切削技術(shù)和液壓外文翻譯@中英文翻譯@外文文獻(xiàn)翻譯

1 附錄 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 附錄 B 切削技術(shù)和液壓 1 自動夾具設(shè)計 用作裝配設(shè)備的傳統(tǒng)同步夾具把零件移動到夾具中心上，以確保零件從傳送機上或從備件盤上取出后置于已定位置上。然而在某些應(yīng)用場合、強制零件移動到中 心線上時，可能引起零件或設(shè)備破壞。當(dāng)該零件易損而且小小振動可能導(dǎo)致報廢時，或當(dāng)其位置是由機床主軸或模具來確定時，再或者當(dāng)公差要求很精密時，那寧可讓夾具去適應(yīng)零件位置，而不是相反。為著這些工作任務(wù)，美國俄亥俄州 Elyria 的 Zaytran 公司已經(jīng)開發(fā)了一般性功能數(shù)據(jù)的非同步系列柔順性夾具。因為夾具的作用力和同步化裝置是各自獨立的，該同步裝置可以用精密的滑移裝置來替換而不影響夾具的作用力。夾具規(guī)格范圍是從 0.2 英寸行程、 5英磅夾緊力到 6英寸行程、 400英磅夾緊力。 現(xiàn)代生產(chǎn)的特征是批量變得越來越小而產(chǎn)品 的品種規(guī)格變化更大。因此，生產(chǎn)的最后階段，裝配因生產(chǎn)計劃、批量和產(chǎn)品設(shè)計的變更而顯得特別脆弱。這種情形正迫使許多公司更多地致力于廣泛的合理化改革和前面提到過情況那樣裝配自動化。盡管柔性夾具的發(fā)展很快落后于柔性運輸處理裝置的發(fā)展，如落后于工業(yè)機器人的發(fā)展，但仍然試圖指望增加夾具的柔順性。事實上夾具是重要的裝置 生產(chǎn)裝置的專項投資就加強了使夾具更加柔性化在經(jīng)濟上的支持。 根據(jù)它們的柔順性，夾具可以分為：專用夾具、組合夾具、標(biāo)準(zhǔn)夾具和高柔性夾具。柔性夾具是以它們對不同工件的高適應(yīng)性和以少更換低費用為特征的。結(jié)構(gòu) 形式可變換的柔性夾具裝有可變更結(jié)構(gòu)排列的零件（例如針形頰板，多片式零件和片狀頰板），標(biāo)準(zhǔn)工件的非專用夾持或夾緊元件（例：氣動標(biāo)準(zhǔn)夾持夾具和帶有可移動元件的夾具配套件），或者裝有陶瓷或硬化了的中介物質(zhì)（例如流動粒子床夾具和熱夾緊夾具）。為了生產(chǎn)，零件要在夾具中被固緊，需要產(chǎn)生夾緊作用，其有幾個與夾具柔順性無關(guān)的步驟： 根據(jù)被加工的即基礎(chǔ)的部分和工作特點，確定工件在夾具中所需的位置，接著必須選擇若干穩(wěn)定平面的組合，這些穩(wěn)定平面就構(gòu)成了工件被固定在夾具中確定位置上的夾持狀輪廓結(jié)構(gòu)，均衡所有各力和力矩，而且必須保證 接近工件工作特點。最后，必須計算、調(diào)整、組裝可拆裝的或標(biāo)準(zhǔn)夾具元件的所需的位置，以便使工件牢牢地被夾緊在夾具中。依據(jù)這樣的程序，夾具的輪廓結(jié)構(gòu)和裝合的規(guī)劃和記錄過程可以進行自動化（控制）。 結(jié)構(gòu)造型任務(wù)就是要產(chǎn)生若干穩(wěn)定平面的組合，這樣在這些平面上的各夾緊力將使工13 件和夾具穩(wěn)定。按慣例，這個任務(wù)可用人 -機對話即幾乎完全自動化的方式來完成。以人 -機對話即以自動化方式確定夾具結(jié)構(gòu)造型的優(yōu)點是可有組織有規(guī)劃進行夾具設(shè)計，減少所需的設(shè)計人員，縮短研究周期和能更好地配置工作條件。簡言之，可成功地達(dá)到顯著提高夾具生產(chǎn)效率 和經(jīng)濟效益。 在充分準(zhǔn)備了構(gòu)造方案和一批材料情況下，在完成首次組裝可以成功實現(xiàn)節(jié)約時間達(dá)60。因此夾具結(jié)構(gòu)造型過程的目的是產(chǎn)生合適的編程文件。 2 加工基礎(chǔ) 作為產(chǎn)生形狀的一種方法，機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。機械加工過程是一個產(chǎn)生形狀的過程，在這過程中，驅(qū)動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，使用移動式裝備來實現(xiàn)加工，但大多數(shù)的機械加工是通過既支承工件又支承刀具的裝備來完成。 機械加工在制造過程中具有兩方面。小批生產(chǎn)低費用。對于鑄造、鍛 造和壓力加工，每一個要生產(chǎn)的具體工件形狀，即使是一個零件，幾乎都要花費高額的加工費用?？亢附觼懋a(chǎn)生的結(jié)構(gòu)形狀，在很大程度上取決于有效的原材料的形狀。一般來說，通過利用貴重設(shè)備而又無需特種加工條件下，幾乎可以從任何種類原材料開始，借助機械加工把原材料加工成任意所要求的結(jié)構(gòu)形狀，只要外部尺寸足夠大，那都是可能的。因此對于生產(chǎn)一個零件，甚至于當(dāng)零件結(jié)構(gòu)及要生產(chǎn)的批量大小上按理都適于用鑄造、鍛造或壓力加工來生產(chǎn)的，但通常寧可選擇機械加工。 嚴(yán)密的精度和良好表面光潔度。機械加工的第二方面用途是建立在高精度和可能的表面 光潔度基礎(chǔ)上。許多零件，如果用別的其他方法來生產(chǎn)屬大批量生產(chǎn)的話，那么在機械加工中則是屬低公差且又能滿足要求的小批量生產(chǎn)了。另方面，許多零件靠較粗的生產(chǎn)加工工藝提供其一般表面形狀，而僅僅是在需要高精度的且選擇過的表面上才進行機械加工。例如內(nèi)螺紋，除了機械加工之外，幾乎沒有別的加工方法能進行加工。又如已鍛工件上的小孔加工，也是被鍛后緊接著進行機械加工才完成的。 3 基本的機械加工參數(shù) 切削中工件與刀具的基本關(guān)系是以下四個要素來充分描述的：刀具的幾何形狀，切削速度，進給速度，和吃刀深度。切削刀具必須用一種合適的 材料來制造，它必須是強固、韌性好、堅硬而且耐磨的。刀具的幾何形狀 以刀尖平面和刀具角為特征 對于每一種切削工藝都必須是正確的。 14 切削速度是切削刃通過工件表面的速率，它是以每分鐘英寸來表示。為了有效地加工，切削速度高低必須適應(yīng)特定的工件 刀具的配合。一般來說，工件材料越硬，速度越低。 進給速度是刀具切進工件的速率。若工件或刀具作旋轉(zhuǎn)運動，進給量是以每轉(zhuǎn)轉(zhuǎn)過的英寸數(shù)目來度量的。當(dāng)?shù)毒呋蚬ぜ魍鶑?fù)運動時，進給量是以每一行程走過的英寸數(shù)度量的。一般來說，在其他條件相同時，進給量與切削速度成反比。吃刀深度 以 英寸計 是刀具進入工件的距離。它等于旋削中的切屑寬度或者是等于線性切削中的切屑的厚度。粗加工比起精加工來，吃刀深度較深。 4 切削參數(shù)的改變對切削溫度的影響 金屬切削操作中，熱是在主變形區(qū)和副變形區(qū)發(fā)生的。這結(jié)果導(dǎo)致復(fù)雜的溫度分布遍及刀具、工件和切屑。圖中顯示了一組典型的等溫曲線，從中可以看出：像所能預(yù)料的那樣，當(dāng)工件材料在主變形區(qū)被切削時，沿著整個切屑的寬度上有著很大的溫度梯度，而當(dāng)在副變形區(qū)，切屑被切落時，切屑附近的前刀面上就有更高的溫度。這就導(dǎo)致了前刀面和切屑離切削刃很近的地方切削溫度最高。 實質(zhì)上 由于在金屬切削中所做的全部功都被轉(zhuǎn)化為熱，那就可以預(yù)料：被切離金屬的單位體積功率消耗增加的這些因素就將使切削溫度升高。這樣刀具前角的增加而所有其他參數(shù)不變時，將使被切離金屬的單位體積所耗功率減小，因而切削溫度也將降低。當(dāng)考慮到未變形切屑厚度的增加和切削速度，這情形就更是復(fù)雜。未變形切屑厚度的增加勢必導(dǎo)致通過工件的熱的總數(shù)上產(chǎn)生比例效應(yīng)，刀具和切屑仍保持著固定的比例，而切削溫度變化傾向于降低。然而切削速度的增加，傳導(dǎo)到工件上的熱的數(shù)量減少而這又增加主變形區(qū)中的切屑的溫升。進而副變形區(qū)勢必更小，這將在該區(qū)內(nèi)產(chǎn)生 升溫效應(yīng)。其他切削參數(shù)的變化，實質(zhì)上對被切離金屬的單位體積功率消耗上并沒有什么影響，因此實際上對切削溫度沒有什么作用。因為事實已經(jīng)表明：切削溫度即使有小小的變化對刀具磨損率都將有實質(zhì)意義的影響作用。這表明如何從切削參數(shù)來確定切削溫度那是很合適的。 為著測定高速鋼刀具溫度的最直接和最精確的方法是 W T法，這方法也就是可提供高速鋼刀具溫度分布的詳細(xì)信息的方法。該項技術(shù)是建立在高速鋼刀具截面金相顯微測試基礎(chǔ)上，目的是要建立顯微結(jié)構(gòu)變化與熱規(guī)律圖線關(guān)系式。當(dāng)要加工廣泛的工件材料時， Trent已經(jīng)論述過測定高速 鋼刀具的切削溫度及溫度分布的方法。這項技術(shù)由于利用電子顯微掃描技術(shù)已經(jīng)進一步發(fā)展，目的是要研究將已回過火和各種馬氏體結(jié)構(gòu)的高速鋼再回火引起的微觀顯微結(jié)構(gòu)變化情況。這項技術(shù)亦用于研究高速鋼單點車刀和麻花鉆的溫15 度分布。 5 刀具的磨損 從已經(jīng)被處理過的無數(shù)脆裂和刃口裂紋的刀具中可知，刀具磨損基本上有三種形式：后刀面磨損，前刀面磨損和 v形凹口磨損。后刀面磨損既發(fā)生在主刀刃上也發(fā)生在副刀刃上。關(guān)于主刀刃，因其擔(dān)負(fù)切除大部金屬切屑任務(wù)，這就導(dǎo)致增加切削力和提高切削溫度，如果聽任而不加以檢查處理，那可能導(dǎo)致刀具和工件生 振動且使有效切削的條件可能不再存在。關(guān)于副刀刃，那是決定著工件的尺寸和表面光潔度的，后刀面磨損可能造成尺寸不合格的產(chǎn)品而且表面光潔度也差。在大多數(shù)實際切削條件下，由于主前刀面先于副前刀面磨損，磨損到達(dá)足夠大時，刀具將失效，結(jié)果是制成不合格零件。 由于刀具表面上的應(yīng)力分布不均勻，切屑和前刀面之間滑動接觸區(qū)的摩擦應(yīng)力，在滑動接觸區(qū)的起始處最大，而在接觸區(qū)的尾部為零，這樣磨蝕性磨損在這個區(qū)域發(fā)生了。這是因為在切屑卡住區(qū)附近比刀刃附近發(fā)生更嚴(yán)重的磨損，而刀刃附近因切屑與前刀面失去接觸而磨損較輕。這結(jié)果離切削刃一定距 離處的前刀面上形成麻點凹坑，這些通常被認(rèn)為是前刀面的磨損。通常情況下，這磨損橫斷面是圓弧形的。在許多情況中和對于實際的切削狀況而言，前刀面磨損比起后刀面磨損要輕，因此后刀面磨損更普遍地作為刀具失效的尺度標(biāo)志。然而因許多作者已經(jīng)表示過的那樣在增加切削速度情況下，前刀面上的溫度比后刀面上的溫度升得更快，而且又因任何形式的磨損率實質(zhì)上是受到溫度變化的重大影響。因此前刀面的磨損通常在高速切削時發(fā)生的。 刀具的主后刀面磨損帶的尾部是跟未加工過的工件表面相接觸，因此后刀面磨損比沿著磨損帶末端處更為明顯，那是很普遍的。這 是因為局部效應(yīng)，這像未加工表面上的已硬化層，這效應(yīng)是由前面的切削引起的工件硬化造成的。不只是切削，還有像氧化皮，刀刃產(chǎn)生的局部高溫也都會引起這種效應(yīng)。這種局部磨損通常稱作為凹坑性磨損，而且偶爾是非常嚴(yán)重的。盡管凹坑的出現(xiàn)對刀具的切削性質(zhì)無實質(zhì)意義的影響，但凹坑常常逐漸變深，如果切削在繼續(xù)進行的話，那么刀具就存在斷裂的危機。 如果任何進行性形式的磨損任由繼續(xù)發(fā)展，最終磨損速率明顯地增加而刀具將會有摧毀性失效破壞，即刀具將不能再用作切削，造成工件報廢，那算是好的，嚴(yán)重的可造成機床破壞。對于各種硬質(zhì)合金刀具和對于 各種類型的磨損，在發(fā)生嚴(yán)重失效前，就認(rèn)為已到達(dá)刀具的使用壽命周期的終點。然而對于各種高速鋼刀具，其磨損是屬于非均勻性磨損，已經(jīng)發(fā)現(xiàn)：當(dāng)其磨損允許連續(xù)甚至到嚴(yán)重失效開始，最有意義的是該刀具可以獲得重磨使16 用，當(dāng)然，在實際上，切削時間遠(yuǎn)比使用到失效的時間短。以下幾種現(xiàn)象之一均是刀具嚴(yán)重失效開始的特征：最普遍的是切削力突然增加；在工件上出現(xiàn)燒損環(huán)紋和噪音嚴(yán)重增加等。 6 表面精整加工機理 對已加工表面進行精整加工的機理，基本上有五個方面，它們是： 1)切削過程的基本幾何結(jié)構(gòu)。例如在工件每轉(zhuǎn)一轉(zhuǎn)，單點車刀將軸向前進一 個等距。當(dāng)垂直對著走刀運動方向觀察時，結(jié)果在工件表面上有一系列基本形狀一樣，即似切割刀具刀尖形狀復(fù)制而成的三角槽紋。 2）切削加工的效率。已經(jīng)論述過，帶不穩(wěn)定切屑瘤的切削加工將產(chǎn)生含有硬切屑瘤碎屑的表面，這些碎屑將導(dǎo)致表面光潔度的破壞（降級）。已經(jīng)證明，在采用進給量大，前角小，切削速度低的不利情況下，除了產(chǎn)生不穩(wěn)定的切屑瘤外，切削過程也會不穩(wěn)定。同時，在切削區(qū)里