加工工藝工裝夾具基本加工工序和切削技術(shù)外文文獻(xiàn)翻譯@中英文翻譯@外文翻譯

附錄 Basic Machining Operations and Cutting Technology Machine tools have evolved from the early foot-powered lathes of the Egyptians and John Wilkinsons boring mill. They are designed to provide rigid support for both the workpiece and the cutting tool and can precisely control their relative positions and the velocity of the tool with respect to the workpiece. Basically, in metal cutting, a sharpened wedge-shaped tool removes a rather narrow strip of metal from the surface of a ductile workpiece in the form of a severely deformed chip. The chip is a waste product that is considerably shorter than the workpiece from which it came but with a corresponding increase in thickness of the uncut chip. The geometrical shape of workpiece depends on the shape of the tool and its path during the machining operation. Most machining operations produce parts of differing geometry. If a rough cylindrical workpiece revolves about a central axis and the tool penetrates beneath its surface and travels parallel to the center of rotation, a surface of revolution is produced, and the operation is called turning. If a hollow tube is machined on the inside in a similar manner, the operation is called boring. Producing an external conical surface uniformly varying diameter is called taper turning, if the tool point travels in a path of varying radius, a contoured surface like that of a bowling pin can be produced； or, if the piece is short enough and the support is sufficiently rigid, a contoured surface could be produced by feeding a shaped tool normal to the axis of rotation. Short tapered or cylindrical surfaces could also be contour formed. Flat or plane surfaces are frequently required. They can be generated by radial turning or facing, in which the tool point moves normal to the axis of rotation. In other cases, it is more convenient to hold the workpiece steady and reciprocate the tool across it in a series of straight-line cuts with a crosswise feed increment before each cutting stroke. This operation is called planning and is carried out on a shaper. For larger pieces it is easier to keep the tool stationary and draw the workpiece under it as in planning. The tool is fed at each reciprocation. Contoured surfaces can be produced by using shaped tools. Multiple-edged tools can also be used. Drilling uses a twin-edged fluted tool for holes with depths up to 5 to 10 times the drill diameter. Whether the drill turns or the workpiece rotates, relative motion between the cutting edge and the workpiece is the important factor. In milling operations a rotary cutter with a number of cutting edges engages the workpiece. Which moves slowly with respect to the cutter. Plane or contoured surfaces may be produced, depending on the geometry of the cutter and the type of feed. Horizontal or vertical axes of rotation may be used, and the feed of the workpiece may be in any of the three coordinate directions. Basic Machine Tools Machine tools are used to produce a part of a specified geometrical shape and precise I size by removing metal from a ductile material in the form of chips. The latter are a waste product and vary from long continuous ribbons of a ductile material such as steel, which are undesirable from a disposal point of view, to easily handled well-broken chips resulting from cast iron. Machine tools perform five basic metal-removal processes: I turning, planning, drilling, milling, and grinding. All other metal-removal processes are modifications of these five basic processes. For example, boring is internal turning； reaming, tapping, and counter boring modify drilled holes and are related to drilling； bobbing and gear cutting are fundamentally milling operations； hack sawing and broaching are a form of planning and honing； lapping, super finishing. Polishing and buffing are variants of grinding or abrasive removal operations. Therefore, there are only four types of basic machine tools, which use cutting tools of specific controllable geometry: 1. lathes, 2. planers, 3. drilling machines, and 4. milling machines. The grinding process forms chips, but the geometry of the abrasive grain is uncontrollable. The amount and rate of material removed by the various machining processes may be I large, as in heavy turning operations, or extremely small, as in lapping or super finishing operations where only the high spots of a surface are removed. A machine tool performs three major functions: 1. it rigidly supports the workpiece or its holder and the cutting tool； 2. it provides relative motion between the workpiece and the cutting tool； 3. it provides a range of feeds and speeds usually ranging from 4 to 32 choices in each case. Introduction of Machining Machining as a shape-producing method is the most universally used and the most important of all manufacturing processes. Machining is a shape-producing process in which a power-driven device causes material to be removed in chip form. Most machining is done with equipment that supports both the work piece and cutting tool although in some cases portable equipment is used with unsupported workpiece. Low setup cost for small Quantities. Machining has two applications in manufacturing. For casting, forging, and press working, each specific shape to be produced, even one part, nearly always has a high tooling cost. The shapes that may he produced by welding depend to a large degree on the shapes of raw material that are available. By making use of generally high cost equipment but without special tooling, it is possible, by machining； to start with nearly any form of raw material, so tong as the exterior dimensions are great enough, and produce any desired shape from any material. Therefore .machining is usually the preferred method for producing one or a few parts, even when the design of the part would logically lead to casting, forging or press working if a high quantity were to be produced. Close accuracies, good finishes. The second application for machining is based on the high accuracies and surface finishes possible. Many of the parts machined in low quantities would be produced with lower but acceptable tolerances if produced in high quantities by some other process. On the other hand, many parts are given their general shapes by some high quantity deformation process and machined only on selected surfaces where high accuracies are needed. Internal threads, for example, are seldom produced by any means other than machining and small holes in press worked parts may be machined following the press working operations. Primary Cutting Parameters The basic tool-work relationship in cutting is adequately described by means of four factors: tool geometry, cutting speed, feed, and depth of cut. The cutting tool must be made of an appropriate material； it must be strong, tough, hard, and wear resistant. The tool s geometry characterized by planes and angles, must be correct for each cutting operation. Cutting speed is the rate at which the work surface passes by the cutting edge. It may be expressed in feet per minute. For efficient machining the cutting speed must be of a magnitude appropriate to the particular work-tool combination. In general, the harder the work material, the slower the speed. Feed is the rate at which the cutting tool advances into the workpiece. Where the workpiece or the tool rotates, feed is measured in inches per revolution. When the tool or the work reciprocates, feed is measured in inches per stroke, Generally, feed varies inversely with cutting speed for otherwise similar conditions. The depth of cut, measured inches is the distance the tool is set into the work. It is the width of the chip in turning or the thickness of the chip in a rectilinear cut. In roughing operations, the depth of cut can be larger than for finishing operations. The Effect of Changes in Cutting Parameters on Cutting Temperatures In metal cutting operations heat is generated in the primary and secondary deformation zones and these results in a complex temperature distribution throughout the tool, workpiece and chip. A typical set of isotherms is shown in figure where it can be seen that, as could be expected, there is a very large temperature gradient throughout the width of the chip as the workpiece material is sheared in primary deformation and there is a further large temperature in the chip adjacent to the face as the chip is sheared in secondary deformation. This leads to a maximum cutting temperature a short distance up the face from the cutting edge and a small distance into the chip. Since virtually all the work done in metal cutting is converted into heat, it could be expected that factors which increase the power consumed per unit volume of metal removed will increase the cutting temperature. Thus an increase in the rake angle, all other parameters remaining constant, will reduce the power per unit volume of metal removed and the cutting temperatures will reduce. When considering increase in unreformed chip thickness and cutting speed the situation is more complex. An increase in undeformed chip thickness tends to be a scale effect where the amounts of heat which pass to the workpiece, the tool and chip remain in fixed proportions and the changes in cutting temperature tend to be small. Increase in cutting speed； however, reduce the amount of heat which passes into the workpiece and this increase the temperature rise of the chip m primary deformation. Further, the secondary deformation zone tends to be smaller and this has the effect of increasing the temperatures in this zone. Other changes in cutting parameters have virtually no effect on the power consumed per unit volume of metal removed and consequently have virtually no effect on the cutting temperatures. Since it has been shown that even small changes in cutting temperature have a significant effect on tool wear rate it is appropriate to indicate how cutting temperatures can be assessed from cutting data. The most direct and accurate method for measuring temperatures in high -speed-steel cutting tools is that of Wright &. Trent which also yields detailed information on temperature distributions in high-speed-steel cutting tools. The technique is based on the metallographic examination of sectioned high-speed-steel tools which relates microstructure changes to thermal history. Trent has described measurements of cutting temperatures and temperature distributions for high-speed-steel tools when machining a wide range of workpiece materials. This technique has been further developed by using scanning electron microscopy to study fine-scale microstructure changes arising from over tempering of the tempered martens tic matrix of various high-speed-steels. This technique has also been used to study temperature distributions in both high-speed -steel single point turning tools and twist drills. Wears of Cutting Tool Discounting brittle fracture and edge chipping, which have already been dealt with, tool wear is basically of three types. Flank wear, crater wear, and notch wear. Flank wear occurs on both the major and the minor cutting edges. On the major cutting edge, which is responsible for bulk metal removal, these results in increased cutting forces and higher temperatures which if left unchecked can lead to vibration of the tool and workpiece and a condition where efficient cutting can no longer take place. On the minor cutting edge, which determines workpiece size and surface finish, flank wear can result in an oversized product which has poor surface finish. Under most practical cutting conditions, the tool will fail due to major flank wear before the minor flank wear is sufficiently large to result in the manufacture of an unacceptable component. Because of the stress distribution on the tool face, the frictional stress in the region of sliding contact between the chip and the face is at a maximum at the start of the sliding contact region and is zero at the end. Thus abrasive wear takes place in this region with more wear taking place adjacent to the seizure region than adjacent to the point at which the chip loses contact with the face. This result in localized pitting of the tool face some distance up the face which is usually referred to as catering and which normally has a section in the form of a circular arc. In many respects and for practical cutting conditions, crater wear is a less severe form of wear than flank wear and consequently flank wear is a more common tool failure criterion. However, since various authors have shown that the temperature on the face increases more rapidly with increasing cutting speed than the temperature on the flank, and since the rate of wear of any type is significantly affected by changes in temperature, crater wear usually occurs at high cutting speeds. At the end of the major flank wear land where the tool is in contact with the uncut workpiece surface it is common for the flank wear to be more pronounced than along the rest of the wear land. This is because of localised effects such as a hardened layer on the uncut surface caused by work hardening introduced by a previous cut, an oxide scale, and localised high temperatures resulting from the edge effect. This localised wear is usually referred to as notch wear and occasionally is very severe. Although the presence of the notch will not significantly affect the cutting properties of the tool, the notch is often relatively deep and if cutting were to continue there would be a good chance that the tool would fracture. If any form of progressive wear allowed to continue, dramatically and the tool would fail catastrophically, i. e. the tool would be no longer capable of cutting and, at best, the workpiece would be scrapped whilst, at worst, damage could be caused to the machine tool. For carbide cutting tools and for all types of wear, the tool is said to have reached the end of its useful life long before the onset of catastrophic failure. For high-speed-steel cutting tools, however, where the wear tends to be non-uniform it has been found that the most meaningful and reproducible results can be obtained when the wear is allowed to continue to the onset of catastrophic failure even though, of course, in practice a cutting time far less than that to failure would be used. The onset of catastrophic failure is characterized by one of several phenomena, the most common being a sudden increase in cutting force, the presence of burnished rings on the workpiece, and a significant increase in the noise level. Mechanism of Surface Finish Production There are basically five mechanisms which contribute to the production of a surface which have been machined. These are: 1 The basic geometry of the cutting process. In, for example, single point turning the tool will advance a constant distance axially per revolution of the workpiecc and the resultant surface will have on it, when viewed perpendicularly to the direction of tool feed motion, a series of cusps which will have a basic form which replicates the shape of the tool in cut. 2 The efficiency of the cutting operation. It has already been mentioned that cutting with unstable built-up-edges will produce a surface which contains hard built-up-edge fragments which will result in a degradation of the surface finish. It can also be demonstrated that cutting under adverse conditions such as apply when using large feeds small rake angles and low cutting speeds, besides producing conditions which lead to unstable built-up-edge production, the cutting process itself can become unstable and instead of continuous shear occurring in the shear zone, tearing takes place, discontinuous chips of uneven thickness are produced, and the resultant surface is poor. This situation is particularly noticeable when machining very ductile materials such as copper and aluminum. 3 The stability of the machine tool. Under some combinations of cutting conditions； workpiece size, method of clamping ,and cutting tool rigidity relative to the machine tool structure, instability can be set up in the tool which causes it to vibrate. Under some conditions this vibration will reach and maintain steady amplitude whilst under other conditions the vibration will built up and unless cutting is stopped considerable damage to both the cutting tool and workpiece may occur. This phenomenon is known as chatter and in axial turning is characterized by long pitch helical bands on the workpiece surface and short pitch undulations on the transient machined surface. 4 The effectiveness of removing swarf. In discontinuous chip production machining, such as milling or turning of brittle materials, it is expected that the chip (swarf) will leave the cutting zone either under gravity or with the assistance of a jet of cutting fluid and that they will not influence the cut surface in any way. However, when continuous chip production is evident, unless steps are taken to control the swarf it is likely that it will impinge on the cut surface and mark it. Inevitably, this marking besides looking. 5 The effective clearance angle on the cutting tool. For certain geometries of minor cutting edge relief and clearance angles it is possible to cut on the major cutting edge and burnish on the minor cutting edge. This can produce a good surface finish but, of course, it is strictly a combination of metal cutting and metal forming and is not to be recommended as a practical cutting method. However, due to cutting tool wear, these conditions occasionally arise and lead to a marked change in the surface characteristics. Limits and Tolerances Machine parts are manufactured so they are interchangeable. In other words, each part of a machine or mechanism is made to a certain size and shape so will fit into any other machine or mechanism of the same type. To make the part interchangeable, each individual part must be made to a size that will fit the mating part in the correct way. It is not only impossible, but also impractical to make many parts to an exact size. This is because machines are not perfect, and the tools become worn. A slight variation from the exact size is always allowed. The amount of this variation depends on the kind of part being manufactured. For examples part might be made 6 in. long with a variation allowed of 0.003 (three-thousandths) in. above and below this size. Therefore, the part could be 5.997 to 6.003 in. and still be the correct size. These are known as the limits. The difference between upper and lower limits is called the tolerance. A tolerance is the total permissible variation in the size of a part. The basic size is that size from which limits of size arc derived by the application of allowances and tolerances. Sometimes the limit is allowed in only one direction. This is known as unilateral tolerance.Unilateral tolerancing is a system of dimensioning where the tolerance (that is variation) is shown in only one direction from the nominal size. Unilateral tolerancing allow the changing of tolerance on a hole or shaft without seriously affecting the fit.When the tolerance is in both directions from the basic size it is known as a bilateral tolerance (plus and minus). Bilateral tolerancing is a system of dimensioning where the tolerance (that is variation) is split and is shown on either side of the nominal size. Limit dimensioning is a system of dimensioning where only the maximum and minimum dimensions arc shown. Thus, the tolerance is the difference between these two dimensions. 基本加工工序和切削技術(shù) 機(jī)床是從早期的埃及人的腳踏動(dòng)力車和約翰 威爾金森的鏜床發(fā)展而來的。它們?yōu)楣ぜ偷毒咛峁﹦傂灾尾⒖梢跃_控制它們的相對位置和相對速度?；旧现v，金屬切削是指一個(gè)磨尖的鍥形工具從有韌性的工件表面上去除一條很窄的金屬。切屑是被廢棄的 產(chǎn)品，與其它工件相比切屑較短，但對于未切削部分的厚度有一定的增加。工件表面的幾何形狀取決于刀具的形狀以及加工操作過程中刀具的路徑。 大多數(shù)加工工序產(chǎn)生不同幾何形狀的零件。如果一個(gè)粗糙的工件在中心軸上轉(zhuǎn)動(dòng)并且刀具平行于旋轉(zhuǎn)中心切入工件表面，一個(gè)旋轉(zhuǎn)表面就產(chǎn)生了，這種操作稱為車削。如果一個(gè)空心的管子以同樣的方式在內(nèi)表面加工，這種操作稱為鏜孔。當(dāng)均勻地改變直徑時(shí)便產(chǎn)生了一個(gè)圓錐形的外表面，這稱為錐度車削。如果刀具接觸點(diǎn)以改變半徑的方式運(yùn)動(dòng)，那么一個(gè)外輪廓像球的工件便產(chǎn)生了；或者如果工件足夠的短并且支撐是十分剛硬的 ，那么成型刀具相對于旋轉(zhuǎn)軸正常進(jìn)給的一個(gè)外表面便可產(chǎn)生，短錐形或圓柱形的表面也可形成。 平坦的表面是經(jīng)常需要的，它們可以由刀具接觸點(diǎn)相對于旋轉(zhuǎn)軸的徑向車削產(chǎn)生。在刨削時(shí)對于較大的工件更容易將刀具固定并將工件置于刀具下面。刀具可以往復(fù)地進(jìn)給。成形面可以通過成型刀具加工產(chǎn)生。 多刃刀具也能使用。使用雙刃槽鉆鉆深度是鉆孔直徑 5-10 倍的孔。不管是鉆頭旋轉(zhuǎn)還是工件旋轉(zhuǎn)，切削刃與工件之間的相對運(yùn)動(dòng)是一個(gè)重要因數(shù)。在銑削時(shí)一個(gè)帶有許多切削刃的旋轉(zhuǎn)刀具與工件接觸，工件相對刀具慢慢運(yùn)動(dòng)。平的或成形面根據(jù)刀具的幾何形狀和進(jìn)給 方式可能產(chǎn)生。可以產(chǎn)生橫向或縱向軸旋轉(zhuǎn)并且可以在任何三個(gè)坐標(biāo)方向上進(jìn)給。 基本機(jī)床 機(jī)床通過從塑性材料上去除屑片來產(chǎn)生出具有特別幾何形狀和精確尺寸的零件。后者是廢棄物，是由塑性材料如鋼的長而不斷的帶狀物變化而來，從處理的角度來看，那是沒有用處的。很容易處理不好由鑄鐵產(chǎn)生的破裂的屑片。機(jī)床執(zhí)行五種基本的去除金屬的過程：車削，刨削，鉆孔，銑削。所有其他的去除金屬的過程都是由這五個(gè)基本程序修改而來的，舉例來說，鏜孔是內(nèi)部車削；鉸孔，攻絲和擴(kuò)孔是進(jìn)一步加工鉆過的孔；齒輪加工是基于銑削操作的。拋光和打磨是磨削和去除磨 料工序的變形。因此，只有四種基本類型的機(jī)床，使用特別可控制幾何形狀的切削工具 1.車床， 2.鉆床， 3.銑床， 4.磨床。磨削過程形成了屑片，但磨粒的幾何形狀是不可控制的。 通過各種加工工序去除材料的數(shù)量和速度是巨大的，正如在大型車削加工，或者是極小的如研磨和超精密加工中只有面的高點(diǎn)被除掉。 一臺(tái)機(jī)床履行三大職能： 1.它支撐工件或夾具和刀具 2.它為工件和刀具提供相對運(yùn)動(dòng) 3.在每一種情況下提供一系列的進(jìn)給量和一般可達(dá) 4-32 種的速度選擇。 機(jī)械加工介紹 作為產(chǎn)生形狀的一種方法，機(jī)械加工是所有制造過程中最普遍使用的而 且是最重要的方法。機(jī)械加工過程是一個(gè)產(chǎn)生形狀的過程，在這過程中，驅(qū)動(dòng)裝置使工件上的一些材料以切屑的形式被去除。盡管在某些場合，工件無支承情況下，使用移動(dòng)式裝備來實(shí)現(xiàn)加工，但大多數(shù)的機(jī)械加工是通過既支承工件又支承刀具的裝備來完成。 小批量，低成本。機(jī)械加工在制造業(yè)上有兩個(gè)應(yīng)用。是鑄造，鍛造和壓力工作，產(chǎn)生每一個(gè)特殊形狀，甚至一個(gè)零件，幾乎總有較高的模具成本。焊接的形狀很大程度上取決于原材料。通過利用總成本高但沒有特殊模具的設(shè)備，加工是有可能的；從幾乎任何形式的原材料開始，只要外部尺寸足夠大，由任意材料設(shè)計(jì)形狀 。因此加工是首選的方法，當(dāng)生產(chǎn)一個(gè)或幾個(gè)零件甚至在大批量生產(chǎn)時(shí)，零件的設(shè)計(jì)在邏輯上導(dǎo)致鑄造，鍛造或沖壓制品 。 高精度，表面精度。機(jī)械加工的第二個(gè)應(yīng)用是基于可能的高精度和表面精度的。如果在其他工序中大批量生產(chǎn)，很多低量零件會(huì)產(chǎn)生出低的但可接受的公差。另一方面，許多零件由一些大變形過程產(chǎn)生一般的形狀，并且只在具有很高精度的選定面加工。舉例來說，內(nèi)線流程是很少產(chǎn)生任何方式以外的其他機(jī)械加工并且緊接著壓力操作后零件上的小洞可能被加工。 主要的切削參數(shù) 在切削時(shí)基本工具工作的關(guān)系充分描述的方法有 4 個(gè)因素：刀具幾何形 狀，切削速度和切削深度。刀具必須由適當(dāng)?shù)牟牧献龀?；它必須有一定的?qiáng)度，粗糙度，硬度和抗疲勞度。 刀具幾何形狀由面和角度描述，對每一種切削操作都是正確的。切削速度是指切削刃通過工作面的速度，它已每分鐘通過的英尺數(shù)表示。 對于加工效率，切削速度相對于特殊工作組合必須具有適當(dāng)規(guī)模。一般來講，工件越硬，速度越小。進(jìn)給是刀具進(jìn)入工件的速率。當(dāng)工件或刀具旋轉(zhuǎn)時(shí)，進(jìn)給量的單位是英寸每轉(zhuǎn)。 當(dāng)?shù)毒呋蚬ぜ鶑?fù)移動(dòng)時(shí)，進(jìn)給量的單位是英寸沒次，總的來說，在其他相似情況下進(jìn)給量與切削速度成反比。切削速度用英寸表示，是刀具進(jìn)入工件的 距離表示的，它是指車削時(shí)屑片的寬度或是直線切削時(shí)屑片的厚度。粗加工時(shí)切削深度比精加工的切削深度大。 切削參數(shù)的改變對切削溫度的影響 在金屬切削作業(yè)中熱量產(chǎn)生于主要和第二變形區(qū)而這些結(jié)果導(dǎo)致了復(fù)雜溫度遍布于刀具，工件和屑片。一個(gè)典型的等溫先如圖所示，它可以看出正如預(yù)測的，當(dāng)工件材料經(jīng)歷主要變形，被減切時(shí)，有一個(gè)非常大溫度梯度遍布于屑片的整個(gè)寬度。當(dāng)?shù)诙冃螀^(qū)的屑片還有一小段距離就達(dá)到了最大溫度。 因?yàn)閹缀跛械墓ぷ鞫家越饘偾邢鬓D(zhuǎn)化為熱量而完成，可以預(yù)測去除每一單位體積的金屬所增加的能量消耗