CNC FEED DRIVESAkhil Krishnan G

M.Tech

1

CONTENTS

1.Introduction

2.Requirements of CNC feed drives

3.Servo motor

3.1 Servo drive control

3.2 Components of servo controlled cnc.

3.3 servo-drive control

4.DC Servo Motor

5. AC Servo Motor

6.Comparison between D.C. And A.C. Servomotor

7. Stepping Motor

7.1 Construction

7.2 Working

7.3 Advantages

7.4 Disadvantages

8.Reference2

1.INTRODUCTION

The driving system is an important component of a CNC machine as

the accuracy and repeatability depend very much on the characteristics

and performance of the driving system.

The driving system is classified as spindle drives and feed drives.

The spindle drives are used to provide angular motion to the work

piece or a cutting tool.

The feed drives are used to drive the slide or a table.

The feed drive consists of an electromotor and mechanical

transmission elements.3

4

Fig 2.Working principle of CNC feed drive

Fig 1.working principle of spindle drives

o This system usually uses electric motors although hydraulic motors

are sometimes used for large machine tools.

Electric Motors

o An electric motor is an electric machine that converts electrical

energy into mechanical energy

o Most electric motors operate through the interaction between an

electric motor's magnetic field and winding currents to generate

force within the motor.

Hydraulic motor

o A hydraulic motor is a mechanical actuator that converts hydraulic

pressure and flow into torque and angular displacement (rotation)5

The main purpose is to move the working parts of machine tool

(working table, tool unit, spindle unit etc.)

The requirement is that the driving system has to response

accurately according to the programmed instructions.

The motor is coupled either directly or through a gear box to the

machine lead screw to moves the machine slide or the spindle. Two

types of electrical motors are commonly used.

I. Servomotors.

1. DC Servomotors.

2. AC Servomotors.

II. Stepper motors.

6

2.REQUIREMENTS OF CNC FEED DRIVES.

High torque-to-weight ratio

Low armature or rotor inertia

Low electrical and mechanical time constants

Permanent magnet construction

Maximum speed up to 3000rpm

High peak torque for quick response

The required constant torque for overcoming frictional and

working force must be provided. 7

3.SERVO MOTOR

o Positions mechanical elements within a given time and with a given

precision.

o Operates at low power ranges.

o Factors to be considered in servo action are :

o Dynamics

o Position exactness

o Peak torque

o Speed regulation range etc

8

3.1 SERVO DRIVE CONTROL

9

3.2 COMPONENTS OF SERVO CONTROLLED CNC

10

Motor speed control

Motor lead screw rotation table moves

position sensed by encoderfeedback

3.3 SERVO-DRIVE CONTROL

Working of a servo motor :

o The desired position is given as input to the system. The actual

position is measured by the encoder is given to position controller as

feedback.

o This results in the desired speed which is compared with actual speed

feedback by the Tachogenerator.

o This comparison gives out the output of the desired current to be

compared with the feedback of actual current.

o The current controller gives the output of control voltage to the

motor.

o A six pulse-bridge supplied with a 3-phase system gives the direct

voltage depending on the control voltage.

11

4.DC SERVO MOTOR

This is the most common type of feed motors used in CNC machines

The principle of operation is based on the rotation of an armature winding in

a permanently energized magnetic field.

The armature winding is connected to a commutator, which is a cylinder of

insulated copper segments mounted on the shaft

DC current is passed to the commutator through carbon brushes, which are

connected to the machine terminals.

The change of the motor speed is by varying the armature voltage and the

control of motor torque is achieved by controlling the motor's armature

current.

In order to achieve the necessary dynamic behavior it is operated in a closed

loop system equipped with sensors to obtain the velocity and position

feedback signals.12

4.1 FEATURES OF DC SERVOMOTOR

o Smooth rotation at speed less than one rpm.

o Bush life of more than 4000 rpm.

o Sensor less switching.

o Transistor controller and diode rectifier.

o Resolver for speed control and position control and switching

control.

o Resolver ® performs the task of both tachogenerator and pulse-

generator (p-encoder).

13

14

Fig 3.Working of DC Servomotor

15

Fig4. Control of DC Servomotor

5. AC SERVO MOTOR

In an AC servomotor, the rotor is a permanent magnet while the

stator is equipped with 3-phase windings

The speed of the rotor is equal to the rotational frequency of the magnetic

field of the stator, which is regulated by the frequency converter.

AC motors are gradually replacing DC servomotors

The main reason is that there is no commutator or brushes in AC servomotor

so that maintenance is virtually not required

Furthermore, AC servos have a smaller power-to-weight ratio and faster

response.

16

17

Fig 5.Working principle of AC Servo motor

6.COMPARISON BETWEEN D.C. AND A.C.

SERVOMOTOR

18

7. STEPPING MOTOR

A stepping motor is a device that converts the electrical pulses

into discrete mechanical rotational motions of the motor shaft

This is the simplest device that can be applied to CNC machines

since it can convert digital data into actual mechanical displacement

19

7.1 CONSTRUCTION

o Stator has a winding made of concentrated coils on distinct poles.

o Rotor has a permanent magnet cylinder.

o The stator windings are inserted on the periphery of the yoke and this

is slide on the permanent magnet pole cylinder.

o The rotor shaft is located at the center of the yoke, mounted on

bearings

o Examples of stepping motor application are the magnetic head of

floppy-disc drive and hard disc drive of computer, daisy-wheel type

printer, X-Y tape control, and CNC EDM Wire-cut machine

20

21

Fig 6 STEPPER MOTOR.

7.2 WORKING

22

The rotation of the magnet inside the yoke will

energize the poles

Fig 7 working of stepper motor

7.3 ADVANTAGES

o It is not necessary to have any analog-to-digital converter nor

feedback device for the control system. They are ideally suited to

open loop systems

o Low cost

o Ruggedness

o Simplicity in construction

o High reliability

o No maintenance

23

7.4 DISADVANTAGES

Slow speed

Low torque

Low resolution

Easy to slip in case of overload

24

8.REFERENCE

http://www.ignou.ac.in/

http://www.kimla.pl/en/feeddrives

http://nptel.ac.in/courses/112103174/pdf/mod4.pdf

http://accessengineeringlibrary.com/

http://electrical4u.com/electrical-drives/

25

26