数控技术外文文献翻译、中英文翻译

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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.

外文资料翻译

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.

外文资料翻译

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

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摘要：

外文资料翻译NCandCNCTheHistoryofNCandCNCDevelopmentNumericalControl(NC)isanymachiningprocessinwhichtheoperationsareexecutedautomaticallyinsequencesasspecifiedbytheprogramthatcontainstheinformationforthetoolmovements.TheNCconceptwasproposedinthelate1940sbyJohnParsonsofTraverseCity,Michigan.Parsonsrecommend...

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