1. CNC FEED DRIVES

2. 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.Reference

3. 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.

4. Fig 1.working principle of spindle drives
Fig 2.Working principle of CNC feed drive

5. This system usually uses electric motors although hydraulic motors are sometimes used for large machine tools.
Electric Motors
An electric motor is an electric machine that converts electrical energy into mechanical energy
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
A hydraulic motor is a mechanical actuator that converts hydraulic pressure and flow into torque and angular displacement (rotation)

6. 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.
Servomotors.
DC Servomotors.
AC Servomotors.
Stepper motors.

7. 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.

8. 3.Servo motor
Positions mechanical elements within a given time and with a given precision.
Operates at low power ranges.
Factors to be considered in servo action are :
Dynamics
Position exactness
Peak torque
Speed regulation range etc

9. 3.1 servo drive control

10. 3.2 Components of servo controlled cnc
Motor
lead screw rotation
table moves
Motor speed control
feedback
position sensed by encoder

11. 3.3 servo-drive control Working of a servo motor :
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.
This results in the desired speed which is compared with actual speed feedback by the Tachogenerator.
This comparison gives out the output of the desired current to be compared with the feedback of actual current.
The current controller gives the output of control voltage to the motor.
A six pulse-bridge supplied with a 3-phase system gives the direct voltage depending on the control voltage.

12. 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.

13. 4.1 Features of dc servomotor
Smooth rotation at speed less than one rpm.
Bush life of more than 4000 rpm.
Sensor less switching.
Transistor controller and diode rectifier.
Resolver for speed control and position control and switching control.
Resolver ® performs the task of both tachogenerator and pulse-generator (p-encoder).

14. Fig 3.Working of DC Servomotor

15. Fig4. Control of DC Servomotor

16. 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.

17. Fig 5.Working principle of AC Servo motor

18. 6.comparison between d.c. and a.c. servomotor

19. 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

20. 7.1 construction
Stator has a winding made of concentrated coils on distinct poles.
Rotor has a permanent magnet cylinder.
The stator windings are inserted on the periphery of the yoke and this is slide on the permanent magnet pole cylinder.
The rotor shaft is located at the center of the yoke, mounted on bearings
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

21. Fig 6 STEPPER MOTOR.

22. Fig 7 working of stepper motor
The rotation of the magnet inside the yoke will energize the poles
Fig 7 working of stepper motor

23. 7.3 advantages
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
Low cost
Ruggedness
Simplicity in construction
High reliability
No maintenance

24. 7.4 disadvantages Slow speed Low torque Low resolution
Easy to slip in case of overload

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