Numerical control is a form of prog

Numerical control is a form of programmable auto¬mation in which a machine is controlled by numbers (and other symbols) that have been coded on punched paper tape or an alternative storage medium. The initial appli¬cation of numerical control was in the machine tool in¬dustry, to control the position of a cutting tool relative to the work part being machined. The NC part program represents the set of machining instructions for the par¬ticular part. The coded numbers in the program specify x-y-z coordinates in a Cartesian axis system, defining the various positions of the cutting tool in relation to the work part. By sequencing these positions in the program, the machine tool is directed to accomplish the machining of the part. A position feedback control system is used in most NC machines to verify that the coded instruc¬tions have been correctly performed. Today a small com¬puter is used as the controller in an NC machine tool. Since this form of numerical control is implemented by computer, it is called computer numerical control, or CNC. Another variation in the implementation of nu¬merical control involves sending part programs over tel-ecommunications lines from a central computer to indi¬vidual machine tools in the factory. This form of numeri¬cal control is called direct numerical control, or DNC.
Many applications of numerical control have been de¬veloped since its initial use to control machine .tools. Other machines using numerical control include compo¬nent-insertion machines used in electronics assembly, drafting machines that prepare engineering drawings, coordinate
measuring machines that perform accurate inspections of parts. In these applications coded numeri¬cal data are employed to control the position of a tool or workhead relative to some object. Such machines are used to position electronic components (e.g., semiconductor chip modules) onto a printed circuit board (PCB). It is basically an x-y positioning table that moves the printed circuit board relative to the part-insertion head, which then places the individual component into position on the board. A typical printed circuit board has dozens of in¬dividual components that must be placed on its surface; in many cases, the lead wires of the components must be inserted into small holes in the board, requiring great precision by the insertion machine. The program that controls the machine indicates which components are to be placed on the board and their locations. This informa¬tion is contained in the product-design database and is typically communicated directly from the computer to the insertion machine.
Automated assembly
Assembly operations have traditionally been per¬formed manually, either at single assembly workstations or on assembly lines with multiple stations. Owing to the high labour content and high cost of manual labour, greater attention has been given in recent years to the use of automation for assembly work. Assembly opera¬tions can be automated using production line principles if the quantities are large, the product is small, and the design is simple (e.g., mechanical pencils, pens, and ciga¬rette lighters). For products that do not satisfy these conditions, manual assembly is generally required.
Automated assembly machines have been developed that operate in a manner similar to machining transfer lines, with the difference being that assembly operations, instead of machining, are performed at the workstations. A typical assembly machine consists of several stations, each equipped with a supply of components and a mecha-nism for delivering the components into position for as¬sembly. A workhead at each station performs the actual attachment of the component. Typical workheads include automatic screwdrivers, welding heads and other join¬ing devices. A new component is added to the partially completed product at each workstation, thus building up the product gradually as it proceeds through the line. Assembly machines of this type are considered to be ex¬amples of fixed automation, because they are generally configured for a particular product made in high volume. Programmable assembly machines are represented by the component-insertion machines employed in the electron¬ics industry.

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数控是一种可编程自动¬信息，机器控制的数字（或其他符号）已在穿孔纸带或其他存储介质编码。数值控制的初步应用¬阳离子在¬工业机床，控制一个相对工作部分被加工刀具的位置。数控程序的一部分，占票面¬特殊零件的加工指令的集合。编码数字程序指定x-y-z坐标在直角坐标系统，定义与工作部分的刀具不同的位置。通过在程序中这些位置排序，机床是针对完成零件的加工。位置反馈控制系统大多采用数控机床来验证代码的指令，¬已正确执行。今天，一个小¬COM计算机作为数控机床的控制器。由于这种形式的数值控制是通过计算机实现，它就被称为计算机数控，数控。在新¬数值控制实现的另一个变化是发送部分节目的电话交流线路中央计算机对¬守寡的机床厂。这种形式的数值¬CAL控制称为直接数字控制，或DNC。
数控许多应用程序已经开发的¬自控制机器初次使用的工具。利用数值控制其他机器的组成部分包括¬插入机用于电子装配，绘图机，准备工程图纸，坐标测量
完成零件的精确检验机。在这些应用中编码的数值¬CAL数据用于控制刀具或工件相对于对象的位置。这种机器是用来定位电子元件（例如，半导体芯片模块）到印刷电路板（PCB）。它基本上是一个X-Y定位平台，将印刷电路板相对于部分插入头部，然后将各个组件在板的位置。