• On completion of this session, the delegates
would have understood about
The history of NC and CNC machine
The components of CNC machine
Working of a CNC machine
Application and limitations of CNC machine
Merits and demerits of adopting CNC
What is a machine tool?
• The main function of a machine tool is to remove
metal from a piece of work to give the required
shape and size.
• This is accomplished by achieving proper relative
motion between the work-piece and the tool.
Conventional Machine Tool
• In conventional lathe the job is fixed between
centers and made to rotate by powering the
• The tool motion is achieved by operating the
hand wheels and the feed rates are manually
• The productivity and quality of the product
depends on the operator’s skill and attitude.
• These machines are characterized by low speed,
feed and metal removal rate.
• Producing complex part will be difficult
• In, recent times, the main requirement of the machining
industry is that the machine tool must be flexible, generally
suitable for batch production, capable of carrying out many
operations on single setting and high metal removal rate.
• The conventional machines don’t meet these requirements;
hence the machine tool with new concept was invented
• Numer ically cont r olled (NC) is t he t er m t o
descr ibe t he cont r ol of machine movement s
and var ious ot her f unct ions by inst r uct ions
expr essed as a ser ies of number s, let t er s or
symbols and init iat ed in an elect r oniccont r olled syst em.
Computer numerical control
• Comput er ized numer ical cont r ol (CNC) is t he
t er m used when t he cont r ol syst em includes a
comput er in t he loop (1970s)
History of NC machines
• 1947-U.S. air force has great need for automated machining of free
• Late 1940’s- john person devices method involving drilling holes at
locations specified on punch cards.
• 1951 - Servo-mechanism Lab at MIT subcontracted to refine
• 1952 lab demonstrates modified milling machine (first NC
machine).MEEM4403 Computer Aided Design Methods.
• 1970-computer controlled developed (called computer numerical
control – CNC
• 1980’s- direct numerical control
• One mainframe
History of NC machine
• 1980’s – Dist r ibut ed numer ical cont r ol
• Mainf r ame comput er sends pr ogr ams t o
PC cont r olling each NC machine.
Benefits of CNC
• Improved automation
• The operator invention related to producing work piece
can be reduced or eliminated.
• Many CNC machine can run attached during their entire
machining cycle ,freeing operator to do other tasks.
• Reduced operator fatigue
• Consistent and predictable machining time for each work
• Today’s CNC machines boast almost unbelievable accuracy
and repeatability specifications.
• CNC machine tools are flexible. Since these machines are
run from program, running a different work piece is almost
as easy as loading a different program.
Any limitations of CNC?
Relatively few but…
• Initial cost and subsequent installation.
• Continuous working to meet cost.
• Steady flow of work is must.
• Operator skill and management.
• Lots of preplanning and high caliber
Schematic of an NC machine tool
• MCU – machine control unit- controls the
motion of the NC machine tool
• DPU – data processing unit –reads and
interprets the part program; sends immediate
commands to CLU.
• CLU- control loop unit – reads position sensors
and sends control signals to motors.
Punched tape used for Transfer of Part
Program to NC Machine
CNC machine Basic Working
• Motion Control- The heart of CNC
- All forms of CNC equipment have to or more
direction of motion called axes.
- These axes can be precisely and automatically
positioned along their length of travel
• The two most common axis types are:
- Linear (driven along a straight path)
- Rotary (driven along a circular path)
Conventional Machine - Working
• A conventional machine’s slide is moved by an
operator turning the hand wheel
• Accurate positioning is accomplished by the
operator accounting the number of
revolutions made on the hand wheel, plus the
graduation on the dial
In CNC Machines
• CNC machines allow motions to be actuated
by servomotors under control of CNC, and
guided by part program.
• Generally speaking, the motion type (rapid,
linear and circular), the axes to move, the
amount of motion and notion rate (feed rate )
are programmable with almost CNC machine
• The latest developments in the CNC machine
tools are the versatile machining center.
• This is a single machine capable of doing a
number of operations such as drilling, reaming
tapping, milling and boring.
• All types of tools are mounted on a drum/ chain
or egg box type magazine which are put into the
spindle by automatic tool changer (ATC) under
the control of tool selection instruction.
Establishing coordinate system
• Establish coordinate systems- Tool path coordinate system must
match machine tool coordinate system
• Z-direction-: same as tool spindle rotation or work-piece rotation
axis. + ve --- tool moves away from work-piece
• Y- direction: chosen to give right hand coordinate system
Some machine tools have secondary slide motion labeled u, v, and w.
• Some machine tool have rotational motion about x, y, and z axes
labeled a, b, and c respectively
• 2- axis machine allows controlling two motion simultaneously a 3axis machine allows controlling three motion simultaneously ,etc.
• 2-axis, 3-axis ,4-axis and 5-axis machines are most popular; 9 axis is
• Drive motor are required to perform the
• To drive the main spindle (spindle drive)
• To drive the saddle or carriage(Axis drive)
• In CNC machines, large variation in cutting speed is
• The cutting may vary from 10 meters per minutes to
1000meters per minutes
• The cutting speed are provided by rotation of main spindle
with the help of an electric motor through suitable gear
• To obtained optimum cutting speed and feeds, the drive
mechanism should be such as to provide infinitely variable
speed between the upper and the lower limits.
• The infinitely variable speed system used CNC machines
employ either electrical motors (AC or DC) or fluid motors
• The drive may be direct from the motor to
machine spindle or indirect, through belt or gear
• If belt drive is employed, Toothed Belts are used
and if gear drives are employed, is constant
meshing type of gear box where gears giving
various ratio are usually in constant mesh and are
operated by remote controlled electro magnetic
or hydraulic clutches
• Stepper motor drives have limited use in CNC
AC induction motors are used to drive main spindle directly
The AC induction motor are more reliable, easily
Maintainable and less costly compared to other electrical motors
Speed variation in AC motors can be obtained by pole change method
The step-less speed variation can also be obtained by varying the
frequency of the help of frequency converters
DC motors are being extensively used for step less speed variation of
The step-less variation of speed is obtained by varying the D.C. voltage
applied to the motor
Fluid motor are also being used for driving the spindle
Pressurized oil or air supplier by a pump running at constant speed is
directed on to the blades of the motor, which are capable of giving very
high rotational speed
• By the end of session ,the delegates would
• Principles of grinding
• Classification of grinding processes
• Grinding wheel specifications
• Detail of abrasive wheel parameters
• Different wheel forms
• Dressing of grinding wheels
Definition: Another material removal process, in
which abrasive particles are contained in bonded
grinding wheel, that operates at very high surface
the grinding wheel is usually in disk shaped and
is precisely balanced for high rotational speeds.
• Dimensional accuracy: 0.3 to 0.5 µm
• Surface finish: Ra = 0.15 to 1.25 µm
• Specific energy for grinding: 50 j/mm3 (other
processes: 2 to 5 j/mm3)
• In grinding, the chips are small but are formed by the
same basic mechanism of compration and shear.
• Burning chips are the sparks observed during grinding
with no cutting fluid, because the chips have heat
energy to burn or melt I the atmosphere.
• The feed and the depth of cut in grinding are small,
while the cutting speed is high.
grinding may be classified as non-precision or
precision, according to the purpose and procedure.
The common form called, snagging and off-hand grinding.
Both are done primarily to remove stock that can not be
taken of as conveniently by other methods.
The work is pressed hard against the wheel or vice versa.
The accuracy and surface finish are of secondary
It’s concern with producing good surface finished and
3 types of precision grinding exists
External cylindrical grinding
internal cylindrical grinding
• A grinding wheel is made of abrasive grins held
together by bond.
• These grains cut like teeth when the wheel is
revolved at high speed and is brought to bear
against a work piece.
• The properties of a wheel that determine how it
acts are the kind and size of abrasive, how closely
the grains packed together and amount of
Grinding wheel specifications
• All grinding wheel manufacturers use substantially the
same standard wheel making system.
• This system uses numbers and letters to specify
abrasive type, grit size, grade structure, and bonding
• However, properties of the wheels are determined to a
large extent by the way the wheels are made.
• The processes vary from one plant to another, and
wheels carrying the same symbols but made by
different manufacturers are not necessarily identical.
American national standard institute's
marking system for standard wheels
Prefix-Abrasive type-Abrasive grain size-Grade-structureBond-type-Manufacture record
Prefix : Manufacturer symbol indicating exact kind of
abrasive (use optional)
Abrasive type: A: aluminum oxide
C: silicon carbide
B: boron nitride
Grain size: coarse:8-24,medium:30-60,fine:70-180,very
American national standard institute's marking
system for standard wheels(continue….)
Grade : ranges from A-Z, where A represent soft , Z
represent hard wheel grade.
Structure : Scale is numerical. 1: very dense and 15 very
Bond type : B : Resinoid
E : Shellac
R : Rubber
S : Silicate
V : Vitrified
Manufacturer record : Manufacture’s private marking to
identify the wheel.
Correctly Mounted Wheel
Bench or pedestal mounted grinders
• Different abrasive materials are appropriate
for grinding different work material.
• Abrasive are hard substances used in various
forms as tool for grinding and other surface
• They are also able to cut materials which are
too hard for other tools and give better
finishes and hold closer tolerances.
Common abrasive materials
1. Aluminum oxide (Al2O3) known as Alundum or
Aloxide. Various substances may be added to
enchance hardness, toughness, etc. plain is Al2O3
white, and used to grind steel ferrous , high
2. Silicon carbide (SiC) known in trade as
Carborundum and Crystalon. Harder than Al2O3,
but not tough. Used to grind : aluminum, brass,
stainless steel, cast iron, certain brittle ceramics
Common abrasive materials(continue)
• Boron Nitride in the forms of single-crystal
cubic boron nitride (CBN) and microcrystalline
cubic boron nitride (MCBN) under trade
names such as Boraszon or Borpax . Used for
hard materials such as hardened tool steels
and aerospace alloys.
• Diamond a pure form of carbon. Used on hard
materials such as ceramics cemented carbide
To get wide range of properties needed in grinding
wheels, abrasive materials bonded by using
organic or inorganic materials.
Inorganic bonds :
1. Vitrified bond: clay bond melted to a porcelain or
glass like consistency.
it can be made strong and rigid for heavy grinding
and not affected by water ,oil , acids.
2. Silicate bond: essentially water glass hardened
by baking. It holds grains more loosely than a
vitrified bond gives closer cut .
Wheel shape and sizes
The principal dimensions that designate the size
of grinding wheel are the outside diameter,
width, hole diameter. Standard wheel shapes
are made in certain sizes only, but variety is
• Honing is a low abrading which uses bonded
abrasive wheel sticks for removing stock from
metallic and non metallic cylindrical surfaces as
well as flat surface.
• Performed as a final operation to correct the
errors that have occurred from the previous
Correction of geometrical accuracy – out of
roundness, taper, axial distortion
Principles of grinding
Classification of grinding processes
Grinding wheel specifications
Details of abrasive wheel parameters
Different wheel forms
Dressing of grinding wheels
A brief outline of honing process
have been discussed