文献翻译-数控机床的组成部分

N/C Machine Tool Elements

N/C machine tool elements consist of dimensioning systems, control systems, servomechanisms and open-or close-loop systems. It is important to understand each element prior to actual programming of a numerically controlled part.

The team measuring system in N/C refers to the method a machine tool uses to move a part from a reference point to a target point. A target point may be a certain location for drilling a slot, or other machining operation. The two measuring systems used on N/C machines are the absolute and incremental. The absolute (also called coordinate) measuring system uses a fixed reference point (origin). It is on this point that all positional information is based. In other words, all the locations to which a part will be moved must be given dimensions relating to that original fixed reference point. The incremental measuring system (also called delta) has a floating coordinating system. With the inference point each time the part is moved .Notice that with this system, each new location bases its values in X and Y from the preceding location. One disadvantage to this system is that any errors made will be repeated throughout the entire program, if not detected and corrected.

There are two types of control systems commonly used on N/C equipment: point-to-point and continuous path. A point-to-point controlled N/C machine tool, sometimes referred to as a positioning control type, has the capability of moving only along a straight line. However, when two axes are programmed simultaneously with equal values a 45°angle will be generated .Point-to-point systems are generally found on drilling and simple milling machine where hole location and straight milling jobs are performed. Point-to-point systems can be utilized to generate arcs and angles by programming the machine to move in a series of small steps. Using this technique, however, the actual path machined is slightly different from the cutting path specified.

Machine tools that have the capability of moving simultaneously in two or more axes are classified as continuous-path or contouring. These machines are used for machining arcs, radii, circles, and angles of any size in two or three dimensions. Continuous-path machines are more expensive than point-to-point systems and

generally require a computer to aid programming when machining complex contours.

N/C servomechanisms are devices used for producing accurate movement of a table or slide along an axis. Two types of servos are commonly used on N/C equipment: electric stepping motors and hydraulic motors. Stepping motor servos are frequently used on less expensive N/C equipment. These motors are generally high-torque power servos and mounted directly to a lead screw of a table or tool slide. Most stepping motors are actuated by magnetic pulses from the stator and rotor assemblies. The net result of this action is that one rotation of the motor shaft produces 200 steps. Connecting the motor shaft to a 10-pitch lead screw allows 0.0005-in, movements to be made (1/200×1/10=0.0005 in.).Hydraulic servos produce a fluid pressure that flows through gears or pistons to effect shaft rotation. Mechanical motion of lead screws and slides is accomplished through various valves and controls from these hydraulic motors. Hydraulic servos produce more torque than stepping motors. However, they are more expensive and very noisy. Larger N/C machines use hydraulic servos.

N/C machines that use an open-loop system contain no-feedback signal to ensure that a machine axis has traveled the required distance. That is, if the input received was to move a particular table 1.000 in, the servo unit generally moves the table 1.000 in. There is no means for comparing the actual table movement with the input signal, however. The only assurance that has actually moved 1.000 in is the reliability of the servo system used. Open- loop systems are, of course, less expensive than close-loop systems. A close-loop system compares the actual output (the table movement of 1.000 in.) with the input signal and compensates for errors. A feedback unit actually compares the amount the table has been moved with the input signal. Some feedback units on close-loop systems are transducers, electrical or magnetic scales, and synchros. Close-loop systems greatly increase the reliability of N/C machines.

数控机床的组成部分

数控机床的组成部分包括测量系统，控制系统，伺服系统及开环或闭环系统，在对数控零件进行程序设计之前，了解各组成部分是重要的。

数控中，测量系统这一术语指的是机床将一个零件从基准点移动到目标点的方法。目标点可以是钻一个孔，铣一个槽或其它加工操作的一个确定的位置。用于数控机床的两种测量系统是绝对测量系统和增量测量系统。绝对测量系统（亦称坐标测量系统）采用固定基准点（原点），所有位置信息正是以这一点为基准。换句话说，必须给出一个零件运动的所有位置相对于原始固定基准点的尺寸关系。增量测量系统有一个移动的坐标系统。运用增量系统时，零件每移动一次，机床就建立一个新的原点（基准点）。注意，使用这个系统时，每个新的位置在X和Y轴上的值都是建立在前一个位置之上的。这种系统的一个缺点是，如果产生的任何错误没有被发现与校正，则错误会在整个过程中反复存在。

用于数控设备的控制系统通常有两类，即点位控制系统和连续控制系统。点位控制数控机床（有时称为位置控制型数控机床）只有沿直线运动的能力。然而，当沿两轴以等值同时编程时，会形成45°斜线。点位控制系统常用于需确定孔位的钻床和需进行直线铣削加工的简单铣床上。点位控制系统可通过程序控制机床，以一系列小步运动形成弧线和斜线。然而，用这种方法时，实际加工轨迹与规定的切削轨迹略有不同。

具有在两个或多个坐标轴方向同时运动的能力的机床，归属连续轨迹控制或轮廓类机床。这些机床用于加工两维或三维空间中各种不同大小的弧形，圆角圆及斜角。连续轨迹控制的数控机床比点位控制的机床贵得多，在加工复杂轮廓时，一般需要计算机辅助程序设计。

数控伺服机构是使工作台或滑座沿坐标轴准确运动的装置。用于数控设备的伺服机构通常有两种：步进电机和液压马达。步进电机伺服机构常用于不太贵重的数控设备上。这些电机通常是大转矩的伺服机构，直接安装在工作台或刀座的丝杠上。大多数步进电机是来自定子和转子组件的磁力脉冲驱动的，这种作用的结果是电机轴转一转产生200步距。把电机轴接在10扣/英寸的丝杠上，每步能产生0.0005英寸的移动（1/200×1/10=0.0005英寸）。液压伺服马达使压力液体流过齿轮或柱塞，从而使轴转动。丝杠和滑座的机械运动是通过各种阀和液压马

达的控制来实现的。液压伺服马达产生比步进电机更大的转矩，但比步进电机贵，且噪音很大。大多数大型数控机床使用液压伺服机构。

使用开环系统的数控机床，没有反馈信号来确保机床的坐标轴是否运动了所需的距离。即，如果接收的输入信号是使一特定坐标轴移动1.000英寸，伺服装置通常使工作台运动1.000英寸，但无法将工作台的实际运动与输入信号加以比较。使工作台实际了1.000英寸的唯一保证是所用的伺服机构的准确性。当然，开环系统比闭环系统便宜。闭环系统能将实际输出（工作台一英寸的运动量）与输入信号加以比较，并对任何误差进行补偿。反馈装置真实地将工作台已运动的量与输入信号进行了比较。用于闭环系统的一些反馈装置是传感器，电尺或磁尺以及同步器。闭环系统大大增加了数控机床的准确性。