机械外文翻译

精品文档 1欢迎下载 Lathes Lathes are machine tools designed primarily to do turning facing and boring Very little turning is done on other types of machine tools and none can do it with equal facility Because lathes also can do drilling and reaming their versatility permits several operations to be done with a single setup of the work piece Consequently more lathes of various types are used in manufacturing than any other machine tool The essential components of a lathe are the bed headstock assembly tailstock assembly and the leads crew and feed rod The bed is the backbone of a lathe It usually is made of well normalized or aged gray or nodular cast iron and provides s heavy rigid frame on which all the other basic components are mounted Two sets of parallel longitudinal ways inner and outer are contained on the bed usually on the upper side Some makers use an inverted V shape for all four ways whereas others utilize one inverted V and one flat way in one or both sets They are precision machined to assure accuracy of alignment On most modern lathes the way are surface hardened to resist wear and abrasion but precaution should be taken in operating a lathe to assure that the ways are not damaged Any inaccuracy in them usually means that the accuracy of the entire lathe is destroyed The headstock is mounted in a foxed position on the inner ways usually at the left end of the bed It provides a powered means of rotating the word at various speeds Essentially it consists of a hollow spindle mounted in accurate bearings and a set of transmission gears similar to a truck transmission through which the spindle can be rotated at a number of speeds Most lathes provide from 8 to 18 speeds usually in a geometric ratio and on modern lathes all the speeds can be obtained merely by moving 精品文档 2欢迎下载 from two to four levers An increasing trend is to provide a continuously variable speed range through electrical or mechanical drives Because the accuracy of a lathe is greatly dependent on the spindle it is of heavy construction and mounted in heavy bearings usually preloaded tapered roller or ball types The spindle has a hole extending through its length through which long bar stock can be fed The size of maximum size of bar stock that can be machined when the material must be fed through spindle The tailsticd assembly consists essentially of three parts A lower casting fits on the inner ways of the bed and can slide longitudinally thereon with a means for clamping the entire assembly in any desired location An upper casting fits on the lower one and can be moved transversely upon it on some type of keyed ways to permit aligning the assembly is the tailstock quill This is a hollow steel cylinder usually about 51 to 76mm 2to 3 inches in diameter that can be moved several inches longitudinally in and out of the upper casting by means of a hand wheel and screw The size of a lathe is designated by two dimensions The first is known as the swing This is the maximum diameter of work that can be rotated on a lathe It is approximately twice the distance between the line connecting the lathe centers and the nearest point on the ways The second size dimension is the maximum distance between centers The swing thus indicates the maximum work piece diameter that can be turned in the lathe while the distance between centers indicates the maximum length of work piece that can be mounted between centers Engine lathes are the type most frequently used in manufacturing They are heavy duty machine tools with all the components described previously and have power drive for all tool movements except on the compound rest They commonly range in size from 305 to 610 mm 12 to 24 inches swing and from 610 to 1219 mm 24 to 48 inches center distances but swings up to 精品文档 3欢迎下载 1270 mm 50 inches and center distances up to 3658mm 12 feet are not uncommon Most have chip pans and a built in coolant circulating system Smaller engine lathes with swings usually not over 330 mm 13 inches also are available in bench type designed for the bed to be mounted on a bench on a bench or cabinet Although engine lathes are versatile and very useful because of the time required for changing and setting tools and for making measurements on the work piece thy are not suitable for quantity production Often the actual chip production tine is less than 30 of the total cycle time In addition a skilled machinist is required for all the operations and such persons are costly and often in short supply However much of the operator s time is consumed by simple repetitious adjustments and in watching chips being made Consequently to reduce or eliminate the amount of skilled labor that is required turret lathes screw machines and other types of semiautomatic and automatic lathes have been highly developed and are widely used in manufacturing 2 Numerical Control One of the most fundamental concepts in the area of advanced manufacturing technologies is numerical control NC Prior to the advent of NC all machine tools ere manually operated and controlled Among the many limitations associated with manual control machine tools perhaps none is more prominent than the limitation of operator skills With manual control the quality of the product is directly related to and limited to the skills of the operator Numerical control represents the first major step away from human control of machine tools Numerical control means the control of machine tools and other manufacturing systems through the use of prerecorded written symbolic instructions Rather than operating a machine tool an NC technician writes a program that issues operational instructions to the machine tool For a 精品文档 4欢迎下载 machine tool to be numerically controlled it must be interfaced with a device for accepting and decoding the programmed instructions known as a reader Numerical control was developed to overcome the limitation of human operators and it has done so Numerical control machines are more accurate than manually operated machines they can produce parts more uniformly they are faster and the long run tooling costs are lower The development of NC led to the development of several other innovations in manufacturing technology Electrical discharge machining Laser cutting Electron beam welding Numerical control has also made machine tools more versatile than their manually operated predecessors An NC machine tool can automatically produce a wide of parts each involving an assortment of widely varied and complex machining processes Numerical control has allowed manufacturers to undertake the production of products that would not have been feasible from an economic perspective using manually controlled machine tolls and processes Like so many advanced technologies NC was born in the laboratories of the Massachusetts Institute of Technology The concept of NC was developed in the early 1950s with funding provided by the U S Air Force In its earliest stages NC machines were able to made straight cuts efficiently and effectively However curved paths were a problem because the machine tool had to be programmed to undertake a series of horizontal and vertical steps to produce a curve The shorter the straight lines making up the steps the smoother is the curve Each line segment in the steps had to be calculated This problem led to the development in 1959 of the Automatically Programmed Tools APT language This is a special programming language for NC that uses statements similar to English language to define the part geometry describe the cutting tool configuration and specify the 精品文档 5欢迎下载 necessary motions The development of the APT language was a major step forward in the fur ther development from those used today The machines had hardwired logic circuits The instructional programs were written on punched paper which was later to be replaced by magnetic plastic tape A tape reader was used to interpret the instructions written on the tape for the machine Together all of this represented a giant step forward in the control of machine tools However there were a number of problems with NC at this point in its development A major problem was the fragility of the punched paper tape medium It was common for the paper tape containing the programmed instructions to break or tear during a machining process This problem was exacerbated by the fact that each successive time a part was produced on a machine tool the paper tape carrying the programmed instructions had to be rerun through the reader If it was necessary to produce 100 copies of a given part it was also necessary to run the paper tape through the reader 100 separate tines Fragile paper tapes simply could not withstand the rigors of a shop floor environment and this kind of repeated use This led to the development of a special magnetic plastic tape Whereas the paper carried the programmed instructions as a series of holes punched in the tape the plastic tape carried the instructions as a series of magnetic dots The plastic tape was much stronger than the paper tape which solved the problem of frequent tearing and breakage However it still left two other problems The most important of these was that it was difficult or impossible to change the instructions entered on the tape To made even the most minor adjustments in a program of instructions it was necessary to interrupt machining operations and make a new tape It was also still necessary to run the tape through the reader as many times as there were parts to be produced Fortunately computer technology became a reality and soon solved the problems of NC associated with punched paper and plastic tape 精品文档 6欢迎下载 The development of a concept known as direct numerical control DNC solved the paper and plastic tape problems associated with numerical control by simply eliminating tape as the medium for carrying the programmed instructions In direct numerical control machine tools are tied via a data transmission link to a host computer Programs for operating the machine tools are stored in the host computer and fed to the machine tool an needed via the data transmission linkage Direct numerical control represented a major step forward over punched tape and plastic tape However it is subject to the same limitations as all technologies that depend on a host computer When the host computer goes down the machine tools also experience downtime This problem led to the development of computer numerical control 3 Turning The engine lathe one of the oldest metal removal machines has a number of useful and highly desirable attributes Today these lathes are used primarily in small shops where smaller quantities rather than large production runs are encountered The engine lathe has been replaced in today s production shops by a wide variety of automatic lathes such as automatic of single point tooling for maximum metal removal and the use of form tools for finish on a par with the fastest processing equipment on the scene today Tolerances for the engine lathe depend primarily on the skill of the operator The design engineer must be careful in using tolerances of an experimental part that has been produced on the engine lathe by a skilled operator In redesigning an experimental part for production economical tolerances should be used Turret Lathes Production machining equipment must be evaluated now more than ever before this criterion for establishing the production qualification of a specific method the turret lathe merits a high rating 精品文档 7欢迎下载 In designing for low quantities such as 100 or 200 parts it is most economical to use the turret lathe In achieving the optimum tolerances possible on the turrets lathe the designer should strive for a minimum of operations Automatic Screw Machines Generally automatic screw machines fall into several categories single spindle automatics multiple spindle automatics and automatic chucking machines Originally designed for rapid automatic production of screws and similar threaded parts the automatic screw machine has long since exceeded the confines of this narrow field and today plays a vital role in the mass production of a variety of precision parts Quantities play an important part in the economy of the parts machined on the automatic screw machine Quantities less than on the automatic screw machine The cost of the parts machined can be reduced if the minimum economical lot size is calculated and the proper machine is selected for these quantities Automatic Tracer Lathes Since surface roughness depends greatly on material turned tooling and feeds and speeds employed minimum tolerances that can be held on automatic tracer lathes are not necessarily the most economical tolerances In some cases tolerances of 0 05mm are held in continuous production using but one cut groove width can be held to 0 125mm on some parts Bores and single point finishes can be held to 0 0125mm On high production runs where maximum output is desirable a minimum tolerance of 0 125mm is economical on both diameter and length of turn 精品文档 8欢迎下载 车床 车床主要是为了进行车外圆 车端面和镗孔等项工作而设计的机床 车削很少在 其他种类的机床上进行 而且任何一种其他机床都不能像车床那样方便地进行车削加 工 由于车床还可以用来钻孔和铰孔 车床的多功能性可以使工件在一次安装中完成 几种加工 因此 在生产中使用的各种车床比任何其他种类的机床都多 车床的基本部件有 床身 主轴箱组件 尾座组件 溜板组件 丝杠和光杠 床身是车床的基础件 它能常是由经过充分正火或时效处理的灰铸铁或者球墨铁 制成 它是一个坚固的刚性框架 所有其他基本部件都安装在床身上 通常在床身上 有内外两组平行的导轨 有些制造厂对全部四条导轨都采用导轨尖朝上的三角形导轨 即山形导轨 而有的制造厂则在一组中或者两组中都采用一个三角形导轨和一个 矩形导轨 导轨要经过精密加工以保证其直线度精度 为了抵抗磨损和擦伤 大多数 现代机床的导轨是经过表面淬硬的 但是在操作时还应该小心 以避免损伤导轨 导 轨上的任何误差 常常意味着整个机床的精度遭到破坏 主轴箱安装在内侧导轨的固定位置上 一般在床身的左端 它提供动力 并可使 工件在各种速度下回转 它基本上由一个安装在精密轴承中的空心主轴和一系列变速 齿轮 类似于卡车变速箱 所组成 通过变速齿轮 主轴可以在许多种转速下旋转 大 多数车床有 8 12 种转速 一般按等比级数排列 而且在现代机床上只需扳动 2 4 个手 柄 就能得到全部转速 一种正在不断增长的趋势是通过电气的或者机械的装置进行 无级变速 由于机床的精度在很大程度上取决于主轴 因此 主轴的结构尺寸较大 通常安 装在预紧后的重型圆锥滚子轴承或球轴承中 主轴中有一个贯穿全长的通孔 长棒料 可以通过该孔送料 主轴孔的大小是车床的一个重要尺寸 因此当工件必须通过主轴 孔供料时 它确定了能够加工的棒料毛坯的最大尺寸 尾座组件主要由三部分组成 底板与床身的内侧导轨配合 并可以在导轨上作纵 向移动 底板上有一个可以使整个尾座组件夹紧在任意位置上的装置 尾座体安装在 底板上 可以沿某种类型的键槽在底板上横向移动 使尾座能与主轴箱中的主轴对正 精品文档 9欢迎下载 尾座的第三个组成部分是尾座套筒 它是一个直径通常大约在 51 76mm 2 3 英寸 之 间的钢制空心圆柱体 通过手轮和螺杆 尾座套筒可以在尾座体中纵向移入和移出几 个英寸 车床的规格用两个尺寸表示 第一个称为车床的床面上最大加工直径 这是在车 床上能够旋转的工件的最大直径 它大约是两顶尖连线与导轨上最近点之间距离的两 倍 第二个规格尺寸是两顶尖之间的最大距离 车床床面上最大加工直径表示在车床 上能够车削的最大工件直径 而两顶尖之间的最大距离则表示在两个顶尖之间能够安 装的工件的最大长度 普通车床是生产中最经常使用的车床种类 它们是具有前面所叙的所有那些部件 的重载机床 并且除了小刀架之外 全部刀具的运动都有机动进给 它们的规格通常 是 车床床面上最大加工直径为 305 610mm 12 24 英寸 但是 床面上最大加工直 径达到 1270mm 50 英寸 和两顶尖之间距离达到 3658mm 的车床也并不少见 这些车 床大部分都有切屑盘和一个安装在内部的冷却液循环系统 小型的普通车床 车床床 面最大加工直径一般不超过 330mm 13 英寸 被设计成台式车床 其床身安装在工 作台或柜子上 虽然普通车床有很多用途 是很有用的机床 但是更换和调整刀具以及测量工件 花费很多时间 所以它们不适合在大量生产中应用 通常 它们的实际加工时间少于 其总加工时间的 30 此外 需要技术熟练的工人来操作普通车床 这种工人的工资高 而且很难雇到 然而 操作工人的大部分时间却花费在简单的重复调整和观察切屑过 程上 因此 为了减少或者完全不雇用这类熟练工人 六角车床 螺纹加工车床和其 他类型的半自动和自动车床已经很好地研制出来 并已经在生产中得到广泛应用 2 数字控制 先进制造技术中的一个基本的概念是数字控制 NC 在数控技术出现之前 所 有的机床都是由人工操纵和控制的 在与人工控制的机床有关的很多局限性中 操作 者的技能大概是最突出的问题 采用人工控制是 产品的质量直接与操作者的技能有 关 数字控制代表了从人工控制机床走出来的第一步 数字控制意味着采用预先录制的 存储的符号指令来控制机床和其他制造系统 一个数控技师的工作不是去操纵机床 而是编写能够发出机床操纵指令的程序 对于 一台数控机床 其上必须安有一个被称为阅读机的界面装置 用来接受和解译出编程 指令 精品文档 10欢迎下载 发展数控技术是为了克服人类操作者的局限性 而且它确实完成了这项工作 数 字控制的机器比人工操纵的机器精度更高 生产出零件的一致性更好 生产速度更快 而且长期的工艺装备成本更低 数控技术的发展导致了制造工艺中其他几项新发明的 产生 电火花加工技术 激光切割 电子束焊接数字控制还使得机床比它们采用有人工 操的前辈们的用途更为广泛 一台数控机床可以自动生产很多类的零件 每一个零件都可以有不同的和复杂的 加工过程 数控可以使生产厂家承担那些对于采用人工控制的机床和工艺来说 在经 济上是不划算的产品生产任务 同许多先进技术一样 数控诞生于麻省理工学院的实验室中 数控这个概念是 50 年代初在美国空军的资助下提出来的 在其最初的价段 数控机床可以经济和有效地 进行直线切割 然而 曲线轨迹成为机床加工的一个问题 在编程时应该采用一系列的水平与竖 直的台阶来生成曲线 构成台阶的每一个线段越短 曲线就越光滑 台阶中的每一个 线段都必须经过计算 在这个问题促使下 于 1959 年诞生了自动编程工具 APT 语言 这是一个专门 适用于数控的编程语言 使用类似于英语的语句来定义零件的几何形状 描述切削刀 具的形状和规定必要的运动 APT 语言的研究和发展是在数控技术进一步发展过程中的 一大进步 最初的数控系统下今天应用的数控系统是有很大差别的 在那时的机床中 只有硬线逻辑电路 指令程序写在穿孔纸带上 它后来被塑料带所取代 采用带阅 读机将写在纸带或磁带上的指令给机器翻译出来 所有这些共同构成了机床数字控制 方面的巨大进步 然而 在数控发展的这个阶段中还存在着许多问题 一个主要问题是穿孔纸带的易损坏性 在机械加工过程中 载有编程指令信息的 纸带断裂和被撕坏是常见的事情 在机床上每加工一个零件 都需要将载有编程指令 的纸带放入阅读机中重新运行一次 因此 这个问题变得很严重 如果需要制造 100 个某种零件 则应该将纸带分别通过阅读机 100 次 易损坏的纸带显然不能承受严配 的车间环境和这种重复使用 这就导致了一种专门的塑料磁带的研制 在纸带上通过采用一系列的小孔来载有 编程指令 而在塑料带上通过采用一系列的磁点眯载