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数控技术

数字控制与计算机数字控制的发展历史

数字控制是按照含有机床(刀具)运动信息程序所指定的顺序自动执行操作的加工过程。数控这一概念是由密歇根洲特拉华城的约翰·帕森于20世纪四十年代后期提出的。为了在工件上加工光滑的轮廓，帕森提出了一种自动的机床控制方式，它能够引导铣床刀具加工出一种“过轴曲线”。

1949年，美国空军与帕森签署了合同，要求开发一种能够提高生产率的新型机床。帕森委托麻省理工学院(MIT)来开发一种新概念机床，麻省理工学院的科学家和工程师研制出了一种用穿孔纸带作为输入媒介的二轴联动铣床控制系统。在较短时间内，当时所有主要的机床生产商都生产了一些数控机床，但直到20世纪七十年代后期，基于计算机的数字控制才被得到广泛的使用。只有价格低廉且功能强大的微处理芯片代替了计算机数控系统中的硬连线逻辑发生器后，NC才真正成为一门自动化技术。

当数控机床在计算机监控下工作时，它就被称为计算机数控机床(CNC)。计算机是CNC机床的控制单元，它们内嵌于数控机床或者通过通讯渠道与数控机床联接，当程序员编程时，通过纸带或磁盘将一些信息输入，计算机将对一些必要的数据进行计算的完成工作。

由于第一台数控机床的数据是由纸带控制的，因此数控系统被称为纸带控制机床。它们只能控制由输入到机床内的纸带或磁带所规定的单一操作，输入到机床内的程序是不能被编辑的，要改变程序必须重做新纸带。

当今的系统都由计算机来控制数据，因而称之为计算机数控机床(简称CNC机床)。NC和CNC系统两者的工作原理一样，仅仅控制执行的方式不同。新型的数控系统通常速度更快、功率更大、功能更齐全。

数字控制与计算机数字控制的应用

数控技术自创立以来就得到了广泛的应用，包括车床和车削中心、铣床和加工中心、冲床、电火花(EDM)加工机床、线切割机床、磨床以及测试检测装置等。最复杂的计算机数控机床是车削中心，图4—1所示一个具有十转位的刀架能进行

快速换刀的现代车削中心，立式加工中心如图4—2所示(刀具库在机床的左边。右边的控制面板可由操作者通过转臂转动)，以及卧式加工中心，通过按下按钮每把刀可在数秒钟内定位。如图4—3所示(配有自动换刀系统。刀具库储备有200把切削工具)。

当为某项操作编程时，程序员必须选择传统的加工切削数据推荐值。这些切削数据包括切削速度、进给率、刀具和刀具几何形状等。当程序员正确选择所有必要信息后，操作人员将程序载入机床并按下按钮，切削循环就开始。

数控技术是一种利用程序实现自动控制的技术，加工制造设备采用数控技术后能由数字、字符和符号等进行控制。这些数字、字符和符号等被编码成按一定格式定义的指令程序用于一个特定的加工或工件，这些指令可以采用两种二进制编码的数字系统中的任意一种进行定义，这两种二进制编码数字系统分别为电工协会代码(EIA)和美国标准信息交换代码(ASCII)。一般来说，ASCII编码的机床控制系统不能接受EIA编码的指令，反之亦然。当然，这样的问题已经逐渐得到解决。数控加工制造目前已经广泛地应用于几乎所有的金属加工机床：车床，铣床，钻床，镗床，磨床，回转冲床，电火化，线切割和焊接机床，甚至弯管机也采用数控加工技术。

数控技术的基本组成

一个数控系统主要由以下3个部分组成：

(1)程序指令

(2)加工控制单元

(3)制造装备

程序指令是由一条一条的详细指令所组成，制造装备按要求执行这些指令。最常用的指令有：可以按要求使机床刀具主轴位于工作台上的具体位置，工作台是用于固定加工零件的，许多更高级的指令还包括用于主轴速度的选择、刀具速度的选择及其他一些功能。

加工控制单元(MCU)包括一些用于阅读和解释程序指令并将其转换为机床刀具或其他制造装备的机械动作的电子和控制硬件。

制造装备是一种进行金属加工的数控技术装备，在常用的数控技术领域中，制造装备用于进行机械制造。制造装备包括工作台、主轴、电机及控制驱动单元。

数控技术的类型

数控技术系统主要有两种类型：点对点数控系统和轮廓线数控系统。

点对点数控系统也称为位置数控系统，比轮廓线数控系统简单，其主要的原理是移动刀具或工件从一个程序控制点到另一一个控制点，通常像钻床这样的加工功能，每个点帮司以通过NC程序中的指令进行控制。点对点数控系统适用于像钻孔、沉孔加工、沉孔镗孔、铰孔和攻丝等。其他冲孔机床、点焊机和装配机床等也都采用点对点数控系统。

轮廓线数控系统也称为轮廓线路径数控系统，定位和切割操作都是以不同的速度沿着控制的路径进行的。由于刀具沿路径进行切削，因此刀具的运动和速度的精确控制和同步性能是非常重要的。轮廓线数控系统经常应用于车床、铣床、磨床、焊接机床和加工中心中。刀具沿着路径的运动，或称为插补逐渐出现了几个不同的方法。有许多类型的插补方法用于处理轮廓线数控系统中生成光滑的轮廓线时遇到的各种问题。几种比较常用的方法有线性插补，圆形插补，螺旋形插补、抛物线插补和立方插补等，在所有的插补方法中，路径控制是以刀具的旋转中心为标准，对于不同类型、不同直径的刀具，加工过程中的不同刀具磨削量在数控程序中获得不同的补偿。

数控系统的编程

一个数控系统(NC)的程序包括使数控(NC)机床进行操作和加工的一系列指令。数控程序可以由数控机床内部的程序库开发生成，也可以从外面采购获得。另外，程序可以通过手工编写，也可以进行计算机辅助编程。

数控程序包括一系列指令系统和命令系统。几何类指令用于定义刀具和工件之间的相对位置和运动：加工类指令用于定义主轴转速、进给、刀具转速等：传送类指令用于定义刀具或工作台的运动速度和插补的类型等；开关类指令用于冷却液供给、主轴旋转、主轴旋转方向选择、换刀、工件进给、夹具等的开关。第一个用于数控编程的数控编程语言是20世纪50年代由麻省理工学院数控编程系统开发小组专家开发的，并被命名为自动编程工具(APT)。

直接数字控制(DNC)和计算机数字控制(CNC)

数控技术的发展在批量生产和车间生产加工中，不管是在技术上还是在商业上都获得了巨大的成功。目前，已经有两种数控技术系统得到了发展，分别是：

(1)直接数字控制(DNC)。

(2)计算机数字控制(CNC)。

直接数字控制(DNC)可以被定义为这样一个生产制造系统，该制造系统有许多台加工机床，而相互之间由一台计算机采用直接连接，进行实时控制。这样，在传统的数字控制技术中采用的磁带阅读器在直接数字控制中被取消，从而保证了系统的可靠性。不使用磁带阅读器，被加工的零件程序就从计算机的存储器中直接传送到进行加工的刀具上。从原理上讲，一台计算机可以控制多达100台加工机床，(在20世纪70年代一个商业使用的DNC系统宣称可以控制多达256个机床刀具)。直接数字控制计算机用于按要求提供加工指令给每个进行加工的刀具上，当机床需要控制指令时，计算机就可以马上将指令传送到机床上。

随着直接数字控制(DNC)技术和计算机技术的飞速发展，数字式计算机的尺寸和价格的大幅度的降低，数字式计算机的计算能力的大大提高，大量传统的以硬件线路为基础的加工控制单元被以数字计算机为基础的数字控制单元所替代。最初在20世纪70年代使用了小型计算机。后来，随着计算机的进一步小型化，早期的小型计算机逐渐被现在的微型计算机所取代。

计算机数控(CNC)使用专用的微型计算机作为加工控制单元。因为数字计算机都用于计算机数控(CNC)和直接数控(DNC)，所以应该注意两者之间的区别，可以从3个方面来加以区分。

(1) DNC计算机是将指令数据发送到许多机床去或从许多机床中收集数据，而CNC计算机每次只控制一台或几台机床。

(2) DNC计算机一般位于距机床一定距离的位置，而CNC计算机一般都位于距机床较近的位置。

(3) DNC计算机开发的软件不仅可以用于控制单件生产，而且可以用于一个企业制造部门的管理信息系统，而CNC计算机开发的软件一般只用于某个特殊加工的工具。

数控铣床

数控铣床是在一般铣床的基础上发展起来的,两者的加工工艺基本相同,结构也有些相似,但数控铣床是靠程序控制的自动加工机床,所以其结构也与普通铣床有很大区别.

数控铣床的组成

主轴箱包括主轴箱体和主轴传动系统，用于装夹刀具并带动刀具旋转，主轴转速范围和输出扭矩对加工有直接的影响。进给伺服系统：由进给电机和进给执行机构组成，按照程序设定的进给速度实现刀具和工件之间的相对运动，包括直线进给运动和旋转运动。控制系统：数控铣床运动控制的中心，执行数控加机床基础件：通常是指底座、立柱、横梁等，它是整个机床的基础和框架。夹具：数控铣床主要用于加工形状复杂的零件，但所使用夹具的结构往往并不复杂。数控铣床夹具的选用可首先根据生产零件的批量来确定。对单件、小批量、工作量较大的模具加工来说，一般可直接在机床工作台面上通过调整实现定位与夹紧，然后通过加工坐标系的设定来确定零件的位置。数控铣床的组成：数控铣床的基本组成，它由床身、立柱、主轴箱、工作台、滑鞍、滚珠丝杠、伺服电机、伺服装置、数控系统等组成。床身用于支撑和连接机床各部件。主轴箱用于安装主轴。主轴下端的锥孔用于安装铣刀。当主轴箱内的主轴电机驱动主轴旋转时，铣刀能够切削工件。主轴箱还可沿立柱上的导轨在Z 向移动，使刀具上升或下降。工作台用于安装工件或夹具。工作台可沿滑鞍上的导轨在X 向移动，滑鞍可沿床身上的导轨在Y 向移动，从而实现工件在X和Y 向的移动。无论是X、Y 向，还是Z 向的移动都是靠伺服电机驱动滚珠丝杠来实现的。伺服装置用于驱动伺服电机。控制器用于输入零件加工程序和控制机床工作状态。控制电源用于向伺服装置和控制器供电。

数控铣床的工作原理

根据零件形状、尺寸、精度和表面粗糙度等技术要求制定加工工艺，选择加工参数。通过手工编程或利用CAM 软件自动编程，将编好的加工程序输入到控制

器。控制器对加工程序处理后，向伺服装置传送指令。伺服装置向伺服电机发出控制信号。主轴电机使刀具旋转，X、Y 和Z向的伺服电机控制刀具和工件按一定的轨迹相对运动，从而实现工件的切削。

数控铣床加工的特点

（1）用数控铣床加工零件，精度很稳定。如果忽略刀具的磨损，用同一程序

加工出的零件具有相同的精度。

（2）数控铣床尤其适合加工形状比较复杂的零件，如各种模具等。

（3）数控铣床自动化程度很高，生产率高，适合加工批量较大的零件。

NC and CNC

The History of NC and CNC Development

Numerical Control (NC) is any machining process in which the operations are executed automatically in sequences as specified by the program that contains the information for the tool movements. The NC concept was proposed in the late 1940s by John Parsons of Traverse City, Michigan. Parsons recommended a method of automatic machine control that would guide a milling cutter to produce a "thru-axis curve" in order to generate smooth profiles on work pieces.

In 1949, The U.S. Air Force awarded Parsons a contract to develop a new type of machine tool that would be able to speed up production methods. Parsons commissioned the Massachusetts Institute of Technology (M.I.T.) to develop a practical implementation of his concept. Scientists and engineers at M.I.T. built a control system for a two-axis milling machine that used a perforated paper tape as the input media. In a short period of time, all major machine tool manufacturers were producing some machines

with NC, but it was not until the late 1970s that computer-based NC became widely used. NC matured as an automation technology when inexpensive and powerful microprocessors replaced hard-wire logic-making computer-based NC systems.

When Numerical Control is performed under computer supervision, it is called Computer Numerical Control (CNC). Computers are the control units of CNC machines, they are built in or linked to the machines via communications channels. When a programmer input some information in the program by tape and so on, the computer calculates all necessary data to get the job done.

On the first Numerically Controlled (NC) machines were controlled by tape, and

because of that, the NC systems were known as tape-controlled machines. They were able to control a single operation entered into the machine by punched or magnetic tape. There was no possibility of editing the program on the machine. To change the program, a new tape had to be made.

Today's systems have computers to control data; they are called Computer Numerically Controlled (CNC) machines. For both NC and CNC systems, work principles are the same. Only the way in which the execution is controlled is different. Normally, new systems are faster, more powerful, and more versatile

The Applications of NC/CNC

Since its introduction, NC technology has found many applications, including lathes and turning Centers, milling machines and machining centers , punches , electrical discharg machines(EDM) Flame cutters,grinders,and inspection equipment. the most complex CNC machine

tools are the turning center,shown in Fig.4-1(Amodern turning center with a ten-station turret that accepts quick-chang tools.Each tool can be positioned in Seconds with the press of a button).And the machine center shown in Fig.4-2(Vertical machining center,the tool magazine is on the machine.the control panel on the right can be swiveled by the operator)and Fig.4-3(horizontal machining center,equipped with an automatic tool changer .tool magazines can store 200 ctting tools.

When preparing a progam for a particular operation ,the prommer must select all cutting data using recommendations for conventional machining .this includes proper

Selection of cutting speeds,feedrate,tools and tool geometry,and so on.when the programmer has chosen all of the necessary information properly,the operator loads the programme into the machine and presses a button to start the cutting crycle .the CNC machine moves automatically from one maching operation to another , changing the cutting tols and applying the coolent.in a surprisingly short time ,the workpiece is Machined according to the highest quality stangards. But that is not all.no matter how big the work series is,all of the parts will be almost identical in size and surface finishing. At this time of advanced technology,with its high demands for surface finishing and tolerances of components in,for example ,aerospace,nuclear,and medical equipment manufacturing,only CNC machines provide successful results.

Numerical control (NC) is a form of programmable automation in which the processing equipment is controlled by means of numbers, letters, and other symbols. The numbers, letters, and symbols are coded in an appropriate format to define a program of instructions for a particular

workpart or job. The instructions are provided by either of the two binary

coded decimal systems: the Electronic Industries Association (EIA) code,

or the American Standard Code for Information Interchange (ASCII).

ASCII-coded machine control units will not accept . EIA coded instructions

and vice versa. Increasingly, however, control units are being made to

accept instructions in either code. 121Automation operation by NC is

readily adaptable to the

operation of all metalworking machines. Lathes, milling machines,

drill presses, boring machines, grinding machines, turret punches, flame

or wire-cutting and welding machines, and even pipe benders are available

with numerical controls.

Basic Components of NC

A numerical control system consists of the following three basic components:

(1) Program instructions

(2) Machine control unit

(3) Processing equipment

The program instructions are the detailed step by step commands that

direct the processing equipment. [31In its most common form, the commands

refer to positions of a machine tool spindle with respect to the worktable

on which the part is fixtured. More advanced instructions include selection

of spindle speeds, cutting tools, and other functions.

The machine control unit (MCU) consists of the electronics and control

hardware that reads and interprets the program of instructions and convert

it into mechanical actions of the machine tool or other processing equipment.

The processing equipment is the component that performs metal process.

In the most common example of numerical control, it is used to perform

machining operations. The process-ing equipment consists of the worktable

and spindle as well as the motors and controls needed to drive them.

Types of NC

There are two basic types of numerical control systems: point to point

and contouring.

Point to point control system, also called positioning, is simpler than

contouring control system. Its primary purpose is to move a tool or

workpiece from one programmed point to another. Usually the machine

function, such as a drilling operation, is also activated at each point

by command from the NC program. Point to point systems are suitable for

hole machining operations such as drilling, countersinking, couterbofing,

reaming, boring and tapping. Hole punching machines, spotwelding machines,

and assembly machines also use point to point NC systems.

Contouring system, also known as the continuous path system,

positioning and cutting operations are both along controlled paths but at

different velocities. Because the tool cuts as it travels along a

prescribed path, accurate control and synchronization of velocities and

movements are important. The contouring system is used on lathes, milling

machines, grinders,incrementally, by one of several basic methods. There

are a number of interpolation schemes that have been developed to deal with

the various problems that are encountered in generating a smooth continuous

path with a contouring type NC system. They include linear interpolation,circular interpolation, helical interpolation, parabolic interpolation and cubic interpolation. In all

interpolations, the path controlled is that of the center of rotation of the tool. Compensation for different tools, different diameter tools, or tools wear during machining, can be made in the NC .

Programming for NC

A program for numerical control consists of a sequence of directions that causes an NC machine to carry out a certain operation, machining being the most commonly used process. Programming for NC may be done by an internal programming department, on the shop floor, or purchased from an outside source. Also, programming may be done manually or with computer assistance.

The program contains instructions and commands. Geometric instructions pertain to relative movements between the tool and the workpiece. Processing instructions pertain to spindle speeds, feeds, tools, and so on. Travel instructions pertain to the type of interpolation and slow or rapid movements of the tool or worktable. Switching commands pertain to on/off position for coolant supplies, spindle rotation, direction of spindle rotation, tool changes, workpiece feeding, clamping, and so on. The first NC programming language was developed by MIT developmental work on NC programming systems in the late 1950s and called APT(Automatically Programmed Tools).

DNC and CNC

The development of numerical control was a significant achievement in batch and job shop manufacturing, from both a technological and a commercial viewpoint. There have been two enhancements and extensions of NC technology, including:

(1)Direct numerical control

(2) Computer numerical control

Direct numerical control can be defined as a manufacturing system in which a number of machines are controlled by a computer through direct connection and in real time. The tape reader is omitted in DNC, thus relieving the system of its least reliable component. Instead of using the tape reader, the part program is transmitted to the machine tool directly from the computer memory. In principle, one computer can be used to control more than 100 separate machines. (One commercial DNC system during the 1970s boasted a control capability of up to 256 machine tools.) The DNC computer is designed to provide instructions to each machine tool on demand. When the machine needs control commands, they are communicated to it immediately.

Since the introduction of DNC, there have been dramatic advances in computer technology. The physical size and cost of a digital computer has been significantly reduced at the same time that its computational capabilities have been substantially increased. In numerical control, the result of these advances has been that the large hard-wired MCUs of conventional

NC have been replaced by control units based on the digital computer. Initially, minicomputers were utilized in the early 1970s. As further miniaturization occurred in computers, minicomputers were replaced by today's microcomputers.

Computer numerical control is an NC system using dedicated microcomputer as the machine control unit. Because a digital computer is used in both CNC and DNC, it is appropriate to distinguish between the two types of system. There are three principal differences:

1) DNC computers distribute instructional data to, and collect data from, a large number of machines. CNC computers control only one machine, or a small number of machines.

2) DNC computers occupy a location that is typically remote from the machines under their control. CNC computer are located very near their machine tools.

3) DNC software is developed not only to control individual pieces of production equipment, but also to serve as part of a management information system in the manufacturing sector of the firm. CNC software is developed to augment the capabilities of a particular machine Tool.

CNC milling machine CNC milling machine milling in general developed on the basis of, both processing the same, somewhat similar structure, but the CNC milling machine is programmed by automatic machine tools, so its structure is verydifferent with ordinary milling machine.

The composition of CNC milling machine

Including the spindle headstock spindle box and transmission, and drive tool for rotating clamping tool, spindle speed range and output torque has a direct impact on the process. Feed servo system: from the feed motor and feed the implementation of institutions, in accordance with the procedures set feed rate to achieve tool and the workpiece relative motion between, including the linear feed motion and rotation. Control system: CNC milling machine motor control center, the implementation of CNC machine tools based on additional pieces: usually refers to the base, columns, beams, etc., it is the basis and framework for the entire machine. Clamp: CNC milling machine is mainly used for processing of complex shape parts, but the use of the structure is often complicated jig. CNC milling machine fixtures used to manufacture parts under the first batch to determine. Single piece,

small batch and volume of the larger mold, the general directly in the machine table surface by adjusting the positioning and clamping to achieve, and then processed to determine the coordinates of the set location of parts. The composition of CNC milling machine: CNC milling machine elements, which consists of bed, column, spindle boxes, tables, slide saddle, ball screws, servo motors, servo devices, composed of CNC system. Bed machine for support and to connect the various parts. Headstock spindle for the installation. Bottom of the cone spindle hole cutter for installation. When the spindle inside the spindle motor drive spindle, the cutter can cut the workpiece. Spindle box also on the rail along the column in the Z to move the tool up or down. Table for mounting the workpiece or fixture. Table can slide along the rails on the saddle to move in X, the saddle can slip along the bed rail body to move in the Y to achieve the workpiece in the X and Y to the mobile. Both X, Y direction, or Z to the mobile are driven by ball screw servo motor to achieve. Servo devices for driving servo motors. Controller for input processing and control machine parts working condition. Control power for the power supply to the servo unit and controller.

CNC milling machine works

According to the part shape, size, accuracy and surface roughness and other technical requirements to develop processing technology, select the machining parameters. Programming by hand or using CAM software, automatic programming, processing will be programmed input to the controller. Controller of the processing procedures, and send commands to the servo device. Servo devices send control signals to the servo motor. Tool spindle motor to rotate, X, Y and Z servo motor control to the tool and workpiece relative motion trajectory of a certain order to achieve the workpiece cutting.

CNC milling machine features

(1) with a CNC milling machine parts, precision is stable. If you ignore the tool wear, machined with the same procedure with the same precision

parts.

(2), CNC milling machine is particularly suitable for processing more complex shape parts, such as various molds.

(3) a high degree of automation, CNC milling, high productivity, large quantities of parts for processing.