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
NC: Numerical Control
↓
( 1970's )
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
idea :
punched cards for the many calculations
↘
Punched card
（穿孔卡）
data
used to position
the machine tool
first attempts: (automatic position control)
punch card tabulating machines
（to calculate the positions）
+
manual milling machine
（to position the tool ）
( two operators,
one to move each axis )
↓
tens of times more accurate
still time-consuming
punch card tabulating machine
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 cabinet
（电气控制柜）
electrical relay （继电器）
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 hardwares
4…------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
1.2
Concept of NC and 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
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 drawing
Part program
Control computer
Servo-drive unit
Machine tool
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
submission(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）
advantages: high accuracy,
easy implementation,
compatible with digital signals;
1
2
3
4
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 windup
Advantage: 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 horizontal
Vertical : primarily for flat parts; mold and die work;
Machining centers: vertical or horizontal
Horizontal : 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 Applications
characteristics:
(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