CNC Machine for Engraving and Drilling

DESIGN AND FABRICATION OF A

CNC MACHINE FOR ENGRAVING AND DRILLING

Group Members:

MUHAMMAD ZAEEM ABBAS (090404)

SADDAM JAVED (090407)

UBADAH MEHTAB (090450)

AUMAIR AZEEM MALIK (090456)

AMMAR IKRAM (090474)

BE MECHATRONICS (2009-2013)

Project Supervisor

Liaquat Ali Khan

Assistant Professor

DEPARTMENT OF MECHATRONICS ENGINEERING

FACULTY OF ENGINEERING

AIR UNIVERSITY, ISLAMABAD

ii

DESIGN AND FABRICATION OF A CNC MACHINE FOR

ENGRAVING AND DRILLING

FINAL YEAR PROJECT REPORT

(2009-2013)

DEPARTMENT OF MECHATRONICS ENGINEERING

iii

DESIGN AND FABRICATION OF A CNC MACHINE

FOR ENGRAVING AND DRILLING

Submitted By:

MUHAMMAD ZAEEM ABBAS (090404)

SADDAM JAVED (090407)

UBADAH MEHTAB (090450)

AUMAIR AZEEM MALIK (090456)

AMMAR IKRAM (090474)

Project Supervisor

____________________________

Assistant Professor Liaquat Ali Khan

iv

ACKNOWLEDGEMENT

We express our humblest gratitude to Allah Almighty who has given us the

direction to accomplish our goals. The love and support provided by our parents

and family has been a source of strength and comfort through our work. We are

thankful to Assistant Professor Liaquat Ali Khan for providing us with valuable

material and also guiding us on this project. He has been indeed a source of

inspiration, support and encouragement for us.

v

ABSTRACT

The aim of our project is to design and fabricate a 3 axis CNC machine for

engraving, drilling and particularly routing PCB (Printed Circuit Boards).

Keeping in mind the requirements of a modern CNC router micro steeping and

frictionless linear guide mechanism is integrated into the machine to ensure

optimum engraving results without compromising on the speed of operation.

vii

List of figures

Figure 1.1: A CNC Engraver………………………………………………………………………………….1

Figure 1.2: Improper Design of Circuits……………….………………………………………………..3

Figure 1.3: Circuits in Non-Corrosion Resistant Material Casing……………….….4

Figure 1.4: Untidy Design of Circuit Boards………………………………………………………….4

Figure 1.5: Linear Guide Mechanism…………………………………………………………………….5

Figure 2.6: Flow Chart of Typical CNC Machine………………………………………………….9

Figure 2.7: Typical CNC Machine………………………………………………………………….…..10

Figure 2.8: Close-Up Views of Ball Screw Mechanism………………………………….…..13

Figure 2.9: Sectioned Ball Screw Mechanism………………………………………………….…14

Figure 2.10: V - Shaped Cutting Tool………………………………………………………………...15

Figure 2.11: Unipolar Winding Arrangement……………………………………………………..19

Figure 2.12: Bipolar Winding Arrangement………………………………………………………..19

Figure 2.13: Engraving Using Absolute Coordinate System………………………………..24

Figure 2.14: DB-25 Pin Diagram…………………………………………………………………………25

Figure 4.15: AutoCAD Model Showing Isometric View……………………………………..36

Figure 4.16: Top View of AutoCAD Model………………………………………………………..37

Figure 4.17: Front View of AutoCAD Model………………………………………………………38

Figure 4.18: Side View of AutoCAD Model……………………………………………………….39

Figure 5.19: Application Specific 3-Axes CNC Controller………………………………….41

Figure 5.20: Overall System Block Diagram……………………………………………………….42

vii

List of Tables

Table 2.1: PMDC Comparison……………………………………………………………………………16

viii

Nomenclature

BLDC Brushless Direct Current Motor

CAD Computer Aided Drawing

CAM Computer Aided Manufacturing

DC Direct Current

mm millimeter (10-3)

NC Numerical Control

PCB Printed Circuit Board

PM Permanent Magnet

PMDC Permanent Magnet Direct Current Motor

rpm revolutions per minute

VR Variable Reluctance

μ Micron (10-6)

ix

Table of Contents:

Chapter 1: Introduction 1

1.1 Background 1

1.2 Objectives 2

1.3 Drawbacks in pre-existing CNC Machine 2

1.4 Improvements 6

1.5 Methodology 6

Chapter 2: Description 7

2.1 Introduction to Numerical Control 7

2.2 Advantages of CNC Machines 8

2.3 Disadvantages of CNC Machines 8

2.4 CNC Operation Flow Diagram 9

2.5 Structure of Typical CNC Engraver 10

2.5.1 Base, Frame and Y Axis Gantry 11

2.5.2 Three Axes Linear Motion System 11

2.5.3 Three Axes Linear Drive System 12

2.5.4 Ball Screw Mechanism 13

2.5.5 Cutting Tool and Spindle 15

2.5.6 Actuators 16

2.5.6.1 DC Motor 16

2.5.6.2 Stepper Motor 18

2.6 Software and Human Machine Interface 22

x

2.7 CNC Programming 23

2.8 Coordinates 24

2.8.1 Absolute Coordinates 24

2.8.2 Relative Coordinates 25

2.9 Interfacing 26

2.9.1 Serial Communication 26

2.9.2 Parallel Communication 26

Chapter 3: Mathematical Modeling 29

3.1 Mechanical Modeling 29

3.2 Electrical Modeling 29

3.3 Spindle Motor Power Calculations 31

3.4 Spindle Motor Modeling 32

3.5 Ball Screw Calculations 35

Chapter 4: Design 36

4.1 Mechanical Model 36

4.2 Mechanical Drawings 37

4.3 Mechanical Features of Machine 40

4.4 Application Specific 3- Axes CNC Controller 41

4.5 Overall System Block Diagram 42

Chapter 5: Conclusions and Recommendations 43

5.1 Conclusions 43

5.2 Recommendations 43

xi

Appendix A

Appendix B

Bibliography

1

Chapter # 1: Introduction

1.1 Background:

Engraving is the process in which a design is inscribed on to a hard, usually flat

surface, by cutting grooves into that surface. Historically, it was an important

method of producing images on paper.

In manufacturing of PCB, etching is a very important old process and it also takes

very long time. This machine is a substitution to the old manual process. In the

era of rapid technological development PCB’s are very important components as

they are attached to every electronic device as well as mechanical too, to make

them work efficiently.

An example image of a PCB engraving and drilling machine is shown below:

Figure 1.1: A CNC Engraver

INTRODUCTION

2

Etching is a standard process for making PCB’s but it has several drawbacks:

High production cost

Lengthy process

Accuracy and effectiveness problems

Human errors

Corrosive action of FeCl3 (etching chemical liquid)

On the other hand this automatic engraving machine is a substitute to the manual

process and one can reduce production cost in terms of labor cost and time cost.

1.2 Objectives

Our main goal is the improvement of a pre-existing CNC machine made by a

previous batch (2006-2010) of Department of Mechatronics Engineering.

1.3 Drawbacks in the Pre-Existing Machine:

The method of job clamping used is quite primitive; drilling in four corners of job

is done manually and then with the help of screws tightened on the wooden table.

Play is also found in linear guide motion of machine during inspection of

mechanical structure. The pitch of the lead screws used is also high while the

same lead screws of low pitch can easily be fabricated and installed. This would

greatly improve its precision in addition to this the guiding/connecting rods of

linear guide motion are also of Iron which is non-corrosion resistant material. The

performance of drilling tool can also be improved by changing the DC motor with

one of high power rating. The control unit of the machine is made of Iron which is

Design & Fabrication of a CNC

Machine for Engraving & Drilling

3

a non-corrosion resistant material. PCB’s are poorly designed with lots of jumper

wires used.

In a nutshell the problems found were:

This CNC machine is capable of engraving on wood but with a very low

accuracy due to flaws in mechanical design.

The stepper motors and DC motors needs to be replaced.

Electronic control unit is poorly routed with results in poor performance of

actuators.

Primitive method of job clamping is employed.

The speed of cut is not up to the mark.

Incapability of engraving complex contours.

Some pictures of the pre-existing CNC machine are shown on next page

emphasizing on the problems discussed above:

Figure 1.2: Improper Design of Circuits

INTRODUCTION

4

Figure 1.3: Circuits in Non-Corrosion Resistant Material Casing

Figure 1.4: Untidy Design of Circuit Boards



5

Figure 1.5: Linear Guide Mechanism

There are several key points to be explored in this project:

Designing the CNC router mechanical motion and driving system

Improving the accuracy and precision by improving the pitch of lead screws

Generating CAM file from the design software and interpretation into co-

ordinates

Coding of serial data communication.

Implementing a user friendly Human Machine Interface.

Synchronizing the whole system

INTRODUCTION

6

1.4 Improvements to be made:

To improve the accuracy of the machine by employing low pitch ball screw

mechanism.

To improve the overall mechanical design.

To replace old actuators with more application specific ones for high speed

engraving.

To design and fabricate better and neatly routed electronic circuits.

To introduce a mechanism of job clamping.

To introduce a mechanism for automatic chip removal from workspace.

To ensure the capability of CNC machine to engrave complex contour.

1.5 Methodology:

Research and consultation with lectures and course work.

Brainstorming of ideas for electrical, mechanical and programming.

Design of mechanisms.

Implementation of efficient power supply.

Design of efficient CNC Control Unit

Fabrication of mechanical components like lead screws etc.

Chapter # 2: Description

2.1 Introduction to Numerical Control:

The concept of numerical control was proposed in 1947. John C. Pearson of

Pearson Corporation, a rotor blade manufacturer. This concept was further

carried in 1949 when the United States Air Force realized that parts of war

planes and missiles were becoming complex with time and with improvements

in the older designs the production was sluggish. United States Air Force

decided to grant a study contract to Pearson Corporation. Subcontractor was

the famous Servo Mechanism Laboratory at Massachusetts Institute of

Technology (MIT). The prototype of first NC machine, was successfully

demonstrated in 1952.

These early servomechanisms were integrated with analog and digital systems

which gave birth to modern CNC (Computer Numeric Control) system which

was a revolutionary change in the machining process. In modern CNC systems

the component design is highly automated using computer-aided

design (CAD) and computer-aided manufacturing (CAM) programs like Fanuc

Control, Siemens, GSK and Mach 3 etc. With the help of these programs a file

is produced that interprets the commands (in most cases G-codes and M-

codes) to operate the particular machine via a postprocessor. These commands

are generated automatically by the software in present day CNC systems.

Many different types of CNC machines are used in industry such as:

Machining Centers

Turning Centers

Mills

Lathes

Surface grinders

DESCRIPTION

8

Drilling Machines

Hot wire foam cutters

EDM Sinker and wire cut Machines

Water Jet Profilers

Flame and Laser-Cutting Machines

2.2 Advantages of CNC Machines:

High Repeatability and Precision e.g. PCB routing and Aircraft parts etc.

High volume of production.

Complex shapes and contours can be manufactured.

Safer and automated operation.

Better quality than conventional machining.

Less paper work and faster prototype production.

2.3 Disadvantages of CNC Machine:

High initial setup cost.

Requires skilled operators.

Computer programming knowledge is a pre-requisite for operator.

Maintenance is comparatively difficult.



9

2.4 CNC Operation Flow Diagram:

Figure 2.6: Flow Chart of Typical CNC Machine

DESCRIPTION

10

2.5 Structure of a Typical CNC Engraver:

A typical CNC Engraving Machine consists of the following parts:

X-axis linear drive system

Y axis linear drive system

Z-axis linear drive system (plunge arm)

Y-axis gantry

Cutting table

Spindle

Base and Frame

CNC Control Unit

Computer

Figure 2.7: Typical CNC Machine



11

2.5.1 The Base, Frame and Y-axis Gantry Design:

Mostly the CNC routers are of two types:

1. Stationary gantry with mobile bed

2. Mobile gantry with stationary bed

The first type i.e. stationary gantry with mobile bed is not preferred as far as

industrial applications are concerned. This type of design works well for small

applications i.e. for smaller workspaces but with the increase in size of the

horizontal axis the efficiency decreases. The main advantage of this type of

design is that the weight of the gantry does not affect the performance of the

machine.

The second type is mostly preferred for industrial applications since it is not

limited to smaller workspaces but it requires an optimal gantry design. The

design and material selection of the gantry plays a crucial role in the efficiency

of machine. The gantry must be light so that actuators and the drive circuits

could perform well and within the torque limits.

Our design is based on the second type. By doing proper analysis of the forces,

bending moment, torques and material selection we can reduce the stress and

damage to the components like bearing, lead screws motors and other

components that are subjected to forces. This analysis will enable us to assess

the durability and longevity of the machine.

2.5.2 3-axis Linear Motion System:

The linear drive system of the CNC is probably the most important part in the

mechanical fabrication and it must be very smooth and frictionless. The linear

drive system can be of many types like V-groove wheels and track roller

systems, it can be shaft guiding with rods and bushings but we are making use

of re-circulating rolling elements.

DESCRIPTION

12

These rolling elements are actually ball or roller bearings that are positioned

between rail and block. These rolling elements are advantageous because it

reduces coefficient of friction by converting sliding friction into rolling

thereby providing high positional accuracy, long lifetime, high speed motion,

equal loading in all directions, easy installation and easy lubrication.

2.5.3 3-axis Linear Drive System:

The driving system works in harmony with the linear motion system. Both

these systems have to perfect in order to ensure vibration less and smooth

operation. The basic idea behind driving system is converting rotational

movement of the actuator into translational movement.

The drive system can fabricated with different styles e.g. threaded rod or lead

screw drive system. The disadvantage is low precision due to backlash.

Backlash is defined as the number of linear gaps between the screw and the

nut. By making use of Acme Thread this problem can be minimized to a good

extent. The acme screw has broader, thicker and stronger thread. It has good

power transmission capabilities and can carry heavy loads due to trapezoidal

threading but it also has a high coefficient of friction between the screw and

nut that requires high starting torque.

Another type of drive system can be fabricated by using Rack, Pinion and

Gears but this type has some limitations in eliminating backlash effect. An

alternative is to use Pulleys and Timing belts but that’s too has a disadvantage

of slip when given high frequency movement.

The system we are going to use is Ball Screw Mechanism. This mechanism

ensures high precision, high efficiency, high accuracy and smooth movement.



13

2.5.4 Ball Screw Mechanism:

This mechanism is actually a mechanical linear actuator which consists of

threaded shaft which provides a helical raceway for ball bearings which act as

a nut while shaft act as a screw. Due to its ability to withstand high thrust

loads in addition to other benefits it’s a first choice for industrial machinery. It

is used in aircrafts, missiles, robotics, electric fly by wire systems and car

power steering etc.

The relation of torque ‘𝑇’ applied to screw or nut with linear force ‘𝐹’, length

of ball screw ‘𝑙’ and efficiency ‘𝜂’ is given below:

𝑇 =𝐹𝑙

2𝜋𝜂

Figure 2.8: Close-Up Views of Ball Screw Mechanism

DESCRIPTION

14

Figure 2.9: Sectioned Ball Screw Mechanism

Advantages:

High mechanical efficiency as compared to its alternatives due to low

friction in ball screws.

A typical ball screw has a mechanical efficiency of about 90% as

compared to 50% efficiency of Acme lead screw of same size.

It reduces down time for maintenance and demand of lubrication which

increases operational cost.

Extends the life span of the system on which it is used due to lack of

sliding friction between nut and screw.

All these factors ultimately contribute towards an optimal system in

which power requirements are greatly reduced.



15

Disadvantages:

Manufacturing cost is high as compared to Acme lead screws.

Due to very less internal friction ball screws can be back-driven due to

low internal friction especially in hand fed systems, but in our system

that is not the case.

Specifications of the ball screw mechanisms that we used are:

Material: mild steel (density = 7390g/m3)

Mass: 0.750 Kg = 750 g

Lead: L = 5mm

Pitch: P = 5mm

Pitch diameter = d – 0.649519 P = 14.768 mm

Major diameter: d = 17mm = 1.7cm

2.5.5 Cutting Tool (Spindle):

The cutting tool is installed on the shaft of motor which is then used for

machining. The spindle is directly involved in the machining. The material of

cutting tool depends upon the material which is being engraved like in our

case it is Bakelite or Fiber board. While choosing a spindle factors considered

are speed (rpm), rated values of load, size of spindle and power requirements.

Figure 2.10: V - Shaped Cutting Tool

DESCRIPTION

16

2.5.6 Actuators:

Two main types of actuators used are the Brushed DC motors and stepper

motors. The cutting tool mounted on the spindle is driven by a DC motor.

While the movement along the three axes is controlled by stepper motors.

2.5.6.1 DC Motor:

DC Motors can be categorized based on field excitation:

Brushed DC Motors (BDC)

Permanent Magnet (PMDC)

Shunt Wound

Series Wound

Compound Wound

Brushless DC Motors (BLDC)

Characteristics Permanent

Magnet

Shunt Series Compound

Cost Low Moderate Moderate High

Loss of

Magnetism

Worst None None None

Torque vs.

Speed

Good at low

speed but

less at high

speed

Consistent

at low speed

Great at

low speed

Best at low

speed

Safety (Motor

Runaway)

No chance High

Chance

Hig

h Chance

Low chance

Speed Control Excellent Excellent Poor Great

Table 2.1: PMDC Comparison



17

During selection of the DC motors some important parameters were kept in

mind:

1. Supply Voltage: It is the maximum voltage needed to operate the motor

2. Rated Current: This is the current at stall conditions at applied rated

voltage.

3. Back EMF Constant or Voltage Constant: It is the constant that defines

the magnitude of back EMF generated in the armature. In a PMDC motor

this constant is determined by driving the motor as an unloaded generator

and then measuring the voltage divided by the speed in revolutions per

minute.

4. Motor Constant (Km): It is the ratio of the torque to input power.

5. Stall Torque: It is the torque under stall conditions.

We have selected Permanent Magnet DC motor for spindle based on the

theory given above. The specifications of the motor are as follows:

Manufacturer: Pittman™, USA

Part/Model No.: 8224

Supply Voltage: 48 V

Rated Current: 2.82 A

Voltage Constant: 4.57 V/k rpm

Motor Constant: 0.011 Nm/√W

Stall Torque: 0.1193 Nm

2.5.6.2 Stepper Motor:

A stepper motor is a brushless DC motor that divides a full rotation into a

small number of equal steps. The motor's position can easily be commanded

through control signals to move or to hold at any of these steps without the use

of any feedback sensor.

DESCRIPTION

18

Types of Stepper Motors:

1. Permanent Magnet (PM) Stepper Motors

2. Variable Reluctance (VR) Stepper Motors

3. Hybrid Stepper Motors

4. Lavet Type Stepping Motor

In Permanent magnet motors there is a permanent magnet in the rotor which

operates on the attraction or repulsion between the rotor permanent magnet

and electromagnets on the stator. Variable reluctance (VR) motors operates on

the principle that minimum reluctance occurs with minimum gap so that the

rotor points are attracted toward the magnet poles of stator. Hybrid stepper

motors employ the combination of both techniques to achieve maximum

power in smallest size.

Types on Basis of Winding Arrangement:

1. Unipolar

2. Bipolar

Unipolar Winding of Stepper Motor:

A unipolar stepper motor has two windings per phase. In this type of

arrangement the magnetic poles can be reversed without the need of reversing

the direction of current. The drive circuit in this case is also very simple. One

end of each winding is made common to have three leads per phase and

therefore for a two phase motor there would be a total of six leads. Out of

these six leads the two commons of both the phases are often joined together

to have a total of five leads for a two phase motor.



19

Figure 2.11: Unipolar Winding Arrangement

A stepper motor drivers can be used to energize the transistors in correct order,

and this ease of operation makes unipolar stepper motors ideal to use. It is the

most inexpensive way to get precise angular movements. A six lead unipolar

stepper motor can also be driven using a bipolar driver circuit. In this

configuration, one of the windings per phase remains useless as current never

flows through it.

Bipolar Winding of Stepper Motor:

A bipolar stepper motor has a single winding per phase. To reverse a magnetic

pole in this configuration the current needs to be reversed, which means that

the driving circuit is comparatively more complex, typically with an H-bridge

arrangement.

Figure 2.12: Bipolar Winding Arrangement

DESCRIPTION

20

Following are the Excitation Modes for stepper motor:

1. Full Step Drive: Uses a four step switching sequence.

2. Half Step Drive: Each step of a full step drive is divided into two steps.

This means that a 100-step motor having a resolution of 1.8°, will now

have a resolution of 200 steps with a resolution of 0.9°.

3. Micro Step Drive: Micro stepping divides motor's steps up to 256 times.

It improves smoothness in low speed and minimizes resonance effects in

low speed but in micro stepping around 30% less torque is produced than

two phase full stepping. The primary goal of micro stepping is to make

the motor to run as smooth as possible and we have the same

requirements for our 3 axes stepper motors.

While making selection we kept following things in mind:

1. Holding Torque: The maximum torque which can be applied to an

energized motor without causing any rotation.

2. Pull-in Torque: Maximum Torque at which a motor can start, stop and

reverse without losing steps.

3. Pull-out Torque: The maximum torque that can be applied to motor

(running at constant speed) which does not cause step missing.

4. Detent Torque: The maximum torque that can be applied to the shaft of

non-energized motor.

5. Maximum Starting Frequency: Maximum pulse rate (frequency) at

which the motor can start and run without missing steps.

6. Maximum Slew Frequency: The maximum rate at which the stepper

motor will run.



21

Following are the specifications of the stepper motor we have selected:

Winding type: Unipolar

Manufacturer: Oriental Motor Company™, Japan

Model: 103-820-2 (Nema 34D)

Mass: 1.2 Kg

Step angle: 2 deg./step

Holding torque: 2.1 N.m

Number of phases: 2 phase

Current per phase: 1.4 A

Resistance per phase: 3.21 ohm

Supply voltage: 4.5 V

2.6 Software and Human Machine Interface:

Software acts as a bridge between the human and machine. The design that

would be created will be communicated to the machine through the software

in the form of special codes that are discussed in detail in CNC programming

section. These codes are generated automatically by software according to the

design. The bridge between the software/computer and machine is CNC

Control Unit.

The software we are going to use is ArtSoft™ Mach3™ as CAM and CNC

control software while Sprint Layout 5.0™ as CAD software which converts

gerber PCB files to Isolation Milling (.plt) files. Isolation milling file isolates

the background from the connection lines that are to be milled away.

DESCRIPTION

22

Features of this software are given below:

Converts a PC to a fully featured 6-axis CNC controller

Allows import of DXF, BMP, JPG, and HPGL files through LazyCam™

Visual G-code display

Generates G-code via LazyCam™ or Wizards

Customizable M-Codes and Macros using VBscript™

Video display of machine

Spindle Speed control

Fully customizable interface

Multiple relay control

Manual pulse generation

Mach3™ is the most widely used software. Other CNC related software are

KCam, EMC2, TurboCNC, Flashcut, MasterCAM, MeshCAM, BobCAD-

CAM etc.

2.7 CNC Programming:

With the successful implementation of numerical control in MIT’s

Servomechanism Laboratory, many implementations have been developed by

commercial and non-commercial organizations. This called for a standardized

version, so in US Electronic Industrial Alliance developed a standardized

version in early 1960’s and the final version approved in 1980’s as RS274. In

European countries other standards are used like DIN 66025 or PN-73M-

55256, PN-93/M-55251. In the rest of the world the standard ISO6983 is

mostly followed.

Many variants have been developed independently by manufacturers, which

mean that the operator of a specific controller must be fully aware of

differences of each manufacturer’s product. Quite few machines use BCL,

which is a standardized version of G-code.

http://en.wikipedia.org/wiki/Deutsches_Institut_f%C3%BCr_Normung

http://en.wikipedia.org/wiki/ISO



23

In CNC programming, commands are used and each command consists of a

letter with a two digit number to signal an action. Such instructions are called

part program commands. Each key alphabet of the command is designated by

a specific set of functions e.g. ‘M’ stands for Miscellaneous Functions, ‘A’

stands for Absolute or incremental position, ‘G’ stands for preparatory /

general commands etc.

G-code is a commonly used name to refer CNC programming language. G-

code is a language of communication between a human operator and CNC

machine in which machine is told what to make and how to do it.

Another very common group of commands are the M-codes; here ‘M’ stands

for miscellaneous group of commands i.e. it performs miscellaneous functions.

2.8 Coordinates:

Coordinates are series of numerical positions according to which the required

pattern or contour from the isolation milling file (*.plt) or DXF file is

engraved. While working with CNC’s we come across two types of coordinate

systems:

2.8.1 Absolute Coordinates:

In this type of coordinate system the numerical positions are calculated from a

fixed point of origin e.g. if we are to engrave a word ‘PAKISTAN’ onto a

sheet of soft material like Bakelite, the machine will engrave the word ‘P’ first

then move to the origin (the point that was set as a reference which is usually

the point from where it started), after that the word ‘A’ will be engraved and

the tool will be given the command to get back to the reference position. This

process will be repeated till the word ‘N’ is engraved. Given below is the

mental picture of the process explained above.

DESCRIPTION

24

Figure 2.12: Engraving Using Absolute Coordinate System

The main advantage of using this coordinate system is the accuracy because

every time when the tool moves to its reference the position errors that are

generated overtime will be nullified. The main disadvantage is that large

amount of time will be consumed in the engraving process.

2.8.2 Relative/Incremental Coordinates:

In incremental coordinate system the series of numerical positions use the

previous position as the point of origin. The main advantage of this type of

system is that it takes very less time to perform the operation. The main

disadvantage is the decreased accuracy. Many ways have been developed to

counter this inaccuracy e.g. using Servo-motors or proper check on number of

steps taken if a stepper motor is used etc.



25

2.9 Interfacing:

There are two methods of interfacing or data communication:

2.9.1 Serial Communication:

In telecommunication and computer science, serial communication is the

process of sending data one bit at a time, sequentially, over a communication

channel or computer bus. Serial communication is used for all long distance

communication and most computer networks, where the cost

of cable and synchronization difficulties makes parallel communication

impractical. Some examples of serial communication architectures are RS432,

RS422, I²C, RS223, RS485, SPI, Avionics Digital Video Bus (ARINC 818),

Universal Serial Bus (USB), Control of electronic musical instruments

(MIDI), Spacecraft communication network (SpaceWire) etc.

2.9.2 Parallel Communication:

In Parallel communication multiple bits are transferred simultaneously.

Examples of Parallel communication are SCSI, ATA, PCI and ISA.

Types of Pins in DB25 Parallel Port:

The parallel port on a PC is DB25 female connector. The 25 pins present on

this connector can be categorized into four groups:

Figure 2.13: DB-25 Pin Diagram

DESCRIPTION

26

Data pins:

The data pins are used to send data in the form of bytes to the machine in such

a manner that each pin is designated one bit of the data. Pins 2 up to 9 are Data

pins.

Status Pins:

These pins sense signals from an external machine. Mostly these pins sense

the outcome of and external switch or read pulses of encoders. In above figure

status pins are 10 up to 15.

Control pins:

Control pins are used to provide control signals for. In above figure 1, 14, 16

and 17 are control pins.

Ground Pins:

In figure ground pins are from 18 to 25. These provide proper ground to the

machine. Without proper grounding, the machine may not work properly or

even get damaged.

The choice of parallel links over serial links is due to these factors:

Speed: The speed of a parallel data link equals to the number of bits sent

multiplied by the bit rate.

Cable length: Interference between the lines worsens with the length of

cable. This places an upper limit on the length of a parallel connection

cable. This limit is usually shorter than that of a serial connection.

Complexity: Creating a parallel port in a computer system is relatively

simple. It requires only a latch to copy data onto a bus. Contrary to that,

conversion into parallel form by a universal asynchronous receiver /



27

transmitter (UART) is required before they may be directly connected to a

data bus.

Due to the above mentioned reasons we are using Parallel communication

protocols. The machine would be linked up with the computer through the

Electronic Control Unit which contains the power supply along with required

drive circuits of 3 axes stepper motor.

MATHEMATICAL MODELING

AND CALCULATIONS

28

Chapter # 3: Mathematical Modeling and

Calculations

3.1 Mechanical Modeling:

∑ 𝜏 = 𝐽 �̈�

𝐽�̈� + 𝑏�̇� = 𝜏net ------------------------------------------------------ (1)

3.2 Electrical Modeling:

Using Kirchoff’s Voltage Law

V Input = 𝐼𝑅 + 𝐿 (𝑑𝑖

𝑑𝑡)

𝑉 = 𝐼 (𝑅 + 𝐿𝐷)

𝐼 =𝑉

(𝑅+𝐿𝐷) ------------------------------------------------------------ (2)

The net torque of stepper motor is

𝜏 = 𝐾m 𝐼 sin (𝑟∆𝜃) ---------------------------------------------------- (3)

∆𝜃 = 2

𝑟 = 60



29

𝐾m = 𝜏 hold on / 𝐼 rated = 1.5 Nm/A

Substituting (1) in (3)

𝐽�̈� + 𝑏�̇� = 𝐾m 𝐼 sin(𝑟∆𝜃)----------------------------------------------- (4)

Now, Substituting (2) in (4)

𝐽�̈� + 𝑏𝜃 ̇ = 𝐾m sin(𝑟∆𝜃) × 𝑉

(𝑅+𝐿𝐷)

Taking Laplace

𝜃(𝑠)[𝐽𝑠2 + 𝐵𝑠] = 𝐾m sin(𝑟∆𝜃) [𝑉

𝑠(𝑅 + 𝐿𝑠)]---------------------- (5)

𝑉 → 𝑆𝑡𝑒𝑝 𝐼𝑛𝑝𝑢𝑡 𝑜𝑓 4.5 𝑉𝑜𝑙𝑡𝑠

𝐽 = 𝑀𝑅2 (𝜔2) = 𝐼𝜔2

M = 0.32 kg

R = 0.0179 m

𝐽 = 0.318 (0.0179)2 × (6.28)2 = 4.01 × 10−3 𝑁𝑚

Fc = 9.81 N

b = 1.56 N/s


AND CALCULATIONS

30

Substituting values in (5)

𝜃(𝑠)[4.01 × 10−3𝑠2 + 1.5 𝑠]

= 1.55(60 × 2) [4.5

𝑠 (3.21 + 0.05)]

So,

𝜃(𝑡) = 0.17 − 0.209𝑒(−0.64𝑡) + 0.034𝑒(−0.389𝑡)

3.3 Spindle Motor Power Calculations:

RPM = 628.3 𝑟𝑎𝑑

𝑠𝑒𝑐

Input Voltage = 12 V

Current = 0.8 A

Input Power = IV = 12 × 0.8

PInput = 9.6 W

Efficiency = 80%

POutput = 80 × (96

100) W = 7.68 W

P required = 𝜏 required × 𝜔

𝜏 required → Required Torque for routing PCB (Copper + Bakelite

or Fiber)

𝜏 required = 0.012 Nm

𝜔 = 628.3 𝑟𝑎𝑑

𝑠𝑒𝑐



31

P required = 7.54 W

P required < P Output

Motor can be used.

3.4 Spindle Motor Modeling:

RPM = 6000

𝜔 = 628.3 𝑟𝑎𝑑

𝑠𝑒𝑐

tm = K Im ---------------------------------------------------------------- (1)

E = k 𝜔m ---------------------------------------------------------------- (2)

K = 12

628.3 = 0.019 V/s

G = 1

Ro = G Ed / Im = 15 Ohms

Using Newton’s 2nd Law of Motion

∑ 𝜏 = 𝐽 �̈� = 𝜏M - b 𝜔m – 𝜏L --------------------------------------------- (3)

IM = 𝜏M / K


AND CALCULATIONS

32

Using Kirchoff’s Voltage Law

EM = G ED - Ro IM

EF = h 𝜔m

e = ED - EF

EM = Ge + Ro IM ---------------------------------------------------------- (4)

G = 1

So,

𝜏M = K × (ED - h 𝜔m - k 𝜔m) Ro --------------------------------------- (5)

Substituting equation (5) in equation (3)

𝐽𝜔m + b 𝜔m = k/ Ro [Ed - h 𝜔m - k 𝜔m] – 𝜏L ------------------------ (6)

𝜔m = [JD + (B+K2/ Ro + K h/ Ro) = – 𝜏L + K Ed / Ro ------------- (7)

J = 9.54 x 10-5 Nm

FC = 0.02 (2000 RPM)

B= FC/𝜔 = 0.02/628.3 = 𝜏L max / Imax = 0.012 / 0.8 = 0.015

Substituting values in equation (7) gives

𝜔m [(1.28 D +1)] =17 Ed – 6.6 x 104 𝜏L

𝑎, = Static Gain = 17

𝑏, = Disturbance Sensitivity = -6.6 x 104

𝑐 , = Time Constant = 1.28 seconds



33

Natural time constant = J / B = 9.54 ×10−5

3.18 × 10−5 = 3 seconds

Homogenous complimentary solution is

𝜔m (1.28 D + 1) = (17× 2) - 6.6 × 104 (6 × 10-5)

𝜔m (1.28 D + 1) = 200 ----------------------------------------------- (7)

𝜔m (1.28 D + 1) = 0

D = -0.78

𝜔mh = A e-0.78t

Complimentary solution is

𝜔mc = 200

(1.28 𝐷+1)

𝜔mc = 200

𝜔m (t) = A e-0.78t +200

Initial conditions at 𝜔m = 0 and t=0

𝜔m = 0

0 = A +200

A = -200

So,

𝜔m (t) = 200 (1 – e-0.78t)


AND CALCULATIONS

34

3.5 Ball Screw Calculations:

Required Accuracy = 0.0254 mm

Step of motor = 2O

L × (2

360) = 0.0254

L = 4.572 mm (Pitch of Lead Screw)

Percentage of Error = 1.57 %

Can be neglected. OK to use

Chapter # 4: Design

4.1 Mechanical Model:

Figure 4.14: AutoCAD Model Showing Isometric View



37

4.2 Mechanical Drawings:

Figure 4.15: Top View of the Model on AutoCAD

DESIGN

38

Figure 4.16: Front View of the Model on AutoCAD



39

Figure 4.17: Side View of Model on AutoCAD

DESIGN

40

4.3 Mechanical Features of Machine:

Net weight of Machine = 30 kg

Resolution = 0.013 inch / step

Spindle motor RPM = 6000 rpm

Spindle motor power = 9.6W

Stepper motor rated power = 6.3W per phase

Size of the base = 14 x 16 (inches)

Size of the Y-Axis positioning table = 15.3 x 11.5 (inches)

Size of the Z-Axis positioning table = 4.1 x 7.6 (inches)

Max Depth of cut = ¼ of an inch

Max Feed rate = 25.4mm/sec

Accuracy = ±5 %

Stepper motor PPS = 180

Stepper Motor Step Angle = 2O

Work piece material = PCB, Soft Wood, Soft Metals



41

4.4 Application Specific 3-Axis CNC Controller:

Figure 4.18: Application Specific 3-Axes CNC Controller

DESIGN

42

4.5 Overall System Block Diagram:

Figure 4.19: Overall System Block Diagram

Chapter # 5: Conclusion &

Recommendations

5.1 Conclusion:

Working in this project helped us to explore the vast field of industrial

automation and particularly CNC machines. We started with a machine

capable only of point to point motion with many drawbacks in mechanical

design as well as in electronic design. We searched for, observed and

compared the professional CNC machines with the one we currently had.

Along with this we consulted many books on CNC machines.

After making the required improvements the machine is in a position to mill

complex contours on PCB, soft wood and soft metals with considerable

accuracy and speed. Achieving this primary goal is the result of improvements

in both electrical and mechanical design. Exploring new software with better

capabilities also helped us in achieving this goal. The workspace and job

clamping system is also improved.

5.2 Recommendations:

The speed of cut can be improved.

By implementing feedback control systems can greatly improve the

accuracy of machine.

This machine with 3 DOF’s can be made capable of 5 DOF milling.

Appendix A

List of G-codes:

G00 Rapid move

G01 Feed Rate move

G02 Clockwise move

G03 Counter Clockwise move

G04 Dwell time

G08 Spline Smoothing On

G09 Exact stop check, Spline Smoothing Off

G10 A linear feed rate controlled move with a decelerated stop

G11 Controlled stop

G17 XY PLANE

G18 XZ PLANE

G19 YZ PLANE

G28 Return to clearance plane

G33 Threading (Lathe)

G35 Bypass error checking on next line

G40 Tool compensation off

G41 Tool compensation to the left

G42 Tool compensation to the right

G43 Tool length compensation - negative direction

G44 Tool length compensation - positive direction

G49 Tool length compensation cancelled

G53 Cancel work coordinate offsets

G54-G59 Work coordinate offsets 1 through 6

G61 Spline contouring with buffering mode off

G64 Spline contouring with buffering mode on

G65 Mill out rectangular pocket

G66 Mill out circular pocket

G67 Fly cut

G68 Mill out rectangular pocket with radius corners

G70 Inch mode

G71 Millimeter mode

G74 Peck drilling (Lathe)

G81 Drill cycle

G82 Dwell cycle

G83 Peck cycle

G84 Tapping cycle

G85 Boring cycle 1

G86 Boring cycle 2

G88 Boring cycle 3

G89 Boring cycle 4

G90 Absolute mode

G91 Incremental mode

G92 Home coordinate reset

G94 IPM mode (Lathe) default

G95 IPR mode (Lathe)

G96 Constant Surface Feed On (Lathe)

G97 Constant Surface Feed off (Lathe)

G110 Lathe Groove Face

G111 Lathe Groove OD

G112 Lathe Groove ID

G113 Lathe Thread OD

G114 Lathe Thread ID

G115 Lathe Face Rough

G116 Lathe Turn Rough

G120 Mill Outside Square

G121 Mill Outside Circle or Island

G122 Mill out Counter Bore

G123 Mill outside Ellipse pocket

G124 Mill inside Ellipse pocket

G125 Mill outside Slot

G126 Mill inside Slot pocket

G130 3D tool compensation with gouge protection

G131 3D offset parallel to 3D profile

G132 3D tool compensation with gouge protection in the Z axis only

G135 5 axis tool compensation with gouge protection

G136 Included angle limit for gouge protection

G140 3D part rotation and plane tilting

G141 Scale factor for the X axis only

G142 Scale factor for the Y axis only

G143 Scale factor for the Z axis only

G160 Mill 3D Cylinder

G162 Mill 3D Sphere

G163 Mill 3D Ramped Plane

G170 Set soft limits and crash fixture/chuck barriers to defaults

G171 Set backward crash fixture/chuck barriers

G172 Set forward crash fixture/chuck barriers

G181 Bolt hole Drill

G182 Bolt hole Dwell

G183 Bolt hole Peck

G184 Bolt hole Tap

G185 Bolt Hole Bore

Appendix B

List of M-codes:

M02 End of Program

M03 Spindle on Clockwise, Laser, Flame, Power ON

M04 Spindle on Counter Clockwise

M05 Spindle Stop, Laser, Flame, Power OFF

M06 Tool Change

M08 Coolant On

M09 Coolant Off

M10 Reserved for tool height offset

M13 Spindle On, Coolant On

M30 End of Program when macros are used

M91 Readout Display Incremental

M92 Readout Display Absolute

M97 Go to or jump to line number

M98 Jump to macro or subroutine

M99 Return from macro or subroutine

M100 Machine Zero Reset

M199 Mid program start

Bibliography

1. Katsuhiko Ogata, Modern Control Engineering, 4th Edition, Prentice Hall

Of India, New Delhi, 2004

2. Kelly James Floyd, Patrick Hood-Daniel, Build Your Own CNC Machine, 1st

Edition, Springer-Verlag, New York, 2009.

3. Geoff Williams, CNC Robotics: Build Your Own Shop Bot, 1st Edition,

McGraw Hill, New York, 2003.

4. Seames, S. Warren, Computer Numerical Control, 4th Edition, Thomson

Learning Inc., New York, 2002.

5. Overby Alan, CNC Machining Handbook: Building, Programming, and

Implementation, 1st Edition, McGraw Hill, New York, 2010.

6. Madison James, CNC Machining Handbook, 1st Edition, Industrial Press

Inc., New York, 1996.

7. Pabla B.S., Adithan M., CNC Machines, 1st Edition, New Age International,

New Delhi, 1994.

8. Albert Alan, Understanding CNC Routers, 1st Edition, FPInnovations,

Ottawa, 2011.

9. Valentino James , Goldenberg Joseph, Introduction to Computer Numerical

Control, 5th Edition, Prentice Hall, New York, 2012

Date post:	24-Dec-2015
Category:	Documents
Upload:	aumair-malik
View:	46 times
Download:	8 times

CNC Machine for Engraving and Drilling

Documents