1

INDUSTRIAL TRAINING REPORT

COMPUTER NUMERICALLY CONTROLLED MACHINES undertaken at

BHARAT HEAVY ELECTRICALS LTD.

(GOINDWAL,TARN TARAN)

Submitted by

SARAVPREET SINGH DHILLON

UE 95083

Under the Guidance of

Mr.Sanjeev Kumar Mr.Tejinder Singh

HOD

Department of Electronics & Communication Engineering

UNIVERSITY INSTITUTE OF ENGINEERING & TECHNOLOGY

PANJAB UNIVERSITY, CHANDIGARH

JUNE -2013

2

DECLARATION

I hereby declare that the project work entitled COMPUTER NUMERICALLY CONTROLLED

MACHINES is an authentic record of my own work carried out at BHEL (GO+NDWAL) as

requirements of six months Industrial Training for the award of the degree of B.E. at University

Institute of Engineering & Technology, Panjab University, Chandigarh under the guidance of

Mr.Tejinder Singh and Mr.Sanjeev, during January 4,2013 to July 3, 2013

SARAVPREET SINGH DHILLON

UE 95083

Date : June 12, 2013

Certified that the above statement made by the student is correct to the best of our knowledge

and belief.

Mr.Sanjeev Kumar Mr.Tejinder Singh

HOD

+

+

3

INDEX

OVERVIEW OF THE ORGANISATION 1 INTRODUCTION 1

POWER TRANSMISSION & DISTRIBUTION (T & D) 3

INDUSTRIES 4

TRANSPORTATION 4

RENEWABLE ENERGY 5

MISSION 5

INTERNATIONAL OPERATIONS 6

TECHNOLOGY UPGRADATION AND R&D 6

PROJECT REPORT

Introduction 9

Configuration of CNC Systems

12

Input Device 13

Machine Control Unit 15

Machine Tool 15

Driving System 16

Feedback Systems 17

Display Unit 21

CNC PART PROGRAMMING 23

PROGRAMMING SYSTEMS 24

CNC POSITIONING SYSTEM 27

INTERPOLATION 29

PROGRAMMING FORMAT 31

STEPS FOR CNC PROGRAMMING 37

ELECTRONICS OF A CNC MACHINE 41

CNC CONTROLLER COMPONENTS SETUP 42

PROGRAMMABLE LOGIC CONTROLLER 44

4

ANALYSIS AND PROGRAMMING OF A VERTICAL MACHINING

CENTER 48

MACHINE SPECIFICATIONS 51

PROGRAMM USED IN MANUFACTURING OF TURBINE COUPLINGS 53

STEPS IN USING THE MACHINE 55

ADVANTAGES OF CNC MACHINES 56

REFERENCE 57

5

BHARAT HEAVY ELECTRICALS LTD.

BHEL was established more than 50 years ago when its first plant was setup in Bhopal ushering

in the indigenous Heavy Electrical Equipment Industry in India. A dream which has been more

than realized with a well recognized track record of performance it has been earning profits

continuously since 1971-72 and achieved a turnover of Rs 2,658 crore for the year 2007-08,

showing a growth of 17 per cent over the previous year. Bharat Heavy Electricals Limited is

country’s ‘Navratna’ company and has earned its place among very prestigious national and

international companies. It finds place among the top class companies of the world for

manufacture of electrical equipments.

6

BHEL caters to core sectors of the Indian Economy viz., Power Generation's & Transmission,

Industry, Transportation, Telecommunication, Renewable Energy, Defense, etc. BHEL has

already attained ISO 9000 certification for quality management, and ISO 14001 certification for

environment management and OHSAS – 18001 certification for Occupational Health and Safety

Management Systems. The Company today enjoys national and international presence featuring

in the “Fortune International -500” and is ranked among the top 10 companies in the world,

manufacturing power generation equipment. BHEL is the only PSU among the 12 Indian

companies to figure in “Forbes Asia Fabulous 50” list.

BHARAT HEAVY ELECTRICALS LTD.

An Overview:

7

BHEL today is the largest Engineering Enterprise of its kind in India with excellent track record

of performance, making profits continuously since 1971-72.

BHEL's vision is to become a world-class engineering enterprise, committed to enhancing

stakeholder value. The company is striving to give shape to its aspirations and fulfill the

expectations of the country to become a global player.

BHEL business operations cater to core sectors of Indian Economy like.

Power

Industry

Transportation

Transmission

Defenses etc.

The greatest strength of BHEL is its highly skilled and committed employees. Every

participative style of management all these have engendered development of a committed and

motivated workforce setting new benchmarks in terms of productivity, quality and

responsiveness. employee is given an equal opportunity to develop himself and grow in his

career. Continuous training and retraining, career planning, a positive work culture.

POWER TRANSMISSION & DISTRIBUTION (T & D)

BHEL offer wide ranging products and systems for T & D applications. Products

manufactured include power transformers, instrument transformers, dry type transformers, series

– and stunt reactor, capacitor tanks, vacuum – and SF circuit breakers gas insulated switch gears

and insulators.

A strong engineering base enables the Company to undertake turnkey delivery of electric

substances up to 400 kV level series compensation systems (for increasing power transfer

capacity of transmission lines and improving system stability and voltage regulation), shunt

compensation systems (for power factor and voltage improvement) and HVDC systems (for

economic transfer of bulk power). BHEL has indigenously developed the state-of-the-art

8

controlled shunt reactor (for reactive power management on long transmission lines). Presently a

400 kV Facts (Flexible AC Transmission System) project under execution.

INDUSTRIES

BHEL is a major contributor of equipment and systems to industries, cement, sugar, fertilizer,

refinances, petrochemicals, paper, oil and gas, metallurgical and other process industries. The

range of system & equipment supplied includes: captive power plants, co-generation plants DG

power plants, industrial steam turbines, industrial boilers and auxiliaries. Water heat recovery

boilers, gas turbines, heat exchangers and pressure vessels, centrifugal compressors, electrical

machines, pumps, valves, seamless steel tubes, electrostatic precipitators, fabric filters, reactors,

fluidized bed combustion boilers, chemical recovery boilers and process controls.

TRANSPORTATION

BHEL is involved in the development design, engineering, marketing, production, installation,

and maintenance and after-sales service of Rolling Stock and traction propulsion systems. In the

area of rolling stock, BHEL manufactures electric locomotives up to 5000 HP, diesel-electric

locomotives from 350 HP to 3100 HP, both for mainline and shunting duly applications. BHEL

is also producing rolling stock for special applications viz., overhead equipment cars, Special

well wagons, Rail-cum-road vehicle etc., Besides traction propulsion systems for in-house use,

BHEL manufactures traction propulsion systems for other rolling stock producers of electric

locomotives, diesel-electric locomotives, electrical multiple units and metro cars. The electric

and diesel traction equipment on India Railways are largely powered by electrical propulsion

systems produced by BHEL. The company also undertakes retooling and overhauling of rolling

stock in the area of urban transportation systems. BHEL is geared up to turnkey execution of

electric trolley bus systems, light rail systems etc. BHEL is also diversifying in the area of port

handing equipment and pipelines transportation system.

9

RENEWABLE ENERGY

Technologies that can be offered by BHEL for exploiting non-conventional and renewable

sources of energy include: wind electric generators, solar photovoltaic systems, solar lanterns

and battery-powered road vehicles. The Company has taken up R&D efforts for development of

multi-junction amorphous silicon solar cells and fuel based systems.

MISSION

The leading Indian engineering enterprise providing quality products systems and

services in the fields of energy, transportation, infrastructure and other potential areas.

The design and construction of Computer Numerically Controlled(CNC) machines differs

greatly from that of conventional machine tools.This difference arises from the requirements of

higher performance levels.The CNC machines often employ the various mechatronics elements

thathave been developed over the years. However, the quality and reliability of these machines

depends on the various machine elements and subsystems of the machines. There are some of the

important constituents parts and aspectsof CNC machines to be considered in their designing, for

example Machinestructure, Guideways, Feed drives, Spindle and Spindle bearings,

Measuringsystems, Controls, Software and Operator interface, Gauging, Toolmonitoring.The

control of a machine tool by means of stored information

throughthe computer is known as Computer Numerically Controlled. Theinformation stored in

the computer can be read by automatic means andconverted into electrical signals, which operate

the electrically controlledservo systems. Electrically controlled servo systems permits the slides

of amachine tool to be driven simultaneously and at the apporopriate feeds anddirection so that

complex shapes can be cut, often with a single operationand without the need to reorient the

workpiece.Computer Numerically Control can be applied to milling

machines,Lathe machines, Grinding machines, Boring machines, Flame cutters,Drilling

machines etc.

10

International Operations

BHEL has, over the years, established its references in around 60 countries of the world,

ranging for the United States in the West to New Zealand in the Far East. These references

encompass almost the entire product range of BHEL, covering turnkey power projects of

thermal, hydro and gas-based types, substation projects, rehabilitation projects, besides a wide

variety of products, like transformers, insulators, switchgears, heat exchangers, castings and

forgings, valves, well-head equipment, centrifugal compressors, photo-voltaic equipment etc.

Apart from over 1110MW of boiler capacity contributed in Malaysia, and execution of four

prestigious power projects in Oman, Some of the other major successes achieved by the

Company have been in Australia, Saudi Arabia, Libya, Greece, Cyprus, Malta, Egypt,

Bangladesh, Azerbaijan, Sri Lanka, Iraq etc.

The Company has been successful in meeting demanding customer's requirements in terms

of complexity of the works as well as technological, quality and other requirements viz extended

warrantees, associated O&M, financing packages etc. BHEL has proved its capability to

undertake projects on fast-track basis. The company has been successful in meeting varying

needs of the industry, be it captive power plants, utility power generation or for the oil sector

requirements. Executing of Overseas projects has also provided BHEL the experience of

working with world renowned Consulting Organisations and inspection Agencies.

In addition to demonstrated capability to undertake turnkey projects on its own, BHEL

possesses the requisite flexibility to interface and complement with International companies for

large projects by supplying complementary equipment and meeting their production needs for

intermediate as well as finished products.

Technology Upgradation and Research & Development

To remain competitive and meet customers' expectations, BHEL lays great emphasis on

the continuous upgradation of products and related technologies, and development of new

products. The Company has upgraded its products to contemporary levels through continuous in

11

house efforts as well as through acquisition of new technologies from leading engineering

organizations of the world. The Corporate R&D Division at Hyderabad, spread over a 140 acre

complex, leads BHEL's research efforts in a number of areas of importance to BHEL's product

range. Research and product development centers at each of the manufacturing divisions play a

complementary role.

BHEL's Investment in R&D is amongst the largest in the corporate sector in India.

Products developed in-house during the last five years contributed about 8.6% to the revenues in

2000-2001.

BHEL has introduced, in the recent past, several state-of-the-art products developed in-

house: low-NQx oil / gas burners, circulating fluidized bed combustion boilers, high-efficiency

Pelton hydro turbines, petroleum depot automation systems, 36 kV gas-insulated sub-stations,

etc. The Company has also transferred a few technologies developed in-house to other Indian

companies for commercialization.

12

PROJECT REPORT

CNC SYSTEMS

The definition of CNC given by Electronic Industry Association (EIA) is as follows:

“A system in which actions are controlled by the direct insertion of numerical

data at some point. The system must automatically interpret at least some

portion of this data.”

In a simple word, a CNC system receieves numerical data, interpret the data and then control the

action accordingly.

A Vertical Machining Center

13

INTRODUCTION

Numerical control (NC) is a method employed for controlling the motions of a machine tool

slide and its auxiliary functions with input in the form of numerical data. A computer numerical

control (CNC) is a microprocessor-based system to store and process the data for the control of

slide motions and auxiliary functions of the machine tools. The CNC system is the heart and

brain of a CNC machine which enables the operation of various machine members such as slides,

spindles, etc. as per the sequence programmed into it, depending on the machining operations.

The main advantage of a CNC system lies in the fact that the skills of the operator hitherto

required in the operation of a conventional machine is removed and the part production is made

automatic.

The CNC systems are constructed with a NC unit integrated with a programmable logic

controller (PLC) and some times with an additional external PLC (non-integrated). The NC

controls the spindle movement and the speeds and feeds in machining. It calculates the traversing

path of the axes as defined by the inputs. The PLC controls the peripheral actuating elements of

the machine such as solenoids, relay coils, etc. Working together, the NC and PLC enable the

machine tool to operate automatically. Positioning and part accuracy depend on the CNC

system's computer control algorithms, the system resolution and the basic mechanical machine

accuracy. Control algorithm may cause errors while computing, which will reflect during

contouring, but they are very negligible. Though this does not cause point to point positioning

error, but when mechanical machine inaccuracy is present, it will result in poorer part accuracy.

Computer Numerical Control (CNC) is a specialized and versatile form of Soft

Automation and its applications cover many kinds, although it was initially developed to

control the motion and operation of machine tools.

Computer Numerical Control may be considered to be a means of operating a machine through

the use of discrete numerical values fed into the machine, where the required 'input' technical

information is stored on a kind of input media such as floppy disk, hard disk, CD ROM, DVD,

USB flash drive, or RAM card etc. The machine follows a predetermined sequence of

machining operations at the predetermined speeds necessary to produce a work piece of the

right shape and size and thus according to completely predictable results. A different product can

14

be produced through reprogramming and a low-quantity production run of different products is

justified.

Hitachi Seiki VA35 CNC Machining Center

15

Hitachi Seiki VA35 CNC Machining Center

16

CONFIGURATION OF THE CNC SYSTEM

Fig. shows a schematic diagram of the working principle of a NC axis of a CNC

machine and the interface of a CNC control.

CNC system

Fig.1 Schematic diagram of a CNC machine tool

A CNC system consists of the following 6 major elements:

a. Input Device

b. Machine Control Unit

c. Machine Tool

d. Driving System

e. Feedback Devices

f. Display Unit

N

C

P

L

C

Servo

Drive

Servo

Motor

Spindle

Head

Work

piece

Table

Encoder

Position Feedback

Tacho

Generator Velocity

Feedbac

k

Tape Reader

Tape Punch

Other Devices

Machine

Elements

Inputs

Output

s

Lead

Screw

Command

value

Proximity

switches

Limit switches

Relay coils

Pressure switches

Float switches

17

Input Devices

a. Floppy Disk Drive

Floppy disk is a small magnetic storage device for CNC data input. It has been the most common

storage media up to the 1970s, in terms of data transfer speed, reliability, storage size, data

handling and the ability to read and write. Furthermore, the data within a floppy could be

easily edited at any point as long as you have the proper program to read it.

However,this method has proven to be quite problematic in the long run as floppies have a

tendency to degrade alarmingly fast and are sensitive to large magnetic fields and as well as the

dust and scratches that usually existed on the shop floor.

b. USB Flash Drive

A USB flash drive is a removable and rewritable portable hard drive with compact

size and bigger storage size than a floppy disk. Data stored inside the flash drive are impervious

to dust and scratches that enable flash drives to transfer data from

place to place. In recent years, all computers support USB flash drives to read and

write data that make it become more and more popular in CNC machine control unit.

c. Serial communication

The data transfer between a computer and a CNC machine tool is often accomplished

through a serial communication port. International standards for serial communications are

established so that information can be exchanged in an orderly way. The most common interface

between computers and CNC machine tools is referred to the EIA Standard RS-232. Most of

the personal computers and CNC machine tools have built in RS232 port and a standard RS-232

cable is used to connect a CNC machine to a computer which enables the data transfer in

reliable way. Part programs can be downloaded into the memory of a machine tool or

USB Flash Drive

18

uploaded to the computer for temporary storage by running a communication program on

the computer and setting up the machine control to interact with the communication software.

SerialCommunication in a Distributed Numerical Control System

Direct Numerical Control is referred to a system connecting a set of numerically

controlled machines to a common memory for part program or machine program

storage with provision for on-demand distribution of data to the machines. (ISO

2806:1980) The NC part program is downloaded a block or a section at a time into the

controller. Once the downloaded section is executed, the section will be discarded to

leave room for other sections. This method is commonly used for machine tools that do not have

enough memory or storage buffer for large NC part programs.

Distributed Numerical Control is a hierarchical system for distributing data between a

production management computer and NC systems. (ISO 2806:1994) The host computer is

linked with a number of CNC machines or computers connecting to the CNC machines

for downloading part programs. The communication program in the host computer can

utilize two-way data transfer features for production data communication including: production

schedule, parts produced and machine utilization etc.

19

Machine Control Unit (MCU)

The machine control unit is the heart of the CNC system. There are two sub-units in the machine

control unit: the Data Processing Unit (DPU) and the Control Loop Unit (CLU).

a. Data Processing Unit

On receiving a part programme, the DPU firstly interprets and encodes the part programme into

internal machine codes. The interpolator of the DPU then calculate the intermediate

positions of the motion in terms of BLU (basic length unit) which is the smallest unit length that

can be handled by the controller. The calculated data are passed to CLU for further action.

b. Control Loop Unit

The data from the DPU are converted into electrical signals in the CLU to control the driving

system to perform the required motions. Other functions such as machine spindle ON/OFF,

coolant ON/OFF, tool clamp ON/OFF are also controlled by this unit according to the internal

machine codes.

Machine Tool

This can be any type of machine tool or equipment. In order to obtain high accuracy and

repeatability, the design and make of the machine slide and the driving lead screw of a CNC

machine is of vital importance. The slides are usually machined to high accuracy and coated with

anti-friction material such as PTFE and Turcite in order to reduce the stick and slip phenomenon.

Large diameter recirculating ball screws are employed to eliminate the backlash and lost motion.

Other design features such as rigid and heavy machine structure; short machine table overhang,

20

quick change tooling system, etc also contribute to the high accuracy and high repeatability

of CNC machines.

Different Machine Tools

Driving System

The driving system is an important component of a CNC machine as the accuracy and

repeatability depend very much on the characteristics and performance of the driving system.

The requirement is that the driving system has to response accurately according to the

programmed instructions. This system usually uses electric motors although hydraulic motors are

sometimes used for large machine tools. 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.

Stepping Motor

A stepping motor is a device that converts the electrical pulses into discrete mechanical

rotational motions of the motor shaft. This is the simplest device that can be applied to CNC

21

machines since it can convert digital data into actual mechanical displacement. It is not

necessary to have any analog-to-digital converter nor feedback device for the control system.

They are ideally suited to open loop systems.

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

drawbacks: slow speed, low torque, low resolution and easy to slip in case of overload. Examples

of stepping motor application are the magnetic head of floppy-disc drive and hard disc drive of

computer, daisy-wheel type printer, X-Y tape control, and CNC EDM Wire-cut machine.

Stepping Motor

FEEDBACK SYSTEMS

Following are the two types of control systems used in the CNCmachines

1. Open loop control system.

2. Closed loop control system.

1. Open loop control system:-

22

In an open loop system the machine slides are displacedaccording to the information loaded from

the part program into thecontrol system. Hence there is no measurement of slide position and

nofeedback signals for comparison with the input signal. The correctmovement of slide entirely d

epends upon the ability of the drivesystems to move the slide through the required exact

distance.The most common method of driving the lead screw is by a stepper motor. The stepper

motors are the simplest way for converting detailelectrical signals into proportional movement.

As there is no check onthe slide position, the system accuracy depends upon the motors abilityto

step through the exact number of steps provided at the input asshown in fig.

2. Closed loop control system:-

A closed loop system is as shown in fig. sends back a signal to

thecontrol unit from a measuring device called as transducer. Thetransducer is attached to the

slide ways. The signal indicates the actualmovement and position of the slides.The control unit

continues to adjust the position of the slide untilit arrives it’s destination, this system has

feedback. Although morecostly and complex than open loop system, these system gives

moreaccurate positioning. For this type of system, servomotors are used

23

24

Feedback Device

In order to have a CNC machine operating accurately, the positional values and

speed of the axes need to be constantly updated. Two types of feed back devices

are normally used, positional feed back device and velocity feed back device.

a. Positional Feed Back Devices

There are two types of positional feed back devices: linear transducer for direct

positional measurement and rotary encoder for angular or indirect linear

measurement.

Linear Transducers - A linear transducer is 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. This device is considered to be of the highest accuracy and

also more expensive in comparison with other measuring devices mounted

on screws or motors.

Linear Transducer (Courtesy of Heidenhain)

Rotary Encoders - A rotary encoder is a device mounted at the end of the

motor shaft or screw to measure the angular displacement. This device

cannot measure linear displacement directly so that error may occur due to

the backlash of screw and motor etc. Generally, this error can be

compensated for by the machine builder in the machine calibration

process.

25

b. 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. DC tachometer is

essentially a small generator that produces an output voltage proportional to the

speed. The voltage generated is compared with the command voltage

corresponding to the desired speed. The difference of the voltages can is then

used to actuate the motor to eliminate the error.

Tachogenerator (Courtesy of Callan)

Display Unit

The Display Unit serves as an interactive device between the machine and the operator. When

the machine is running, the Display Unit displays the present status such as the position of

the machine slide, the spindle RPM, the feed rate, the part programmes, etc. In an advanced CNC

machine, the Display Unit can show the graphics simulation of the tool path so that part

programmes can be verified before the actually

machining. Much other important information about the CNC system can also displayed for

26

maintenance and installation work such as machine parameters, logic diagram of the programmer

controller, error massages and diagnostic data.

A Display Unit

27

CNC PART PROGRAMMING

Objectives:

To understand the Dimension Systems in CNC Part Programming.

To understand the structure of a CNC Part Programme.

To understand the G-codes and other functions of a CNC Part

Programme.

Axis of motion

In generally, all motions have 6 degrees of freedom. In other words, motion can

be resolved into 6 axes, namely, 3 linear axes (X, Y and Z axis) and 3 rotational

axes (A, B, and C axis).

Axis of Motion

x

Y

Z

28

Programming Systems

Two types of programming modes, the incremental system and

the absolute system, are used for CNC. Both systems have

applications in CNC programming, and no system is either right or

wrong all the time.

Positioning reference point Systems:

1- Incremental

2- Absolute

Most controls on machine tools today are capable of handling either incremental or

absolute programming.

Incremental program locations are always given as the distance

and direction from the immediately preceding point .

Command codes which tell the machine to move the table, spindle,

and knee are explained here using a vertical milling machine as

an example:

A workpiece dimensioned in the incremental system mode

29

• A “X plus” (X+) command will cause the cutting tool to be

located to the right of the last point.

• A “X minus” (X-) command will cause the cutting tool to be located

to the left of the last point.

• A “Y plus” (Y+) command will cause the cutting tool to be

located toward the column.

• A “Y minus” (Y-) will cause the cutting tool to be located away

from the column.

• A “Z plus” (Z+) command will cause the cutting tool or spindle

to move up or away from the workpiece.

• A “Z minus” (Z-) moves the cutting tool down or into the workpiece.

In incremental programming, the G91 command indicates to the

computer and MCU (Machine Control Unit) that programming is in

the incremental mode.

30

Absolute program locations are always given from a single fixed

zero or origin point (Fig. 7). The zero or origin point may be a

position on the machine table, such as the corner of the worktable

or at any specific point on the workpiece. In absolute dimensioning

and programming, each point or location on the workpiece is given

as a certain distance from the zero or reference point.

A workpiece dimensioned in the absolute system mode. Note: All dimensions are given

from a known point of referenc

• A “X plus” (X+) command will cause the cutting tool to be

located to the right of the zero or origin point.

• A “X minus” (X-) command will cause the cutting tool to be located

to the left of the zero or origin point.

• A “Y plus” (Y+) command will cause the cutting tool to be

located toward the column.

31

• A “Y minus” (Y-) command will cause the cutting tool to be located away from the

column.

In absolute programming, the G90 command indicates to the

computer and MCU that the programming is in the absolute mode.

Point-to-Point or Continuous Path

CNC programming falls into two distinct categories . The

difference between the two categories was once very distinct.

Now, however, most control units are able to handle both point-topoint

and continuous path machining. A knowledge of both programming

methods is necessary to understand what applications

each has in CNC.

CNC POSITIONING SYSTEM

Point-To-Point OR Positioning

Continuous Path OR Contouring

Point-to-Point Positioning

Point-to-point positioning is used when it is necessary to accurately

locate the spindle, or the workpiece mounted on the machine

table, at one or more specific Iocations to perform such

operations as drilling, reaming, boring, tapping, and punching (Fig.

9). Point-to-point positioning is the process of positioning from one

coordinate (XY) position or location to another, performing the

machining operation, and continuing this pattern until all the

operations have been completed at all programmed locations.

32

The path followed by point-to-point positioning to reach various programmed points

(machining locations) on the XY axis.

In Fig. above, point 1 to point 2 is a straight line, and the machine

moves only along the X axis; but points 2 and 3 require that

motion along both the X and Y axes takes place. As the distance

in the X direction is greater than in the Y direction, Y will reach its

15 position first, leaving X to travel in a straight line for the remaining

distance. A similar motion takes place between points 3 and 4.

Continuous Path (Contouring)

Contouring, or continuous path machining, involves work such as

that produced on a lathe or milling machine, where the cutting tool

is in contact with the workpiece as it travels from one programmed

point to the next. Continuous path positioning is the ability to

control motions on two or more machine axes simultaneously to

keep a constant cutter-workpiece relationship. The programmed

information in the CNC program must accurately position the

cutting tool from one point to the next and follow a predefined

accurate path at a programmed feed rate in order to produce the

form or contour required.(Fig. on next page)

33

Fig. 10 Types of contour

machining

(A) Simple

contour;

(B) complex

contour

Interpolation

The method by which contouring machine tools move from one

programmed point to the next is called interpolation. This ability to

merge individual axis points into a predefined tool path is built into

most of today’s MCUs. There are five methods of interpolation:

linear, circular, helical, parabolic, and cubic. All contouring controls

provide linear interpolation, and most controls are capable of both

linear and circular interpolation. Helical, parabolic, and cubic

34

interpolation are used by industries that manufacture parts which

have complex shapes, such as aerospace parts and dies for car

bodies.

Linear Interpolation

Linear Interpolation consists of any programmed points linked

together by straight lines, whether the points are close together or

far apart . Curves can be produced with linear interpolation

by breaking them into short, straight-line segments. This

method has limitations, because a very large number of points

would have to be programmed to describe the curve in order toproduce a contour

shape.

A contour programmed in linear interpolation requires the coordinate

positions (XY positions in two-axis work) for the start and

finish of each line segment. Therefore, the end point of one line or

segment becomes the start point for the next segment, and so on,

throughout the entire program.

An example of two-axis linear interpolation.

Circular Interpolation

The development of MCUs capable of circular interpolation has

greatly simplified the process of programming arcs and circles. To

program an arc (Fig. 12), the MCU requires only the coordinate

35

positions (the XY axes) of the circle center, the radius of the circle,

the start point and end point of the arc being cut, and the direction

in which the arc is to be cut (clockwise or counterclockwise) See

Fig. . The information required may vary with different MCUs.

For two-dimensional circular interpolation the MCU must be supplied with the XY axis,

radius, start point, end point, and direction of cut.

Programming Format

Word address is the most common programming format used for

CNC programming systems. This format contains a large number

of different codes (preparatory and miscellaneous) that transfers

program information from the part print to machine servos, relays,

micro-switches, etc., to manufacture a part. These codes, which

conform to EIA (Electronic Industries Association) standards, are

in a logical sequence called a block of information. Each block

should contain enough information to perform one machining

operation.

36

Word Address Format

Every program for any part to be machined, must be put in a format that the machine

control unit can understand. The format

used on any CNC machine is built in by the machine tool builder

and is based on the type of control unit on the machine. A variable-

block format which uses words (letters) is most commonly

used. Each instruction word consists of an address character,

such as X, Y, Z, G, M, or S. Numerical data follows this address

character to identify a specific function such as the distance, feed

rate, or speed value.

The address code G90 in a program, tells the control that all

measurements are in the absolute mode. The code G91, tells the

control that measurements are in the incremental mode.

Codes

The most common codes used when programming CNC machines

tools are G-codes (preparatory functions), and M codes

(miscellaneous functions). Other codes such as F, S, D, and T are

used for machine functions such as feed, speed, cutter diameter

offset, tool number, etc.

G-codes are sometimes called cycle codes because they refer to

some action occurring on the X, Y, and/or Z axis of a machine tool,

Fig. 13.

The G-codes are grouped into categories such as Group 01,

containing codes G00, G01, G02, G03. which cause some movement

37

of the machine table or head. Group 03 includes either

absolute or incremental programming, while Group 09 deals with canned cycles.

A G00 code rapidly positions the cutting tool while it is above the

workpiece from one point to another point on a job. During the

rapid traverse movement, either the X or Y axis can be moved

individually or both axes can be moved at the same time. Although

the rate of rapid travel varies from machine to machine, it ranges

between 200 and 800 in./min (5 and 20 m/min).

The G01, G02, and G03 codes move the axes at a controlled

feedrate.

• G01 is used for straight-line movement (linear interpolation).

• G02 (clockwise) and G03 (counterclockwise) are used for arcs

and circles (circular interpolation)

38

G- Codes

Group Code Function

01 G00 Rapid positioning

01 G01 Linear interpolation

01 G02 Circular interpolation clockwise (CW)

01 G03 Circular interpolation

counterclockwise (CCW)

06 G20* Inch input (in.)

06 G21* Metric input (mm)

G24 Radius programming (**)

00 G28 Return to reference point

00 G29 Return from reference point

G32 Thread cutting (**)

07 G40 Cutter compensation cancel

07 G41 Cutter compensation left

07 G42 Cutter compensation right

08 G43 Tool length compensation positive

(+) direction

08 G44 Tool length compensation minus (-)

direction

08 G49 Tool length compensation cancel

39

G84 Canned turning cycle (**)

03 G90 Absolute programming

03 G91 Incremental programming

(*) - on some machines and controls, these may be G70 (inch) and

G71 (metric)

(**) - refers only to CNC lathes and turning centers

M Codes-

Code Function

M00 Program stop

M01 Optional program stop

M02 Program end

M03 Spindle on clockwise

M04 Spindle on counterclockwise

M05 Spindle stop

M06 Tool change

M08 Coolant on

M09 Coolant off

M10 Clamps on

40

M11 Clamps off

M30 Program stop, reset to start

Block of Information

CNC information is generally programmed in blocks of five words.

Each word conforms to the EIA standards and they are written on

a horizontal line. If five complete words are not included in each

block, the machine control unit (MCU) will not recognize the

information, therefore the control unit will not be activated.

Using the example shown in Fig. 17 , the five words are as follows:

N001 represents the sequence number of the operation.

G01 represents linear interpolation

X12345 will move the table 1.2345 in. in a positive direction

along the X axis.

Y06789 will move the table 0.6789 in. along the Y axis.

M03 Spindle on CW.

A complete block of information consists of five words.

41

Steps for CNC Programming and Machining

The following is the procedures to be followed in CNC programming and

machining. The most important point is to verify the programme by test run it on

the machine before the actual machining in order to ensure that the programme

is free of mistakes.

a. Study the part drawing carefully.

b. Unless the drawing dimensions are CNC adapted, select a suitable

programme zero point on the work piece. The tool will be

adjusted to this zero point during the machine set up.

c. Determine the machining operations and their sequence.

d. Determine the method of work clamping (vice, rotary table,

fixtures etc).

e. Select cutting tools and determine spindle speeds and feeds.

f. Write programme (translate machining steps into programme

blocks). If many solutions are possible, try the simplest solution

first. It is usually longer, but better to proceed in this way.

g. Prepare tool chart or diagram, measure tool geometry (lengths,

radii) and note.

h. Clamp work piece and set up machine.

i. Enter compensation value if necessary.

j. Check and test programme. It is a good practice to dry run the

programme (i) without the workpiece, (ii) without the cutting

tools, or (iii) by raising the tool to a safe height.

If necessary, correct and edit programme and check again.

k. Start machining.

42

Programming for Positioning

Before starting to program a job, it is important to become familiar

with the part to be produced. From the engineering drawings, the

programmer should be capable of planning the machining sequences

required to produce the part. Visual concepts must be

put into a written manuscript as the first step in developing a part

program, Fig. 18. It is the part program that will be sent to the

machine control unit by the computer, tape, diskette, or other input

media.

The programmer must first establish a reference point for aligning

the workpiece and the machine tool for programming purposes.

The manuscript must include this along with the types of cutting

tools and work-holding devices required, and where they are to be

located.

Dimensioning Guidelines

The system of rectangular coordinates is very important to the

successful operation of CNC machines. Certain guidelines should

be observed when dimensioning parts for CNC machining. The

following guidelines will insure that the dimensioning language

means exactly the same thing to the design engineer, the technician,

the programmer, and the machine operator.

1. Define part surfaces from three perpendicular reference

planes.

2. Establish reference planes along part surfaces which are

parallel to the machine axes.

3. Dimension from a specific point on the part surface.

43

25

4. Dimension the part clearly so that its shape can be understood

without making mathematical calculations or guesses.

5. Define the part so that a computer numerical control cutter

path can be easily programmed.

Machine Zero Point

The machine zero point can be set by three methods—by the

operator, manually by a programmed absolute zero shift, or by

work coordinates, to suit the holding fixture or the part to be

machined.

MANUAL SETTING - The operator can use the MCU controls to

locate the spindle over the desired part zero and then set the X

and Y coordinate registers on the console to zero.

Stored zero shifts (G54...G59)

Programmed zero shift (G92)

R = Reference point (maximum travel of machine)

M = Machine zero point (X0,Y0,Z0) of machine coordinate system.

W = Part zero point workpiece coordinate system.

The relationship between the part zero and the machine system of coordinate

44

Under G54 ... G59 the actual machine coordinates of part

zero are stored in the stored zero offsets memory and

activated in the part program.

Under G92 the actual machine coordinates are inserted and

used on the G92 line of the part program.

ABSOLUTE ZERO SHIFT - The absolute zero shift can change

the position of the coordinate system by a command in the CNC

program. The programmer first sends the machine spindle to

home zero position by a G28 command in the program. Then

another command (G92 for absolute zero shift) tells the MCU

how far from the home zero location, the coordinate system origin

is to be positioned, Fig. 19.

The sample commands may be as follows:

N1 G28 X0 Y0 Z0 (sends spindle to home zero position)

N2 G92 X4.000 Y5.000 Z6.000 (the position the machine will

reference as part zero)

45

ELECTRONICS OF A CNC MACHINE

The CNC controller components work together to interpret positioning signals created by a

computer and NC software into precise motor control. This page will explain the function of

each of the controller components and how they work together to make a controller system.

The Components

There are three primary CNC controller components that make up a CNC controller, the

power supply unit, the circuitry protection system, and the motor driver.

The Power Supply Unit

When you plug a small flash drive up to your USB port, the computer powers the device

through the port. When you plug a larger device, say a printer for example, you have to

use an external power source because the computer is not capable of supplying sufficient

power. This condition means that you have to plug that device into the computer as usual,

but you also have to plug a second line to an auxiliary power source, such as the 110V

outlet on your wall.

The same conditions are true for CNC devices. They require a low-voltage

communication line, through which the computer tells the machine what to do, and a power

source that provides the power for moving, cutting, and other such operations.

A power converter, usually referred to as the “power supply unit (PSU),” is often used to change

the form of the supplied power from alternating current (AC) from the power grid, to direct

current (DC) that is more easily used by the machine’s drive motors.

The power supply handles large voltages and currents that could be harmful to the NC circuitry.

Therefore, the power source, motor drivers, and motors are often separated from the computer

with a circuitry protection system that isolates surges in electrical power.

46

The Circuitry Protection System

The circuitry protection system contains a breakout board to isolate signals from the

computer, distribute the signals to the desired drivers, and also allows easy hook up of

peripherals such as limit switches that feed information back to the computer. Fuses are

also part of the circuitry protection system. Fuses could save the equipment in case of

electrical spikes, shorts, or faulty wiring.

A low-voltage communication signal passes from the computer through the breakout

board unchanged to the motor drivers. This isolated your computer from the CNC controller

circuit but allows the signals to carry through to your motor drivers.

The motor drivers

The motor drivers receives the communication signal and and then coordinates pulses of

the desired current and voltage to elicit the movement in the drive motors. The motor drivers

may communicate position information one way to the motor (open loop system), or send and

receive position information(closed loop system), depending on the user’s choice of drive

system. More on these systems may be found in the drivers sections.

CNC Controller Components Setup

The figure below shows the typical setup of the CNC controller components, such as that for a 3

axis CNC router. The system is composed of a computer with NC control software, the controller

box, and the drive motors. The controller box includes a breakout board, power source (not

shown), and motor drivers. The “breakout board” provides the circuitry protection and signal

distribution inside the controller box.

In the figure

above, you

47

can see the computer is connected to the breakout board, usually through the printer port (DB25).

There are many types of breakout boards; some of very high quality and great protection, and

lower budget options that do not offer much protection. The best models employ opto-isolators,

which use a light signals to transmit data across an air gap. These devices provide complete

conductive isolation between the controller circuit and your computer.

As a note, the computer is fully capable of connecting directly to the drivers and driving the

motors, but this setup puts your computer at risk.

The first figure is an idealized block diagram of your CNC electrical system. A little more detail

has been added in the figure below to show the conceptual layout of signal wires (light black)

and power wires (heavy black).

Although your computer will run on 110VAC, the CNC machine may run on 110VAC, or

220VAC, or 480VAC, and may be single phase, three phase, etc… This power enters the

controller box and will be distributed by the power supply. For most machines, the power supply

will convert the incoming power from alternating current (AC) to direct current (DC). This DC

supply will be of a lower voltage, such as 12V, 24V, 36V, or higher. The power supplied

depends on you CNC controller components, specifically the motor drivers.

48

PROGRAMMABLE LOGIC CONTROLLER

“A digitally operating electronic apparatus which uses a programmable memory for the internal

storage of instructions by implementing specific functions such as logic sequencing, timing,

counting, and arithmetic to control, through digital or analog input/output modules, various types

of machines or processes

Programmable Logic Controller (PLC)

A PLC matches the NC to the machine. PLCs were basically introduced as replacement for hard

wired relay control panels. They were developed to be reprogrammed without hardware changes

when requirements were altered and thus are reusable. PLCs are now available with increased

functions, more memory and large input/output capabilities. Fig.7 gives the generalized PLC

block diagram.

In the CPU, all the decisions are made relative to controlling a machine or a process. The CPU

receives input data, performs logical decisions based upon stored programs and drives the

outputs. Connections to a computer for hierarchical control are done via the CPU.

The I/O structure of the PLCs is one of their major strengths. The inputs can be push buttons,

limit switches, relay contacts, analog sensor, selector switches, proximity switches, float

switches, etc. The outputs can be motor starters, solenoid valves, position valves, relay coils,

indicator lights, LED displays, etc.

The field devices are typically selected, supplied and installed by the machine tool builder or the

end user. The voltage level of the field devices thus normally determines the type of I/O. So,

power to actuate these devices must also be supplied external to the PLC. The PLC power supply

is designated and rated only to operate the

internal portions of the I/O structures, and not the field devices. A wide variety of voltages,

current capacities and types of I/O modules are available.

49

The principle of operation of a PLC is determined essentially by the PLC program memory,

processor, inputs and outputs.

The program that determines PLC operation is stored in the internal PLC program memory. The

PLC operates cyclically, i.e. when a complete program has been scanned, it starts again at the

beginning of the program. At the beginning of each cycle, the processor examines the signal

status at all inputs as well as the external timers and counters and are stored in a process image

input (PII). During subsequent program scanning, the processor the accesses this process image.

To execute the program, the processor fetches one statement after another from the

programming memory and executes it. The results are constantly stored in the process image

output (PIO) during the cycle. At the end of a scanning cycle, i.e. program completion, the

processor transfers the contents of the process image output to the output modules and to the

external timers and counters. The processor then begins a new program scan.

Fig.7 Generalized PLC block diagram

Processor

Logic

memory

Storage

memory

Power

Supply

Inputs

Output

s

Power

Supply

Programmer

Field

Devices

50

The "Signals"

The signal lines coming from the computer

operate on 5V DC supplied by the computer

communication port, and is a square wave form

called a Transistor-to-Transistor Logic (TTL)

signal. This signal is essentially a series of

small pulses from 0V to +5V that represent 0’s

and 1’s in a binary computer language. This

signal is a form of a Pulse-Width Modulated

(PWM) signal where the length of the pulse is varied to indicate information. The width of the

pulse determines the binary code sent; either a “0” or a “1” as communicated by the computer

and interpreted by the motor driver. More on the signals may be found in the signals page.

The signal from the computer to the breakout board is the same as that from the breakout board

to the motor driver. Remember, the breakout board provides circuit protection and signal

distribution. Therefore, the signal coming out of the breakout board is also a 5V TTL signal of

the same form. However, as discussed previously, the signal after the driver has been

conditioned as needed to provide the large “move” voltage and current needed to drive the

machine.

51

Breakout Boards

Breakout boards are a common electrical components

that take a bundled cable and “breaks out” each

conductor to a terminal that can easily accept a hook-up

wire for distribution to another device. They are a

common item in electronic projects and enable easy,

clean installation of electronic devices. The image at

right shows a simple DB25 breakout circuit board

from Winford Engineering. The breakout board is

positioned between your computer or indexer and the

motor drivers and serves two purposes in the CNC control system: circuit protection and signal

distribution. Here we will describe the board’s function in the CNC control system and what you

need to know about how to select one to suit your need

52

Analysis And Programming Of A Vertical Machining System

CNC MACHINING CENTER

HITACHI SEIKI VA35 CNC MACHINING CENTER

The Hitachi Seiki VA35 CNC (Computer Numeric Controlled) machining center that

belongs

to the Department of Production Engineering is frequently used for accurate and

automated

machining of metals as well as for wood, plastic and other materials. The machining

accuracy

of the machine is 0.001mm. Figure 4-2 shows various parts of the machine tool

53

Hitachi Seiki VA35 CNC Machining Center

54

S Y

X

Z

Z

EEPROM OR

TAPE INPUT

Control Microprocessor ROM

Motor

Driver

Spindle

Motor

Spindle

Position

X,Y,Z Positions

X,Y,Z Spindle Motors

Numerical Control Of The Machine

55

MACHINE SPECIFICATIONS

GENERAL INFORMATION

� Manufacturer Hitachi Seiki Co. Ltd., Japan

� Model VA 35–II

� Control unit Fanuc System 6M–B

� Weight 4000 kg

TABLE

� Working area 1000*355 mm2

� Maximum carrying capacity 500 Kg

STROKES

� x-axis stroke in the crosswise

direction of the table 560 mm

� y-axis stroke in the longitudinal

direction of the table 350 mm

� z-axis stroke in the vertical

direction of the spindle head 400 mm

� Distance between the spindle nose

and top of the table 150-550 mm

SPINDLE HEAD

� Spindle nose contour NT 40

� Spindle speed 60-600 rpm

� Spindle speed change Stepless (s 4 digit)

� Spindle motor AC 5.5 kW (30 min)

56

FEED

� Least increment 0.001 mm

� Cutting feed rate 3600 mm/min

� Rapid traverse 13000 mm/min

AUTOMATIC TOOL CHANGE (ATC)

� No. of tools 30

� Shank type BT 40, CAT 40

� Maximum tool diameter 95 mm

� Maximum tool length 250 mm

� Maximum tool weight 10 kg

� Tool selection method Random shortest course

� Pull stud type MAS - 1

G CODE AND M CODE

The entire functioning of the machine is based on G Code and M Code specifications.

G Codes define the preparatory functions of the machine. In simple terms, they control

the

movement and machining related functions of the machine tool. For example, the code

"G76"

followed by some related arguments is used for fine boring. "G00" with X,Y,Z arguments

rapidly moves the bed and the spindle head to the position specified by the arguments.

M Codes are known as auxiliary functions. They control specific behaviors of the

machine.

For example "M08" turns on the coolant, M05 stops the spindle.

57

A Program Used In manufacturing Of Turbine Couplings

The listing given below is a program which was used to bore holes in couplings of two

turbines which were manufactured in the Engineering Workshops. It is written in G and

M

Codes.

Unless otherwise stated, all the dimensions are in mm.

1 G28 G91 Z0;

2 G28 X0 Y0;

3 G40 G49;

4 G90;

5 G92 X253.087 Y177.818 Z343.05; 21

6 G00 x131.25;

7 G00 Z5.0 F10;

8 M03 S150;

9 M98 P151;

10 M05;

11 M09;

12 G28 G91 Z0;

13 G28 X0 Y0;

14 M30;

15 %

The meaning of each line is given below.

1 Return to reference point, Incremental programming, Z=0 is the reference point (Z

movement only)

2 Return to reference point, X=0 and Y=0 (X and Y movements only)

3 Tool diameter compensation cancel, Tool length offset cancel

58

4 Absolute programming

5 Programming of absolute zero point, X=253.087, Y=177.818, Z=343.05

6 Positioning (rapid), X=131.25, Y=0

7 Positioning (rapid), Z=5.0, Feed rate set to 10 mm/min

8 Spindle rotation CW, speed=150 rpm

9 Sub program (o0151) call-out

10 Spindle stop

11 Mist/coolant off

12 Return to reference point, Z=0, Incremental programming (Z movement only)

13 Return to reference point, X=0 and Y=0

14 End of program, Control unit reset

15 Just display the end of current listing

Line 9 in the above program calls the sub program o0151. This sub program is

the actual part

of the program which bore holes and is listed below.

1 G76 G98 X131.25 Y0.0 Z-52.0 Q0.5 R2.0;

2 X119.903 Y53.384;

3 (some more x and y values)

4 M99;

The meaning of each line is as follows:

1 Fine boring; Return to initial level in canned cycle after finishing; Starting X,Y

coordinate: X=131.25, Y=0.0; Final Z coordinate = -52.0; Before boring tool is taken

out, move it 0.5 away from the bored wall of the workpiece; Radius of boring = 2.0

(This value does not have any effect on boring since the tool determines the actual

radius.).

2 Repeat boring for X=119.903 and Y=53.384.

3 Repeat the same in line 2.

4 End of sub program.

59

STEPS IN USING THE MACHINE

The distinct operations involved in using the CNC machine are listed below in sequence

they

are done.

1. Generating the program (in G & M Codes)

2. Sending it to the machine

3. Running the program

First a drawing of the machined workpiece is created using AutoCAD in a PC. Then

using a

special routine of AutoCAD, the contours of the cutting tool are generated. This is finally

stored as a text file in the hard drive of the PC.

Next, the CNC machine is set to retrieve this file. Through the coaxial cable which links

the

PC and CNC machine, it is then fed into the machine tool. A numeric name for the

program is

given at the beginning of the file retrieval to figure out the starting point (or the address

in the

memory) of the retrieving program from earlier read programs.

Using this numeric name of the program, it is taken to the front from other programs in

the

memory and it stays waiting to run. Pressing the "Start" button sequentially executes the

listing.

60

ADVANTAGES OF CNC MACHINES

CNC machines are widely used in the metal cutting industry and are best used to produce the

following types of product:

• Parts with complicated contours

• Parts requiring close tolerance and/or good repeatability

• Parts requiring expensive jigs and fixtures if produced on conventional machines

• Parts that may have several engineering changes, such as during the development stage of

a prototype

• In cases where human errors could be extremely costly

• Parts that are needed in a hurry

• Small batch lots or short production runs

Some common types of CNC machines and instruments used in industry are as following:

• Drilling Machine

• Lathe / Turning Centre

• Milling / Machining Centre

• Turret Press and Punching Machine

• Wirecut Electro Discharge Machine (EDM)

• Grinding Machine

• Laser Cutting Machine

• Water Jet Cutting Machine

And Most important For Electronics industry is that CNC Milling and Drilling is used for

Manufacturing Of PCB .( Printed Circutiry Board)..