INTRODUCTION TO CNC MACHINES

• Controlling a machine tool by means of prepared program, which consists of blocks, or series of commands/numbers, is known as numerical control. Numerical Control [NC] for machine tools was introduced in 1950 by Prof. John T Parsons. The first CNC machine was built at the Massachusetts institute of technology [MIT] in 1953 by joint efforts of US Air force, MIT and the Parsons Corporation.

The CNC machine basically entails three main regions:-• The control system• The drives (driving elements) and • The machine tool

FUNCTIONS OF A CONTROL SYSTEM

Some important functions, controlled by control system.• Displacement of machine slides• Angular rotation of circular table• Start / Stop of the main spindle• Changing of spindle speed• Reverse spindle direction• Changing the feed rate of machine slides• Rotate tool turret• Change tool • Coolant ON / OFF• Lock table in position

ADVANTAGES OF CNC TECHNOLOGY

• REDUCED LEAD TIME• GREATER FLEXIBILITY• USER WRITTEN PROGRAMS• PROGRAM EDITING AT THE MACHINE SITE

CONSTRUCTIONAL FEATURES OF CNC MACHINES

• Slideways • Spindle drive• Feed drives• Position measuring transducers• Tool magazines and automatic tool changers• Automatic work changers• Lubrication system• Coolant system• Chip handling system

CNC SYSTEM FEATURES

• A CNC system can be interfaced to machine tools in order to effect control over machine functions. In order to perform these functions, a CNC system is characterized by a number of features.

• Control system features• Memory features• Programming features• PLC features• Diagnostic features

CLASSIFICATION OF CNC SYSTEMS

The CNC system can be classified on the basis of • Control of path motions • Feed backCLASSIFICATION BASED ON CONTROL OF PATH MOTIONS• Point to Point control system• Continuos path control systemCLASSIFICATION BASED ON FEEDBACK SYSTEM • Open loop control system• Closed loop control system

Block Diagram of a typical CNC system

Set of commands

Peripheral to Machine (PMI)Interface

Operator Control Paneland CRT

MicroprocessorMemory andAssociated H / W

Panel Interface

Axis interface

I / O interface

Axis Drive

Feedback

M

To electrical panel of machine

ELEMENTS OF A CNC SYSTEM

The basic elements of a CNC system are• Control panel• Control circuits • Power supply

In addition to the above basic elements the CNC system must be interfaced with following devices.

• Driving devices• Feedback devices

THE FAGOR 8030 MGI CNC SYSTEM

The FAGOR series of CNC control systems are of high accuracy, low cost, high performance and fixed software with high-speed microprocessors and semiconductor devices.

FAGOR 8030 MGI CNC system is basically contains following PCBs •Power supply PCB•CRT PCB•The main CPU PCB•Input/output PCB•Feedback adaptor PCB•Keyboard

Features of FAGOR CNC system

• Axis control• Operation• Programming features• Other features

Block diagram of FAGOR 8030 CNC

CENTRAL UNIT MONITOR/KEYBOARD

OPERATOR PANEL

DRIVES

Drives are devices by which the speed of a motor can be controlled. These are essential for the CNC machines for precise axis movement and spindle rotation. ELECTRONICALLY CONTROLLED DRIVE

SPEEDREGULATOR

CURRENTREGULATOR

FIRINGCIRCUIT

POWERBRIDGE M

FIELD

T

Ref. Signal (from CNC) AC inputVelocity feedback

PROGRAMMABLE LOGIC CONTROLLER

Programmable logic controller (PLC System) is a computer system, basically designed to perform logical decision making for industrial control applications. The PLC system is composed of electronic circuitry with a microprocessor centered. In PLC systems, the construction of the controller and wiring are independent of program execution.