Unit 1:Introduction to Numerical Control

1.1 History of Numerical Control

1.2 Concept of NC and CNC

1.3 Basic Components of NC Machine Tools

1.4 Classification of NC Machine Tools

1.5 NC Features and Applications

NC and CNC

( 1970's )

NC: Numerical Control

↓

1.1 History of NC ( 数控 )

(1) Concept for NC → 1940s United States Air Force

and aerospace industry （ complex parts ： manual machining

+ inspected by templates

∣ manual methods ） ↓↓

time consuming and inaccurate

John Parsons： inspection templates for helicopter rotor blades

Parsons' method:

calculating the coordinate points （ intermediate points ） ︱manually moving to each of these points ↓ accuracy of templates improved

Punched card （穿孔卡）

idea :

punched cards for the many calculations

data used to position the machine tool

↘

punch card tabulating machine

first attempts: (automatic position control)

punch card tabulating machines

+ manual milling machine

（ to calculate the positions）

（ to position the tool ）

( two operators, one to move each axis )

↓ tens of times more accurate

still time-consumingstill time-consuming

Parsons envisioned the following system:

a computer (calculate the path)

+ punched cards (store information)

+ a reader ( read cards)

+ the machine control (control motors attached to each axis)

• In 1952, a three-axis milling machine was demonstrated

vacuum tube ( 电子管 )

electrical relay （继电器）

electrical cabinet（电气控制柜）

The first NC machine

(2)Development of NC

1 generation: vacuum tube ( 电子管 )

(1952) +electrical relay ( 继电器） +machine control interface ( 接口 )2 generation: improved miniature electronic tubes (transistor)

(1959) ( 晶体管 )3 generation: small scale integrated circuits( 小规模集成电路 )

(1965)

4 generation: improved integrated circuits ( 大规模集成电路 )

(1970s)

1.2.3----hardwared NC: functions by hardwares4…------computer used→CNC

integrated circuits

ROM

improved integrated circuit

------microprocessor (1980s) (machines are strengthened with advances:)

Part program memory storage

(零件程序存储 )；Reduction of printed circuits boards

(印刷电路板 )

Programmable interface

(可编程接口 )

Faster memory access

(快速程序存取 )

Parametric subroutines

(参数化子程序 )

Macro capabilities

(virtually: 固定循环 )

microprocessor

Since the 1980s,

no hardwired NCs have been produced.

Today, when the term NC is mentioned, it normally means

CNC

(1) NC: a form of programmable automation in which the

mechanical actions of a machine tool or other equipment are controlled by a program containing coded alphanumeric data ( 字母数字信息 ).

• alphanumeric data: relative positions

between a workhead ( 加工部分 , 工作头 ) and a workpart( 工件 )

+ instructions needed to operate the machine

1.2 Concept of NC and CNC

suitable for low and medium production

(2) CNC: A CNC machine:

an NC machine + an on-board computer

↓ machine control unit or MCU

★ NC and CNC

NC: hard wired ( machine functions by physical

electronic elements)

CNC: ‘soft’ wired ( machine functions are encoded into

the computer ) more flexible than NC: a memory, calculations, decisions detect problems, communicate with operator and external devices

Part program

Servo-drive unit

Machine tool

Part drawing

Control computer

Part

1.3 Basic Components of NC Machine Tools

five fundamental units of typical NC machine tool

• 1)the input media （输入介质） ;

• 2)the machine control unit （机床控制单元） ;

• 3)the servo-drive unit （伺服驱动单元） ;

• 4)the feedback transducer （反馈传感）

• 5)the mechanical machine tool unit

（机床本体） .

1) The input media:

program of instructions （ a part program ） →direct actions

Instructions: position commands + additional instructions

The program is coded on a suitable medium for su

bmission(传送 ) to the machine control unit.

Types of medium :punched tape, magnetic tape, diskette,

electronic transfer from computer.

2) machine control unit (MCU) :

a microcomputer

+related control hardware

+ system software,

Function of MCU: stores program,

Converts commands into mechanical actions,

Executes program

3) servo-drive unit:

drive motors + servo-amplifiers

4) feedback transducer(传感器 ):

sensors measure actual position to minimize this difference.

5) the machine tool :

performs useful work

worktable and spindle

1.4 CLASSIFICATIONS OF NC MACHNINES

NC machines are classified in different way:

(1) the types of NC motion control system,

(2) the type of servo-drive system,

(3) application of NC.

(1) Types of NC Motion Control System

two types:

point-to-point

continuous path

﹡Point-to-Point Control Systems

(Positioning Control Systems)

move to a programmed location without regard for the path Movement from one point to the next is non-machining

The rate of movement is usually maximum for the slide

﹡ Contouring Control Systems

(continuous path control systems)

To follow any path at any prescribed feed-rate

To control two or more axes simultaneously to get desired shape (angular surfaces, two-dimensional curves,

or three-dimensional contours)

▲parallel to one of major axes →straight-cut NC simultaneous control of two or more axes →contouring NC

(2) Types of NC Servo-Drive Systems

based on how the NC system accomplishes positioning,

Three types of drives:

open-loop( 开环 ) drive system,

closed-loop( 闭环 ) drive system ， half closed-loop( 半闭环 ) drive system.

● Open-Loop Servo-Drive （开环伺服驱动）simplest and least cost form of servo-drive

An open-loop control system

Feature:

lacks feedback ( no sensing device to confirm the action )

( 反馈 ) ↓ actuator will not have the intended effect

→ (disadvantage )

motor : stepping motor （步进电机）1

2

3

4

advantages: high accuracy, easy implementation, compatible with digital signals;

disadvantages: low torque ( 转矩，扭矩） , limited speed and risk of missed pulse under load （重载）

application is restricted to smaller machines

∣light duty machinery ( no problems of instability,

no requirements of high precision)

→ economical NC system

●Half-Closed-Loop Servo-Drive （半闭环伺服驱动）

one of the feedback control system

A half-closed-loop control system

Feature:

indirect feedback （间接反馈） monitors the output of servomotor （伺服电机） ↓ a feedback sensor is attached to servomotor axis or lead-screw (measures the rotary angle of motor or leadscrew)

↓Disadvantage: unable to sense backlash or lead-screw windupAdvantage: convenient to adjust , a good stability

motor : Servo motor (DC or AC) ( 直流或交流伺服电机 )

●Closed-Loop Servo-Drive （闭环伺服驱动） feedback control system

A closed-loop control system

Feature:

direct feedback （直接反馈） monitors the actual position of worktable

↓

A feedback sensor directly measures

the position of worktable

↓

Advantage: more accurate

Disadvantage: implementation costs higher

motor : Servo motor (DC or AC) ( 直流或交流伺服电机 )

(3) Application of CNC applications divided into two categories:

(A) machine tool applications

(B) non-machine tool applications

Machine tool applications are those usually associated with the metalworking industries.

Non-machine tool applications comprise a diverse

group of operations in other industries.

(A) Machine Tool Applications

(Machining : manufacturing process, in which the geometry of work is produced by removing excess material.

four common types of machining operations:

(1) turning (performed on a lathe),

(2) drilling (on a drilling press),

(3) milling (on a milling machine),

(4) grinding (on a grinding machine).

NC lathe: (horizontal or vertical )

two-axis continuous path control

NC boring mill: (horizontal or vertical spindle)

Operation: continuous path, two-axis control

Boring is similar to turning, except that an internal cylinder is created instead of an external cylinder

NC drill press: point-to-point control of workhead (spindle containing the

drill bit)

and two axis (X-Y) control of the worktable.

NC milling machine: require continuous path control to perform straight cut or contouring operations

NC grinding machine:

intended for finishing treatment

includes

cylindrical-( 外圆磨床 ),

surface- ( 平面磨床 ),

internal- ( 内圆磨床 ),

spindle- ( 心轴磨床 ),

thread- ( 螺纹磨床 ),

gear- ( 齿轮磨床 ),

tool-grinding machines ( 工具磨床 )

Tool grinding machine

Machining center: multifunction NC machines

with automatic tool changer and tool storage

( 自动换刀装置 ) ( 刀库 )

major advantages: Increased productivity and versatility

(eliminates the need for a number of individual machine tools,

↓thus reducing capital equipment and labor requirements

Additional savings: materials handling, fixture costs,

and floor space requirements

throughput faster

in-process inventory replaced by work at one machine

Machining centers: vertical or horizontalVertical : primarily for flat parts; mold and die work;

Machining centers: vertical or horizontalHorizontal : for large, boxy, and heavy parts.

• 1- 数控柜 control cabinet

• 2- 刀库 tool magazine

• 3- 主轴箱 headstock

• 4- 操作面板 operation panel

• 5- 驱动电源 driving power supply

• 6- 工作台 worktable 7- 滑枕 ram

• 8- 床身 machine bed

• 9- 进給伺服电机 feed servo motor

• 10- 换刀机械手 tool change manipulator

Pivot insertion tool changer

(B) Non-Machine Tool Applications

Punch presses for sheet metal hole punching ( 板金冲床 )

Presses for sheet metal bending （弯板机）Welding machine （焊机）Thermal cutting machine （热加工机）Tube bending machine （弯管机）

1.5 NC Features and Applicationscharacteristics:

(1) batch production in small or medium lot sizes;

(2) repeat orders at random or periodic intervals;

(3) complex part geometry;

(4) Much metal needs to be removed from part;

(5) many separate machining operations on the part

(6) the part is expensive

↓

NC benefits and advantages

(economic savings for the user company)

Advantages and Disadvantages of NC

Advantages ：Greater accuracy and repeatability ；More complex part geometries are possible ； Nonproductive time is reduced

Lower scrap rates （废品率）Inspection requirements are reduced

Engineering changes can be accommodated more gracefully Simpler fixtures are needed

Shorter manufacturing lead times （订货至交货的时间）Reduced parts inventory （库存）

Less floor space （房屋面积） required

Operator skill-level requirements are reduced

Disadvantages of NC ：

Higher investment cost

Higher maintenance effort

Part program

Higher utilization of NC equipment