1、 本科生毕业设计（论文） 外文翻译 外文题目： Numerically Controlled (NC) Machines 译文题目： 数控机床 学 生 姓 名： 张 龙 专 业： 机械设计制造及其自动化 指导教师姓名： 张 凯 评 阅 日 期： Numerically Controlled (NC) Machines With automatics, programming is expensive and can be justified only for long production runs. However, with machines incorporating feedback
2、control, programs can be provided in the form of punched tapes or punched cards, which are relatively inexpensive to produce compared with disc and drum cams. These machines are known as numerically controlled (NC) machines and can be used economically in small-batch production. As the name implies,
3、 numerical control involves control on the basis of numerical information that specifies the relative position of the tool and workpiece. From the block diagram for a machine-tool control system in Fig. 12 it can be seen that two essential elements are added to an otherwise standard machine. The fir
4、st added element is a means of driving the machine table or toolholder by a servomotor, and hence the motion of the tool or workpiece depends on the signal passed to the servomotor. The second added element is a transducer that continuously monitors the position of the tool or workpiece. The signal
5、from the transducer is compared with that obtained from the tape, and any difference (or error) is converted to analog form, amplified, and used to drive the servomotor until the tool or workpiece position agrees with the position specified by the information on the tape. Fig. 12: Feedback loop for
6、one axis of a machine-tool control system There are two basic types of NC systems: the point-to-point, or positioning, system and the continuous-path, or contouring, system. The point-to-point system would be applied, for example, to a vertical-drilling machine. If control of the two horizontal-moti
7、on axes of the table supporting the workpiece is arranged, the machine can be programmed to locate and then drill a specified pattern of holes. In the point-to-point system the path of the tool relative to the workpiece between holes is not important, and only the coordinates of the end point of eac
8、h motion of the table are specified. The continuous-path system would be applied, for example, to a vertical-milling machine that was required to end mill a complicated shape, such as cam or pocket in a workpiece. In the continuous-path system the position of the tool relative to the workpiece must
9、be continuously controlled while workpieces are being machined. With continuous-path, or contouring, systems the position of the tool relative to the workpiece is specified by a series of coordinates, and the control system is designed to follow a path between these points by interpolation. Some mac
10、hines follow a straight-line path (linear interpolation)； others follow a curved path (circular or parabolic interpolation). Numerical control can be applied to motions along or about any axis, but two or three-axis control systems are the most common. In general, vertical-milling machines and lathe
11、s utilize continuous-path, or contouring control. Vertical-drilling machines jig borers, and small milling machines often use positional control. One sophisticated form of NC machine is known as the machining center. This machine is generally a vertical-milling machine with several axes of control a
12、nd with automatic tool-changing facilities. The tools are usually held in a rotary magazine, and tool changes are commanded by the punched tape. Thus, with a machining center a complicated workpiece can be completely machined on all faces except the base through a combination of milling, drilling, b
13、oring, facing, reaming, and tapping operations. This type of system is therefore most suitable for the batch production of main components. A further refinement of numerical control is adaptive control. This type of system can adapt itself to the prevailing circumstances. These circumstances are mea
14、sured by the system itself and might include the power required for the machining operation, the wear of the cutting tool or grinding wheel, the forces generated, or the onset of chatter or instability. The system ideally would be designed for automatic adjustment of the feed, speed, or tool positio
15、n to produce components at minimum cost and within the tolerance specified. Such a system would be very expensive and has not yet found wide application. A relatively simple adaptive control system would automatically vary the cutting speed and feed in such a way as to maximize metal-removal rates without exceeding predetermined cutting forces and power consumption. Systems of this type are relatively inexpensive and can machine under near-optimum conditions.
