Unit 3 Computer Numerical Control

Industrial Control and
Automation
Unit-3 Computer Numerical
Control

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 suryaprakash.s.eee@kct.ac.in 2

Introduction to CNC Systems
• CNC is defined as an NC system whose microcontroller is
based on a dedicated microcomputer rather than on a hard-
wired controller.
• The numerical data required to produce a part is provided
to a machine in the form of program, called part program or
CNC (computer numerical control)
• The program is translated into the appropriate electrical
signals 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 coordinate
axis values in the NC part program into relative
positions of the tool and work part during processing.
• The worktable moves linearly by means of a rotating
lead screw, which is driven by a stepping motor or
servomotor.
• The velocity of the worktable, which corresponds to
the feed rate in a machining operation, is determined
by 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 position
achieved in the move is same as the desired
position
• Cost is less than the Closed loop system and are
appropriate when the force resisting the actual
motion is minimal

• It uses a stepping motor to rotate the lead
screw a stepping motor is driven by a series of
electrical pulses, which are generated by the
MCU in an NC system.
• Each pulse causes the motor to rotate a fraction
of one revolution called the step angle.
• The possible step angles must be consistent
with 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 pulse
generator at a certain frequency, which drives the
worktable at a corresponding velocity or feed rate in
the direction of the lead screw axis.
• The rotational speed of the lead screw depends on the
frequency of the pulse train .
• The table travel speed in the direction of lead screw
axis is determined by the rotational speed.

Closed Loop System
• It uses feedback measurements to confirm that the
final position of the worktable is the location
specified in the program.
• Mostly used for continuous path operations such as
milling or turning, in which there are significant
forces resisting the forward motion of the cutting
tool

• It uses servomotors and feedback measurements to
ensure that the worktable is moved to the desired
position
• Commonly used feed back sensor is Optical
Encoder
• optical encoder consists of a light source and a
photo detector on either side of a disk
• The disk contains slots uniformly spaced around
the outside of its face. These slots allow the light
source to shine through and energize the photo
detector.

• 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 lead
screw pitch and the gear reduction between the
encoder shaft and the lead screw.
• The velocity of the worktable, which is normally the
feed rate in a machining operation, is obtained from
the frequency of the pulse train.

• The pulse train generated by the encoder is compared
with the coordinate position and feed rate specified in
the part program and the difference is used by the
MCV to drive a servomotor, which in turn drives the
worktable.
• A digital-to-analog converter converts the digital
signals used by the MCU into a continuous analog
current that powers the drive motor.
• Metal cutting machine tools that perform continuous
path 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 Devices
1. Linear Transducer
2. Rotary Encoder
• Velocity Feed back Devices

Encoders

Tachometer

CNC Part Programming
• planning and documenting the sequence of processing
steps to be performed on an NC machine.
• Traditional input medium dating back to the first NC
machines in 1950s is l-inch wide punched tape
• More recently the use of magnetic tape and floppy
disks 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

For feed back and suggestion mail me to
suryaprakash.s.eee@kct.ac.in

Unit 3 Computer Numerical Control

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