CNC Machine & Its Components

Numerical Control Machine(NC)

Form of programmable automation.

Mechanical actions of machine tool are controlled by program.

The program is in form of alphanumeric data.

After a job is finished the program of instructions can be changed to process a new job.

Numerical Control Machine(NC)

Advantages

Reduces non productive time

Reduces manufacturing lead time

Greater manufacturing flexibility

Improves quality control

Reduced inventory

Disadvantages

High investment cost

High maintenance effort.

Part programming

Higher utilization of NC equipments

CNC Machines

To overcome the shortcomings of the NC machines CNC machines evolved.

CNC machines are the NC machines whose MCU is based on the micro computer rather than the hardwired controller.

Elements of CNC machines

arePart program Machine Control

Unit Machine tool

Features of CNC machines

are:

Storage of more than one part

program

In-process compensation

Improved programming and operating

features

Adaptive control adjustments

Common types of CNC machines

CNC Lathe machine

CNC Milling machine

CNC Drilling machine

CNC Grinding machine

CNC Laser cutting machine

Water jet cutting machine

Electro discharge machine

Advantages Higher accuracy

Reduce lead time

Higher flexibility

Reduce scrap rate

Reliable operation

Consistent quality

Reduced manpower

Increased productivity

Reduced non productive time

Applications

• Parts needed in a hurry• Parts with complicated contours• Parts requiring expensive jigs and fixtures• Parts those have several engineering changes• Cases where human errors can be extremely costly• Parts requiring close tolerance or good repeatability

Components of CNC

Electrical components

Mechanical components

Electrical components

Stepper motor

Servo motors

Stepper Motor Stepper Motor

Dc brushless motor

Digital pulse is converted into rotary motion.

It is an open loop system

Operating principle

A stepper motor is a type of DC motor which has a full rotation divided in an equal number of steps.

There is stator winding and a permanent magnet at the centre of windings.

When electric current is passed through winding it starts acting as an electromagnet and the permanent magnet moves.

The position of the rotor is controlled by the attractive and repulsive forces.

Advantages

Control is achieved at low cost

High torque at startup and low speed

Simple construction

Can operate in open loop control system

Low maintenance

Disadvantage

Low torque at high speed

Requires a dedicated control circuit

Should not be used for high performance or high load applications.

Servo Motor

Servo MotorsA servomotor is a simple electrical motor controlled

with the help of servomechanism.

Servo motors are functional with the sensor feedback.

The input to its control is signal either in the analog or digital form

Servo motors are primarily used where speed, power , noise level as well as velocity and positional accuracy.

Servo motors

DC brush servo motor

AC servo motors

DC Brush Servo motors:The brush refers to the brushes which passes the electric current to the rotor , the rotating part of the motor. In DC Servo motor each coil is angularly displaced from one another so when the torque drops off from one coil it automatically switches to the another coil as to produce the maximum torque. This switching is done by the brushes and a commutator present in the motor.

AC Servo motors:Its physical construction is similar to that of the DC Servo motor but there are no magnets in the AC Servo motor. But both the rotor and the stator are constructed from coils. These are capable of delivering very high torque at very high speed. There are no brushes in this type of motors which means they are maintenance free.

Working Principle

Motor is controlled by a feedback signal generated by comparing output signal and the reference input signal.

Reference input signal is applied to the system and compared with output signal of the system and thus a third signal is generated by feedback system.

This third signal acts as input signal to the device. This input signal to the device is present as long as there is a logical difference between reference input signal and output signal of the system.

After achieving the desired output there will be no longer logical difference between the input signal and the output signal of the system.

Now the feedback signal is not enough to produce further output until the next reference input signal is provided.

Advantages

Low Cost

Smooth rotations

High speed attainable

Wide variety is available

High peak torque available

Disadvantages

Thermally inefficient

Require maintenance

Can be demagnetised

DC Servo motors

AC servo motorsAdvantages

High Performance Maintenance Free

High torque availableCannot be demagnetised Accurate position information

Disadvantages

Expensive

Added feedback component

Increased control complexity

Encoders

An encoder is a device, circuit, transducer, software program, algorithm that converts information from one format or code to another, for the purposes of standardization.

ENCODERS

INCREMENTAL ABSOLUTE ROTARY AND LINEAR

Mechanical components

Recirculating ball screw

Planetary roller screws

Recirculating roller screws:

Recirculating ball screw

The recirculating ball steering mechanism contains a worm gear inside a block with a threaded hole in it.

This block has gear teeth cut into the outside to engage the shaft.

They convert sliding friction into rolling friction.

Advantages

wear will be reduced

reliability of the system will be increased

the efficiency of ball screws may reach up to 90%.

Roller screw

Roller screw

Roller screw

This type of screw provide backlash-free movement

Their efficiency is same as that of ball screws.

These are capable of providing more accurate position control.

Cost of the roller screws are more compared to ball screws.

The thread form is triangular with an included angle of 90 degrees.

Roller screw

Planetary roller screw

Recirculating roller screw

Planetary roller screw

These are capable of transmitting high load at fast speed.

There is no axial movement of the rollers relative to the nut.

The rollers are threaded with a single start thread.

Recirculating roller screw

The rollers in this case are not threaded.

The rollers are provided with a circular groove and are positioned circumferentially by a cage.

Recirculating roller screws are slower in operation.

They are capable of transmitting high loads with greater accuracy