切削技术和液压外文翻译@中英文翻译@外文文献翻译

1 附录 A CUTTING TECHNOLOGY&HYDRALICS 1 Automatic Fixture Design Traditional synchronous grippers for assembly equipment move parts to the gripper centre-line, assuring that the parts will be in a known position after they are picked from a conveyor or nest. However, in some applications, forcing the part to the centre-line may damage either the part or equipment. When the part is delicate and a small collision can result in scrap, when its location is fixed by a machine spindle or mould, or when tolerances are tight, it is preferable to make a gripper comply with the position of the part, rather than the other way around. For these tasks, Zaytran lnc. Of Elyria, Ohio, has created the GPN series of non- synchronous, compliant grippers . Because the force and synchronization systems of the grippers are independent, the synchronization system can be replaced by a precision slide system without affecting gripper force. Gripper size range from 5lb gripping force and 0.2 in. stroke to 4001b gripping force and 6in stroke. Production is characterized by batch-sizes becoming smaller and smaller and greater variety of products. Assembly, being the last production step, is particularly vulnerable to changes in schedules, batch-sizes, and product design. This situation is forcing many companies to put more effort into extensive rationalization and automation of assembly that was previously the case. Although the development of flexible fixtures fell quickly behind the development of flexible handling systems such as industrial robots, there are, nonetheless, promising attempts to increase the flexibility of fixtures. The fact that fixtures ae the essential product-specific investment of a production system intensifies the economic necessity to make the fixture system more flexible. Fixtures can be divided according to their flexibility into special fixtures, group fixtures modular fixtures and highly flexible fixtures. Flexible fixtures are characterized by their high adaptability to different work pieces, and by low change-over time and expenditure. Flexible fixtures with form variability are equipped with variable from elements (e.g., needle-check, multileaf, and lamella-cheek), modular work piece nonspecific holding or clamping-elements (e.g., pneumatic modular holding -fixtures and fixtures kits with moveable elements), or with fictile and hardening media (e.g., particulate- fluidized-bed-fixtures and 2 thermal clamping-fixtures). Independent of the flexibility of a fixture, there are several steps required to generate a fixture, in which a work piece is fixed for a production task. The first step is to define the necessary position of the work piece in the fixture, based on the submachine or base part, and the working features. Following this, a combination of stability planes must be selected. These stability planes constitute the fixture configuration in which the workpiece is fixed in the defined position, all the forces or torques are compensated, and the necessary access to the working features is ensured. Finally, the necessary positions of moveable or modular fixture elements must be calculated, adjusted, or assembled, so that the workpiece is firmly fixed in the fixture. Through such a procedure the planning and documentation of the configuration and assembly of fixture can be automated. The configuration task is to generate a combination of stability planes, such that fixture forces in these planes will result in workpiece and fixture stability. This task call be accomplished conventionally, interactively or in a nearly fully automated manner. The advantages of an interactive or automated configuration determination are a systematic fixture design process, a reduction of necessary designers, a shortening of lead time and better match to the working conditions. In short, a significant enhancement of fixture productivity and economy can be achieved. With the full preparation of construction plans and a bill of materials, a time saving of up to 60% in achieving the first assembly can be realized. Hence, an aim of the fixture configuration process is the generation of appropriate documents. 2 Introduction of Machining Machining as a shape producing method is the most universally ased 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, befogging, and press working, each specific shape to be produced, even one part, nearly 3 always has a high tooling cost. The shapes that may be 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 long as the exterior dimensions are great enough, and produce any desired shape from any material. Therefore, machining is usually the preerred 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 quantities 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. 3 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 tools 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 4 operations, the depth of cut can be larger than for finishing operations. 4 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 this 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,wilI reduce the power per unit volume of metal removed and the cutting temperatures will reduce. When considering increase in undeformed 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 in 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 temperatuers 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 5 examination of sectioned high-speed-steel tools which relates microstructural changes to thermal history. Trent has described measuerments 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-scaIe microstructural changes arising from over tempering of the tempered martensitic 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. 5 Wears of Cutting Tool Discounting brittle fracture and edge chipging, 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,this results in increased cutting forces and higher temperatures which if left unchecked cad 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 a nd surface finish, flank wear can result in an oversized product which bas poor surface finish. Under most practical cutting conditions,the tool will fail due to 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 results in localised pitting of the tool face some distance up the face which is usually referedto as crarering 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 fac e 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. 6 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 rcst of the wear land. This is because of loealised 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 ternperatures 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, eventually the wear rate would increase dramatically and the tool would fail catastrophicaIly,i.e. the tool wouId be no longer capable of cutting and, at best,the workpiece would be scrapped whilst,at worst, damagecould 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 couse,in practice a cutting time far less than that to failure would be used. The onset of catasteophie failure is charactcriscd 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. 6 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 workpiece 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 forln which replicates the shape of the tool in cut. 2) The efficiency of the curling 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 resah in a degradation of the surface finish. It can also be demonstrated that cutting 7 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 cuttong 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. 3)The stability of the machine tool. Under some conbinations of cutting conditions: workplece 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 a steady considerable damage to both the cutting tool and workpiece may occur. This phenomenon is known as chatter and in axial turning is characterised by long pitch helical bands on the workpiece surface and short pitch undulations on the transient machined surface. 4) The effectiveness of removing swarf, ln 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 unattractive, often results in a poorer surface finishing. 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 occationally arise and lead to a marked change in the surface charateristics. 7 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 it 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 8 impractical to make many parts to an exact size. This is because machines are net 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 example,a 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 are derived hy 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 tolcrancing 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 are shown.Thus,the tolerance is the difference between these two dimensions. 8 Surface Finishing and DimensionaI Control Products that have been completed to their proper shape and size frequently require some type of surface finishing to enable them to satisfactorily fulfill their function.In some cases， it is necessary to improve the physical properties of the surface material for resistance to penetration or abrasion， In many manufacturing processes ， the product surface is left with dirt,chips,grease,or other harmful materia upon it Assemblies that are made of different materials， or from the the same materials processed in different manners， may require some special surface treatment to provide uniformity of appearance. Surface finishing may sometimes become an intermediate step processing. For instance, cleaning and polishing are usually essential before any kind of plating process. Some of the cleaning procedures are also used for improving surface smoothness on mating parts and for removing burrs and sharp corners,which might be harmful in later use. Another important need 9 for surface finishing is for corrosion protection in a variety of enviromnents. The type of protection procedure will depend largely upon the anticipated exposure, with due consideration to the material being protected and the economic factors involved. Satisfying the above objectives necessitates the use of main surface-finishing methods that involve chemical change of the surface mechanical work affecting surface properties, cleaning by a variety of methods, and the application of protective coatings, organic and metallic. In the early days of engineering, the mating of parts was achieved by machining one part as nearly as possible to the required size,machining the mating part nearly to size, and then completing its machining,continually offering the other part to it, until the desired relationship was obtained. If it was inconvenient to offer one part to the other part during machining, the final work was done at the bench by a fitter, who scraped the mating parts until the desired fit was obtained, the fitter therefore being a fitter in the literal sense, It is obvious chat the two parts would have tc remain together, and in the event of one having to be replaced, the fitting would have to be done all over again. In these days, we expect to be able to purchase a replacement for a broken part, and for it to function correctly without the need for scraping and other fitting operations. When one part can be used off the shelf to replace another of the same dimension and material specification, the parts are said to be interchangeable. A system of interchangeability usually lowers the production costs as there is no need for an expensive, fiddling operation, and it benefits the customer in the event of the need to replace worn parts. 9 Variable Speed Hydraulic Systems It is particularly important on many hydraulic systems,as on machine tools, to be able to vary the speed of operation at will. This can be carried out in the following ways, sometimes more than one way being combned: a By varying the pump output manually； b By using several pumps in combinations； c By restricting or throttling the output of a automatically variable delivery pump, or a pump accumulator system,or by throttling the inlet； d By by passing part of the pump output with a flow dividing valve； e By varying the volume of the operating jack. 10 1) Variation in Pump Delivery. Pump delivery can be varied by a Alteration in its speed； b Alteration of its stroke in a variable stroke type of pump； c Using two or more pumps of different delivery in parallel so that by stopping and starting the pumps in various combinations different total deliveries can obtained. The first system is an easy one when tile pump is electrically driven,although the electric motor involved is comparatively complicated for normal requirements. Mechanical variable speed gear boxes have been used successfully with constant speed electric drive.Several of the pump mechanisms previously described can readily be adapted to give a varying output by reducing the working strok manually by means of a control wheel, etc. The third system is simple enough,but varies the output in fixed steps. Two pumps itl parallel can give three ranges of output corresponding to Pump A, Pump B, Pump A plus B. Three pumps in parallel can give seven steps corresponding to Pump A, Pump A plus B, Pump B plus C. Pump B, Pump A plus C, pump A plus B plus C. Pump C. Since, however, variable stroke pumps are readily available,such a complication as three pumps in parallel hardly seems worthwhile although the two-pump system is probably excellent for such duties as presses,etc. ,where a great part of the working stroke is at low pressure,where a relatively cheap type of pump can be used ,cutting out in favour of a smaller delivery high pressure pump lot the finai working stroke. Automatic isolation of the low pressure pump can be effected by a valve. Any normal type of automatic cut out will operate in the Iow-pressure system to by-pass it, without interference from the other pump. 2) Restriction of Pump Output. With a variable delivery pump the flow of oil to the system proper can be metered through a restriction, the dellvery of the pump automatically adjusting itself to the reduced flow. An automatic flow control valve or throttle is to be preferred to a simple restrictor. This is an extremely simple system ,but is liable to variation of speed owing to change in viscosily of the oil ,temperature effects,etc. ,and the metering restriction may have to be adjusted from time to time to keepthe speed constant. On the other hand,it is possible to 11 evolve a restriction conlpensated for changes. By fitting the flow control valve in either jack line ,control in one directions only can be exercised,but note that as the volumes of the jack returning to tank may not be the same in both directions,the degree cf speed control may not be similar. 3) Use of Flow dividing Valves. The flow dividing valves of various types are used to control the speed of a system by by-passing part of the pump output, even if at the expense of a slight wastage of power. It is possible to use a selector incorporating several ports, which in turn control the flow of fluid past several different flow dividing valves, giving different rates of flow for each position of the selector. 4) Variation in Jack Volume. Another means of obtaining Variable speed from a constant delivery pump is to use jacks of different volumes(i, e. at defferent pressures) ,either in parallel,or using a muhi-volume construction. If, for example, the machine tool slide , etc. ,is fitted with two operating jacks, by suitable selection varying speed of operation can be obtained corresponding to a Use of jack A； b Use of jack B； c Use of jack A and B together. If B=2A , the speeds are in the order 1,2,3. The comhinstion of two jacks and two pumps can obviously give 9 speeds, but at the expense of considerably more complication than would appear to be present with a variable delivery pump. 12 附录 B 切削技术和液压 1 自动夹具设计 用作装配设备的传统同步夹具把零件移动到夹具中心上，以确保零件从传送机上或从备件盘上取出后置于已定位置上。然而在某些应用场合、强制零件移动到中 心线上时，可能引起零件或设备破坏。当该零件易损而且小小振动可能导致报废时，或当其位置是由机床主轴或模具来确定时，再或者当公差要求很精密时，那宁可让夹具去适应零件位置，而不是相反。为着这些工作任务，美国俄亥俄州 Elyria 的 Zaytran 公司已经开发了一般性功能数据的非同步系列柔顺性夹具。因为夹具的作用力和同步化装置是各自独立的，该同步装置可以用精密的滑移装置来替换而不影响夹具的作用力。夹具规格范围是从 0.2 英寸行程、 5英磅夹紧力到 6英寸行程、 400英磅夹紧力。 现代生产的特征是批量变得越来越小而产品 的品种规格变化更大。因此，生产的最后阶段，装配因生产计划、批量和产品设计的变更而显得特别脆弱。这种情形正迫使许多公司更多地致力于广泛的合理化改革和前面提到过情况那样装配自动化。尽管柔性夹具的发展很快落后于柔性运输处理装置的发展，如落后于工业机器人的发展，但仍然试图指望增加夹具的柔顺性。事实上夹具是重要的装置 生产装置的专项投资就加强了使夹具更加柔性化在经济上的支持。 根据它们的柔顺性，夹具可以分为：专用夹具、组合夹具、标准夹具和高柔性夹具。柔性夹具是以它们对不同工件的高适应性和以少更换低费用为特征的。结构 形式可变换的柔性夹具装有可变更结构排列的零件（例如针形颊板，多片式零件和片状颊板），标准工件的非专用夹持或夹紧元件（例：气动标准夹持夹具和带有可移动元件的夹具配套件），或者装有陶瓷或硬化了的中介物质（例如流动粒子床夹具和热夹紧夹具）。为了生产，零件要在夹具中被固紧，需要产生夹紧作用，其有几个与夹具柔顺性无关的步骤： 根据被加工的即基础的部分和工作特点，确定工件在夹具中所需的位置，接着必须选择若干稳定平面的组合，这些稳定平面就构成了工件被固定在夹具中确定位置上的夹持状轮廓结构，均衡所有各力和力矩，而且必须保证 接近工件工作特点。最后，必须计算、调整、组装可拆装的或标准夹具元件的所需的位置，以便使工件牢牢地被夹紧在夹具中。依据这样的程序，夹具的轮廓结构和装合的规划和记录过程可以进行自动化（控制）。 结构造型任务就是要产生若干稳定平面的组合，这样在这些平面上的各夹紧力将使工13 件和夹具稳定。按惯例，这个任务可用人 -机对话即几乎完全自动化的方式来完成。以人 -机对话即以自动化方式确定夹具结构造型的优点是可有组织有规划进行夹具设计，减少所需的设计人员，缩短研究周期和能更好地配置工作条件。简言之，可成功地达到显著提高夹具生产效率 和经济效益。 在充分准备了构造方案和一批材料情况下，在完成首次组装可以成功实现节约时间达60。因此夹具结构造型过程的目的是产生合适的编程文件。 2 加工基础 作为产生形状的一种方法，机械加工是所有制造过程中最普遍使用的而且是最重要的方法。机械加工过程是一个产生形状的过程，在这过程中，驱动装置使工件上的一些材料以切屑的形式被去除。尽管在某些场合，工件无支承情况下，使用移动式装备来实现加工，但大多数的机械加工是通过既支承工件又支承刀具的装备来完成。 机械加工在制造过程中具有两方面。小批生产低费用。对于铸造、锻 造和压力加工，每一个要生产的具体工件形状，即使是一个零件，几乎都要花费高额的加工费用。靠焊接来产生的结构形状，在很大程度上取决于有效的原材料的形状。一般来说，通过利用贵重设备而又无需特种加工条件下，几乎可以从任何种类原材料开始，借助机械加工把原材料加工成任意所要求的结构形状，只要外部尺寸足够大，那都是可能的。因此对于生产一个零件，甚至于当零件结构及要生产的批量大小上按理都适于用铸造、锻造或压力加工来生产的，但通常宁可选择机械加工。 严密的精度和良好表面光洁度。机械加工的第二方面用途是建立在高精度和可能的表面 光洁度基础上。许多零件，如果用别的其他方法来生产属大批量生产的话，那么在机械加工中则是属低公差且又能满足要求的小批量生产了。另方面，许多零件靠较粗的生产加工工艺提供其一般表面形状，而仅仅是在需要高精度的且选择过的表面上才进行机械加工。例如内螺纹，除了机械加工之外，几乎没有别的加工方法能进行加工。又如已锻工件上的小孔加工，也是被锻后紧接着进行机械加工才完成的。 3 基本的机械加工参数 切削中工件与刀具的基本关系是以下四个要素来充分描述的：刀具的几何形状，切削速度，进给速度，和吃刀深度。切削刀具必须用一种合适的 材料来制造，它必须是强固、韧性好、坚硬而且耐磨的。刀具的几何形状 以刀尖平面和刀具角为特征 对于每一种切削工艺都必须是正确的。 14 切削速度是切削刃通过工件表面的速率，它是以每分钟英寸来表示。为了有效地加工，切削速度高低必须适应特定的工件 刀具的配合。一般来说，工件材料越硬，速度越低。 进给速度是刀具切进工件的速率。若工件或刀具作旋转运动，进给量是以每转转过的英寸数目来度量的。当刀具或工件作往复运动时，进给量是以每一行程走过的英寸数度量的。一般来说，在其他条件相同时，进给量与切削速度成反比。吃刀深度 以 英寸计 是刀具进入工件的距离。它等于旋削中的切屑宽度或者是等于线性切削中的切屑的厚度。粗加工比起精加工来，吃刀深度较深。 4 切削参数的改变对切削温度的影响 金属切削操作中，热是在主变形区和副变形区发生的。这结果导致复杂的温度分布遍及刀具、工件和切屑。图中显示了一组典型的等温曲线，从中可以看出：像所能预料的那样，当工件材料在主变形区被切削时，沿着整个切屑的宽度上有着很大的温度梯度，而当在副变形区，切屑被切落时，切屑附近的前刀面上就有更高的温度。这就导致了前刀面和切屑离切削刃很近的地方切削温度最高。 实质上 由于在金属切削中所做的全部功都被转化为热，那就可以预料：被切离金属的单位体积功率消耗增加的这些因素就将使切削温度升高。这样刀具前角的增加而所有其他参数不变时，将使被切离金属的单位体积所耗功率减小，因而切削温度也将降低。当考虑到未变形切屑厚度的增加和切削速度，这情形就更是复杂。未变形切屑厚度的增加势必导致通过工件的热的总数上产生比例效应，刀具和切屑仍保持着固定的比例，而切削温度变化倾向于降低。然而切削速度的增加，传导到工件上的热的数量减少而这又增加主变形区中的切屑的温升。进而副变形区势必更小，这将在该区内产生 升温效应。其他切削参数的变化，实质上对被切离金属的单位体积功率消耗上并没有什么影响，因此实际上对切削温度没有什么作用。因为事实已经表明：切削温度即使有小小的变化对刀具磨损率都将有实质意义的影响作用。这表明如何从切削参数来确定切削温度那是很合适的。 为着测定高速钢刀具温度的最直接和最精确的方法是 W T法，这方法也就是可提供高速钢刀具温度分布的详细信息的方法。该项技术是建立在高速钢刀具截面金相显微测试基础上，目的是要建立显微结构变化与热规律图线关系式。当要加工广泛的工件材料时， Trent已经论述过测定高速 钢刀具的切削温度及温度分布的方法。这项技术由于利用电子显微扫描技术已经进一步发展，目的是要研究将已回过火和各种马氏体结构的高速钢再回火引起的微观显微结构变化情况。这项技术亦用于研究高速钢单点车刀和麻花钻的温15 度分布。 5 刀具的磨损 从已经被处理过的无数脆裂和刃口裂纹的刀具中可知，刀具磨损基本上有三种形式：后刀面磨损，前刀面磨损和 v形凹口磨损。后刀面磨损既发生在主刀刃上也发生在副刀刃上。关于主刀刃，因其担负切除大部金属切屑任务，这就导致增加切削力和提高切削温度，如果听任而不加以检查处理，那可能导致刀具和工件生 振动且使有效切削的条件可能不再存在。关于副刀刃，那是决定着工件的尺寸和表面光洁度的，后刀面磨损可能造成尺寸不合格的产品而且表面光洁度也差。在大多数实际切削条件下，由于主前刀面先于副前刀面磨损，磨损到达足够大时，刀具将失效，结果是制成不合格零件。 由于刀具表面上的应力分布不均匀，切屑和前刀面之间滑动接触区的摩擦应力，在滑动接触区的起始处最大，而在接触区的尾部为零，这样磨蚀性磨损在这个区域发生了。这是因为在切屑卡住区附近比刀刃附近发生更严重的磨损，而刀刃附近因切屑与前刀面失去接触而磨损较轻。这结果离切削刃一定距 离处的前刀面上形成麻点凹坑，这些通常被认为是前刀面的磨损。通常情况下，这磨损横断面是圆弧形的。在许多情况中和对于实际的切削状况而言，前刀面磨损比起后刀面磨损要轻，因此后刀面磨损更普遍地作为刀具失效的尺度标志。然而因许多作者已经表示过的那样在增加切削速度情况下，前刀面上的温度比后刀面上的温度升得更快，而且又因任何形式的磨损率实质上是受到温度变化的重大影响。因此前刀面的磨损通常在高速切削时发生的。 刀具的主后刀面磨损带的尾部是跟未加工过的工件表面相接触，因此后刀面磨损比沿着磨损带末端处更为明显，那是很普遍的。这 是因为局部效应，这像未加工表面上的已硬化层，这效应是由前面的切削引起的工件硬化造成的。不只是切削，还有像氧化皮，刀刃产生的局部高温也都会引起这种效应。这种局部磨损通常称作为凹坑性磨损，而且偶尔是非常严重的。尽管凹坑的出现对刀具的切削性质无实质意义的影响，但凹坑常常逐渐变深，如果切削在继续进行的话，那么刀具就存在断裂的危机。 如果任何进行性形式的磨损任由继续发展，最终磨损速率明显地增加而刀具将会有摧毁性失效破坏，即刀具将不能再用作切削，造成工件报废，那算是好的，严重的可造成机床破坏。对于各种硬质合金刀具和对于 各种类型的磨损，在发生严重失效前，就认为已到达刀具的使用寿命周期的终点。然而对于各种高速钢刀具，其磨损是属于非均匀性磨损，已经发现：当其磨损允许连续甚至到严重失效开始，最有意义的是该刀具可以获得重磨使16 用，当然，在实际上，切削时间远比使用到失效的时间短。以下几种现象之一均是刀具严重失效开始的特征：最普遍的是切削力突然增加；在工件上出现烧损环纹和噪音严重增加等。 6 表面精整加工机理 对已加工表面进行精整加工的机理，基本上有五个方面，它们是： 1)切削过程的基本几何结构。例如在工件每转一转，单点车刀将轴向前进一 个等距。当垂直对着走刀运动方向观察时，结果在工件表面上有一系列基本形状一样，即似切割刀具刀尖形状复制而成的三角槽纹。 2）切削加工的效率。已经论述过，带不稳定切屑瘤的切削加工将产生含有硬切屑瘤碎屑的表面，这些碎屑将导致表面光洁度的破坏（降级）。已经证明，在采用进给量大，前角小，切削速度低的不利情况下，除了产生不稳定的切屑瘤外，切削过程也会不稳定。同时，在切削区里