Unit 3 Computer Numerical Control

Industrial Control andAutomation

Unit-3 Computer NumericalControl

Contents

• Introduction to CNC Systems

• Types –Analogue & Digital CNC System

• Absolute and Incremental

• Open Loop System and Closed Loop System

• CNC Drives

• Classification and Operation

• Feedback Devices-Adaptive Control

• CNC Part Programming.

19-09-2016 [email protected] 2

Introduction to CNC Systems

• CNC is defined as an NC system whose microcontroller isbased on a dedicated microcomputer rather than on a hard-wired controller.

• The numerical data required to produce a part is providedto a machine in the form of program, called part program orCNC (computer numerical control)

• The program is translated into the appropriate electricalsignals for input to motors that run the machine.


Milestones in the development of CNC Machine

1775• John Wilkinson- cannon boring machine (lathe)

1881•Eli Whitney- milling machine

1947•Mr. John Parsons began experimenting for using 3-axis curvature data to

control the machine tool motion for the production for aircraft components.

1949•parsons- first NC machine

1951•MIT was involved in the project

1955•after refinements NC became available in industry

2016•Today, modern machinery are CNC milling machines and lathes.


Features of CNC Machine

• Storage of more than one part program

• Various form of program input

• Program editing in machine tool

• Fixed cycles and program subroutines

• Interpolation

• Positioning feature for setup

• Cutter length and tool compensation

• Acceleration and deceleration calculation

• Communication interface


• Diagnostic tools• Control start-up diagnostics

• malfunction and failure analysis

• Extended diagnostics for individual components

• Tool life monitoring

• Preventive maintenance notices

• Programming diagnostics


Machine control unit for CNC


Subsystems of MCU for CNC

• Central Processing Unit (CPU)Control section

Arithmetic-logic unit

Immediate Access Memory

• Memory

• Input/ Output Interface

• Control for Machine Tool Axes and Spindle Speed

• Sequence Control for other machine tool function

• Personal Computer and the MCU


Analysis on Positioning Systems

• The NC positioning system converts the coordinateaxis values in the NC part program into relativepositions of the tool and work part during processing.

• The worktable moves linearly by means of a rotatinglead screw, which is driven by a stepping motor orservomotor.

• The velocity of the worktable, which corresponds tothe feed rate in a machining operation, is determinedby the rotational speed of the lead screw,


• There are two types of positioning systems used in NC systems:

(a) open loop and

(b) closed loop,


Open Loop Positioning System

• Operates without verifying the actual positionachieved in the move is same as the desiredposition

• Cost is less than the Closed loop system and areappropriate when the force resisting the actualmotion is minimal


• It uses a stepping motor to rotate the leadscrew a stepping motor is driven by a series ofelectrical pulses, which are generated by theMCU in an NC system.

• Each pulse causes the motor to rotate a fractionof one revolution called the step angle.

• The possible step angles must be consistentwith the following relationship:


• The angle through which the motor shaft rotates is given by

• The motor shaft is generally connected to the lead screw through a gear box, which reduces the angular rotation of the lead screw.


• The linear movement of the worktable is given by the number of full and partial rotations of the lead screw multiplied by its pitch:

• Control pulses are transmitted from the pulsegenerator at a certain frequency, which drives theworktable at a corresponding velocity or feed rate inthe direction of the lead screw axis.

• The rotational speed of the lead screw depends on thefrequency of the pulse train .

• The table travel speed in the direction of lead screwaxis is determined by the rotational speed.


Closed Loop System

• It uses feedback measurements to confirm that thefinal position of the worktable is the locationspecified in the program.

• Mostly used for continuous path operations such asmilling or turning, in which there are significantforces resisting the forward motion of the cuttingtool


• It uses servomotors and feedback measurements toensure that the worktable is moved to the desiredposition

• Commonly used feed back sensor is OpticalEncoder

• optical encoder consists of a light source and aphoto detector on either side of a disk

• The disk contains slots uniformly spaced aroundthe outside of its face. These slots allow the lightsource to shine through and energize the photodetector.


• The flashes are converted into an equal number of electrical pulses.

• By counting the pulses and computing the frequency of the pulse train, worktable position and velocity can be determined


• In the basic optical encoder, the angle between slots in the disk must satisfy the following requirement:

• The pulse count can he used to determine the linear x-axis position of the worktable by factoring in the leadscrew pitch and the gear reduction between theencoder shaft and the lead screw.

• The velocity of the worktable, which is normally thefeed rate in a machining operation, is obtained fromthe frequency of the pulse train.


• The pulse train generated by the encoder is comparedwith the coordinate position and feed rate specified inthe part program and the difference is used by theMCV to drive a servomotor, which in turn drives theworktable.

• A digital-to-analog converter converts the digitalsignals used by the MCU into a continuous analogcurrent that powers the drive motor.

• Metal cutting machine tools that perform continuouspath cutting operations, such as milling and turning,fall into this category


CNC Software• Operating System Software- to interpret the NC part programs and

generate the corresponding control signals to drive the machine tool

axes.

1. Editor -which permits the machine operator to input and edit NC part

programs and perform other file management functions

2. Control Program- which decodes the part program instructions, performs

interpolation and acceleration /deceleration calculations, and

accomplishes other related functions to produce the coordinate control

signals for each axis.

3. Executive Program-which manages the execution of the CNC software as

well as the I/O operations of the MCU.


• Machine Interface Software-to operate the communication link between the

CPU and the machine tool to accomplish the CNC auxiliary functions

• Application Software- applications in the user's plant.


Driving System

• It must response accurately according to the programmed instruction

• Mostly electric motor for small tools and hydraulics for large tools

• Motor is coupled directly to the gear box to the machine lead screw to

slide or spindle

• Four types of electrical driving system used are

DC Servo Motor

AC Servo Motor

Stepping Motor

Linear Motor


DC Servo Motor


AC Servo Motor


Stepping Motor


Linear Motor


Feed back Devices

• In order to operate accurately machine should be updated with good positional and velocity feedback devices.

• Positional Feedback Devices1. Linear Transducer

2. Rotary Encoder

• Velocity Feed back Devices


Encoders


Tachometer


CNC Part Programming

• planning and documenting the sequence of processingsteps to be performed on an NC machine.

• Traditional input medium dating back to the first NCmachines in 1950s is l-inch wide punched tape

• More recently the use of magnetic tape and floppydisks were used due to high data intensity.

• The methods are:1. manual part programming,

2. computer-assisted part programming,

3. part programming using CAD/CAM. and

4. manual data input.


NC Coding System

• Low Level Languages- Binary Number

- Binary Coded Decimal

• High Level Languages-APT( Automated Programming tools)


EIA and ISO Codes



Types of Axes

• 3 Linear Axis

• 3 Rotational Axis


Dimension system

• Incremental System• Sequence of Points

• Error will be accumulated


• Absolute System• All the reference are made to origin


Definition of Programming

“all machining data is compiled and where the data are translated into language which can be

understood by the control system of machine tool”

Machine Sequence- classification of process, tool start point, cutting depth , tool path etc.,

Cutting Condition spindle speed, feed rate, coolant, etc

Selecting of cutting tools


Programming Structure



Commonly Used Functions








Steps for Part Programming



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Unit 3 Computer Numerical Control