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专题：高速切削的刀具材料及切削技术的应用 一、前言高速切削的研究历史，可以追溯到二十世纪30年代由德国Carl Salomon博士首次提出的有关高速切削的概念。Salomon博士的研究突破了传统切削理论对切削热的认识，认为切削热只是在传统切削速度范围内是与切削速度成单调增函数关系。而当切削速度突破一定限度以后，切削温度不再随切削速度的增加而增加，反而会随切削速度的增加而降低，即与切削速度在较高速度的范围内成单调减函数。Salomon博士的研究因第二次世界大战而中断。50年代后期开始，高速切削的试验又开始进入各种试验研究，高速切削的机理开始被科学家们所认识。1979年开始由德国政府研究技术部资助、德国Darmstadt大学PTW研究所牵头、由大学研究机构、机床制造商、刀具制造商、用户等多方面共同组成的研究团队对高速铣削展开了系统的研究。除了高速切削机理外，研究团队同步研究解决高速铣削中机床、刀具、工艺参数等多方面的应用解决方案，使高速铣削在加工机理尚未得到完全共识的情况下首先在铝合金加工和硬材料加工等领域得到应用，解决模具、汽车、航空等领域的加工需求，从而取得了巨大的经济效益。 根据1992年国际生产工程研究会(CIRP)年会主题报告的定义，高速切削通常指切削速度超过传统切削速度510倍的切削加工。因此，根据加工材料的不同和加工方式的不同，高速切削的切削速度范围也不同。高速切削包括高速铣削、高速车削、高速钻孔与高速车铣等，但绝大部分应用是高速铣削。目前，加工铝合金已达到20007500m/min；铸铁为9005000m/min；钢为6003000m/min；耐热镍基合金达500m/min；钛合金达1501000m/min；纤维增强塑料为20009000m/min。二、高速切削的特点实践表明，高速切削具有以下加工特点：切削力降低；工件热变形减少；有利于保证零件的尺寸、形位精度；已加工表面质量高；工艺系统振动减小；显著提高材料切除率；加工成本降低； 高速切削的上述特点，反映了在其适用领域内，能够满足效率、质量和成本越来越高的要求，同时，解决了三维曲面形状高效精密加工问题，并为硬材料和薄壁件加工提供了新的解决方案。三、高速切削加工刀具材料选用 铝合金 易切削铝合金 该材料在航空航天工业应用较多，适用的刀具有K10、K20、PCD，切削速度在20004000m/min，进给量在312m/min，刀具前角为1218，后角为1018，刃倾角可达25。 铸铝合金 铸铝合金根据其Si含量的不同，选用的刀具也不同，对Si含量小于12%的铸铝合金可采用K10、Si3N4刀具，当Si含量大于12%时，可采用PKD(人造金刚石)、PCD(聚晶金刚石)及CVD金刚石涂层刀具。对于Si含量达16%18%的过硅铝合金，最好采用PCD或CVD金刚石涂层刀具，其切削速度可在1100m/min，进给量为0.125mm/r。 铸铁 对铸件，切削速度大于350m/min时，称为高速加工，切削速度对刀具的选用有较大影响。当切削速度低于750m/min时，可选用涂层硬质合金、金属陶瓷；切削速度在5102000m/min时，可选用Si3N4陶瓷刀具；切削速度在20004500m/min时，可使用CBN刀具。铸件的金相组织对高速切削刀具的选用有一定影响，加工以珠光体为主的铸件在切削速度大于500m/min时，可使用CBN或Si3N4，当以铁素体为主时，由于扩散磨损的原因，使刀具磨损严重，不宜使用CBN，而应采用陶瓷刀具。如粘结相为金属Co，晶粒尺寸平均为3m，CBN含量大于90%95%的BZN6000在V=700m/min时，宜加工高铁素体含量的灰铸铁。粘结相为陶瓷(AlNAlB2)、晶粒尺寸平均为10m、CBN含量为90%95%的Amborite刀片，在加工高珠光体含量的灰铸铁时，在切削速度小于1100m/min时，随切削速度的增加，刀具寿命也增加。 普通钢 切削速度对钢的表面质量有较大的影响，根据德国Darmstadt大学PTW所的研究，其最佳切削速度为500800m/min。 目前，涂层硬质合金、金属陶瓷、非金属陶瓷、CBN刀具均可作为高速切削钢件的刀具材料。其中涂层硬质合金可用切削液。用PVD涂层方法生产的TiN涂层刀具其耐磨性能比用CVD涂层法生产的涂层刀具要好，因为前者可很好地保持刃口形状，使加工零件获得较高的精度和表面质量。 金属陶瓷刀具占日本刀具市场的30%，以TiC-Ni-Mo为基体的金属陶瓷化学稳定性好，但抗弯强度及导热性差，适于切削速度在400800m/min的小进给量、小切深的精加工；Carboly公司用TiCN作为基体、结合剂中少钼多钨的金属陶瓷将强度和耐磨两者结合起来，Kyocera公司用TiN来增加金属陶瓷的韧性，其加工钢或铸铁的切深可达23mm。CBN可用于铣削含有微量或不含铁素体组织的轴承钢或淬硬钢。 高硬度钢 高硬度钢(HRC4070)的高速切削刀具可用金属陶瓷、陶瓷、TiC涂层硬质合金、PCBN等。 金属陶瓷可用基本成分为TiC添加TiN的金属陶瓷，其硬度和断裂韧性与硬质合金大致相当，而导热系数不到硬质合金的1/10，并具有优异的耐氧化性、抗粘结性和耐磨性。另外其高温下机械性能好，与钢的亲和力小，适合于中高速(在200m/min左右)的模具钢SKD加工。金属陶瓷尤其适合于切槽加工。 采用陶瓷刀具可切削硬度达HRC63的工件材料，如进行工件淬火后再切削，实现“以切代磨”。切削淬火硬度达HRC4858的45钢时，切削速度可取150180m/min，进给量在0.30.4min/r，切深可取24mm。粒度在1m，TiC含量在20%30%的Al2O3-TiC陶瓷刀具，在切削速度为100m/min左右时，可用于加工具有较高抗剥落性能的高硬度钢。 当切削速度高于1000m/min时，PCBN是最佳刀具材料，CBN含量大于90%的PCBN刀具适合加工淬硬工具钢(如HRC55的H13工具钢)。 高温镍基合金 Inconel 718镍基合金是典型的难加工材料，具有较高的高温强度、动态剪切强度，热扩散系数较小，切削时易产生加工硬化，这将导致刀具切削区温度高、磨损速度加快。高速切削该合金时，主要使用陶瓷和CBN刀具。 碳化硅晶须增强氧化铝陶瓷在100300m/min时可获得较长的刀具寿命，切削速度高于500m/min时，添加TiC氧化铝陶瓷刀具磨损较小，而在100300m/min时其缺口磨损较大。氮化硅陶瓷(Si3N4)也可用于Inconel 718合金的加工。 加拿大学者M.A.Elbestawi认为，SiC晶须增强陶瓷加工Inconel 718的最佳切削条件为：切削速度700m/min，切深为12mm，进给量为0.10.18mm/z。 氮氧化硅铝(Sialon)陶瓷韧性很高，适合于切削过固溶处理的Inconel 718(HRC45)合金，Al2O3-SiC晶须增强陶瓷适合于加工硬度低的镍基合金。 钛合金(Ti6Al6V2Sn) 钛合金强度、冲击韧性大，硬度稍低于Inconel 718，但其加工硬化非常严重，故在切削加工时出现温度高、刀具磨损严重的现象。日本学者T.Kitagawa等经过大量实验得出，用直径10mm的硬质合金K10两刃螺旋铣刀(螺旋角为30)高速铣削钛合金，可达到满意的刀具寿命，切削速度可高达628m/min，每齿进给量可取0.060.12mm/z，连续高速车削钛合金的切削速度不宜超过200m/min。 复合材料 航天用的先进复合材料(如Kevlar和石墨类复合材料)，以往用硬质合金和PCD，硬质合金的切削速度受到限制，而在900以上高温下PCD刀片与硬质合金或高速钢刀体焊接处熔化，用陶瓷刀具则可实现300m/min左右的高速切削。四、高速切削刀具技术 高速切削刀具不仅在耐用度和可靠性方面比常规加工有更高的要求，在刀具系统的安全性方面也有特殊的要求。图3 刀具伸出量对耐用度的影响从提高耐用度和可靠性角度，需要考虑：刀具基体与涂层材料刀尖几何结构刀刃数和刀杆伸出量切削用量走刀方式冷却条件刀具与工件材料匹配从提高使用安全性方面，需要考虑：刀具系统强度与尺寸刀杆与机床的夹持方式刀片夹紧方式刀具动平衡图4 球头铁刀不同铣削方式对耐用度的影响 由于高速切削高转速和快进给等特点，除了良好的耐磨性和高的强度韧性的先进刀具材料，优良的刀具涂层技术，合理的几何结构参数和高同心度的刀刃精度质量等因素外，还需特别注意其它因素对刀具耐用度的影响。图3为不同刀具伸出量对切削路径长度的影响，可见伸出量越短，耐用度越高。一般情况下，顺铣的耐用度高于逆铣，而往复铣的耐用度最低(见图4)。图4中向下进实际反映刀具顶着进给方向进刀，而向上进反映刀具拖着进给方向进刀，对耐用度也有较大影响。铝合金高速铣削通常用双刃铣刀，过多的刀刃会减少容屑空间，容易引起切屑粘刀。为避开共振频率，也可采用三刃铣刀以增加冲击频率。铝合金加工容易产生积屑瘤，这对高速铣削非常有害。要减少积屑瘤的产生，刀具表面要平滑；避免采用物理气相沉积(PVD)涂层刀具，因为TiAlN涂层很易与铝产生化学反应，可以选用非涂层刀具，细晶金刚石涂层或类金刚石涂层刀具；如有可能，尽量采用油雾刀具内冷进行冷却润滑。 高速铣削刀具结构对刀具耐用度和安全性均有很大影响，关键要点包括刀具系统的平衡设计；减少径向和轴向跳动；控制动平衡精度；与机床联接普遍采用HSK刀柄或类似双面接触短锥刀柄；刀具的夹紧最新趋势是采用冷缩式夹紧结构(或称热装式)，装夹时利用感应或热风加热使刀杆孔膨胀，取出旧刀具，装入新刀具，然后采用风冷使刀具冷却到室温，利用刀杆孔与刀具外径的过盈配合夹紧，这种结构刀具的径向跳动在4m，刚性高，动平衡性好，夹紧力大，高转速下仍能保持高的夹紧可靠性，特别适用于更高转速的高速铣削加工。五、高速切削工艺技术 高速切削工艺主要包括：适合高速切削的加工走刀方式，专门的CAD/CAM编程策略，优化的高速加工参数，充分冷却润滑并具有环保特性的冷却方式等等。 高速切削的加工方式原则上多采用分层环切加工。直接垂直向下进刀极易出现崩刃现象，不宜采用。斜线轨迹进刀方式的铣削力是逐渐加大的，因此对刀具和主轴的冲击比垂直下刀小，可明显减少下刀崩刃的现象。螺旋式轨迹进刀方式采用螺旋向下切入，最适合型腔高速加工的需要。 CAD/CAM编程原则是尽可能保持恒定的刀具载荷，把进结速率变化降到最低，使程序处理速度最大化。主要方法有：尽可能减少程序块，提高程序处理速度；在程序段中可加人一些圆弧过渡段，尽可能减少速度的急剧变化；粗加工不是简单的去除材料，要注意保证本工序和后续工序加工余量均匀，尽可能减少铣削负荷的变化；多采用分层顺铣方式；切入和切出尽量采用连续的螺旋和圆弧轨迹进行切向进刀，以保证恒定的切削条件；充分利用数控系统提供的仿真验证的功能。零件在加工前必须经过仿真，验证刀位数据的正确性，刀具各部位是否与零件发生干涉，刀具与夹具附件是否发生碰撞，确保产品质量和操作安全。 高速铣削加工用量的确定主要考虑加工效率、加工表面质量、刀具磨损以及加工成本。不同刀具加工不同工件材料时，加工用量会有很大差异，目前尚无完整的加工数据。通常，随着切削速度的提高，加工效率提高，刀具磨损加剧，除较高的每齿进给量外，加工表面粗糙度随切削速度提高而降低。对于刀具寿命，每齿进给量和轴向切深均存在最佳值，而且最佳值的范围相对较窄。高速铣削参数一般的选择原则是高的切削速度、中等的每齿进给量fz、较小的轴向切深ap和适当大的径向切深ae。 在高速铣削时由于金属去除率和切削热的增加，冷削介质必须具备将切屑快速冲离工件、降低切削热和增加切削界面润滑的能力。常规的冷却液及加注方式很难进入加工区域，反而会加大铣刀刃在切入切出过程的温度变化，产生热疲劳，降低刀具寿命和可靠性。现代刀具材料，如硬质合金、涂层刀具、陶瓷和金属陶瓷、CBN等具有较高的红硬性，如果不能解决热疲劳问题，可不使用冷却液。 微量油雾冷却一方面可以减小刀具切屑工件之间的摩擦，另一方面细小的油雾粒子在接触到刀具表面时快速气化的换热效果较冷却液热传导的换热效果方式能带走更多的热量，目前已成为高速切削首选的冷却介质。 氮气油雾冷却介质在钛合金的高速铣削中取得了很好的效果。氮气油雾冷却介质除具有空气油雾的冷却润滑作用外，还具有抗氧化磨损等作用，在33m/min的铣削速度时，相比较空气油雾冷却，刀具耐用度提高60%，铣削力可降低20%30%。六、结语 高速切削是一项先进的、正在发展的综合技术，必须将高性能的高速切削机床、与工件材料相适应的刀具和对于具体加工对象最佳的加工工艺技术相结合，充分发挥高速切削技术的优势。高速切削工具技术也是一项关键技术，为了适应和推动我国高速切削技术的发展，我们应该充分认识到，工具制造是一个高技术含量的行业，应加强该领域的基础研究、工程研究和应用研究；迅速发展的高速切削技术极大的刺激高性能刀具的需求，我国工具行业应重点在刀具的耐磨性、精度和可靠性方面加强研发力度，提高刀具的竞争能力；刀具的竞争力应集中在高性能带来的整体经济效益，在应用领域推广使用高性能刀具；提供个性化技术服务；根据我国目前的实际情况，建议重点发展涂层技术(如耐磨(硬、软)涂层、复合涂层、纳米结构涂层等)，刀具质量保障技术和刀具数据库。附录:外文文献Numerical ControlOne of the most fundamental concepts in the area of advanced manufacturing technologies is numerical control.Controlling a machine tool using a punched tape or stored program is known as numerical control (NC). NC has been defined by the Electronic Industries Association (EIA) as “ a system in which actions are controlled by the direct insertion of numerical dada at some point .the system must automatically interpret at least some portion of this data.” the numerical data required to produce a part is known as a part program.A numerical control machine tool system contains a machine control unit (MCU) and the machine tool itself. The MCU is further divided into two elements: the data processing unit (DPU) and the control loops unit (CLU). The DPU processes the coded data from the tape or other media and passes information on the potions of each axis, required direction of motion, feed rate, and auxiliary function control signals to the CLU. The CLU operates the drive mechanisms of the machine, receives feed back signals concerning the actual position and velocity of each of the axes, and signals the completion of operation. The DPU sequentially reads the data. When each line has completed execution as noted by the CLU, anther line of data is read.A data processing units consists of some or all of the following parts:1)Data input device such as a paper tape reader, magnetic tape reader, RS232-C port, etc2)Data-reading circuits and parity-checking logic3)Decoding circuits for distributing data among the controlled axes4)An interpolator, which supplies machine-motion commands between data points for tool motionA control loops unit, on the other hand consists of the following:1)Position control loops for all the axes of motion, where each axis has a separate control loop2)Velocity control loops, where feed control is required 3)Deceleration and backlash take up circuits4)Auxiliary functions control, such as coolant on/off, gear change, spindle on/off controlGeometric and kinematic data are typically fed from the DPU to the CLU.The CLU then governs the physical system based on the data from the DPU.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:lElectric discharge machining lLaser-cutting lElectron beam weldingNumerical control has also made machine tools more versatile than their manually operated predecessors. An NC machine tool can automatically produce a wide variety 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 tools and processes. Historical Development of NCLike 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 make 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 shown in the close up in Fig.2.17 had to be calculated. This was a cumbersome approach that had to be overcome if NC was to develop further.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 necessary motions. The development of the APT language was a major step forward in the development of NC technology. The original NC systems were vastly different 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 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 times. Fragile paper tapes simply could not withstand the rigors of a shot floor environment and this kind of repeated use.This led to the development of a special magnetic plastic tape. Whereas the paper tape 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 those was that it was difficult or impossible to change the instructions entered on the tape. To make 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 problem of NC associated with punched paper and plastic tape.1)Advent of Direct Numerical ControlThe 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 (Fig 2.18). Programs for operating the machine tools are stored in the host computer and fed to the machine tool as 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, the machine tools also experience downtime. This problem led to the development of computer numerical control.2)Advent of Computer Numerical ControlThe development of the microprocessor allowed for the development of programmable logic controllers (PLCs) and microcomputer. These two technologies allowed for the development of computer numerical control (CNC). With CNC, each machine tool has a PLC or a microcomputer that serves the purpose. This allows programs to be input and stored at each individual machine tool. It also allows programs to be developed off-line and downloaded at the individual machine tool. CNC solved the problems associated with downtime of the host computer, but it introduced another problem known as data management. This is a problem all work settings dependent on microcomputers have. The same program might be loaded on ten different microcomputers with no communication among them. This problem is the process of being solved by local area networks that connect microcomputers for better data management. The problem of data management led to the development of distributed numerical control.3)Advent of Distributed Numerical ControlDistributed numerical control (also called DNC) takes advantage of the best aspects of direct numerical control and computer numerical control. With distributed numerical control there are both host computers and local computers at the individual machine tools (Fig 2.19). This allows the programs to be stored in the host computers and, thereby, better managed. However, it also allows them to be downloaded to local microcomputers or PLCs. It also allows for local input and interaction through microcomputers or PLCs at the machine levels.NC Machine ComponentsThere are four components in any NC machine:lThe actual NC toollThe machine control unit (MCU)lThe communication interface between the NC machine and the MCUlA variety of accessories for performing specific jobs on the NC machineThe actual NC machine may be a milling machine, lathe, drill, or any other type of machine tool. The MCU is the control unit that holds the programs that instruct the NC machine. The MCU also has various devices available for operator input. Information contained in the MCU is carried to the activators on the NC machine through the communication interface. These activators receive the electronic signals from the MCU and cause the mechanical apparatus of the NC machine to operate.Less sophisticated NC machines have open-loop activators. An open-loop activator can receive a signal and carry out the instructions contained in that signal, but cannot feed back to the MCU to show that instructions carried in the signal have been properly completed. More sophisticated NC machine use closed-loop activators. A closed-loop activator can receive and carry out a signal and feed data back to the MCU showing that the signal has been carried out and to what extent. The more sophisticated closed-loop systems are been used more and more because they allow closer monitoring and immediate corrective action when problems with executing a program arise. The accessories are special tools required to carry out a specific job.NC ProgrammingThese are four ways to program an NC machine: manual programming, digitizing, written programs, and graphic programs (Fig 2.20). Manual programming is the most cumbersome of the four. It involves calculating numerical values that identify tool location and specify tool direction. Once these values have been calculated, recorded and feed into the MCU. Digitizing is a process frequently used in computer-aided design and drafting, whereby a drawing of a part is traced electronically. As it is traced, the various points on the drawing are converted into X-Y coordinates and stored in the computer. Once the drawing has been completely traced, the stored X-Y coordinates define the part and can be fed to an NC machine to provide instructions on tool positioning and movement.Written programs are similar to those developed for use with any computer. With such programs, English language-type statements are written to describe tool positions and movement, as well as speed and feed rates. Such programs are fed into the MCU, where they are translated into machine language and forwarded to the NC machines activators.The most modern, sophisticated method of programming an NC machine is by using a three-dimensional model of the part to provide the data that guide the NC machine in producing the part. As NC technology continues to develop, this programming method will eventually be used more than any other.Classifications of NC MachinesNumerical control machines are classified in different ways. An early method was to categorize them as being either point-to-point or continues-path machines. Point-to-point machines, as the name implies, move in a series of steps from one point to the next (Fig 2.21). Point-to-point machines are less sophisticated and less precise than continuous-path machines. Continuous-path machines move uniformly and evenly along the cutting path rather than through a series of horizontal and vertical steps. Such machines are more sophisticated and require move memory in the MCU than point-to-point machines. Fig 2.22 illustrates the type of cutting paths performed by continuous-path machine.Anther way to classify NC machines is as positioning or contouring machines. Point-to-point machines are considered positioning machines. Continuous-path machines are considered contouring machines. Positioning machines have as few as two axes: the X axis and the Y axis. Contouring machines must have at least three axes: the X, Y and Z axes. Fig 2.23 illustrates the movements governed by the X, Y and Z axes. Note that X represents the longitudinal axes, Y the transverse axis, and Z the up-and-down or vertical axis. Fig 2.24 is a simply line diagram of a typical three-axis machine tool showing how movement is accomplished. On some machines, movement is accomplished by positioning the spindle, and thus the tool, longitudinally along the X axis, transversely long the Y axis, and vertically along the Z axis. The work-piece is affixed to the table. With other machines, both the spindle and table (thus the work-piece) can be moved.Positioning machines work well for drilling applications. Milling operations are more likely to be contouring machines to allow for three-dimensional control.Some of the more sophisticated positioning machines are able to accomplish angular cuts known as slopes. These are cuts that move across the quadrants formed by the intersection of the X and Y axes at angles other than 90 degree to either the X or Y axis (Fig 2.25). Slopes are generally imprecise and inaccurate. However, there are instances in which the ability to make angular cutting paths is important. In these cases, slopes can be an important feature, particularly where the cut surfaces do not have to mate with anther surface. When precise, accurate angular cutting paths must be made, a contouring machine is needed.Pros and Cons in Justifying NCJustifying a conversion from manual to NC machines and processes can be difficult. From a strictly business perspective, such a conversion entails a great deal of risk, primarily centering around the fact that manually operated machines use fewer consumable tools per hour of operation, but they also require more hours to produce the same amount of work. Therefore, in the long run, the overall tool consumption costs for NC are actually the same as for manual machines for a given run length.MDI ControlOne of the limitations of traditional NC is an inability to program a machine on the shop floor. This problem is solved to a great extent by manual data input (MDI) control. With MDI control, an operator can enter data via a computer at the machine on the shop floor. This allows the operator to do a limited amount of programming at the machine site. The hardware for MDI control is small and not obtrusive. Manual data input is easy to use and does not require the skills of a professional part programmer. The operator enters the programming data via keyboard and monitors his entries by watching a CRT terminal. Manual data input control is so simple that it amounts to little more than an operator viewing a terminal display and filling the blanks as necessary.Manual data input control can be used with a wide range of machines from low level machines to highly sophisticated machining centers. However, the most common uses are the less sophisticated three-axis milling machines and two-axis lathes.Computer-Assisted NC ProgrammingThe computer is being widely used in NC programming setting. Computers offer operators several advantages over manually preparing programmed instructions for NC machines. It takes less time to prepare a part program using a computer than to prepare the same program manually. Because the computer performs any necessary calculations rather than the calculations being performed manually, there are fewer errors in the final program. And, because less time is required in using a computer and fewer errors appear in the final program, the overall programming costs are usually lower with computer-assisted NC programming.When using a computer to write an NC program, the operator describes the operations to be performed by the machine tool using English language-like commands. These commands are transformed into a language the NC machine can understand by a post-processor, a special computer program that converts general instructions to machine-specific instructions. The most widely used language for computer-assisted NC programming is the APT language.The computer offers the NC part programmer the same advantages and benefits it offers other technical workers. By performing complicated mathematical calculations, it reduces the time involved in preparing a program and decreases the number of errors made in producing a program. When errors are made, they are easier to correct. It simplifies the input of programs to be stored , retrieved, and used continually without having to rerun a tape each time a machine operation is to be performed.Benefits and Gains from NCNumerical control has been in use for almost 40 years. Since its inception, it has been improved continually. Each improvement has added a new benefit or improved an existing benefit of NC as compared with traditional manual machine operation. During this period, a body of knowledge has developed from the actual benefits that can be derived from the NC. There is a general consensus among manufacturing professionals that the principal benefits derived from NC are the following:lBetter planninglGreater-flexibilitylEasier schedulinglLess setup, lead, and processing timelBetter machine utilizationlLower overall tooling costslGreater uniformity in cuttinglGreater accuracy in cuttinglA higher degree of interchangeability of parts and toolslMore accurate cost estimates lPermanent memory of how a pert was madeThese are the same types of benefits generally associated with any manufacturing process that has successfully moved from a manual format to a fully or partially automated format. The extent of the benefit depends on how successfully the transition has been carried out and how well developed the associated technologies have become. With the advent of DNC, and CNC in numerical control, all of the benefits listed above have evolved into bona fide, document-able benefits.Overview of Direct Numerical ControlDirect numerical control is an advanced form of numerical control. Traditional NC involved imprinting programmed instructions on punched tape or magnetic plastic tape and feeding the tape through a reader that then interprets the instructions and transmits them to the NC machine. With direct numerical control, the tape media are eliminated. Instead, NC machines are connected to a host computer via a data transmission interface (Fig2.26). Programs are stored in the host computer and downloaded to the individual NC machines via the data transmission interface as needed.In addition to eliminating the cumbersome tape medium for carrying programmed instructions, direct numerical control allows NC machines at remote locations to be connected to the host computer. It also allows programmers to develop programs at any location and enter them into the host computer. This means that programmers are no longer physically tied to the NC machine and NC machines are not tied down with the traditional tape media.Historical Development of Direct Numerical ControlDirect numerical control originated in the mid 1960s. Its original purpose was to reduce the amount of hardware required to provide NC. One host computer could serve as the controller instead of having a controller for each individual NC machine. The requirement of having one controller per NC machine was an expensive requirement in the mid 1960s because microelectronic technology had not yet developed to the point where it could be as inexpensive as it is today. As microelectronic technology and computer technology continued to evolve over the years, becoming more sophisticated but less expensive, the original rational for direct numerical control has become less critical.Direct numerical control never worked completely as it was originally envisioned. Elimination of tape as the input medium and of controllers at each individual NC machine were never fully realized because of breakdowns in the data transmission interface and downtime of the host computer. When the host computer is down, it is necessary to have individual controllers at each NC machine and tape to input the instructions on a backup basis. It was only after the advent of computer numerical control (CNC) that the true value of direct numerical control began to be realized.1Direct Computer and Distributed Numerical ControlDistributed numerical control is a concept that combines the best of direct and computer numerical control. The advent of CNC allowed the placement of a microcomputer controller at each individual NC machine. This computer could be used to develop programs, store programs, and input programs to the NC machines. This capability, coupled with the host computer capability of direct numerical control, paved the way for the distributed numerical control concept. In distributed numerical control, there is a host computer as in direct numerical control, as well as microcomputer controllers for individual NC machines (Fig2.27).The host computer in such a system is still used as the main storage point for programs. These programs are downloaded from the host computer to the microcomputers, where they can be stored or transmitted to the NC machines. In this way, the need for the medium and reader is eliminated. The microcomputer controllers can also serve as backup memory when the host computer is down. This concept of distributed control, which combines direct and computer numerical control, is becoming widely accepted that the acronym “DNC” is being used more and more to mean distributed numerical control instead of direct numerical control.2Data Transmission in DNCDependable data transmission is the key to successful DNC. A poor data transmission network can cause NC machines to be idle while waiting for instructions to be transmitted from the host computer. This can also result in operators waiting for NC machines to respond to the instructions from the host computer. Therefore, it is critical to have an effective data transmission network from the host computer to the intermediate microcomputer controllers and to the NC machines. One key to having effective data transmission network is to ensure that the data transmission interface is compatible, not just with the host computer, but with each individual microcomputer controller and the NC machines.This means the host computer must be able to feed the intermediate microcomputer controller only as much of a program as they are able to hold in storage. The DNC system must also be able to download only those portions of a program that a given NC machine is capable of accepting and carrying out. The system, should be able to accommodate revisions to programs in order to optimize the program of a given part without replacing the original program with the optimized program.3Advantage of DNCOne key advantage of DNC is the ability to produce and print reports that provide valuable information to system managers. This information can be studied and used to improve the performance of an overall DNC system. The types of reports that can be produced from a DNC system include the following:lProduction schedule reportslRunning times of various reportslInventory of tools required to produce a given part lOperator instructionslProgram listing contained on a given disk lReports showing when a program was used last and how often a program is usedlMachine utilization reportslReports showing downtime for machineThe ability to produce management reports such as these, coupled with the other advantages of eliminating the tape medium, is making DNC the norm in NC settings.4Best Applications of DNCIn spite of these advantages, DNC is not always the most appropriate NC methodology. Its best applications are in settings that require the types of management reports that a DNC system can produce and in flexible manufacturing settings. Applications that require large amount of control information are also appropriate for DNC. These are applications that use many part programs, thus much program management is necessary.DNC is also ideally suited for control of flexible manufacturing system. In flexible manufacturing system, a central host computer is needed to direct the flow of parts through the system and to download NC programs to microcomputer controllers of individual NC machines. As the technology continues to develop, DNC network will expand to include not just machine tools, but also computer-aided design and drafting systems and other computer-based systems tied to production.附录:中文翻译数字控制先进制造技术领域中的最基本的观念之一是数字控制。使用一卷被打孔的纸带或使用储存的程序来控制机床设备被称为数字控制(NC)。 数字控制被电子工业协会 (EIA) 定义为 “ 一个系统在某点的动作被直接插入的数值所控制 ， 该系统一定能至少自动地理解这些数据的一部分” 这些生产一个零件所必需的数据资料被称为一组零件程序。一个数控机床系统包含一机床控制单元 (MCU) 和工作机床它本身。 MCU 比较进一步被区分为二种: 数据处理单元 (DPU) 和控制循环单元 (CLU)。DPU 处理来自纸带或其他的媒介和传递各个轴的一组信息，所需的运动方向,进给速度，和辅助功能控制信号给CLU。 CLU 操作机器的驱动装置，接收关于每一个轴的实际位置和速度的反馈信号，并且发出完成操作的信号。 DPU 则继续地读取数据。 当每条线都按 CLU 所记录的实行完成时,另一组数据已被读取。一组数据处理单元由以下所诉的所有或某些部分组成:1)数据输入装置例如纸带读卡机，磁带播放机,RS232- C 端口等2)数据读取线路和正负判断逻辑线路3)在受控轴之间分配数据的解码线路4)一个校对机, 为刀具在工作指令点之间运动提供动作指令另一方面，控制循环单元由下列各项组成:1）所有轴的运动的位置控制循环线路, 各个轴有单独的控制循环线路2）速度控制循环线路，及必备的进给控制3)减速和反冲吸收线路4)辅助功能控制，例如 冷却液的开/关 ，换档变速，主轴开/关控制几何的和运动的数据典型地从 DPU 反馈到 CLU 。然后，CLU根据来自 DPU 的数据实际控制系统。数控已经发展到克服了人类操作员的限制，并且已经这么做了。 数控机床比用手能更精确地操作机器,他们能生产更统一的零件, 加工更快速, 能长时间不停的加工，且费用更低。 数控技术的发展导致在制造技术方面一些其他的革新:l 电火花切割加工l 激光切割加工l 电子光束焊接数控也已经使机床设备比以前的手动机床更加通用化。一个NC加工设备能自动地生产多样化的零件,每一个簇零件包括一类复杂多样的加工方法。数控允许制造商能保证产品的质量，这从经济学的观点来看用手工机床加工是无法实现的。 2.2.1数控机床的发展历史如同许多先进的技术一样,数控技术诞生在麻省理工学院。 数控的观念在十九世纪五十年代早期由美国空军提供赞助而被研发。 在它最早的阶段中，数控机床能够有效率的而且准确地作直线的切割。然而, 曲线轨迹是一个问题因为加工机床不得不用程序来保证一系列的水平线和垂直的阶梯线来形成一个曲线。如果构造阶梯的直线越短,则曲线就越平滑。（Fig2.17) 各段直线阶梯成形之间的关系如下图2。17所示。 如果数控技术要进一步发展，这是必须被克服的荆棘之路。这个问题导致1959年自动编程语言(APT)的发展。 这是一种NC专用的编程语言，它使用与英语相似表诉来定义零件的几何外形，描述刀具结构，而且列出所有必须的运动。 编程语言的发展是数控技术迅速发展中关键性一步。 最初的数控系统与今天所用的有很大的不同。 机床有复杂的逻辑电路。指令程序被写在打孔的纸带上, 这种纸带稍后被磁性的塑料音带替换。 一个磁带播放机用于翻译写在音带上的指令。所有的这些都推动机床控制技术向前发展。 然而，在数控的发展中还伴随着许多的问题。一个主要的问题是打孔纸带非常脆弱。 加工过程中，指令纸带断裂或是撕破，这是很常见的。这个问题在实际加工中更严重。因为在一个零件完整的连续的加工中，指令纸带必须完整的并且连续的通过译码器。 如果一定眼生产100件相同的零件,纸带也必须连续的通过译码器100次。对于易碎的纸带来说，要承受外界的点击以及如此重复的使用，是不可能的。这导致一种特别的磁性的塑料带的产生。不同的是纸带携带的程序指令如被打卡上班纸带的一系列的 洞,而塑料的磁带携带的指令是一系列的有磁性的点。塑料带比纸带更加坚韧,这解决了时常发生的扯裂和破坏的问题。 然而,它仍然留下了其他的两个问题。其中最重要的就是它很难或是不可能改变在磁带上被输入的指令。就算是要在指令中做一较小的调整 ,也是必须暂停机床操作而且要制做一卷新的磁带。当零件在批量生产时，它也仍然必需同样地在译码器上跑许多次。幸运地，计算机技术变成现实并且很快解决了数控机床与纸带和磁带相关的一些问题。1)直接数字控制的来到直接数字控制(DNC)观念的发展解决了与数控技术有关的纸带和塑料磁带的问题，使磁带不再作为运载程序指令的媒介了。 在直接数字控制中，机床加工设备,经由一个数据传输链,联接到一个主机上.(Fig 2.18) 操作机床设备的程序被储存在主机中并且通过数据传输链给所需要的加工机床。 直接数字控制从打孔纸带和塑料磁带基础上向前发展了一大步。然而,它同所有仰赖主机的技术一样,机床设备也经历停工时间的限制。 这个问题导致了计算机数字控制的发展。2)计算机数字控制的出现微处理器的发展使可编程逻辑控制器 (PLCs) 和微计算机的发展成为可能。 这二种技术则实现了计算机数字控制的发展(CNC)。 藉由 CNC ，每台机床都有 PLC 或一部伺服的微计算机。 这允许程序可以输入并且储存在每个单独的机床上。 它也允许程序被脱机使用而且可在个别的机床上下载。CNC 解决了与主机有关的停工时间的问题，但是它引入了另一个称为数据管理的问题。这是一个所有工作设备基于微计算机所共有的问题。 由于没有在微机之间的沟通，相同的程序可能在十部不同的微计算机上被装载。这个问题通过用局域网联结微计算机来更好地管理数据得到比较好的解决。 数据管理的问题导致分布式数字控制的发展。3)分布式数字控制的到来分布式数字控制( 也叫做 DNC）采用直接数字控制和计算机数字控制最优的方面。分布式数字控制在单个的机床处既有主机也有地方性计算机。(Fig 2.19) 这使得程序被储存在主机中同时,能够比较好的管理。然而,它也允许程序被下载到地方性的微计算机或 PLCs 。它也允许地方性的数据输入和在机床层面上微计算机或 PLCs 之间的相互交流。2.2.2数控机床的组成任何的数控机床中有一下四个组成部份:l 实际的数控工具l 机床控制单位 (MCU)l 在数控机床和MCU之间的沟通接口l 为在NC机床上进行特种加工的各种辅助用件实际的NC机床可能是铣床，车床，钻床，或是其他任何类型的机床设备。 MCU 是控制单位，保存控制机床的程序。 MCU 也有让操作员输入可用的各种不同的装置。MCU 中的数据信息通过沟通接口被带到数控机床的伺服装置上。 这些伺服装置接受来自 MCU 的电子信号而且引起NC机床的机械装置运作。比较简单的NC机床有开环伺服系统。一个开环伺服系统能接受信号并且执行包含在信息里的指令,但是不能反馈回 MCU 展现信号中的指令是否有完全地执行。比较复杂的NC机床使用闭环控制系统。闭环控制系统能接受和执行信息还能发出反馈信号给MCU以显示指令是否执行和实行到什么程度。更复杂的闭环控制系统被越来越多的应用因为当运行一个程序出现问题的时侯,该系统有比较可靠的监测和及时的纠正行动。其他的配件是实行特种加工时所需的特殊工具。2.2.3数控机床的编程这里有四种方法对NC机床进行编程: 手工编程，数字化编程,电脑编程, 和图标编程(Fig 2.20)。手工编程是四种方法中最麻烦的一种。 它包括计算确定刀具位置和叙述刀具使用方法工作方向。 一旦这些数值被计算出来, 则记录和存入 MCU 之内。数字化是在计算机辅助设计和规划中时常用到一种方法, 它将一个零件的草图用电子扫描出来。当它被扫描的时候,图画上的各种不同的点被转换成 X- Y 坐标相应的数值并且在计算机中储存。一经草图完全地被扫描,被储存的 X- Y 坐标的数值能描绘出零件，并且提供给NC机床关于刀具加工位置和运动的规定指令。书面的编制与那些使用任何计算机编程的相似。这种编程，采用英文的形式描述刀具的位置和运动，以及速度和进
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