Unit_9_NC_Machines

Unit 9 NC Machines

第9单元数控机床

Machine may be classified according to the number of axes of numerically controlled movements with respect to Cartesian X-Y-Z coordinates. There may be other movement not numerically controlled. A two-axis machine would have the table moved lengthwise and crosswise in a horizontal plane; a three-axis machine would have an additional vertical movement of the spindle,for example, Four-, five-, or six-axis machines provide additional linear

or rotary movements.

机器根据多轴数控运动方面笛卡尔某某坐标可分为多种。有可能是其他运动的数字控制。一种双轴机将表的纵向和横向移动在水平面；三轴机将有额外的主轴垂直运动，例如，四-，五-，或提供额外的线性轴机或旋转运动。

Three classes of NC systems are commonly recognized. One of these is called point-to-

point or positioning NC, where a cutting tool and work piece are positioned with respect to

each other before a cut is taken, as exemplified by an NC drill press~. The path between

points is of little concern and is not particularly controlled. Another class,known as a

straight-cut NC system, involves movements between points like point-to-point NC but along straight or curved paths determined by the machine ways or slides. An NC turret lathe may

Be of the second class. A third class is continuous-path NC, or more formally continuous tool Path control, which does contouring or profiling of lines, curves, or surfaces, of all shapes.

One way of achieving continuous path NC is to move the work piece or cutter from point to

point along a straight line or curve (a parabolic path between points is used on some machines) with many points spaced closely enough together so that the composition path of the

Cut approximates a desired curve within limits required~. Another way is to drive machine members, say a saddle and the table on it,along coordinate axes at varying velocities controlled so that the resultant motion is along a particular line or curve.

在三类数控系统中。其中一类被称为逐点或位置调节数控，在这类数控系统中，进行切削之前，切削刀具和工件彼此之间的位置就被确定了，数控钻床就是一个例子。点之间的路径不太重要，不是特别控制。另一类，称为数控纵切系统，涉及运动点像点位数控但沿直线或曲线路径决定的方式或过程。数控转塔车床可能是二类。三分之一类是连续轨迹数控，或更正式的连续轨迹控制，其轮廓或轮廓线，曲线或表面，各种形状。得到数控连续路径的一种方式是使工件或刀具沿一直线或曲线做点到点的运动（在某些机床上，点之间采用抛物线路径）

许多点的间隔足够小，一边在要求的限度内使切削组合的路径接近一预想的曲线。另一种方法是驱动机器的部件，像一个鞍状的结构，沿坐标轴受控于不同的速度，由此产生的运动是一个特定的直线或曲线。

An NC machine may be controlled through an open-loop or closed-loop circuit as depicted in Fig. 9.1. The open-loop system is the simplest and cheapest but does not assure accuracy. A signal that is an order for a certain action (such as to turn on the coolant, start the spindle, or move the table to a certain position) is issued by the control unit. This travels through the drive mechanism which imposes the action upon the controlled member of the machine.The basic drive mechanism for a table or other machine member in an open-loop system is a stepping motor, also called a digital or pulse motor. It has, for example,49 poles around the armature inside of 50 poles around the stat or. Two poles are always aligned. Poles are energized to align the next two poles and turn the armature through a definite small angle each time a pulse of electricity is received. The motor turns the lead screw, which moves the table a corresponding distance. If, for example, a pulse stands for a 0.001mm movement, and a movement of 0.05 mm is desired, the control unit issues50 pulses; the motor turns through 50 steps and turns the lead screw, and the table (or other member) is driven the 0.05 mm required. The rate at which the pulses are issued determines the rate of feed. The power of a stepping motor is limited, and if the resistance to movement is large, the motor may just stall and miss steps. There is no feedback to report the omission, and accuracy is lost. Direct drive with a stepping motor is confined to light service at fractional horsepower. For more power and heavy service, a stepping motor is matched with a hydraulic motor through a servo valve for table drives up to7.5kW (10 hp) and more.

数控机床可控制通过一个开环或闭环电路如图9.1所。开环系统是最简单和便宜但不保证准确性。一个信号是一个特定操作(例如打开冷却剂,启动主轴,或的表移动到某一个位置)的控制单元。这个过程,通过驱动机制规定的行为来控制机器的部件。在开口回路系统中，一工作台或其他机器构件的基本的驱动机制是步进电动机驱动,也称为数字或脉冲发动机。比如,它在电枢内部49极点在定子内部有50的极点。两个极点总是一致的。驱动有足够的力量来调整下两个极点,将通过电枢每次收到的脉冲电产生的一个明确的小角度来进行控制。马达转动的距离是对应于移动表的距离。比如说,控制单元都为50脉冲时，一个脉冲代表0.001毫米的运动和一个脉冲代表0.05毫米的运动效果是很明显的，马达转动到50步产生的0.05毫米的距离是自身驱动所需要的。速度进给是由脉冲的速率决定的。步进电机的力量是有限的,如果要带动大型汽车运动是不可能的。这种方式与步进电

机直接驱动相比仅限于光服务在分马力。如果需要更多的力和完成更重的工作，,一个步进电机配个通过伺服阀液压马达就可以驱动7.5千瓦(10点)的力，甚者更多的力。

It is common in closed-loop systems for the pulsed command signal for the movement of the table(or other member) of the machine to be converted to a steady analog signal.That signal turns on the power to the drive mechanism. Hydraulic and ac but mostly dc electric motors are all used for NC machine drives. The intensity of the signal is determined by the feed velocity ordered. For a higher feed, pulses are issued at a faster rate and/or the analog signal is larger, which turns on more power to the motor to drive the table faster. A tachometer in the motor drive sends back a signal that is compared to the feed rate order to assure that the required feed rate is being delivered and to make corrections if it is not. Commonly, a resolver is driven by the lead screw and returns a signal showing how far the table moves. More accuracy for a longer period of time is obtained with the feedback device attached directly to the table, but such devices are more costly. The feedback signal is compared to the order, and where the feedback equals the order and signifies the required movement has been completed, the power is shutoff, and the movement stops. Since they are controlled through separate channels, all movements may act at the same time. Indeed, it is necessary that they act simultaneously and be synchronized accurately for contouring.

这是脉冲信号指挥的常见的闭环系统的运动，其他的机器需要转换到一个稳定的模拟信号来进行运动。信号是打开电源的驱动机制。大多数的数控机床都是采用直流电机驱动的。发出脉冲的频率能确定进给的频率。要实现一个较高的进给需要脉冲发出的速度更快和模拟信号更强，这将使更大功率电机驱动的更快。转速是电机驱动发送的信号，不是为了保证所需的相对于进给率的进给也不是为了保证与交付的配合。通常一个解析器是由丝杠和返回信号共同作用。反馈装置直接连接到表使时间反馈的更长更准确，但这种装置是更昂贵。反馈信号是顺序的当反馈等于输时，意味着所需的运动已经完成，然后将电源关闭运动停止。通过控制不同的渠道，所有的运动可在同一时间进行，他们的运动同时同步而且准确的进行在实际情况中这是很有必要的