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外文资料CNC machine toolsWhile the specific intention and application for CNC machines vary from one machine type to another, all forms of CNC have common benefits. Here are hut a few of the more important benefits offered by CNC equipment.The first benefit offered by all forms of CNC machine tools is improved automation. The operator intervention related to producing workpieces can be reduced or eliminated. Many CNC machines can run unattended during their entire machining cycle, freeing the operator to do other tasks. This gives the CNC user several side benefits including reduced operator fatigue, fewer mistakes caused by human error, and consistent and predictable machining time for each workpiece. Since the machine will he running under program control, the skill level required of the CNC operator (related to basic machining practice) is also reduced as compared to a machinist producing workpieces with conventional machine tools.The second major benefit of CNC technology is consistent and accurate workpieces. Todays CNC machines boast almost unbelievable accuracy and repeatability specifications. This means that once a program is verified, two, ten, or one thousand identical workpieces can be easily produced with precision and consistency.A third benefit offered by most forms of CNC machine tools is flexibility. Since these machines are run from programs, running a different workpiece is almost as easy as loading a different program. Once a program has been verified and executed for one production run, it can be easily recalled the next time the workpiece is to be run. This leads to yet another benefit, fast change over. Since these machines are very easy to set up and run, and since programs can be easily loaded, they allow very short setup time. This is imperative with todays just-in-time (JIT) product requirements.Motion control - the heart of CNCThe most basic function of any CNC machine is automatic, precise, and consistent motion control. Rather than applying completely mechanical devices to cause motion as is required on most conventional machine tools, CNC machines allow motion control in a revolutionary manner2. All forms of CNC equipment have two or more directions of motion, called axes. These axes can be precisely and automatically positioned along their lengths of travel. The two most common axis types are linear (driven along a straight path) and rotary (driven along a circular path).Instead of causing motion by turning cranks and handwheels as is required on conventional machine tools, CNC machines allow motions to be commanded through programmed commands. Generally speaking, the motion type (rapid, linear, and circular), the axes to move, the amount of motion and the motion rate (feedrate) are programmable with almost all CNC machine tools.A CNC command executed within the control tells the drive motor to rotate a precise number of times.The rotation of the drive motor in turn rotates the ball screw. And the ball screw drives the linear axis (slide). A feedback device (linear scale) on the slide allows the control to confirm that the commanded number of rotations has taken place3.Though a rather crude analogy, the same basic linear motion can he found on a common table vise. As you rotate the vise crank, you rotate a lead screw that, in turn, drives the movable jaw on the vise. By comparison, a linear axis on a CNC machine tool is extremely precise. The number of revolutions of the axis drive motor precisely controls the amount of linear motion along the axis.How axis motion is commanded - understanding coordinate systemsIt would be infeasible for the CNC user to cause axis motion by trying to tell each axis drive motor how many times to rotate in order to command a given linear motion arnount4. (This would he like having to tgure out how many turns of the handle on a table vise will cause the movable jaw to move exactly one inch!) Instead, all CNC controls allow axis motion to he commanded in a much simpler and more logical way by utilizing sonic forni of coordinate system. The two most popular coordinate systems used with CNC machines arc the rectangular coordinate system and the polar coordinate system. By far, the more popular of these two is the rectangular coordinate system.The program zero point establishes the point of reference for motion commands in a CNC program. This allows the programmer to specify movements from a commt)fl location. If program zero is chosen wisely. usually coordinates needed for the program can be taken directly from the print.With this technique, if the programmer wishes the tool to he sent to a position one inch to the right of the program zero point, X1.0 is commanded. lithe programmer wishes the tool to move to a position one inch above the program zero point, Y 1.0 is commanded. The control will automatically deteniiine how many times to rotate each axis drive motor and ball screw to make the axis reach the commanded destination point . This lets the programmer command axis motion in a very logical manner.All discussions to this point assume that the absolute mode of programming is used. The most common CNC word used to designate the absolute mode is G90.In the absolute mode, the end points for all motions will be specified from the program zero point. For beginners, this is usually the best and easiest method of specifying end points for motion commands. However, there is another way of specifying end points for axis motion.In the incremental mode (commonly specified by G9 1), end points for motions are specified from the tools current position, not from program zero. With this method of commanding motion, the programmer must always he asking “How far should I move the tool?”While there are times when the incremental mode can be very helpful, generally speaking, this is the more cumbersome and difficult method of specifying motion and beginners should concentrate on using the absolute mode.Be careful when making motion commands. Beginners have the tendency to think incrementally. If working in the absolute mode (as beginners should), the programmer should always be asking “To what position should the tool be moved?” This position is relative to program zero, NOT from the tools current position.Aside from making it very easy to determine the current position for any command, another benefit of working in the absolute mode has to do with mistakes made during motion commands. In the absolute mode, if a motion mistake is made in one command of the program, only one movement will be incorrect. On the other hand, if a mistake is made during incremental movements, all motions from the point of the mistake will also be incorrect.Assigning program zeroKeep in mind that the CNC control must be told the location of the program zero point by one means or another. How this is done varies dramatically from one CNC machine and control to another. One (older) method is to assign program zero in the program. With this method, the programmer tells the control how far it is from the program zero point to the starting position of the machine. This is commonly done with a G92 (or G50) command at least at the beginning of the program and possibly at the beginning of each tool.Another, newer and better way to assign program zero is through some form of offset.Commonly machining center control manufacturers call offsets used to assign program zero fixture offsets. Turning center manufacturers commonly call offsets used to assign program zero for each tool geometry offsets.Flexible manufacturing cellsA flexible manufacturing cell (FMC) can he considered as a flexible manufacturing subsystem. The following differences exist between the FMC and the FMS:I. An FMC is not under the direct control of the central computer. Instead, instructions from the central computer are passed to the cell controller.2. The cell is limited iii the number of part families it can manufacture.The following elements are normally found in an FMC: Cell controller Programmable logic controller (PLC) More than one machine tool A materials handling device (robot or pallet)The FMC executes fixed machining operations with parts flowing sequentially between operations.High speed machiningThe term High Speed Machining (HSM) commonly refers to end milling at high rotational speeds and high surface feeds. For instance, the routing of 1xckets in aluminum airframe sections with a very high material removal rate . Over the past 60 years, HSM has been applied to a wide range of metallic and non-metallic workpiece materials, including the production of components with specific surface topography requirements and machining of materials with hardness of 50 HRC and above. With most steel components hardened to approximately 32-42 HRC, machining options currently include: Rough machining and semi-finishing of the material in its soft (annealed) condition heat treatment to achieve the final required hardness = 63 HRC machining of electrodes and Electrical Discharge Machining (EDM) of specific parts of dies and moulds (specifically small radii and deep cavities with limited accessibility for metal cutting tools) finishing and super-finishing of cylindrical/flat/cavity surfaces with appropriate cemented carbide, cermet, solid carbide, mixed ceramic or polycrystalline cubic boron nitride (PCBN)For many components, the production process involves a combination of these options and in the case of dies and moulds it also includes time consuming hand finishing. Consequently, production costs can be high and lead times excessive.It is typical in the die and mould industry to produce one or just a few tools of the same design. The process involves constant changes to the design, and because of these changes there is also a corresponding need for measuring and reverse engineering.The main criteria is the quality level of he die or mould regarding dimensional, geometric and surface accuracy. If the quality level after machining is poor and if it cannot meet the requirements, there will be a varying need of manual finishing work. This work produces satisfactory surface accuracy,but it always has a negative impact on the dimensional and geometric accuracy.One of the main aims for the die and mould industry has been, and still is, to reduce or eliminate the need for manual polishing and thus improve the quality and shorten the production costs and lead times.Main economical and technical factors for the development of HSMSurvivalThe ever increasing competition in the marketplace is continually setting new standards. The demands on time and cost efficiency is getting higher and higher. This has forced the development of new processes and production techniques to take place. HSM provides hope and solutions.MaterialsThe development of new, more difficult to machine materials has underlined the necessity to find new machining solutions. The aerospace industry has its heat resistant and stainless steel alloys. The automotive industry has different bimetal compositions, Compact Graphite Iron and an ever increasing volume of aluminum3. The die and mould industry mainly has to face the problem of machining high hardened tool steels, from roughing to finishing.QualityThe demand for higher component or product quality is he result of ever increasing competition. HSM. if applied correctly, offers a number of solutions in this area. Substitution of manual finishing is one example, which is especially iniportant on dies and moulds or components with a complex 3D geometry.ProcessesThe demands on shorter throughput times via fewer setups and simplified flows (logistics) can in most cases, be solved by HSM. A typical target within the die and mould industry is to completely machine fully hardened small sized tools in one setup. Costly and time consuming EDM processes can also he reduced or eliminated with HSM.Design & developmentOne of the main tools iii todays competition is to sell products on the value of novelty. The average product life cycle on cars today is 4 years, computers and accessories 1 .5 years, hand phones 3 months. One of the prerequisites of this development of fast design changes and rapid product development time is the HSM technique.Complex productsThere is an increase of multi-functional surfaces on components. such as new design of turbine blades giving new and optimized functions and features. Earlier designs allowed polishing by hand or with robots (manipulators). Turbine blades with new, more sophisticated designs have to be finished via machining and preferably by HSM . There are also more and more examples of thin walled workpieces that have to be machined (medical equipment, electronics, products for defence, computer parts)Production equipmentThe strong development of cutting materials, holding tools, machine tools, controls and especially CAD/CAM features and equipment, has opened possibilities that niust be met with new production methods and tcchniqucs.Definition of HSMSalomons theory. “Machining with high cutting speeds.” on which, in 1931, took out a German patent, assumes that “at a certain cutting speed (5-10 times higher than in conventional machining), the chip removal temperature at the cutting edge will start to decrease.”Given the conclusion:” . seems to give a chance to improve productivity in machining with conventional tools at high cutting speeds.”Modern research, unfortunately, has not been able to verify this theory totally. There is a relative decrease of the temperature at the cutting edge that starts at certain cutting speeds for different materials.The decrease is small for steel and cast iron. But larger fir aluminum and other non-ferrous metals. The definition of HSM must be based on other factors.Given todays technology. “high speed” is generally accepted to mean surface speeds between I and 10 kilomewrs per minute or roughly 3 300 to 33 000 feet per minute. Speeds above 10 km/min are in the ultra-high speed category, and are largely the realm of experimental metal cutting. Obviously, the spindle rotations required to achieve these surface cutting speeds are directly related to the diameter of the tools being used. One trend which is very evident today is the use of very large cutter diameters for these applications - and this has important implications for tool design.There are many opinions, many myths and many different ways to define HSM.中文译文数控机床虽然各种数控机床的功能和应用各不相同，担它们有着共同的优点。这里是数控设备提供的比较重要的几个优点。各种数控机床的第一个优点足自动化程度提高了。零件制造过程中的人为干预减少或者免除了。整个加工循环中，很多数控机床处于几无人照看状态，这使操作员被解放出来，可以干别的工作。数控机床用户得到的儿个额外好处是：数控机床减小了操作员的疲劳程度，减少了人为误差，工件加工时间一致而且可顶测。由于机床在程序的控制下运行，与操作普通机床的机械师要求的技能水平相比，对数控操作员的技能水平要求（与基本加工实践相关）也降低了。数控技术的第二个优点是工件的一致性好，加工精度高。现在的数控机床宣称的精度以及重复定位精度几乎令人难以置信。这意味着，一旦程序被验证是正确的，可以很容易地加工出 2 个、 10 个或 1000 个相同的零件，而且它们的精度高，一致性好。大多数数控机床的第三个优点是柔性强。由于这些机床在程序的控制下工作，加工不同的工件易如在数控系统中装载一个不同的程序而己。一旦程序验证正确，并且运行一次，下次加工工件的时候，可以很方便地重新调用程序。这又带来另一个好处可以快速切换不同工件的加工。由于这些机床很容易调整并运行，也由于几很容易装载加工程序，因此机床的调试时间很短。这是当今准时生产制造模式所要求的。运动控制一CNC 的核心任何数控机床最基本的功能是其有自动、精确、一致的运动控制。大多数普通机床完全运用机械装置实现其所需的运动，而数控机床是以一种全新的方式控制机床的运动。各种数控设备有两个或多个运动方向，称为轴。这些轴沿着其长度方向精确、自动定位。最常用的两类轴是直线轴（沿直线轨迹）和旋转轴（沿圆形轨迹）。普通机床需通过旋转摇柄和手轮产生运动，而数控机床通过编程指令产生运动。通常，几乎所有的数控机床的运动类型（快速定位、穴线插补和圆弧插补）、移动轴、移动距离以及移动速度（进给速度）都是可编程的。数控系统中的 CNC 指令命令驱动电机旋转某一精确的转数，驱动电机的旋转随叩使滚珠丝杠旋转，滚珠丝杠将旋转运动转换成直线轴（滑台）运动。滑台上的反馈装置（直线光栅尺）使数控系统确认指令转数己完成 。普通的台虎钳上有着同样的基本直线运动，尽管这是相当原始的类比。旋转虎钳摇柄就是旋转丝杠, 丝杠带动虎虎钳钳口移动。与台虎钳相比，数控机床的直线轴是非常精确的，轴的驱动电机的转数精确控制直线轴的移动距离。轴运动命令的方式理解坐标对 CNC 用 户来说，为了达到给定的直线移动量而指令各轴驱动电机旋转多少转，从而使坐标轴运动，这种方法是不可行的。（这就好像为了使钳口准确移动l英寸需要计算出台虎钳摇柄的转数！）事实上，所有的数控系统都能通过采用坐标系的形式以一种较为简单而且合理的方式来指令轴的运动。数控机床上使用最泛的两种坐标系是直角坐标系和极坐标系。目前用得较多的是直角坐标系。编程零点建立数控程序中运动命令的参考点。这使得操作员能从一个公共点开始指定轴运动。如果编程零点选择恰当，程序所需坐标通常可从图纸上直接获得。如果编程员希望刀具移动到编程零点右方1英寸（ 25 . 4 毫米）的位置，则用这种方法指令 X1.0 即可。如果编程员希望刀其移动到编程零点上方 1 英寸的位置，则指令 YI . 0 。数控系统会自动确定（计算）各轴驭动电机和滚珠丝扫要转动多少转，使坐标轴到达指令的目标位置。这使编程员以非常合理的方式命令轴的运动。理解绝对和相对运动至此，所有的讨论都假设采用的是绝对编程方式。用于指定绝对方式的址常用的数控代码是 G90 。绝对方式下，所有运动终点的指定都是以编程零点为起点。对初学者来说，这通常是较好也是址容易的指定轴运动终点的方法，但还有另外一种指定轴运动终点的方法。 增量方式（通常用 G91 指定）下，运动终点的指定是以刀具的当前位置为起点，而不是编程零点。用这种方法指定轴运动，编程员往往会问“我该将刀其移动多远的即离？ ，尽管增最方式多数时候很有用，但一般说来，这种方法指定轴运动较麻烦、困难，初学者应该重点使用绝对方式。指令轴运动时一定要小心。初学者往往以增量方式思考问题。如果工作在绝对方式（初学者应该如此），编程员应始终在问刀具应该移到什么位置？” ，这个位置是相对于编程零点这个固定位置而言，而不是相对于刀具当前位置。绝对工作方式很容易确定指令当前位置，除此之外，它的另外一个好处涉及轴运动中的错误。绝对方式下，如果程序的一个轴运动指令出错，则只有一个运动是不止确的。而另一方面，如果在增量运动过程中出错，则从出错的那一点起，所有的运动都是不止确的。指定编程零点记住必须以某种方式对数控系统指定编程零点的位置。指定编程零点的方式随数控机床和数控系统的不同而很不相同。（较老的）一种方法是在程序中指定编程零点。用这种方法，编程员告诉数控系统从编程零点到机床起始点的即离。通常用 G92 （或 G50 ）在程序的一开始指定，很能在各把刀具的开头指定编程零点。另一种较新、更好的指定编程零点的方法是通过偏置的形式，。通常，加工中心用于指定编程零点的偏置被称作夹具偏置，车削中心上用于指定编程零点的偏置被称作刀具几何偏置。柔性制造单元柔性制造单元 （ FMC ）被认为是柔性制造子系统。以下是 FMC 和 FMS 之间的别：1.FMC 不受中央计算机的直接拎制，中央计算机发出的指令被传送到单元控制器。 2.FMC 能制造的零件族的数口有限。 FMC 一般由下列部分组成：单元控制器 . 可编程逻辑控制器（ PLC ） . 一台以上的机床物流设备（机器人或托盘） FMC 按顺序对零件流执行固定的加工操作。高速加工术语“高速加工 ( HMS ) ”一 般是指在高转速和大进给量下的立铣。例如，以很高的金属切除率对铝合金飞机翼架的凹处进行切削。在过去的 60 年中，高速加工己经广泛应用于金属与非金属材料，包括有特定表面形状要求的零件生产和硬度高于或等于 HRC 50 的材料切削。对于大部分淬火到约为 HRC 32- 42的钢零件，当前的切削选项包括：在软（退火）工况下材料的粗加工和半精加工达到最终硬度要求为 HRC 63 的热处理模具行业的某些零件的电极加工和放电加工 ( EDM ) （特别是金属切削刀具难以加工的小半径圆弧和深凹穴）用适合的硬质合金、金属陶瓷、整体硬质合金、混合陶瓷或多晶立方氮化硼（ PCBN ）刀具进行的圆柱平面 凹穴表面的精加工和超精加工。对于许多零件，生产过程牵涉到这些选项的组合，在模具制造案例中，它还包括费时的精加工，结果导致生产成本高和准备时间长。在模具制造业中典型的是仅生产一个或几个同一产品。生产过程中，产品的设计不断改变，由于产品改变，模具制造中需要测量与反求工程。加工的主要标准是模具的尺寸和表面粗糙度方面的质量水平。如果加工后的质量水平低，不能满足要求，就需手工精加工。手工精加工可产生令人满意的表面粗糙度，但是对尺寸和几何精度总是产生不好的影响。模具制造业的主要目标之一，一直是并且仍然是减少或免除手工抛光，从而提高质量、降低生产成木和缩短准备时间。影响高速加工发展的主要经济和技术因素生存日益激烈的市场竞争导致不断设立新的标准，对时间和成本效率的要求越来越高，这就迫使新工艺和生产技术不断发展。高速加工提供了希望和解决方案 材料新型难加工材料的开发迫切需要寻找新的切削解决方案。航空航天业使用耐热合金钢和不锈钢，汽车工业使用了不同的双金属材料、小石墨铸铁，并增加了铝用量。模具制造业必须面对切削高硬度的淬决钢的问题从粗加工到精加工。质量对质量的高要求是空前激烈竟争所导致的结果。高速加工如果使用得正确，可以在这个领域提供一些解决方案。替代手工精加工是一个例子，这对有复杂 3D 几何形状的模其尤为重要。工艺通过减少装卡次数和简化物流（后勤）来缩短产品产出时间的要求在大部分情况下可由 高速加工解决。模具制造业内的一个典型目标是在一次装卡中完成所有完全淬火小零件的切削。使用高速切削，可以减少和免除费时、费钱的放电加工（EDM)。设计与发展今竞争中的主要方法之一是销售新奇的产品。现在小汽车的平均生命周期是 4 年，计算机和配件 1 年半，手机 3 个月 这种快速的产品设计周期和开发周期的先决条件是高速切削技术。复杂产品零件多功能表面增加了，例如新设计的涡轮叶片有新的、优化的功能与特性。早期的设计允许用手工或机器人（机械手）来抛光。新型、形状复杂的涡轮叶片必须通过切削来完成精加工，最好是用高速切削完成。薄壁工件必须用切削进行精加的例子越来越多（医疗设务、电子、国防产品 、计算机零件）。产品设备切削材料、刀柄刀具、机床、数控系统，特别是 CAD / CAM功能和设备的巨大发展己经使采用新的生产方法和技术成为可能和必须。高速加工的原始定义 1931 年 Salomom 的高速加工理论获得了一项德国专利，他认为“在高于常规切削速度 5 一10 倍的切削速度下，刀刃的切削温度将开始下降 由以上得出结论：“ 用常规刀具以高切削速度加工，从而提高生产率，这是可能的 ”可惜，现代研究还没能全面验证这个理论。对于不同的材料，从某一切削速度开始切削刃上的温有所降低。对于钢和铸铁来说，这种温度降低不大。但是对铝和其他非金属来说则是大的。高速切削的定义须依据其他因素。按照现在的技术，普遍认为“高速”，是指表面速度在1- 10 千米分钟（ k /min ) ，或者约 3300 一 330 英尺分钟（ ft / min ）。 10 千米分钟以上的速度属于超高速范畴，还在实验室金属切削范围显然获得这些表面切削速度所要求的主轴转速直接与使用的刀具直径有关。当前较显著的趋势是采用大直刀具这对刀具的设计有着重要的启发。关于高速切削的定义，存在许多观点、许多谜团和许多不同的方法。
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