Numerical control (NC) machine - Introduction, working, elements

Numerical Control Machine – 

NC machine stand for numerical control machine
Numerical control machine may be defined as a method of automation in which machine tools is compelled by letters, numbers and symbols. Combinations of these letters, number and symbol is called program. The program consists instructions about the process of manufacture and movements of tool. Programme describes tool path, feed, speed etc. of the tool. Since program is the controlling point for job manufacturing, the machine becomes versatile and can be used for any part. Functions of any cnc machine tool is controlled through electronically, hydraulically and pneumatically.

Classification of NC machine-

NCsystem can be divided in 3 parts based on their position              control system. These three systems are as below-
(1) point to point system (2) straight cut system (3) contouring system


 (1) point to point system-  This is the simplest control system of NC machine in it, tool travel                     parallel to one axis ( x or y axis ) by using, this control system simple machining operation                 like drilling operation.
(2)  Straight cut / straight live system - In this system, tool travels along with a straight live in any             particular axis. Straight cut milling is done by this system. Some of uses machining  live face               milling, pocket cutting, step turning is also control by this system.

(3) Contouring system - In this control system, tool moves according to required shape of job.                  Tool  job both will be in inter-related motions on different co-ordinate  axis. In this system,                   control unit monitor all the movement of machine by closed loop feed mechanism. This system          is used to design complex job shape & size.

 Main Element of NC machine-

Main parts of NC machine is as below -
 (1) Input unit- used to send command from operator to control unit of machine. Input unit                      change the signal into digital from by using A/D converters. Input unit stored the data in the                paper  tape till not required of the machine control unit. Instructions is fed up in machine in                  the form of programme.
 (2) Memory unit- It stored the data received from input in the form of code is called programme.                                                                                                                         RAM is used to store data.
 (3) Punched Tape- It is used to store the data in machine magnetic tape, disk is used in magnetic                 device
(4) Control unit- This unit receives the instructions from the memory unit of machine and inter                 prates them. This is the main part of NC machine. It process the all data receive from input unit           to do work operations. It controls the machine speed.

(5) Algorithm unit- This is main unit of the machine.

 (6) Output unit- It receive the data from memory unit on the permission of control unit and then                convert digital signal into analogue then control the servomotors axis drive. Output unit is mainly          used to move positions of axis.

CNC Machine



Control System of NC machine-

Two types of control system is used in NC machine which are as below-

(1) Open Loop System
(2) Closed Loop System

(1) Open Loop System - This type machine system has controlled on the movement of slide. No feed back system is present in open loop system. So it is not possible to put slide position right position through feed drive. In this system we can not compere the output of machine with input. In this system, tape reader reads the all instructions which are stored in taper through decoding. Then tape reader converts two all information into electrical signal. This signal is send to machine control unit. Machine control unit sends signal according to instructions / programme to the driving control unit. Driving motor rotates according to electrical pulses signal. Machine table is rotated with the help of lead serew. Every electrical signal rotates the pulses motor by friction that is called stepping angle. Machine table is also controlled by these electrical pulses signal. Open loop system is used in point to point system.
(2) Closed loop system-
                                                This system has feedback control system. Closed loop control system is used high setting accurated machine. Slide displacement is measured by a monitoring device. Slide position is regulated by servo system when it comes in required position and then compare with input information. Machine table's displacement is compared with input through feedback sensor. In closed loop system AC. DC and hydraulic servo motor is used to work

Terms Used in cnc machine programming-