Elements of a CNC system: Input Device, Central Processing Unit/ Machine Control Unit, Machine Tool, Driving System, Feedback Devices, Display Unit, Ball Lead Screws, CNC Programming, Programming Key Letters, G Codes and M Codes

Elements of a CNC system

Input Device

Central Processing Unit/ Machine Control Unit

Machine Tool

Driving System

Feedback Devices

Display Unit

Input Devices

Floppy Disk Drive

USB Flash Drive

Serial Communication

Ethernet communication

Conversational Programming

Central Processing Unit/ Machine Control Unit

The CPU is the heart of a CNC system.

It accepts the information stored in the memory as part program.

This data is decoded and transformed into specific position control and velocity signals.

It also oversees the movement of the control axis or spindle and whenever this does not match with the programmed values, a corrective action as taken.

Machine Tool

Most are made from high speed steel (HSS), tungsten carbide or ceramics.

Tools are designed to direct waste away from the material.

Some tools need coolant such as oil to protect the tool and work.

Driving System

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.

Three types of electrical motors are commonly used:

1. DC Servo motor

2. AC Servo motor

3. Stepping motor as explained ahead.

1.DC Servo Motor

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.

2. 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.

3. Stepping Motor

The stepper motor is known by its property to convert a train of input pulses (typically square wave pulses) into a precisely defined increment in the shaft position.

Each pulse moves the shaft through a fixed angle.

Multiple “toothed” electromagnets arranged around a central gear-shaped piece of iron.

The electromagnets are energized by an external driver circuit or a micro controller. In that way, the motor can be turned by a precise angle.

What does Stepper means?

To make the motor shaft turn, first, one electromagnet is given power, which magnetically attracts the gear’s teeth.

When the gear’s teeth are aligned to the first electromagnet, they are slightly offset from the next electromagnet.

This means that when the next electromagnet is turned on and the first is turned off, the gear rotates slightly to align with the next one.

From there the process is repeated. Each of those rotations is called a “step”, with an integer number of steps making a full rotation.

However, stepping motors are not commonly used in machine tools due to the following drawbacks:

slow speed,

low torque,

low resolution

easy to slip in case of overload.

Ball Lead Screws

Ball lead screw is the heart of the drive system.

Advantages of ball lead screw are:

Precise position and repeatability

High Speed capability

LessWear

Longer life

Feedback Devices

Two types of feed back devices normally used are:

1. Positional Feed Back Devices

1.1 Linear Transducers – a device mounted on the machine table to measure the actual displacement of the slide in such a way that backlash of screws; motors etc would not cause any error in the feed back data.

1.2 Rotary Encoders: a device to measure the angular displacement. It cannot measure linear displacement directly so that error may occur due to the backlash of screw and motor etc.

2. Velocity Feedback Device

The actual speed of the motor can be measured in terms of voltage generated from a tachometer mounted at the end of the motor shaft.

The voltage generated by the DC tachometer is compared with the command voltage corresponding to the desired speed.

The difference of the voltages is used to actuate the motor to eliminate the error.

Display Unit

Interface between the machine and the operator.

The Display Unit displays:

position of the machine slide

spindle RPM

feed rate

part programs

graphics simulation of the tool path.

Interpolation Methods

1. Linear interpolation Straight line between two points in space

2. Circular interpolation Circular arc defined by starting point, end point, centre or radius, and direction.

3. Helical interpolation Circular plus linear motion

4. Parabolic and cubic interpolation Free form curves using higher order equations

Circular Interpolation

Approximation of a curved path in NC by a series of straight line segments, where tolerance is defined on only the outside of the nominal curve.

CNC Programming

Programming consists of a series of instructions in form of letter codes

Preparatory Codes:

G codes- Initial machining setup and establishing operating conditions

N codes- specify program line number to executed by the MCU

Axis Codes: X,Y,Z – Used to specify motion of the slide along X, Y, Z direction

Feed and Speed Codes: F and S- Specify feed and spindle speed

Tool codes: T – specify tool number

Miscellaneous codes – M codes For coolant control and other activities

Programming Key Letters

O – Program number (Used for program identification)

N – Sequence number (Used for line identification)

G – Preparatory function

X – X axis designation

Y – Y axis designation

Z – Z axis designation

R – Radius designation

F – Feed rate designation

S – Spindle speed designation

H – Tool length offset designation

D – Tool radius offset designation

T – Tool Designation

M – Miscellaneous function

Table of Important G Codes

G codes are instructions describing machine tool movement

G00: Rapid Transverse

G01: Linear Interpolation

G02: Circular Interpolation, CW

G03: Circular Interpolation, CCW

G17: XY Plane, G18: XZ Plane,G19: YZ Plane

G20/G70: Inch units

G21/G71: Metric Units

G40: Cutter compensation cancel

G41: Cutter compensation left

G42: Cutter compensation right

G43: Tool length compensation (plus)

G44: Tool length compensation (minus)

G49: Tool length compensation cancel

G80: Cancel canned cycles

G81: Drilling cycle

G82: Counter boring cycle

G83: Deep hole drilling cycle

G90: Absolute positioning

G91: Incremental positioning

Table of Important M codes

M Codes are instructions describing miscellaneous functions like calling the tool, spindle rotation, coolant on/off etc.,

Table of Important M codes

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