)硬度選擇（硬度越小，切削速度越高）。加工工件硬度的增加對

10、刀具的使用壽命產(chǎn)生不利影響。對于既定的切削速度和硬度，切削應(yīng)力和溫度的只能更加使刀具的使用壽命減少。對于鉆孔和車削，切削溫度的增加是對刀具壽命不利的，因為它產(chǎn)生的額外熱量使刀具邊緣磨損加速。在切削過程中，材料硬度的增加使負(fù)荷增加，導(dǎo)致刀具邊緣過早的磨鈍。</p><p>　　屈服應(yīng)力：拉伸測試是比較金屬材料物理性質(zhì)的方法。拉伸測試可以得出屈服強度、拉伸強度和其他熱處理金屬材料多的物理性質(zhì)。還有，這種測試經(jīng)常用于比

11、較不同的金屬材料的物理性質(zhì)。拉伸測試要采用一個圓柱棒或軸，在液壓機上用一個逐漸增大的力從另一端拉伸。</p><p>　　在測試開始前，在圓柱棒上二英寸和八英寸處各做一記號，當(dāng)圓柱棒受到逐漸增大的力作用時，兩個記號的距離變大。如果負(fù)載可以使圓柱棒和記號回到原來的位置，則材料在彈性變形區(qū)域。測試有這樣一個過程，如果負(fù)載使圓柱棒的記號回不到原來位置，則發(fā)生永久變形。</p><p>　　屈服強

12、度的測量就在永久變形前那個點。圖2展示屈服強度如何測試。</p><p>　　屈服強度的測試在永久變形發(fā)生點之前。屈服強度用每平方英寸多少磅（PSI）來描述。永久變形之前過渡區(qū)橫截面積除以負(fù)載來決定。這種材料特性視具體情況而定，因為它的屈服強度在熱處理后可以改變。硬度增大導(dǎo)致屈服強度增大。因為硬度增加，它需要更大的力來產(chǎn)生永久變形。屈服強度不能與彎曲強度和拉伸強度混淆，因為這些特性與屈服強度不同，與當(dāng)前狀態(tài)無關(guān)。

13、</p><p>　　通過定義可以知道，具有高屈服強度（產(chǎn)生永久變形的力）的材料在加工作業(yè)中切削開始時需要一個更大的力。這表明一個材料的屈服強度增加，產(chǎn)生更強切削應(yīng)力的刀頭形狀和切削更小的切削尺寸來抵御加工區(qū)域增加的負(fù)荷。因此具有相對較高屈服強度的金屬比中行等屈服強度的金屬更難加工，刀具的使用壽命更短。</p><p>　　拉伸強度：一個材料的拉伸強度隨著熱處理使其屈服強度的增加而增加。這

14、個物理材料特性也是有一個拉伸測試確定。拉伸強度（或極限強度）由拉伸測試產(chǎn)生的最大負(fù)荷除以測試樣品過渡區(qū)域橫截面積來定義。因此，拉伸強度像屈服強度一樣用PSI來表示。這樣定義的好處是由于一種材料的當(dāng)前特性不止與物理特性有關(guān)，就像屈服強度、硬度一樣，經(jīng)過熱處理，值可以改變。因此，在材料確定的情況下，每一種硬度讀數(shù)對應(yīng)著不同的拉伸強度和屈服強度。</p><p>　　在加工作業(yè)中正如屈服強度的增加，則切削力增加一樣，拉

15、伸強度增加，切削力增加。同樣的，加工工件拉伸硬度的增加，為了生產(chǎn)效率和刀具使用壽命則需要產(chǎn)生更強切削力的切削刀頭幾何形狀。</p><p>　　3.加工材料的物理特性</p><p>　　某個材料的物理特性包括彈性模量、熱膨脹、加工硬化等物理特性。</p><p>　　彈性模量：在前面提到的條件下，以同樣的方式進(jìn)行可拉伸試驗來定義彈性模量。然而與硬度、屈服強度和拉伸

16、強度不同，彈性模量是某種材料的特定屬性，因此不會受到熱處理的影響。這種特定的屬性是衡量材料受到外力發(fā)生繞度變形的指標(biāo)。這種特性用PSI來描述。主要用處是為金屬提供了幾百萬的PSI。2、4或8尺6長的木棒支撐兩端，在中間掛兩百磅的負(fù)荷，下降的尺寸高度是同樣長度尺寸、掛同樣負(fù)荷鋼材的17倍。不是因為鋼鐵更硬和結(jié)實，而是鋼鐵的彈性模量是木材的17倍。</p><p>　　生產(chǎn)實踐表明，具有相對較低彈性模量的加工工件一般

17、需要前角度數(shù)為正或很大的刀具切割幾何形狀。前角為正的刀具產(chǎn)生較小的切削力，用這種刀具彈性材料的切屑變多了。前角度數(shù)很大的刀具容易發(fā)生咬刀，產(chǎn)生材料的剪切。前角度數(shù)為負(fù)的刀具當(dāng)?shù)毒唛_始切削時，會產(chǎn)生很大的切削力，使加工材料的尺寸發(fā)生局部變形。</p><p>　　熱導(dǎo)率：材料通常被標(biāo)記為導(dǎo)熱材料或絕緣材料。導(dǎo)熱材料容易以較快的速度從更熱或更冷的物體上傳遞熱量，而絕緣體阻止熱量的傳遞。導(dǎo)熱率是描述某種材料傳遞熱量效率

18、的衡量方法。因此，具有相對較高導(dǎo)熱率的材料視為導(dǎo)體，具有相對較低導(dǎo)熱率的材料視為絕緣體。</p><p>　　具有相對較低導(dǎo)熱率的的金屬材料不能較快夠傳遞熱量。因此，加工這些材料時，切削刀具和加工工件變得很熱。這個過程加速了刀具的磨損和減少了刀具的使用壽命。具有底導(dǎo)熱率的金屬材料在切削區(qū)域（在加工工件和切削刀具之間）加足量的冷卻液，來增加刀具的使用壽命。</p><p>　　熱膨脹：許多材

19、料，尤其是金屬材料當(dāng)它們的溫度上升時，尺寸有變大的趨勢。這種物理特性描述為熱膨脹。金屬的膨脹率的多樣化取決于某種金屬或合金的具體情況。金屬的熱膨脹率由熱膨脹系數(shù)決定。具有越大的熱膨脹系數(shù)，金屬溫度上升時或發(fā)生導(dǎo)熱時，產(chǎn)生的熱膨脹變形就越大。例如，在100華氏溫度下100英寸的鋼材棒料，當(dāng)溫度上升時，尺寸變?yōu)?00.065英寸。相同條件的100英寸鋁材棒料，在相同狀況下為100.125英寸。在在這個實驗中，鋁材棒料的改變的尺寸幾乎是鋼材棒

20、料改變尺寸的二倍。這明確的不同表明不同的材料有不同的熱膨脹系數(shù)。</p><p>　　在一般加工方法中，具有較大熱膨脹系數(shù)的材料的加工很難符合尺寸公差要求。因為一個加工工件溫度的微弱增加都會導(dǎo)致尺寸的變化。加工這種類型的金屬需要足夠的冷卻液以保證尺寸的穩(wěn)定性。另外，用前角度數(shù)為正的切削刀具可以減少加工溫度。</p><p>　　加工硬化：許多金屬在冷加工時會有硬度參數(shù)上升的物理特性。冷加工

21、涉及改變金屬物體的形狀，如彎曲、成型、滾壓和造型。當(dāng)金屬被改變形狀，內(nèi)部應(yīng)力的增加使金屬局部硬化。一種金屬與另一種金屬的內(nèi)部硬度改變速率和程度是多種多樣的。熱在金屬加工硬化中也起著重要作用。當(dāng)金屬產(chǎn)生加工硬化時，金屬溫度上升。它的作用像催化劑一樣產(chǎn)出更硬的加工工件。</p><p>　　具有加工硬化特性的加工工件在加工中不應(yīng)用大量冷卻液。另外，切削速度因與之有關(guān)聯(lián)，為了達(dá)到生產(chǎn)效率，不能不計后果的改變材料的切削速

22、度。加工工件在加工過程中，加工硬化決定高速切削產(chǎn)生的額外熱量。薄的切屑應(yīng)避免在這些材料上加工使用，因為加工實踐中由于摩擦產(chǎn)生的熱量會造成前面提到的影響。刀具前角度數(shù)為正、低切削應(yīng)力、中等的切削速度和進(jìn)刀量一般在這種材料上加工很有效率。</p><p><b>　　4.金屬加工</b></p><p>　　術(shù)語“加工性能”是金屬與布氏硬度160、美國鋼鐵協(xié)會B1112高

23、速切削低碳鋼，一種材料的加工難易程度。美國鋼鐵協(xié)會做了這種材料車削180英尺的測試，并與其他幾種材料比較了測試結(jié)果。如果B1112代表100%等級，具有相對較低等級的材料則較難加工，超過100%則較容易加工。</p><p>　　某種材料的加工性能等級評定需要切削速度、表面粗糙度和刀具壽命等綜合考慮到加工性能等級排列。下面的表格多種金屬的加工性能等級排列。</p><p><b>

24、;　　5.加工性能判定</b></p><p>　　影響加工性能的因素已經(jīng)解釋過，下面討論四種判斷加工性能的方法：</p><p>　　刀具使用壽命：金屬切削以及低的速度磨損刀具則認(rèn)為加工性能好，反之亦然。含有小的硬質(zhì)雜物的加工工件材料與耐研磨金屬具有相似的機械加工特性，在切削過程中不需要消耗跟多的能量。還有，這種材料的加工性能比較低，因為它的耐磨特性是加速刀具磨損的主要原因，

25、增加加工成本。</p><p>　　刀具應(yīng)力和能量消耗作為影響工件材料加工性能的因素因為以下兩個原因：</p><p>　　金屬較容易切削即刀具進(jìn)給容易說明加工工件材料具有較好的加工性能等級。</p><p>　　更實用的加工性能概念是與應(yīng)力和能量消耗有關(guān)的每部分加工的最低成本和合適的加工成本。</p><p>　　表面處理：在工件切削加工過

26、程中，表面處理的質(zhì)量有時對判定加工性能等級有很大用處。有些工件像其它工件一樣不用高的精度表面處理。影響表面粗糙度的基本原因是切屑和刀具的切削邊緣增厚。柔軟、韌性材料較容易形成厚的刀具切削邊緣。不銹鋼合金的氣體渦輪和其它有較高硬度的金屬也趨于用較厚的刀具邊緣加工。具有很大剪切取得的材料趨于減少厚度增加的影響。這些材料包括鋁合金、冷加工鋼材、高速切削鋼、黃銅、和鈦合金。如果表面質(zhì)量作為加工性能的唯一指標(biāo)。這些金屬的加工性能等級要高于前面一組

27、。</p><p>　　切屑形狀：有種加工性能等級基于加工過程中產(chǎn)生的切屑形狀類型。這種加工性能可能是由切屑的處置和來判定。產(chǎn)生長線型切屑的材料獲得低等級，就像產(chǎn)生細(xì)粉裝切屑的材料。本來具有細(xì)段切屑、半個或整個螺旋切屑的材料獲得最高等級。切屑的處置十分昂貴。線型切屑對機床和已加工表面質(zhì)量有很大威脅。然而，切屑的形成是與機床和材料有函數(shù)變化關(guān)系。這種方法的等級排定由于斷裂切屑的提供而改變。</p>&

28、lt;p>　　Machinability Of Metals</p><p>　　1.Introduction</p><p>　　The condition and physical properties of the work materials have a direct influence on the machinability of a work material.The

29、 various conditions and characteristics described as‘condition of work material’, individu ally And in combinations,directly influence and determine the machinability. Ope rating conditions,toolmaterial and geo-</p>

30、;<p>　　metry ,and workpiece requirements exercise indirect effects on machinability ang can offen be used to overcome difficult conditions presented by the work material.On the other hand ,they can create situatio

31、ns that increase machining difficulty if they are ignored .A thorough understanding of all of the factors affecting machinability and machining will help in selecting material ang woekpiece designs to achieve the optimum

32、 machining combinations critical to maximum productivity.</p><p>　　2.Condition of Work Material</p><p>　　The following eight factors determine the condition of the work material microstructure,

33、grain size,heat treatmen,chemical, composition ,fabri cation hardness ,</p><p>　　yield ,and tensile .</p><p>　　Microstructure: The microstructure of a metal refers to its crystal or grain struc

34、ture as shown through examination of etched and polished surface under a microscope .Metal whose microstructures are similar have like machining properties .But there can be variations in the mincrostructure of the same

35、workpiece ,that will affect machinability.</p><p>　　Grain Size :Grain size and structure of a metal serve as general indicators of its machinability .A metal with small undistorted grains tends to cut easily

36、 an finish easily .Such a metal is ductile ,but it is also ‘gummy’.Metal of an intermediate grain size represent a compromise that permits both cutting and finishing machinability . Hardness of a metal must be correlated

37、 with grain size and it is generally used as an indicator of machinability.</p><p>　　Heat Treatment:To provide desired properties in metals ,they are sometimes put through a series of heating and cooling ope

38、rtions when in the solid state .A material may be treated to reduce brittleness ,remove stress ,to obtain hardness ,or to make other changes that affect machinability.</p><p>　　Chemical Composition :Chemical

39、 composition of a metal is a major factor in determing its machinability .The effects of composition though ,are not always clear ,because the elements that make up an alloy metal ,work both singly and collectively .Cer

40、tain generalizations about chemical composition of steels in relation to machinability can be made ,but non-ferrous alloy are too numerous and varied to permit such generalizations.</p><p>　　Fabrication: Whe

41、ther a metal has been rolled ,cold rolled ,cold drawn,cast,or forged will affect its grain size ,ductility , hardness ,</p><p>　　structure-and therefore-its machinability .</p><p>　　The term‘wro

42、ught’refers to the hammering or forming of materials into permanfactured shapes which are readily altered into components or products using traditional manufacturing techniques.Wrought metals are defined as that group of

43、 materials which are mechanically shaped into bars,billets, rolls, sheets,plates or tubing.</p><p>　　Casting involves pouring molten metal into a mold to arrive at a near component shape which requires minim

44、al,or in some cases no machining. </p><p>　　Molds for these operations are made from sand ,plaster,metals and a variety of other materials.</p><p>　　Hardness:The textbook definition of hardness

45、is the tendency for a material to resist deformation.Hardness is offten measured using either the Brinell or Rockwell scale. The method used to measure hardness involves embedding a specific size and shaped indentor into

46、 the surface of the test material,using a predetermined load or weight.The distance the indentor penetrates the materials surfacewill correspond to a specific Brinell or Rockwell hardness reading.The greater the indentor

47、 surface penetr</p><p>　　Figure 1 Hardness is measured by depth of indentations made.</p><p>　　The Brinellhardness test involves embedding a steel ball of a specific diameter,using a kilogram lo

48、ad ,in the surface of a test piece. The Brinell Hardness Number (BHN) is determined by dividing the kilogram load by the area(in squre millimeters) of the circel created at the dimple or impression left in the workpiece

49、surface.This standardized approach provides a consistent method to make comparative tests between a variety of workpiece materials or a single material which has undergone various h</p><p>　　The Rockwell tes

50、t can be performed with various indentor sizes and loads.Several different scales exist for the Rockwell methord or hardness testing. The three most popular are outlined below in terms of the actual application the test

51、is designed to address:</p><p>　　Rockwell Testing</p><p>　　Scale Applicatio</p><p>　　A For tungsten carbide and other extremely hard material & thin,

52、hard,sheets.</p><p>　　B For medium hardness low and medium carbon steels in the annealed condition.</p><p>　　C For material ＞ than Rockwell‘B’100</p><p>　　

53、In terms of general machining pratice,low material hardness enhances productivity,since cutting speed is offten selected based on material hardness(the lower the hardness, the higher the speed).Tool life is adversely aff

54、ected by an increase in workpiece hardness,since the ctting loads and temperatures rise for a specific cutting speed with part hardness, thereby reducing with toll life .In drilling and turning ,the added cutting tempera

55、ture is detrimental to tool life ,since it produces excess h</p><p>　　Yield Strengh:Tensile test work is used as a means of comparison of metal material conditions.These tests can establish the yield strengh

56、 ,tensile strengh and many other conditions of a material based on its heat treatment.In addition,these tests are used to compare different workpiece material.The tensile test involves taking a cylindrical rod or shaft,a

57、nd pulling it from opposite ends with a progressively larger force in a hydraulic machine .Prior to the start of the test ,two marks either two</p><p>　　Figure 2 Yield strength is measured by pulling a test

58、specimen as shown.</p><p>　　Yield strengh is measured just prior to the point before permanent deformation takes place.Yield strengh is stated in pounds per square inch(PSI) and is deternined by dividing the

59、 load just prior to permanent deformation by the cross sectional area of the test specimen.This material property has been referred to as a condition ,since it can be altered during heat treatment.Increased part hardness

60、 produces an increase in yield strengh and therefore,as a part becomes harder,it takes a larger force</p><p>　　By definitin,a material with high yield strengh (force required per unit of area to create perma

61、nent deformation) requires a high level of force to initiate chip formation in a machining operation.This implies that as a material’s strength yield increases,stronger insert shapes as well as less positive cutting geo

62、metries are necessary to combat the additional load encountered in the cutting zone.Mareial hardness and yield strength increase simultaneously during heat treatment.Therefore,material</p><p>　　Tensile stren

63、gth:The tensile strength of a material increase along with yield strength as it is heat treated to greater hardness levels.This material condition is also establishde using a tensile test.Tensile strength (or ultimate st

64、rength) is defined as the maximum load that results during the tensile test,divided by the cross-sectional area of the test specimen.Therefore,tensile strength,like yield strength ,is expressed in PSI.This value is refer

65、red to as a material condition rather than a pr</p><p>　　Just as increased yield strength implied higher cutting forces during machining operations,the same could be said for increased tensile strength. Agai

66、n,as the workpiece tensile strength is elevated,stronger cutting edge geometries are required for productive machining and acceptable tool life.</p><p>　　3.Physical Properties of Work Material</p><

67、;p>　　Physical proterties will include those characteristics included in the individual material groups ,such as the modulus of elasticity ,thermal expansion and work hardening.</p><p>　　Modulus of Elastic

68、ity :the modulus of elasticity can be determined during a tensile test in the same manner as the previously mentioned conditions .However ,unlike hardness ,yield or tensile strength ,the modulus of elasticity is a fixed

69、material property and ,therefore,is unaffected by heat treatment .This particular property is an indicator of the rate at which a material will deflect when subjected to an external force .This property is stated in PSI

70、and typical values are several million PSI</p><p>　　General manufacturing practice dictates that productive machining of a workpiece material with a relatively moderate modulus of elasticity normally require

71、s positive or highly positive raked cutting geometries .Positive cutting geometries produce lower cutting enhanced on elastic material using these types of tools . Sharp positive cutting edges tend to bite and promote sh

72、earing of a material ,while blunt negative geometries have a tendency to create large cutting forces which impedd chip format</p><p>　　Thermal Conductivity: Materials are frequently labeled as being either h

73、eat conductors or insulators.Conductors tend to transfer heat from a hot or cold object at a high rate ,while insulators impede the flow of heat .Thermal conductivity is a measure of how efficiently a material which has

74、a relatively high thermal condicitivity would be considered a conductor ,while one with a relatively low level would be regarded as an insulstor .</p><p>　　Metals which exhibit low thermal conductivitis will

75、 not dis- sipate heat freely and therefore ,during the machining of these maerials ,the cutting tool and workpiece become extremely hot .</p><p>　　This excess heat accelerates wear at the cutting edge and re

76、duces tool life .The proper application of sufficient amounts of coolant directly in the cutting zone (between the cutting edge and workpiece) is essential to improving tool life in metals with low thermal conductivities

77、 .</p><p>　　Thermal Expansion :Many materials ,especially metals ,tend to increase in dimensional size their temperature rises .This physical property is referred to as thermal expansion .The rate at which m

78、etals expand varies , depending om the type or alloy of material under conderation .The rate at which metal expands can be determined using the material’s expansion coefficient ,the greater the value of this coefficient

79、,the more a material will expand when subjected to a temperature rise or contract whe</p><p>　　In terms of general machining practice ,those materilas with large thermal expansion coefficient will make holdi

80、ng close finish tolerance extremely difficult ,since a small rise in workpiece temperature will result in dimensinal change .The machining of these types of materials requires adequate coolant supplies for thermal and di

81、mensional stability .In addition,the use of positive cutting geometries on these materials will also reduce machining temperatures .</p><p>　　Work Hardening :Many metals exhibit a physical characteristic whi

82、ch produces dramatic increase in hardness due to cold work .Cold work involves changing the shape of a metal object by bending ,shaping,rolling or forming .As the metal is shaped ,internal stresses develop which act to h

83、arden the part .The rate and magnitude of this internal hardening varies widely from one material to another .Heat also plays an important role in the work hardening of a material .When materials which exhibit work </

84、p><p>　　The machining of workpiece materiaks with work hardening properties should be undertaken with a generous amount of coolant .In additon ,cutting speed should correlate specifically to the material machin

85、ed and should not be recklessly altered to meet a production rate .The excess heat created by unusually high cutting speeds could be extremely by detrimental to the machning process by promoting work hardening of the wor

86、kpiece .Low chip thicknesses should be avoided on these materials ,since this </p><p>　　4.Metal Machining </p><p>　　The term ‘machinability’is a relative measure of how easily a material can be

87、machined when compared to 160 Brinell AISI B1112 free machining low carbon steel .The American Iron and Steel Institute (AISI) ran turning tests of this material at 180 surface feet and compared their results for B1112 a

88、gainst several other materials .If B1112 represents a 100% rating ,then materials with a rating less than this level would be decidedly more difficult to machine ,while those that exceed 100% would be e</p><p&

89、gt;　　The machinability rating of a metal takes the normal cutting speed ,surface finish and tool life attained into consideration to arrive at a final machinability rating .The fllowing chart shows a variety of materials

90、 and their specific machinability</p><p><b>　　Rating:</b></p><p>　　MaterialHardness</p><p>　　Machinability</p><p><b>　　Rating</b></p><

91、;p><b>　　6061-T</b></p><p><b>　　Aluminum-</b></p><p><b>　　190%</b></p><p><b>　　7075-T</b></p><p><b>　　Alumimum-&

92、lt;/b></p><p><b>　　120%</b></p><p><b>　　B1112</b></p><p>　　Steel 160BHN</p><p><b>　　100%</b></p><p>　　416 Stainles

93、s</p><p>　　Steel 200BHN </p><p><b>　　90%</b></p><p>　　1120 Steel 160BHN</p><p><b>　　80%</b></p><p>　　1020 Steel 148BHN</p>

94、<p>　　5.Judging Machinability</p><p>　　The factors affecting machinability have been explained ;four methords used to judge machinability are discussed below:</p><p>　　Tool life :Metals wh

95、ich can be cut without rapid tool wear are generally thought of as being quite machinble ,and vice versa .A workpiece material with many small hard inclusions many appear to have the same mechanical properties as a less

96、abrasive metal .It may require no greater power consumption during cutting . Yet ,machinability of this material would be lower because its abrasive properties are responsible for rapid wear on the tool ,resulting in hig

97、her machining costs.</p><p>　　Tool Force and Power Consumption :The use of tool force or power consumption as a criterion of machinability of the workpiece material comes about fow two reasons .First ,the co

98、ncept of machinability as the case with which a metal is cut ,imples that a metal through which a tool is easily pushed should have a good machinability rating .Second ,the more practical concept of machinability in term

99、s of minimum cost per part machined , relates to forces and power consumption ,and the overhead cost of</p><p>　　Surface Finish ：The quality of the surface finish left on the workpiece during a cutting opeer

100、ation is sometimes useful in determining machinability rating of matel 。Some workpiece will not ‘take a good finish’as well as others .The fundamental reason for surface roughness is the formation and sloughing off of pa