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UNIVERSITY OF NAIROBI
DEPARTMENT OF MECHANICAL AND MANUFACTURING
ENGINEERING
PROJECT TITLE: AUTOMATION OF CENTRE LATHE
MACHINE
AUTHORS:
BETT DAVIES KIPNGETICH F18/35822/2010
MATIKU WAEMA ALBANUS F18/36512/2010
PROJECT CODE: JMO 02/2015
PROJECT SUPERVISOR: PROF. J.M.OGOLA
YEAR: 2014/2015
THIS PROJECT REPORT IS A PARTIAL FULFILMENT OF A
DEGREE COURSE IN BACHELOR OF SCIENCE
(MECHANICAL AND MANUFACTURING ENGINEERING).
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DECLARATION
We declare to the best of our knowledge that this Final Year Project report to be submitted as a
partial fulfilment of the Bachelor of Science ( Mechanical and Manufacturing Engineering)
degree, to be our own original work and has not been presented in this or any other university for
examination, academic or any other purpose.
NAME: BETT DAVIES KIPNGETICH.
REG. NO.: F18/35822/2010
SIGN: ……………………………….
DATE: ………………………………
NAME: MATIKU WAEMA ALBANUS.
REG. NO: F18/36512/2010
SIGN: …………………………………
DATE: ……………………………….
SUPERVISOR:
This project has been submitted for examination with my approval as the university supervisor.
PROF. J.M. OGOLA
SIGN: ………………………………………….
DATE: …………………………………………
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ACKNOWLEDGEMENT
We would like to thank everyone who assisted us with completing this project. The names are
too numerous to list, but they know themselves.
We want to especially thank our project supervisor Prof. J.M. Ogola for his wisdom, friendship,
support and assistance in guiding us throughout this project. He not only gave us the opportunity
to automate a center lathe machine, but also provided the moral and academic support that has
been of great benefit to us in our project. Besides just putting up with us, he was always able to
give us insightful feedback on whatever the problem was.
We also thank Ms. Florence Mbithi from the Mechanical and Manufacturing Engineering
department for her tireless guidance on the entire mechanical and electrical design process. We
would also like to thank the Department of Mechanical and Manufacturing Engineering and all
our lecturers for their academic support that has made this project possible.
We would also like to appreciate the FabLab for being cooperative with us and giving us the
necessary assistance for every consultations we required during our project.
We would also like to thank the entire technical personnel in the Mechanical and Manufacturing
Engineering workshops for every support and assistance at all times during the fabrication and
testing of the model.
Finally we would like to thank our classmates, friends and our families for their endless love and
support throughout the years. We would not have achieved the success that we have without
them.
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ABSTRACT
The main objective of carrying out this project was to automate the turning the facing processes
and as well as to use the above automated processes to develop a cost-effective center lathe
machine suitable desk top operations. There has been an increment in the number of accidents in
the workshops which has demanded for the use of modern technology to improve more on the
safe manufacturing processes .This gave rise to the widespread use of a CNC machine. A CNC
machine is basically made up of the input unit, output units, control unit and the machine
operator interface. Machine movements that are controlled by cams, gears, levers or screws in
conventional machines are directed by computers and digital circuitry in CNC machines.
Despite the fact that a center lathe machine carries out many processes, only facing and turning
processes were automated in a sense that the motions which are associated with the above
operations were computer controlled. Additionally a CAD model was developed using the
Autodesk inventor in order to demonstrate the design and functionality of the automated center
lathe machine.
The components that were needed to come up with the model included the Arduino board,
stepper motor driver, stepper motors, d.c motor, a computer and a universal G-Code sender
software application. A power adapter and a computer was used to power both the Arduino and
the stepper motor. The forward and backward rotary motion of the stepper motor was achieved
with the help of a stepper motor driver. The rotary motion was converted to linear motion with
the help of a threaded nut which meshes with the rotating leadscrew. The threaded nut which was
attached to either the cross slide or the carriage table accomplished the X and Z axis motion
contrast to that of a conventional lathe machine. A universal G-Code sender was used to send
some string of commands to the Arduino telling the Stepper motor to move by certain steps in
certain direction.
The speed of a DC motor was also varied with the help of a computer controlled program which
runs on an Arduino integrated environment in combination with other electrical components
including the transistor, resistors and also the Opto-isolators.
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Having achieved the above objective, the developed model can then be used for desktop
operations such as for learning in higher institutions. Also it can be used to give the conventional
lathe machines a CNC features as opposed to purchasing a new CNC lathe machine.
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NOTATIONS
CNC-Computer Numerical Control
NC-Numerical Control
CAPP-Computer-assisted Part Programming
MDI-Manual Data Input
IDE-integrated Development Environment
ICSP -In Circuit Serial Programming
PWM-Pulse Width Modulation
DAC-Digital to analog conversion
DC-Direct Current
AC-Alternating Current
CAM-Computer Aided Manufacturing
CAD-Computer Aided Drawing
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TABLE OF CONTENTS
DECLARATION......................................................................................................................................... ii
ACKNOWLEDGEMENT ......................................................................................................................... iii
ABSTRACT ................................................................................................................................................ iv
NOTATIONS.............................................................................................................................................. vi
TABLE OF CONTENTS ......................................................................................................................... vii
LIST OF FIGURES ................................................................................................................................... ix
CHAPTER ONE ......................................................................................................................................... 1
1.0 BACKGROUND ............................................................................................................................... 1
1.1 PROBLEM STATEMENT .............................................................................................................. 5
1.2 OBJECTIVES ................................................................................................................................... 6
CHAPTER TWO ........................................................................................................................................ 7
2.0 LITEREATURE REVIEW .............................................................................................................. 7
2.0.1 INTRODUCTION TO COMPUTER NUMERICAL CONTROL SYSTEMS .................... 7
2.0.2 BACKGROUND ........................................................................................................................ 7
2.0.3 NUMERICAL CONTROL ..................................................................................................... 14
2.0.4 ADVANTAGES OF COMPUTER NUMERIC CONTROL OVER MANUAL ................ 14
2.0.5 TYPES OF CONTROL SYSTEMS USED IN NC MACHINES. ............................................ 15
2.0.6 COMMON AXIS ARRANGEMENT OF COMMON MACHINES .................................. 17
2.0.7 NUMERICAL CONTROL PROGRAMMING. ................................................................... 20
2.0.8 METHODS OF NC MACHINE INTERACTION/PROGRAMMING .............................. 21
2.0.9 G-CODE PROGRAMMING .................................................................................................. 23
CHAPTER THREE .................................................................................................................................. 24
3.0 AUTOMATION OF FACING AND TURNING OPERATIONS .............................................. 24
3.0.1 Arduino ..................................................................................................................................... 24
3.0.2 Grbl ........................................................................................................................................... 25
3.0.3 Grbl shield ................................................................................................................................ 26
3.0.4 Universal g-code sender ........................................................................................................... 26
3.0.5 Stepper motors ......................................................................................................................... 27
3.0.6 Stepper motor driver ............................................................................................................... 29
3.0.7 DC motor .................................................................................................................................. 30
3.0.8 Opto-isolators ........................................................................................................................... 32
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CHAPTER FOUR ..................................................................................................................................... 33
4.0 DEVELOPMENT OF CENTER LATHE FOR DESKTOP OPERATIONS ........................... 33
4.0.1 Identification of machining requirements ............................................................................. 34
4.0.2 Choice of the machine type ..................................................................................................... 34
4.0.3 Machine tool structure ............................................................................................................. 34
4.0.4 Selection of cutting speeds and feeds ...................................................................................... 36
4.0.5 Determination of power requirements ................................................................................... 37
4.0.6 Selection of drives..................................................................................................................... 40
4.0.7 Safety devices ............................................................................................................................ 42
4.0.8 CAD model ............................................................................................................................... 42
4.0.9 The physical model construction ............................................................................................ 43
CHAPTER FIVE ...................................................................................................................................... 48
5.0 DISCUSSION .................................................................................................................................. 48
5.1 CONCLUSIONS ............................................................................................................................. 49
5.2 RECOMMENDATIONS ................................................................................................................ 49
REFERENCES .......................................................................................................................................... 50
APPENDIX ................................................................................................................................................ 51
Values of coefficients R1, R2, R3 and the exponents x1, x2, x3 and y1, y2, y3 .................................. 51
Stepper motor axis control ................................................................................................................... 51
Recommended cutting speeds (m/min) for HSS, CARBIDE and OXIDE tools .......................... 52
List of G-code commands ..................................................................................................................... 53
Dc motor speed control program ......................................................................................................... 54
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LIST OF FIGURES
Figure 1 center lathe machine; Source: Google ............................................................................................ 3
Figure 2 Elements of a CNC system ............................................................................................................. 9
Figure 3:Elements of servo mechanism ...................................................................................................... 12
Figure 4: Arduino board, Stepper motor, Limit switch ............................................................................... 13
Figure 5: Closed-Loop System ................................................................................................................... 15
Figure 6: Open-Loop System ...................................................................................................................... 16
Figure 7: Sample CNC Machine ................................................................................................................. 19
Figure 8: Grbl shield ................................................................................................................................... 26
Figure 9: Unipolar stepper motor ................................................................................................................ 27
Figure 10: Bipolar stepper motor ................................................................................................................ 28
Figure 11: circuit diagram of the Arduino, stepper motor and the stepper driver connections ................... 30
Figure 12: electrical design circuit diagram ................................................................................................ 31
Figure 13: Plan view of the carriage and cross slide assembly ................................................................... 44
Figure 14: Bottom view of the carriage and cross slide assembly .............................................................. 45
Figure 15: Plan view of the base ................................................................................................................. 45
Figure 16: Assembly of the base, cross slide and the carriage .................................................................... 46
Figure 17: Electrical circuit diagram ........................................................................................................... 47
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CHAPTER ONE
1.0 BACKGROUND
The global technological advancement has enabled mankind to carry out much more tasks within
a shorter period of time and without having required using much more energy. The CNC lathe
machine is currently being preferred as compared to the conventional lathe machine for use
because it performs everything by commands of computer. The main advantage of using the
CNC machine is that safety as a matter of fact is guaranteed as there is minimal contact between
the machine and the operator. Additionally, accurate dimensions and irregular shapes can be
obtained as compared to those from the conventional lathe machine. (Prakash N. Parmar,
Investigation on Automation of Lathe Machine, 2014)
Machining is one of the most important material removal methods in the technology of
manufacturing. It is basically a collection of material working processes that involves other
processes such as drilling, shaping, sawing, planning, reaming, and grinding among others.
Machining is practically a part of the manufacture of all metals and other materials such as
plastics, and wood as well. An important machine that is useful in machining is the lathe
machine.
A lathe machine is generally used in metalworking, metal spinning, woodturning, and glass
working. The various operations that it can perform include the following: sanding, cutting,
knurling, drilling, and deforming of tools that are employed in creating objects which have
symmetry about the axis of rotation. Some of the most common products of the lathe machine
are crankshafts, camshafts, table legs, bowls, and candlestick holders.
The first lathe machine that was ever developed was the two-person lathe machine which was
designed by the Egyptians in about 1300 BC. Primarily, there were two things that were
achieved in this lathe machine set-up. The first was the turning of the wood working piece
manually by a rope; and the second was the cutting of shapes in the wood by the use of a sharp
tool. Initially it was used for metal cutting and metal shaping purpose but later it expanded its use
towards glass working and wood turning. As civilizations progressed, there have been constant
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modifications and improvements over the original two-person lathe machine, most importantly
on the production of the rotary motion. The production of the rotary motion therefore evolved
according to the following procedures: manual turning by hand by the Egyptians; addition of a
turning bow by the Romans; introduction of the pedal in the Middle Ages; the use of the steam
engines during the Industrial Revolution; the employment of individual electric motors in the
19th
and mid-20th
centuries; and the latest of which is the adoption of numerically controlled
mechanisms in controlling the lathe machine that has given birth to CNC machines.
The wide applications of lathe machines are confined to steel rolling mills, power plants, ship
building, tool rooms, paper mills, workshops repairing shops, automotive, textile, oil, and mining
industries. The most commonly used models of the lathe machines include: Heavy Duty lathe
Machine, light duty lathe machines, roll turning lathes, all geared lathe machines, extra heavy
duty lathe machines and heavy duty roll turning lathe machines
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Figure 1 center lathe machine; Source: Google
For the lathe machine to function and perform its operations effectively, various essential parts
are assembled together and they include the following:
1) Stand (or legs). This is used to hold the lathe machine and elevate the lathe bed to a
particular desirable working height.
2) Bed. This is a horizontal beam that holds the chips and the swerves from the already
machined work piece. It carries units such as the headstock, tail stock and the carriage. It
is made of high grade special cast iron and it is secured rigidly over cabinet leg and end
leg.
3) Headstock. The headstock contains the high precision bearings which hold the horizontal
axle, more commonly known as the spindle. It also houses the necessary transmission
mechanism with speed changing levers to obtain different speeds.
4) Main Spindle. This is a hollow horizontal axle with interior and exterior threads on the
inboard by which the woodworking pieces can be mounted on. The spindle nose has a
Morse taper hole(self-locking taper) which is used to accommodate lathe center or collet
chuck and threaded portion for chuck or face plate
5) Tailstock. This is the counterpart of the headstock which contains a non-rotating barrel
that can slide in and out directly in line with headstock spindle parallel to the axis of the
bed. It is used to support the other end of the work when being machined and to also hold
a tool for performing operations like drilling, reaming, tapping e.t.c
6) Carriage. This is composed of a saddle and an apron and is used as a mount to the cross-
slide and it also slides over the guide ways between the headstock and the tailstock. It
consists of saddle, cross-slide ,tool post, apron and compound rest consisting of a swivel
and top slide
7) Cross-slide. This is a flat piece that sits crosswise on the bed which can be cranked at
right angles with the bed.
8) Tool Post. Sits on top of the cross-slide and holds the cutting tool holders in place. It gets
its movement by the movement of the saddle, cross slide and top slide.
9) Tool Rest. A horizontal area in line with the spindle and the tailstock from which hand
tools are braced against and levered into the work pieces. (Jaychris, 2010)
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Nowadays production of goods by use of modern technology is becoming more popular in
different parts of the continent. This is because there is more and more demand to produce as
many finished products from industries as possible for local sales as well as export to overseas.
This can only be achieved by incorporating and employing the use of computer software,
hardware and even firmware in industries. And as a matter of fact for example, CNC machines
are becoming more and important in modernized industrialization. Therefore since most of the
countries in our continent are still developing, it could be quite expensive to purchase CNC
machines for all the industries and thus its almost next to impossible to wipe out all the manual
machines present there. Therefore the best approach required of them is to convert the present
conventional machines into semi-automatic devices by a process referred to as retrofitting.
(Prakash N. Parmar, Investigation on Automation of Lathe Machine, 2014)
Automation is incorporated in a machine tool or machining system as a whole for higher
productivity with consistent quality aiming meeting the large requirements and overall economy.
For example as of a lathe machine in our case, Such automation enables quick and accurate
auxiliary motions, i.e., handling operations like tool – work mounting, bar feeding, tool indexing
etc. repeatedly with minimum human intervention but with the help of special or additional
mechanism and control systems. These systems may be of mechanical, electro-mechanical,
hydraulic or electronic type or their combination.
Machine tools are classified according to the degree of automation as follows:
• Non automatic-most of the handling operations irrespective of processing operations,
are done manually, like center lathes, turret lathe, capstan lathe etc.
• Semiautomatic
• Automatic-all the handling or auxiliary operations as well as the processing operations
are carried out automatically.
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General purpose machine tools may have both fixed automation and flexible automation where
the latter one is characterized by computer Numerical Control (CNC). Amongst the known
machine tools, lathes are most versatile and widely used.
The conventional general purpose automated lathes can be classified as,
(a) Semi-automatic: Capstan lathe (ram type turret lathe), Turret lathe, multiple spindle turret
lathes and copying (hydraulic) lathe
(b) Automatic: Automatic cutting off lathe, Single spindle automatic lathe, Swiss type
automatic lathe, multiple spindle automatic lathes
The other categories of semi-automatic and automatic lathes are: Vertical turret lathe, Special
purpose lathe, Non-conventional type, i.e flexibly automatic CNC lathes, turning center e.t.c
1.1 PROBLEM STATEMENT
Safety is a major issue of concern in each and every kind of workplaces. Thinking about
accidents which are normally witnessed in the workshops, the first thing that comes into
everyone‟s mind is the safety of the victim i.e. is he or she safe?
In each and every workplace e.g. workshop, office, industries etc. in any corner of this continent,
every one‟s safety is the top most prioritized issue much more even than the profit or any
quantity of output. As a general example someone could be operating machine which doesn‟t
have guards or even safety switches and therefore in case of any loose hanging cloth, he or she
might be pulled away. Also in case of any electrical fault for a machine without safety switch,
then the machine may even blow up or eventually break down. And as a specific example
concerning someone who is operating a convention lathe machine, he or she could have been
having his or her own personal issues and thus once the concentration on the machine disappears,
then that person‟s safety is at risk in many ways. He or she can even get injured by a work piece
getting its way out of the spindle because maybe it was loosely held.
Therefore safety is mandatory to both an individual and the employer also because if one is not
safe, then he or she is not in a position to deliver what he or she is expected of. Also to the
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company, he or she becomes a liability as if he or she gets injured due to own cause ,an expense
is incurred for medical care and other personal benefits like early retirement due to maybe
permanent disability.
This project is intended to improve on the manually controlled machine by incorporating the use
of mechanical electronic circuits. This will in turn modernize the conventional lathe machine and
give it the features of a CNC machine. The anticipated benefits include Enhanced safety and a
higher performance, a lower cost investment rather than purchasing a new machine and rather
lower energy costs.
For safety purposes, the operator himself will be in a position to operate the machine from a
distance away since the control panel interface is not necessarily next to the machine. The
advantage is that in a case where there were to occur an accident it would be minimal since the
operator will not be within the vicinity of operation.
1.2 OBJECTIVES
The main objective of carrying out this project is to automate the existing conventional lathe
machine by providing it with CNC machine features through a process referred to as retro-fitting.
The specific objectives of the project are the following:
(i) To automate facing and turning operations of a center lathe machine.
(ii) Develop a computer simulation model of the automated center lathe parts.
(iii) Construction of a cost-effective automated center lathe suitable for desktop
operations
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CHAPTER TWO
2.0 LITEREATURE REVIEW
2.0.1 INTRODUCTION TO COMPUTER NUMERICAL CONTROL SYSTEMS
Production machinists produce large quantities of one part, especially parts requiring the use of
complex operations and great precision. Many modern machine tools are computer numerically
controlled (CNC).CNC machines control the cutting tool speed and do all necessary cuts to
create a part. The machinist determines the cutting path, the speed and does all necessary cuts to
create a part. He/ She determines the cutting path, the speed of cut and the feed rate by
programming instructions into the CNC machine. Many machinists must be able to use both
manual and computer-controlled machinery in their job.
Generally automating the lathe machine is done in order to: Improve accuracy, Increase safety,
Reduce production cost among other benefits. And in order to achieve all this, the conventional
lathe machine can be automated using the computer numerical control as discussed below.
2.0.2 BACKGROUND
The method of controlling machines by the application of digital electronic computers and
circuitry is referred to as computer numerical control. Machine movements that are controlled by
cams, gears, levers or screws in conventional machines are directed by computers and digital
circuitry in CNC machines.
CNC provides very flexible and versatile control over machine tools. Most machining operations
require that a cutting tool be fed at some speed against a workpiece. In a conventional machine
such as a turret lathe, the turning tool is mounted on a slide with hand-operated in-feed and
cross-feed slides. The operator manually turns a crank that feeds the cutting tool into the work
piece (in-feed) to the desired diameter. Another crank then moves the turning tool along the
longitudinal axis of the machine and produces a cylindrical cut along the workpiece. Sometimes
the cutting tool‟s feed rate is controlled by selecting the feed gears which move the axis slide at
the desired feed. A CNC machine replaces the hand cranks and feed gears with servomotor
systems.
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CNC allows the desired feed rates and cut depths to be keyed in rather than controlled by cranks,
cams, and gears. This thus provides precise, repeatable machine movements that can be
programmed for optimal speeds, feeds and machine cycles. All cutting tool applications whether
on lathe, drill press or machining center have optimum speeds and feeds which are determined
by carefully weighing the economics of tool life, required production rates and operator
attentiveness. With CNC, these parameters are set once and then they are repeated precisely for
each subsequent machine cycle.
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Elements of a CNC system
Figure 2 Elements of a CNC system
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The various parts labeled in the above diagram have the following functions
i. Input unit
It receives all commands from operator interface (operator station containing all the switches,
push buttons display required to operate and monitor machine activities) and feedback in the
form of a.c, d.c and analog signals.
All the input is made compatible to be understood by the control unit. Software are input by
means of paper tape stored in memory until needed by control unit
ii. Control unit
It takes instructions from the memory unit and interoperates them one at a time .it also process
information received from the operator and machine interface. This information is interpreted
and manipulated with hardware logic and computer programs. It then sends appropriate
instructions to other units to cause instructions to other units to cause instructions execution.
iii. Memory unit
It stores instructions and data received from the input and also store the results of arithmetic
operations and supplies information to the output unit. Fixed programs are stored on read only
memory while part programs are stored in random access memory
iv. Arithmetic unit
It performs calculation and makes decision. The results are stored in the memory unit
v. Output unit
It receives data from memory at the command of control unit. The digital signals are first
converted to analog form to control axis drive servomotor. The output signals are used to turn on
and off devices, display information, position axes.
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vi. Operator interface
These are generally several units which it comprises;
Punched tape-commonly used input system for NC system. The instructions for a given
operation are contained in several rows of information called a block. The tape readers may
either be electromechanical or photoelectric type detected light or reflected light type which is
faster than the electromechanical type.
Magnetic devices having the ability to store large amounts of data on a small amount of surface
are also used to feed input to memory unit.
Operator station-used to initiate automatic operation, input data, and to monitor activities using
display devices like cathode ray tube, light emitting diodes, plasma displays among others. In
certain applications a host computer may form operator interface e.g. to carry out computation
while running a part to correct for various machine conditions such as tool wear .The errors in
the leadscrew are measured and stored in the host computer memory in the form of a table. When
a part is made on this machine the host computer modifies a part program while it is still running
by making corrections based on the error table in order to obtain improved accuracy on the part.
vii. Machine interface
Consists of all devices used to monitor and control machine tools like extreme travel limit
switches , miscellaneous position locators , solenoids for hydraulic and air control , control
valves , servomechanisms e.t.c. The limit switches are provided to detect end of travel on any
axis.
A servomechanism is a group of elements which convert the NC output into precision
mechanical displacements. These elements include motors, gear trains and transducers while the
drive to spindle and slides in NC tools is usually provided by either hydraulic or electric motors.
The servomechanisms may be either open or closed loop.
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Figure 3: Elements of servo mechanism
The signals from machine control unit and from the feedback device are fed to command signal
circuits and then to the servomechanism which amplifies it and provide power to move the
desired machine slide or carryout a mechanical movement as required. The servomechanism may
be an electric motor which drives the machine table through a lead screw. Motors may drive the
slides through low-friction lead screws employing circulating ball nuts or through rack-and-
pinion arrangements. High torque electrical motors which can be directly coupled to lead screws
are used. A stepping motor is usually used which is actuated by pulses and moves a fixed angular
unit for each electrical pulse. The motor is usually clamped magnetically at fixed angular
position. The lead screw is almost universally used to convert rotary motion from electric motor
into linear movement of a machine slide. The recirculating ball lead screw is a precision screw
with very low friction. Velocity transducers are used to measure spindle speed and slide velocity
and position transducers are used to measure slide displacement. (Jain, 2009)
13
Stepper motor
Limit switch
Figure 4: Arduino board, Stepper motor, Limit switch
Arduino board
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2.0.3 NUMERICAL CONTROL
Numerical control (NC) is the automation of machine tools that are operated by precisely
programmed commands encoded on a storage medium, as opposed to being controlled manually
via hand wheels or levers, or mechanically automated via cams alone. (Wikipedia, 2014).
The key to the success of numerical control lies in its flexibility. To machine a different part, it is
only necessary to “play” a different tape. NC machines are more productive than conventional
equipment and consequently produce parts at less cost even when higher investment is
considered.NC machines also are more accurate and they produce far less scrap than their
conventional counterparts.
NC is a generic term for the whole field of numerical control and encompasses a complete field
of endeavor. Sometimes CNC, which stands for Computer Numerical Control and applies only to
the control system, is used erroneously as a replacement term for NC.
Albeit a monumental development, use of the term CNC should be confined to installations
where the older hardware control systems have been replaced. (ERIK OBERG, 2000).
2.0.4 ADVANTAGES OF COMPUTER NUMERIC CONTROL OVER MANUAL
CONTROL
The labor rate paid to a CNC machine operator may be lower than the rate paid to the
operator of conventional machines.
Lathe production run requires designing and making a lot of specialized form tools, cams,
fixtures, or jigs, then the economics of CNC machining improves even more because
much of the preproduction work is not required by the nature of the CNC process.
CNC machines can be effective for producing one-of-a-kind jobs if the part is
complicated and requires a lot of different operations that, if done manually, would
require specialized setups, jigs, fixtures, etc.
CNC machines are also especially well-suited for batch jobs where small numbers of
components are produced from an existing part program, as inventory is needed. Once
the part program has been tested, a batch of the parts can be run whenever necessary.
15
CNC machining can help reduce machine idle time. Surveys have indicated that when
machining on manual machines, the average time spent on material removal is only
about40 per cent of the time required to complete a part. On particularly complicated
pieces, this ratio can drop to as low as 10 per cent or even less. The balance of the time is
spent on positioning the tool or work, changing tools, and similar activities. On
numerically controlled machines, the metal removal time frequently has been found to be
in excess of 70 percent of the total time spent on the part. CNC non-machining time is
lower because CNC Machines perform quicker tool changes and tool or work positioning
than manual machines. However, CNC part programs require a skilled programmer and
cost additional preproduction time, (ERIK OBERG, 2000).
Additional advantages of CNC machining are reduced lead time, improved cutting
efficiency and longer tool life, as a result of better control over the feeds and speeds.
2.0.5 TYPES OF CONTROL SYSTEMS USED IN NC MACHINES.
1. Closed-Loop System
Figure 5: Closed-Loop System
Also referred to as a servo or feedback system. It is a control system that issues commands to the
drive motors of an NC machine. The System then compares the results of these commands as
measured by the movement or location of the machine component, such as the table or spindle-
head.
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The feedback devices normally used for measuring movement or location of the component are
called resolvers, encoders, Inductosyns, or optical scales.
Resolvers are normally connected to the lead-screws of NC machines. Linear
measurement is derived from monitoring the angle of rotation of the lead screw
and is quite accurate.
Encoders also are normally connected to the lead screw of the NC machine, and
measurements are in digital form. Pulses, or a binary code in digital form, are
generated by rotation of the encoder, and represent turns or partial turns of the
lead screw. These pulses are well suited to the digital NC system.
The Inductosyns also produces analog signals but is attached to the slide or fixed
part of a machine to measure the position of the table, spindle-head, or other
component. The Inductosyns provides almost twice the measurement accuracy of
the resolver, but is considerably more expensive.
Optical scales generally produce information in digital form and, like the
Inductosyns, are attached to the slide or fixed part of the machine. Optical scale
measurements are more accurate than either resolvers or encoders and, because of
their digital nature, are well-suited to the digital computer in a CNC system.
2. Open-Loop System.
Figure 6: Open-Loop System
A control system that issues commands to the drive motors of an NC machine and has no means
of assessing the results of these commands is known as an open-loop system. In such a system,
17
no provision is made for feedback of information concerning movement of the slide(s), or
rotation of the lead screw(s). Stepping motors are popular as drives for open-loop systems
because they have the advantage that not only can they can be positioned accurately, moved
forward or backwards one 'step' at a time, but also they can rotate continuously.
3. Adaptive Control
Measuring performance of a process and then adjusting the process to obtain optimum
performance is called adaptive control. In the machine tool field, adaptive control is a means of
adjusting the feed and/or speed of the cutting tool, based on sensor feedback information, to
maintain optimum cutting conditions
The sensors used for adaptive control are generally mounted on the machine drive shafts, tools,
or even built into the drive motor. Typically, sensors are used to provide information such as the
temperature at the tip of the cutting tool and the cutting force exerted by the tool.
The information measured by the sensors is used by the control system computer to analyze the
cutting process and adjust the feeds and speeds of the machine to maximize the material removal
rate or to optimize another process variable such as surface finish. For the computer to
effectively evaluate the process in real time (i.e., while cutting is in progress),details such as
maximum allowable tool temperature, maximum allowable cutting force, and information about
the drive system need to be integrated into the computer program monitoring the cutting process.
2.0.6 COMMON AXIS ARRANGEMENT OF COMMON MACHINES
To distinguish among the different motions, or axes, of a machine tool, a system of letter
addresses has been developed. A letter is assigned, for example, to the table of the machine,
another to the saddle, and another to the spindle head.
These letter addresses, or axis designations, are necessary for the electronic control system to
assign movement instructions to the proper machine element. The assignment of these letter
addresses has been standardized on a worldwide basis and is contained in three standards, all of
which are in agreement. These standards are EIA RS-267-B, issued by the Electronics Industries
18
Association; AIA NAS-938, issued by the Aerospace Industries Association; and ISO/R 841,
issued by the International Organization for Standardization.
The standards are based on a “right-hand rule,” which describes the orientation of the motions as
well as whether the motions are positive or negative. If a right hand is laid palm up on the table
of a vertical milling machine, the thumb will point in the positive X-direction, the forefinger in
the positive Y-direction, and the erect middle finger in the positive Z-direction, or up.
The motions are considered from the part programmer's viewpoint, which assumes that the cutter
always moves around the part, regardless of whether the cutter or the part moves. The right-hand
rule also holds with a horizontal-spindle machine and a vertical table, or angle plate. Here,
spindle movement back and away from the angle plate, or work piece, is a positive Z-motion,
and movement toward the angle plate is a negative Z-motion.
Rotary motions also are governed by a right-hand rule, but the fingers are joined and the thumb
is pointed in the positive direction of the axis. Fig. 4shows the designations of the rotary motions
about the three linear axes, X, Y, and Z. Rotary motion about the X-axis is designated as A;
rotary motion about the Y-axis is B; and rotary motion about the Z-axis is C. The fingers point in
the positive rotary directions. Movement of the rotary table around the Y-axis shown in Fig. 4is a
B motion and is common with horizontal machining centers. Here, the view is from the spindle
face looking toward the rotary table. Referring, again, to linear motions, if the spindle is
withdrawn axially from the work, the motion is a positive Z. A move toward the work is a
negative Z.
When a second linear motion is parallel to another linear motion, as with the horizontal boring
mill seen in the figure below, the horizontal motion of the spindle, or quill, is designated as Z
and a parallel motion of the angle plate is W. A movement parallel to the X-axis is U and a
movement parallel to the Y-axis is V. Corresponding motions are summarized as follows:
19
Linear Rotary Linear and Parallel
X A U
Y B V
Z C W
(ERIK OBERG, 2000)
Figure 7: Sample CNC Machine
Source: http://mmsonline.com
20
The same right-hand rule applies to this four-axis lathe, on which each turret moves along its
own two independent axes. Movement of the outside-diameter or upper turret, up and away from
the work piece, or spindle centerline, is a positive Z motion, and movement toward the work
piece is a negative Z-motion. The same rules apply to the U-movement of the inside-diameter, or
boring, turret. Movement of the lower turret parallel to the Z-motion of the outside-diameter
turret is called the W-motion.
A popular lathe configuration is to have both turrets on one slide, giving a two-axis system rather
than the four-axis system shown below:
X-and Z-motions may be addressed for either of the two heads. Upward movement of the boring
head therefore is a positive X-motion.
2.0.7 NUMERICAL CONTROL PROGRAMMING.
A numerical control (NC) program is a list of instructions (commands) that completely describes,
in sequence, every operation to be carried out by a machine.
When a program is run, each instruction is interpreted by the machine controller, which causes
an action such as starting or stopping of a spindle or coolant, changing of spindle speed or
rotation, or moving a table or slide a specified direction, distance, or speed. The form that
program instructions can take, and how programs are stored and/or loaded into the machine,
depends on the individual machine/control system. However, program instructions must be in a
form (language) that the machine controller can understand.
A programming language is a system of symbols, codes, and rules that describes the manner in
which program instructions can be written. One of the earliest and most widely recognized
numerical control programming languages is based on the Standard ANSI/EIARS-274-D-1980.
The standard defines a recommended data format and codes for sending instructions to machine
controllers. Most new controllers also accept instructions written in proprietary formats offered
(specified) by the controller's manufacturer.
21
In programming, before describing the movement of any machine part, it is necessary to
establish a coordinate system(s) as a reference frame for identifying the type and direction of the
motion. A description of accepted terminology used worldwide to indicate the types of motion
and the orientation of machine axes is contained in a separate section (Axis Nomenclature). Part
geometry is programmed with reference to the same axes as are used to describe motion.
2.0.8 METHODS OF NC MACHINE INTERACTION/PROGRAMMING
Manual data input (MDI) permits the machine operator to insert machining instructions
directly into the NC machine control system via push buttons, pressure pads, knobs, or
other arrangements. MDI has been available since the earliest NC machines were
designed, but the method was less efficient than tape for machining operations and was
used primarily for setting up the NC machine. Computer numerical control (CNC)
systems, with their canned cycles and other computing capabilities, have now made the
MDI concept more feasible and for some work MDI may be more practical than
preparing a program. The choice depends very much on the complexity of the machining
work to be done and, to a lesser degree, on the skill of the person who prepares the
program. (ERIK OBERG, 2000)
Conversational part programming is a form of MDI that requires the operator or
programmer to answer a series of questions displayed on the control panel of the CNC.
The operator replies to questions that describe the part, material, tool and machine
settings, and machining operations by entering numbers that identify the material, blank
size and thickness or diameter, tool definitions, and other required data. Depending on
capability, some NUMERICAL CONTROL 1241 controls can select the required spindle
speed and feed rate automatically by using a materials look-up table; other systems
request the appropriate feed and speed data. Tool motions needed to machine a part are
described by selecting a linear or circular motion programming mode and entering
endpoint and intersection coordinates of lines and radius, diameter, tangent points, and
directions of arcs and circles (with some controllers, intersection and tangent points are
calculated automatically). Machined elements such as holes, slots, and bolt circles are
entered by selecting the appropriate tool and describing its action, or with “canned
routines” built into the CNC to perform specific machining operations. On some systems,
22
if a feature is once described, it can be copied and/or moved by: translation (copy and/or
move), rotation about a point, mirror image (copy and rotate about an axis), and scaling
(copy and change size). On many systems, as each command is entered, a graphic image
of the part or operation gives a visual check that the program is producing the intended
results. When all the necessary data have been entered, the program is constructed and
can be run immediately or saved on tape, floppy disk, or other storage media for later use.
Manual part programming describes the preparation of a part program by manually
writing the part program in word addressed format. In the past, this method implied
programming without using a computer to determine tool paths, speeds and feeds, or any
of the calculations normally required to describe the geometry of a part. Today, however,
computers are frequently used for writing and storing the program on disk, as well as for
calculations required to program the part. Manual part programming consists of writing
codes, in a format appropriate to the machine controller, that instruct the controller to
perform a specific action. The most widely accepted form of coding the instructions for
numerically controlled machines uses the codes and formats suggested in the ANSI/EIA
RS-274-D-1980, standard. This type of programming is sometimes called G-code
programming, referring to a commonly used word address used in the RS-274-D
standard. Basic details of programming in this format, using the various codes available,
are discussed in the next section (G-Code Programming).
Computer-assisted part programming (CAPP) uses a computer to help in the
preparation of the detailed instructions for operating an NC machine. In the past, defining
a curve or complicated surface profile required a series of complex calculations to
describe the features in intimate detail. However, with the introduction of the
microprocessor as an integral part of the CNC machine, the process of defining many
complex shapes has been reduced to the simple task of calling up a canned cycle to
calculate the path of the cutter.
Postprocessors (Important)-A postprocessor is computer software that contains a set of
computer instructions designed to tailor the cutter center line location data (CL data),
developed by a computerized part programming language, to meet the requirements of a
particular machine tool/system combination. Generally, when a machine tool is
programmed in a graphical programming environment or any high-level language such as
23
APT, a file is created that describes all movements required of a cutting tool to make the
part. The file thus created is run, or compiled, and the result is a list of coordinates (CL
data) that describes the successive positions of the cutter relative to the origin of the
machine's coordinate system. The output of the program must be customized to fit the
input requirements of the machine controller that will receive the instructions. Cutter
location data must be converted into a format recognized by the control system, such as G
codes and M codes, or into another language or proprietary format recognized by the
controller. Generally, some instructions are also added or changed by the programmer at
this point.
2.0.9 G-CODE PROGRAMMING
Programs written to operate numerical control (NC) machines with control systems that comply
with the ANSI/EIA RS-274-D-1980, Standard consist of a series of data blocks, each of which is
treated as a unit by the controller and contains enough information for a complete command to
be carried out by the machine. Each block is made up of one or more words that indicate to the
control system how its corresponding action is to be performed. A word is an ordered set of
characters, consisting of a letter plus some numerical digits, that triggers a specific action of a
machine tool. The first letter of the word is called the letter address of the word, and is used to
identify the word to the control system. For example, X is the letter address of a dimension word
that requires a move in the direction of the X-axis, Y is the letter address of another dimension
word; and F is the letter address of the feed rate.
A collection of G code commands commonly used in CNC machines is as shown in the
appendix.
24
CHAPTER THREE
3.0 AUTOMATION OF FACING AND TURNING OPERATIONS
Turning is a machining operation in which a cylindrical surface is generated on a rotating work
piece when the feed motion of the tool is axial i.e. parallel to the work piece axis while facing is
a machining operation obtained when the feed motion of the tool is given radially i.e. normal to
the work piece‟s axis of rotation.
Automation refers to the use of technology to operate and control manufacturing systems or
processes with the help of mechanical, electrical, mechatronic, computer and information
technology based systems.
Automation can be of fixed, programmable or flexible type depending on the kind of task the
automation is dedicated to.
In order to automate the above mentioned processes it was necessary to know of what was
required of the whole process. Since part of the aim of automation was to minimize the human-
machine tool interaction, it was fundamental to realize that not every process can be automated
to the latter. Some processes like tool positioning can still be done by the machine operator.
For automation to be done, there has to be a hardware (electromechanical device), a computer
program (software), a microcontroller and a machine-user interface as discussed in Chapter Two.
As for the project, „automation‟ of the facing and turning processes were achieved through
controlling the X and Y motions of the carriage and the cross-slide with the aid of a servomotor
namely a stepper motor of which it‟s pulse were being controlled by a computer program.
In order to accomplish the above mentioned process, the following components were required as
discussed below.
3.0.1 Arduino
This is an open source computer hardware and software company, project and user community
that designs and manufactures kits for building digital devices and interactive objects that can
25
sense and control. For its programming, Arduino platform provides an integrated development
environment (IDE) based on the processing project which includes support for C and C++
programming languages.
The one used in this project is the Arduino Uno. It has 14-digital input/output pins (of which 6
can be used as PWM outputs), 6 analogue inputs, a 16MHz ceramic resonator, a USB
connection, a power jack, an ICSP (In Circuit Serial Programming) header and a reset button.
(Arduino board Uno, 2015)
The board can be powered either via a USB which is connected to a computer or it can be
powered externally via a d.c power jack depending on the current demanded by the device which
is to be connected with the Uno. Arduino and other micro controllers provide analog to digital
(ADC) conversion to convert an input voltage to a digital value. One might think that they also
provide the converse which is digital to analog (DAC) conversion but that is not the case because
they instead provide pulse-width modulated (PWM) outputs.
The Arduino library provides this functionality with a function called analogWrite ().The name
seems to imply DAC functionality but it just controls the PWM output. For many applications,
such as the case of motor control, PWM is sufficient. For other applications, such as creating a
linear voltage or current driver, a real DAC is needed. (ProvideYourOwn, 2015)
3.0.2 Grbl
This is a free, open source, high performance software for controlling the motion of machines
that move, that make things, or that make things move, and will run on a straight Arduino and
written in optimized C.
It is a no-comprise, high performance, low cost alternative to parallel-port-based motion control
for CNC machines such as 3D printers, laser cutters, automatic hand writers and milling
machines.
It accepts standards-compliant g-code and has been tested with the output of several CAM tools
with no problems .Arcs, circles and helical motion are fully supported as well as, all other
primary g-code commands. (Grbl, 2015)
26
3.0.3 Grbl shield
This is a shield that plugs onto the Arduino development platform transforming it into a CNC
controller using the CNC firmware. It is compatible with the uno and other 328p versions of the
Arduino development platform. (grbl shield |synthetos.com, 2015)
Figure 8: Grbl shield
Source: https://www.synthetos.com/project/grblshield/
Each stepper motor received electric signals from a stepper motor on how fast to spin, as well as
control the direction of spin of the motor shaft.
3.0.4 Universal g-code sender
This is one of the CNC special software which is compatible with the GRBL shield .Once it was
installed on the computer, it was then be used to send signals over a USB to the Arduino Uno
board. (winder/Universal-G-Code-Sender · GitHub, 2015)
In order to control the individual stepper motors precisely, proper wiring was necessary in order
to ensure that the computer software interacted well with the available hardware.
27
3.0.5 Stepper motors
These are electromechanical devices which converts electrical pulses into discrete mechanical
movements. The shaft or spindle of a stepper motor rotates in discrete step increments when
electrical command pulses are applied to it in the proper sequence. The sequence of the pulses is
directly related to the direction of the motor shafts rotation.
It is very important to note that a stepper motor can be a good choice whenever a controlled
movement is required. Additionally, they can be used to advantage in applications where one
needs to control the rotational angle, speed, position and synchronism.
In-order to achieve the precise motion of the stepper motor programming was done to have full
control of the cross slide and the carriage using numerical control.
Stepper motors can be classified as either unipolar or bipolar depending on the winding
arrangements for the electromagnetic coils in a two phase current supply.
Unipolar stepper motors
Figure 9: Unipolar stepper motor
28
These motors typically have 6 wires, where two of those wires are usually connected to ground,
also called „common‟ or common ground. This leaves 4 wires for control. When a voltage is
applied to a control wire, current will flow from that wire to ground. When no voltage (0 volts) is
applied to a control wire, no current will flow to or from that control wire. Unipolar motors are
heavier, weaker, and slower than bipolar motors. Unipolar motors is advantageous in that they
require less power than bipolar motors. Powerful power supplies can be expensive, so it may be
slightly cheaper to buy and use unipolar motors.
Bipolar stepper motors
Figure 10: Bipolar stepper motor
These motors usually have 4 wires, but may have 8 wires for very powerful motors. In these
motors, current can flow either direction through the coils, and all coils can be activated at the
same time. This means bipolar motors can be very strong and fast which is thus an added
advantage over the unipolar motors. The only disadvantage is that these motors can require a lot
of power, which means you might need a beefy supply if you want to run the motors at a high
voltage for faster speeds.
The step angle and the step sizes of a stepper motor were obtained as follows. Given that a
stepper motor moved 200 steps per each revolution, then the step angle was expressed as:
8.1200
36011
3601200
step
revolutionsteps
The steps per mm covered by the stepper motor can be given as follows
29
128025.1
8200__
_
_/__
mmpersteps
pitchthread
microstepsdriverrevstepsmmpersteps
This was taking into consideration that the leadscrew had a standard M8 thread of 1.25mm pitch.
In a stepper motor, a torque is developed when the magnetic fluxes of the rotor and stator are
displaced from each other. The torque produced is dependent on: the step rate, the drive current
in the windings and the drive design or type (i.e. wave drive-1 phase on, full step drive-2 phases
on, half step drive-1 and 2 phases on, microstepping-continously varying motor currents).
The performance of a stepper motor system (driver and motor) also highly depends on the
mechanical parameters of the load (i.e. what the motor drives) which is typically frictional,
inertial or a combination of the two.
Friction is a resistance to motion due to unevenness of surfaces which rub together. Friction is
constant with velocity. Increasing a frictional load lowers the top speed, lowers the acceleration
and increase the positional error and the vice versa is also true.
Inertia is the resistance to changes in speed. A high inertial load requires a high inertial starting
torque and the same would apply for braking. If the inertial load is increased, the speed stability
increases also which consequently increases the amount of time it takes to reach a desired speed.
3.0.6 Stepper motor driver
Since the stepper motors needed precise timing to control their movement a stepper motor driver
was used. The driver had to be able to supply the current needed. As a matter of fact, the stepper
driver is basically made up of an H-bridge which enables a voltage to be applied across a load in
either direction. These circuits allows the stepper motors to run forward and backwards.
As for the constructed model, a Grbl shield was used which onto it carried the TI DRV8818
stepper drivers.
30
The following was the resultant circuit diagram of the Arduino, stepper motor and the stepper
driver connections.
Figure 11: circuit diagram of the Arduino, stepper motor and the stepper driver
connections
3.0.7 DC motor
The permanent magnet DC Motor is the most commonly used type of small direct current motor
available producing a continuous rotational speed that can be easily controlled. They are ideal for
use in applications where speed control is required such as in small toys, models, robots and
other related electronic circuits. (Storr, 2015)
A DC motor consists basically of two parts namely: the stator which is the stationary part of the
motor and the rotor which is the inner part that rotates producing the movement
The rotational speed of a DC motor depends upon the interaction between two magnetic fields
i.e. one set up by the stator‟s (stationary) permanent magnets and the other by the armature‟s
rotating electromagnets and therefore by being in a position to control this interaction, it can
control the speed of rotation of which this is what is within the scope of the project.
Stepper driver
Ground
To motor
supply (12v)
Arduino
board
Stepper
Motor
31
If the strength of the armatures electromagnetic field is changed by controlling the current
flowing through the windings, more or less magnetic flux will be produced resulting in a stronger
or weaker interaction and therefore resulting in a faster or a slower speed.
This speed control of the D.C. motor was achieved through Regulation of the amount of voltage
across its terminals; a technique known as pulse width modulation. Pulse width modulation
speed control worked by driving the motor with a series “ON-OFF” pulses and varying duty
cycle, the fraction of the time that the output voltage was “ON” compared to when it was “OFF”,
of the pulses while the frequency was kept constant.
The width of these ON and OFF pulses was varied by varying the average DC voltage applied to
the motor terminals. This was made possible by use of the AnalogWrite () function which is a
function that allows for conversion of an analog input voltage of 5V to a digital value between 0-
255 bits. The 0 bit represented the motor zero speed while 255 bits represented the maximum
motor rpm which was 800 rpm for the one used in the model.
The below circuit diagram also illustrated the summary of the electrical design which was used
to demonstrate the DC motor speed control by use of an Arduino board and the other electrical
components as shown below.
Figure 12: electrical design circuit diagram
12V
Opto isolator Motor
220Ω
220Ω
4.7KΩ
Arduino
Uno
MOSFET
transistor
32
3.0.8 Opto-isolators
They are also often called Opto-couplers and it has a mechanism that allows for transmission of
signals between two circuits or keeping the two paths isolated by use of light. They can also be
used if the circuits are of two different voltage levels and there is a need to protect one part of the
circuit in the event of any electrical damage. When used in the model, they act to isolate the
Arduino from any over voltage damage. At the same time, it also functioned as a digital
switching device as they transferred binary control signals i.e. digital data as wells as analog
signals via pulse width modulation.
33
CHAPTER FOUR
4.0 DEVELOPMENT OF CENTER LATHE FOR DESKTOP OPERATIONS
In order to come up with a center lathe machine for desktop operations, it was necessary to come
up with essential design features as well as a CAD model to show the constituent parts and the
functionality of a proposed automated center lathe machine.
Some of the essential design features that were considered while in the process of coming up
with the model were similar to those of any machine tool which included: static and dynamic
stiffness of the machine structure, operational (cutting and feed) motions, gearboxes, controls
and accuracy considerations. Several of the constructional elements like beds, columns, spindles,
slide ways and bearings are also quite similar in the different types of machine tools.
Since the center lathe machine to be constructed can be scaled up and used to manufacture a
commercial purpose lathe machine, the general performance criteria that were also considered
while coming up with the design included the following:
1. Safety-the machine tool should be totally safe for the operator and all the possibilities of
accidents must be foreseen and guarded against.
2. Ease of operation- Loading of the work piece and unloading of the machined part,
bringing the tool into and out of the engagement with the workpiece, feed control, in
process measurement of dimensions and tool resetting should be made easier through
automation.
3. Production accuracy-the work pieces produced on the machine tool should have the
required degree of geometrical, dimensional and surface accuracy.
4. Reliability and machinability-the probability of breakdowns other than those caused by
the anticipated wear and tear of its replaceable parts of the machine during its designated
service life should be limited to a low value. The machine should also be easy to service.
5. Compactness-the machine tool should be compact as possible as it reflects the degree of
elegance, refinement of construction or sophistication of design.
34
6. Ease of manufacture-it should be easily reproducible depending on the simplicity of
design and the proportion of standardized parts (and components) to the total number of
parts in the machine tool.
7. Initial and operating costs-the overall cost of possession and use of a machine tool
includes the manufacturing cost as well as its periodical operating and maintenance cost.
(G. JAYAKUMAR JESUDOSS, 2011)
4.0.1 Identification of machining requirements
Having been able to be in a position to identify all the operations to be carried out in a general
purpose lathe machine, only facing and turning processes were selected to be automated in line
with the objective. Based on the choice of the work material and its availability together with the
machinability, the proposed machine tool intended to be used for desktop purposes as well as for
small scale production of simple shapes.
4.0.2 Choice of the machine type
Since the intention was to automate the tool and work piece movements, the NC lathe type was a
perfect choice under which the sequence of operations would be carried out under machine
control with no human interference required. This therefore replaced the human operator and
thus on the other hand led to meeting the needs of mass production of interchangeable items.
The floor space occupied by the machine tool also mattered greatly and this largely depended
upon its entire structural design as this dictated the quantity of the machine tools to be installed
in a specific room and also its mobility together with the amount of material to be used during its
manufacturing.
4.0.3 Machine tool structure
The proposed NC lathe structure comprised of a bed and a headstock which was integrated in the
bed. This structure formed the backbone of the NC lathe as it transmitted the cutting loads from
the work piece to the foundation and also provided the required stiffness (against static loads)
and the rigidity against inertial forces.
35
The structural configuration of a NC lathe was determined by the required arrangement of slide
ways ,the length of stroke, the mechanisms that must be housed inside the structural elements,
inspection, adjustment, lubrication and chutes to collect the chips that fall down and coolant that
may splash out.
Rectangular cross sections was chosen for beds and columns having sufficient number of ribs.
Ribs welded or cast with the bed wall enhanced rigidity
A mild steel structure was chosen for the sake of higher stiffness. However cast iron may be
preferable for situations where it is desired to use higher inertia for making the structure stable.
The carriage, tailstock, tool holder, motor bracket and the cross slide were attached to the lathe
bed by mostly bolting and mounting on ways. These elements must have high rigidity and its
interaction with the bed must be made as vibration free as possible.
The carriage and cross-slide carried the cutting tools and work piece respectively along straight
or radial paths. They were mounted on ways that were fixed on another slide or sat directly on
some other element of the lathe machine i.e. bed (housing) or even the carriage itself as in the
case of the cross slide. Slide ways may have V,-inverted V, flat, dovetail or cylindrical.
In the CAD design, both the V-shaped and the dovetail type of slide ways were chosen for the
carriage and the cross slide. This is because of the fact that:
1) They provide the unrestrained guidance(this is for the case of the combination of both the
V and flat ways)
2) They are self-adjusting
3) It is easy to lubricate and
4) It is easy to keep dirt away from them
5) Friction in the slide ways can be minimized by forcing oil under pressure between the
mating surfaces
36
4.0.4 Selection of cutting speeds and feeds
The optimum cutting conditions (i.e. speeds, feed and depths of cut) depended upon the tool and
work material and the required surface finish and the dimensional accuracy. From table 4 in the
appendix, the above optimum cutting conditions were easily obtained.
They were then used to determine the required spindle speeds for the proposed dimensional
information about the work pieces to be machined .For instance, if a cylindrical turning operation
was to be carried out, the following optimum conditions would be met:
Let:
Vmax=maximum cutting speed among the various tool-work combinations (m/min)
Vmin=minimum cutting speed among the various tool-work combinations (m/min)
Dmax=diameter of the largest work piece to be processed on the proposed NC lathe (mm)
Dmin=diameter of the smallest work piece.
Workpiece material-Stainless Steel
Tool material-High speed steel
Dmax and Dmin are taken to be 50mm and 5mm respectively (From the CAD model)
Then the following parameters would be obtained from the table 4 in the appendix as follows:
Feed rate=0.5mm/rev
Depth of cut=3mm
Cutting speed range=25 to 30 m/min
The maximum spindle speed Nmax and the minimum spindle speed Nmin may then be obtained as
follows.
37
rpmD
VN 86.1909
5
3010001000
min
maxmax
rpmD
VN 15.159
50
2510001000
max
minmin
4.0.5 Determination of power requirements
The drive power required for the cutting motion depends upon the cutting speed and the cutting
force. The power needed for the feed drive is also dependent upon the feed force and the feed
rate.
The cutting forces mainly depended upon the work material, the size of cut, tool geometry, and
cutting speed and to extend the tool material.
From the analysis of the forces in turning, it was found that the cutting forces in turning could be
resolved into three components namely:
1. The tangential component Ft-it is in the direction of the cutting velocity vector
2. The radial component Fr-is in the direction of the radius of the job
3. The axial component Fa-is in the direction of the longitudinal feed(i.e parallel to the axis
of the job)
Since there is no radial movement, there is no work done by the radial force. Therefore work is
done by the other two components.
The feed velocity is however so small that the product of the feed force and the feed velocity is
negligible compared to that of the tangential force and the cutting velocity.
The above force components in turning may be represented in the exponential form involving
feed and depth of cut as shown below
11
1
yx
t dfRF
38
22
2
yx
r dfRF
3
33 yx
a dfRF (B L Juneja, 1987)
From table 2 of coefficients in the appendix, the following can be obtained as the force
components
kgfdfRFyx
t 3.31335.07.196 96.085.0
111
kgfdfRFyx
r 33.10735.05.37 26.148.0
222
kgfdfRF yx
a 499.11635.0297.89 69.071.03
33
Therefore the rate of work needed in turning approximately equals to the product of tangential
force component and the cutting velocity
i.e. the horsepower needed for turning,4500
maxVFP t
Kw
hp
56.1
089.2
4500
3030.313
The power rating of the cutting drive is given by
hpP
Pc
RC 984.27.0
089.2
hpP
Pc
RC 458.285.0
089.2
ηc is the mechanical efficiency of kinematic chain comprising of the cutting drive and it normally
varies from 0.7 to 0.85.
39
Also for the feed drive, it was necessary to know its power rating. Therefore the feed force Q
may be calculated for lathe operations from the following relation
)( drca WFFkFQ
Where:
Fa,Fc and Fv are the components of the cutting force in the feed, cutting and radial directions
respectively
k is a recommended factor ranging from 1.1 to 1.4 taking into account the overturning moment
β is the coefficient of friction between the carriage and the guide ways
Wd is the weight of the parts being traversed. (B L Juneja, 1987)
Therefore from the material choice of the both the carriage and the bed to be made up of cast
iron and cast iron, The values of β from the various handbook is 1.1 for clean and dry surfaces
with no motion and 0.15 for the same with motion while that for a lubricated and a greasy
surface with motion is 0.07.
Taking k as 1.2 which is in the accepted range of values and Wd approximately 10kgf, then the
feed force Q can be expressed as
kgfQ
Q
94.169
)1033.10730.313(07.0499.1162.1
The power rating of the feed drive is then obtained from the equation below
WhpP
fQP
RF
f
RF
3.134180.020.04500
1086.19095.094.169 3
Ranging from
40
WhpPRF 93.1402.015.04500
1015.1595.094.169 3
ηf is mechanical efficiency of the feed drive and varies from 0.15 to 0.20.
4.0.6 Selection of drives
The prime mover in all the drives is the electric motor and this power is transmitted to the
spindles, carriages and slides in a manner that the rotary motion of the motor shaft is transmitted
through to the drive elements in the required cutting and feed motions.
Some of the factors that were considered in the selection of drive included:
1) Power loss in the drive should be minimum
2) Cutting speeds and feed rates should be obtained without interrupting the machining
operation
3) Full power should be available at all working speeds
4) It should be able to provide the necessary number of cutting speeds and feed rates.
While choosing the prime movers, electrical motors were used of which they were equivalently
used in building up the model.
For the case of the cutting drive, a geared permanent dc motor was chosen. This is because it
motor provided a fine speed control as compared to ac motor of which this was very much
desirable for the case of lathe processes e.g. turning which demands different working spindle
speeds for different workpiece material-cutting tool combinations.
The power developed by the motor was transmitted to the spindle shaft via a belt and pulley
combination of which it then drives the spindle which is coupled to the 3 or 4-jaw chuck which
then holds the workpiece to be machined.
The pulley-belt combination was included in the CAD model but not in the physical model
developed for presentation because of the fact that the small diameter pulleys and belts would
not be found locally.
41
The pulley-belt combination is much advantageous over the chain-sprocket combination or the
gear system in that:-
It has higher mechanical efficiencies
No need for lubrication
No stretching due to wear
Precision timing with no loss of high torque carrying capabilities
With the above design, it implied that the amount of power produced by the d.c motor was the
same amount that which was transmitted to the spindle shaft. This was because since the chosen
permanent magnet dc motor had a gear ratio of 1:1, it meant that the expected output
programmed speed equaled the dc motor output provided that it lied within the manufacturer‟s
designed speed range. In summary, the output speed of the motor equals the rotational speed of
the workpiece taking into consideration that there are no losses during transmission. However for
practical cases, if power was lost through friction, heat or even noise, then the above was
factored in the design and selection of the driver and driven belt pulleys.
For the case of feed drive, a stepper motor was chosen due to the reason that a controlled
movement could be achieved. Its accuracy and repeatability also was of a great importance since
it was a function of mechanical precision of its parts and assembly. They also have a step angle
accuracy of 3-5% of one step and this error is non-accumulative from step to step. Apart from
having the advantage that they can be positioned accurately, the stepper motors can also rotate
continuously.
Two stepper motors were required for both axes i.e. X and Y or X and Z or Y and Z axes
depending on the choice of the Cartesian co-ordinates. One was used to drive the carriage while
the other was used to drive the cross slide which carried the compound slide onto which the tool
post would be mounted on.
Several mechanisms can be used in order to convert the rotary motion into a linear motion. These
include: Rack and pinion, belt and pulley and other mechanical linkages. The above conversion
was accomplished preferably by means of a threaded nut and lead screw because of its
availability as it was integrated with the stepper motor during purchase.
42
In order to generate linear motion, the threaded nut was kept stationary relative to the rotating
lead screw which was attached to the intermediate rotor half shaft. The lead screw chosen was
that with the square thread waveform type due to the reason that:
It is capable of carrying of large loads
It has least friction and is much efficient as compared to trapezoidal waveform.
4.0.7 Safety devices
In order to enhance safety while using an automated lathe machine, limit switches had to be
fitted in different points including just before the end of the carriage slide towards the chuck and
also just before the right far end towards the tailstock side. These are helpful in that during the
movement of a carriage or even the cross slide in either directions, they help detect the extent to
which they are supposed to travel beyond which, movement is prohibited. Just at the point and
the time of contact, a command is sent to the control system telling it to abort any further
movement. By these, accidents in the workplace would be minimized by a great deal.
4.0.8 CAD model
The above mentioned design features were then used to come up with a full pictorial
representation of a CAD model. The CAD model was drawn using the Autodesk Inventor
software.
In order to come up with a complete assembly of an automated center lathe machine model,
several parts were first drawn.
Each and every single part was drawn in each respective 2-Dimensional work plane. Once the 2-
D sketch was complete, other 3-D sketch features such as extrusion, filleting and chamfering
were then carried out to give the feature or part a 3-D look.
After all the parts had been completely drawn, assembly was then done. This was done by
applying constraints such as mate, flush, rotational, transition, angle and rotational-translational
which would in turn bring the two selected parts together. For example, the mate constraint
43
would bring two surfaces together as one along a certain axis while the angle constraint would
ensure that one surface is mated to another by a certain angle.
The flush constraints were then driven by some distance in either forward or backward direction
in order to ensure that the chosen constraints were fit to be used in the animation of the carriage
and cross slide motion.
The material selection was also done for each component in the material selection library and
saved which would then automatically reflect in the assembly.
An animation of the selected movable parts such as the chuck, cross-slide, carriage and the
tailstock was then done in the Inventor Studio. The suitable constraints used in bringing the
above movable parts were then animated with the help of an animation timeline.an animation
timeline is a platform which allows for a certain constraint in the assembly to be moved or
animated over a certain period of time before another takes place.
The complete assembly and the parts list are as attached below in the next page.
4.0.9 The physical model construction
First, the appropriate sizes of the wood to be used were to be cut. Having measured the
maximum length of the stepper motor lead screw, then that was taken to be the maximum length
of the first two linear rails to be cut to be used for the lower platform which drove the carriage
(taken to be X axis).
The first two blocks of soft board were cut measuring 25cm by 20cm and 25cm by 4cm
respectively .These would form a base over which the linear rails would be held. Also the X axis
stepper motor would be fixed on the larger piece using the recommended screw sizes.
After the marked points at which the linear rails would be fitted on both boards, drilling was then
carried out with the help of a drilling machine using a suitable drill bit.
Four stainless steel linear rails of length approximately 25cm and 0.9cm diameter were then cut
using a hacksaw.
44
A 25cm by 13cm board was cut which would then be used to represent the carriage which is
being carried along when the stepper motor rotates. Using one side of the board, center points
were marked. These points represented those along which the linear guide rails fitted on the side
supports would be moving along thus providing a smooth and restrained movement of the
carriage to and fro. After the four points had been marked, the thin metallic strips of a diameter
nearly similar to that of the bushings to be fitted were then secured with a set of screws.
Bushings were then fitted inside the metallic strips. In the remaining marked center axis, a lead
screw nut was then tightly secured using a similar metallic strip so as to ensure that it doesn‟t
freely rotate with respect to the rotation of the stepper motor lead screw.
As for the Z axis components, similar steps were followed. The only additional thing that was
done was that, the constructed cross slide was attached to the carriage soft board as shown in the
attached pictures below.
By this it implied that whenever the carriage moved in either forward or backward direction with
a given feed, it correspondingly moved together with the cross slide and whenever it came to a
stop for a given command, then the cross slide then independently moved in with the tool and it
thus approached the workpiece at a certain depth of cut.
Plan view of the carriage and cross slide assembly
Figure 13: Plan view of the carriage and cross slide assembly
45
Bottom view of the carriage and cross slide assembly
Figure 14: Bottom view of the carriage and cross slide assembly
Plan view of the base
Figure 15: Plan view of the base
The above components were then assembled all together to form one complete assembly as
shown below.
46
Assembly of the base,cross slide and the carriage
Figure 16: Assembly of the base, cross slide and the carriage
Before fixing the stepper motors onto the soft board, they were first tested while still in the
ground in order to ensure that they were functioning very well under no load. Wires of the same
phase were identified by carrying out the continuity test of the wire using the digital multi meter.
The red and blue, black and green wire pairs were connected to one stepper driver port. Similar
color combination was used for the case of the other motor.
The Grbl stepper shield was then mated with the corresponding pins of the Arduino Uno board.
The Uno was then connected to a 12V d.c external power supply via a power jack. A USB cable
was then connected to the computer to enable serial communication between the two.
Before powering any device, proper wiring was checked using the multi meter in order to verify
the voltages provided to the circuit as recommended by the manufacturer in the data sheets.
47
With everything in order, the operation was good to go. The universal G-code sender application
was then initialized in the computer which would act as a machine interface in the real world.
Several statements of G-codes were then send to control the movement of the stepper motor
either in the reverse or the forward direction.
The respective simple electrical circuit diagram was as shown below
Figure 17: Electrical circuit diagram
The sample programs used to control the stepper motor and the speed of the DC motor are as
shown in the appendix.
Stepper driver
Ground
To motor
supply (12v)
Arduino
board
48
CHAPTER FIVE
5.0 DISCUSSION
The overall design provided for an innovative and cheap solution to the design problem. The
model weighed approximately 7kg and even though small it can handle some work piece which
is supported by a pair of linear rails beneath it to stop it from rotating with the rotating lead
screw. The cost of constructing the model was also cheap with the use of locally available
resources as compared with importing the components since the overall cost of all the materials
used summed up to approximately Ksh.80,000.
Since the model was computer controlled, it was safer to use and its accuracy was high. The
material used to construct the model was wood which was locally available, light and also cheap.
The speed control which uses the gear mechanism in the headstock was also replaced by a
computer controlled program which uses the pulse width modulation to regulate the amount of
the power applied to the motor by varying the width of the “ON” and “OFF” applied pulses and
thereby varying the average DC voltage applied to the motor terminals. By use of d.c motors,
fine speed control was achieved without use of much complex circuitry as compared to a.c
motors. Fine speed control is fundamental in any machining operation because different
operations require different spindle speeds depending on the different workpiece-tool
combinations.
The d.c motor and the other associated electrical components weighed much less as compared to
the gear system which is heavy and also wears out slowly making it expensive to replace. Also
the gears are very expensive to manufacture as compared to the d.c motor which is computer
controlled.
The use of a stepper motor also ensured that good positional accuracy was obtained during the
movements of the carriage and the cross-slide. Since the stepper motor was controlled using a
computer program, it implied that a precise rate of feed motion and the depth of cut would be
achieved since the amount of current to be supplied to each coil of the stepper motor would be
controlled precisely and therefore improving the accuracy.
49
5.1 CONCLUSIONS
The design requirement of the project was to come up with a model to show the automated
processes i.e facing and turning using the locally available materials of which the objective was
met successfully.
In order to demonstrate of the functionality and design of an automated center lathe machine to
be used for desktop operations, the various drawings of a retro-fitted lathe machine parts were
drawn using Autodesk inventor software taking into consideration factors such as: the safety of
the machine operator, cost of maintenance and improved accuracy of the machining processes.
With the above cost analysis and economic considerations, it is quite clear that the conventional
lathe machines can be re-fitted to make use of CNC features and thus cutting down the costs of
production as wells as purchase costs.
5.2 RECOMMENDATIONS
There are several ways in which the design can be improved. First, the remaining processes such
as, knurling, threading, coolant control, spindle control, tapering, boring among others can also
be automated in a similar manner. Secondly, the structural design of the machine tool can also be
designed for maximum performance against rigidity, stiffness, weight and also minimum energy
consumption.
The above model can also be used to come up with more and more models to be used for desktop
purposes such as practical learning in higher education institutions. Furthermore they can also be
used to design a multi-purpose lathe which can be used for even machining of small size woods,
metals, plastics e.t.c in industries.
With the advantage that the model is light and portable, it can thus be easily moved to another
place for use as opposed to the bench lathe which is heavy and fixed on the ground.
50
REFERENCES
1) Arduino board Uno. (2015, March 26). Retrieved from arduino.cc:
http://arduino.cc/en/main/arduinoBoardUno
2) B L Juneja, G. S. (1987). Fundamentals of Metal Cutting and Machine Tools. New Delhi:
New Age International (P) Limited,Publishers.
3) ERIK OBERG, F. D. (2000). Numeric control. In F. D. ERIK OBERG, Machinery's
Handbook (p. 1293). New York: Industrial press.
4) G. JAYAKUMAR JESUDOSS, C. R. (2011). GENERAL MACHINIST THEORY.
5) Grbl. (2015, February 5). Retrieved from github.com: https://github.com/grbl/grbl
6) grbl shield |synthetos.com. (2015, April 3). Retrieved from www.synthetos.com:
https://www.synthetos.com/project/grblshield/
7) Jain, E. R. (2009). Numerical Control. In E. R. Jain, Production technology (p. 740).
Khanha publishers.
8) Jaychris. (2010, January 15). www.brighthubengineering.com. Retrieved from What is a
Lathe Machine? History, Parts, and Operation:
http://www.brighthubengineering.com/manufacturing-technology/59033-what-is-a-lathe-
machine-history-parts-and-operation/
9) Prakash N. Parmar, P. N. (2014). Investigation on Automation of Lathe Machine.
International Journal of Emerging Technology and Advanced Engineering, 6.
10) ProvideYourOwn. (2015, January 16). Analog Output - Convert PWM to Voltage.
Retrieved from www.instructables.com: http://www.instructables.com/id/Analog-Output-
Convert-PWM-to-Voltage/
11) Storr, W. (2015, April 13). Pulse Width Modulation Used for Motor Control. Retrieved
from www.electronics-tutorials.ws: http://www.electronics-tutorials.ws/blog/pulse-width-
modulation.html
12) Wikipedia. (2014, December 8). Numerical control. Retrieved 1 6, 2015, from
Wikipedia: http://en.wikipedia.org/wiki/Numerical_control
13) winder/Universal-G-Code-Sender · GitHub. (2015, April 3). Retrieved from github.com:
https://github.com/winder/Universal-G-Code-Sender
51
APPENDIX
Values of coefficients R1, R2, R3 and the exponents x1, x2, x3 and y1, y2, y3
material Ft Fr Fa
R1 x1 y1 R2 x2 y2 R3 x3 y3
0.2%
carbon
steel
162.4 0.85 0.98 34.3 0.8 1.46 40.5 0.67 0.47
Brass 123.6 0.81 0.96 22.3 0.91 1.43 56.9 0.97 0.38
Stepper motor axis control
52
Recommended cutting speeds (m/min) for HSS, CARBIDE and OXIDE tools
Work
material
Depth of
cut(mm)
0.1-0.4 0.4-2.5 2.5-5.0 5-10
Feed
rate(mm/rev)
0.05-0.15 0.2-0.35 0.4-0-75 0.75-1.25
Cut Light cut Heavy cut
Free
machining
steel
100-15 80-100 50-80 25-50
carbon and
alloy steel
90-150 60-90 40-60 20-40
medium alloy
steel
80-150 50-80 35-50 20-35
High alloy
steel
70-120 45-75 30-45 20-30
Stainless
steels
45-75 30-45 25-30 15-25
Cast
iron(soft)
45-75 35-45 25-40 20-30
Cast irons
(medium
hard)
45-75 35-45 25-35 15-25
Cast
irons(hard)
35-65 25-35 15-25 10-15
Aluminum
alloys
100-150 60-100 40-60 30-40
Copper alloys 100-150 80-100 60-80 40-60
Magnesium
alloys
150-220 100-150 80-100 60-80
53
List of G-code commands
54
Dc motor speed control program
