Unit V CNC Machining

RAMCO INSTITUTE OF TECHNOLOGY
Mr.M.LAKSHMANAN
Assistant Professor (Senior Grade)
Department of Mechanical Engineering

UNIT V CNC MACHINING SYLLABUS
Numerical Control (NC) machine tools – CNC
types, constructional details, special features,
machining centre, and part programming
fundamentals CNC – manual part
programming – micromachining – wafer
machining

1959 CNC Machine: Milwaukee – Matic- II was first
machine with a tool changer

Numerical Control Machine (NC)
If various function of machine tools are
controlled by means of prepared program,
which consists of letters numbers and
symbols, then machine tools are called
numerical control machine tools.

Machine Control Unit (MCU)
It has some electronic hardware elements to
read the NC Program, interprets it and equally
translates it for mechanical actions of the
machine tool. It’s the brain of NC Machine.
MCU converts the information from the tape
program into the desired command signals.
It controls the path of the cutting tool, speed,
feed, tool change and several other functions.

Types of NC Systems based on the Type of
Machine Control
• Traditional Numerical Control (NC)
• Computer Numerical Control (CNC)
• Distributed Numerical Control (DNC)

Traditional Numerical Control (NC)
The original numerical control machines were
referred to as NC machine tool. They have
“hardwired" control, whereby control is
accomplished through the use of punched paper
(or plastic) tapes or cards.
Tapes tend to wear, and become dirty, thus causing
misreadings. Many other problems arise from the
use of NC tapes, for example the need to manual
reload the NC tapes for each new part and the lack
of program editing abilities, which increases the
lead time. The end of NC tapes was the result of
two competing developments, CNC and DNC.

Computer Numerical Control (CNC)
CNC refers to a system that has a local computer to
store all required numerical data. While CNC was
used to enhance tapes for a while, they eventually
allowed the use of other storage media, magnetic
tapes and hard disks.
The advantages of CNC systems include but are not
limited to the possibility to store and execute a
number of large programs (especially if a three or
more dimensional machining of complex shapes is
considered), to allow editing of programs, to execute
cycles of machining commands,etc.

Distributed Numerical Control (DNC)
The development of CNC over many years, along with the
development of local area networking, has evolved in
the modern concept of DNC.
Distributed numerical control is similar to CNC, except a
remote computer is used to control a number of
machines. An off-site mainframe host computer holds
programs for all parts to be produced in the DNC
facility.
Programs are downloaded from the mainframe computer,
and then the local controller feeds instructions to the
hardwired NC machine.
The recent developments use a central computer which
communicates with local CNC computers (also called
Direct Numerical Control)

Coordinates of NC Machine Tool
NC system can be classified on the number of
directions of motion they are capable to control
simultaneously on a machine tool.
Each free body has six degree of freedom, three
positive or negative translations along x, y, and z-
axis, and three rotations clockwise or counter
clockwise about these axes.
Commercial NC system are capable of controlling
simultaneously two, two and half, three, four and
five degrees of freedom, or axes. The NC systems
which control three linear translations (3-axis
systems), or three linear translations and one
rotation of the worktable (4-axis systems) are the
most common.

Coordinates of NC Machine Tool

Positioning Reference point System
• Absolute Positioning
• Incremental Positioning
Absolute Positioning means that the tool
locations are always defined in relation to
the zero point.
Incremental positioning means that the next
tool location must be defined with references
to the previous tool location.

Absolute System mode
Incremental system mode

Types of Control System in NC
• Open loop system
• Closed loop system

Classification of NC Machine Based on Type of Motion
Control (or) Positioning System of NC (or) Positioning
Control System
• Point to Point NC System
• Continuous Path NC System
– Straight cut system
– Contouring system

1. Point to Point NC System
Point-to-point (PTP) is also sometimes called a
positioning system. In PTP, the objective of the
machine tool control system is to move the cutting
tool to a predefined location. The principle function
of the PTP is to position the tool form one point to
another within coordinate system. The positioning
may be linear in the x-y plane or linear and rotary if
the machine has a rotary table. Each tool axis is
controlled independently, therefore; the
programmed motion always in rapid travers. Once
the tool reaches the desired location, the machining
operation is performed at that position (machining
can only take place after positioning is completed).

NC drill presses are a good example of PTP
systems. The spindle must first be positioned
at a particular location on the workpiece. This
is done under PTP control. Then the drilling of
the hole is performed at the location, and so
forth.
Since no cutting is performed between holes,
there is no need for controlling the relative
motion of the tool and workpiece between
hole locations.

Positioning systems are the simplest machine tool
control systems and are therefore the least expensive of
the three types. However, for certain processes, such as
drilling operations, tapping, riveting and spot welding,
PTP is perfectly suited to the task. Example below
illustrate path of three drilled holes.

2. Continuous Path NC System
1. Straight cut system:
Straight cut control systems are capable of moving the
cutting tool parallel to one of the major axes at a
controlled rate suitable for machining . It is therefore
appropriate for performing milling operations to
fabricate workpieces of rectangular configurations. With
this type of NC systems it is therefor appropriate for
performing milling operations to fabricate workpieces of
rectangular configurations . With this type of NC system
it is not possible to combine movements in more than
single axis direction. Therefore angular cuts on the
workpiece would not be possible. An NC machine tool
capable of performing straight cut movements is also
capable of point to point movements.

2. Contouring System:
Contouring is the most complex flexible and the
most expensive type of machine tool control . It
is capable of performing both PTP and straight
cut operations . In addition the distinguishing
feature of the of contouring NC system is their
capacity for simultaneous control of more than
one axis movement of machine tool Figures
illustrate the versatility of continuous path NC.
Milling and Turning are the common examples
of the use of contouring control.

Applications of NC Machines
• Machine tool Applications such as Drilling, turning and
other metalworking.
• Non Machine tool Applications such as assembly,
drafting and inspection.
• Other applications:
• Press working machine tools
• Welding machines
• Inspection machines
• Automatic Drafting
• Assembly Machines
• Tube bending
• Flame cutting
• Industrial robots
• Automatic Riveting

Computer Numerical Control(CNC)
• CNC is a NC System that utilizes a stored program to
perform the basic numerical functions. This NC system
controlled by the computer is known as Computer
Numerical Control.
• CNC is a microprocessor based control system which
accepts a set of program instructions, processes and feeds
the output control information to a machine tool, accept the
feedback information from the machine tool and it assures
the proper motion, speed to perform the operation.
Advantages
• Accuracy is more
• Production time is less
• Complicated part can be manufactured
• Highly skilled and experienced operator is not necessary

Advantages of CNC Machine tool
• Storage of more than one part program
• Various form of program input
• Program editing at the machine tool
• Positioning features for setup
• Tool length compensation
• Acceleration and deceleration calculations
• Communication interface
• Diagnostics

Disadvantages
• The price of CNC machine is very high
• Maintenance is complicated
• Well trained software professionals are
needed for writing part programs.

Manual Vs Computer Assisted Part Programming

Classification of CNC system
• Based on feed back control
a) Open loop system b) Closed loop system
• Based on motion control
a) Point to point b) Straight cut c) Contouring
system
• Based on power drive
a) Hydraulic b) Electric c) Pneumatic
• Based on circuit technology
a) Analog b) Digital
• Based on positioning system
a) Absolute positioning b) Incremental
• Based on axis identification
a) 2-axis b) 3-axis

Classification of CNC based on feed back control
• Open loop control system
• Closed loop control system
Open loop control system:

• Part program:
A part program is a series of
coded instructions
required to produce a part.
It controls the movement
of the machine tool and
on/off control of auxiliary
functions such as spindle
rotation and coolant. The
coded instructions are
composed of letters,
numbers and symbols.

• Program input device:
The program input device is the means for part
program to be entered into the CNC control.
The commonly used program input devices are:
• Punch tape reader
• Magnetic tape reader
• Floppy Disc Drive
• USB Flash Drive
• Serial Communication
• Computer via RS-232-C communication

• Machine Control Unit (MCU):
The machine control unit (MCU) is the heart of a CNC
system. It is used to perform the following
functions:
• To read the coded instructions.
• To decode the coded instructions.
• To implement interpolations (linear, circular, and
helical) to generate axis motion commands.
• To feed the axis motion commands to the amplifier
circuits for driving the axis mechanisms.
• To receive the feedback signals of position and
speed for each drive axis.
• To implement auxiliary control functions such as
coolant or spindle on/off and tool change.

• Drive System:
A drive system consists of amplifier circuits, drive
motors, and ball lead-screws. The MCU feeds the
control signals (position and speed) of each axis to
the amplifier circuits. The control signals are
augmented to actuate drive motors which in turn
rotate the ball lead-screws to position the machine
table.

• Machine Tool:
CNC controls are used to control various types of
machine tools. Regardless of which type of machine
tool is controlled, it always has a slide table and a
spindle to control of position and speed. The
machine table is controlled in the X and Y axes,
while the spindle runs along the Z axis.

• Feed Back System:
The feedback system is also referred to as the
measuring system. It uses position and speed
transducers to continuously monitor the position at
which the cutting tool is located at any particular
instant. The MCU uses the difference between
reference signals and feedback signals to generate
the control signals for correcting position and speed
errors.

Constructional Features of CNC Machine Tool
(or) Parts of CNC Machine Tool
• Machine structure
• Slideways or Guideways
• Spindle/spindle bearing
– Hydrodynamic
– Hydrostatic
– Antifriction
• Spindle drives
– Electrical drives
– Hydraulic drives
– Pneumatic drives
• Feed drives
– Servo motor
– Mechanical transmission system
• Measuring system
– Direct
– Indirect
• Controls, software and user interface
• Gauging
• Tool monitoring

• Machine structure:
• The machine structure is the load carrying and
supporting member of the machine tool. All
motors, drive mechanism and other functional
assemblies of machine tools are aligned to each
other and rigidity to the machine structure.
• Its subjected to static, dynamic and thermal
forces .
• It does not deform or vibrate beyond the
permissible limit under the action of these
forces.

• Slideways or Guideways
Machine tools are provided with tables, slides,
carriages, etc. to carry the workpieces or cutting
tools. These parts are mounted in the ways and slide
along the ways. They are fixed on the parts of the
machine tools such ad a column, housing , bed or
knee.
Types of Slideways used in CNC Machine:
1. Hydrostatic slideways
a. Oil lubricated slideways
b. Air bearing slideways
2. Anti- friction slideways
a. Ball bearing slideways
b. Roller bearing slideways
c. Wear resistant slideways

Wear resistant slideways (or)
Recirculating ball Screw

Advantages of ball screws
• Highly efficient and reliable.
• Less starting torque.
• Lower coefficient of friction compared to sliding type screws
and run at cooler temperatures
• Power transmission efficiency is very high and is of the order of
95%.
• Could be easily preloaded to eliminate backlash.
• The friction force is virtually independent of the travel velocity
and the friction at rest is very small; consequently, the stick-
slip phenomenon is practically absent, ensuring uniformity of
motion.
• Has longer thread life hence need to be replaced less
frequently.
• Ball screws are well-suited to high through output, high speed
applications or those with continuous or long cycle times.
• Smooth movement over full range of travel.

Spindles used in CNC Machine Tools

STEPPER MOTOR
A stepper motor is a pulse-driven motor that
changes the angular position of the rotor in steps.
Due to this nature of a stepper motor, it is widely
used in low cost, open loop position control systems.
Types:
• Permanent Magnet
Employ permanent magnet
Low speed, relatively high torque
• Variable Reluctance
Does not have permanent magnet
Low torque.

Hybrid Stepper Motor
Hybrid stepping motors combine a permanent
magnet and a rotor with metal teeth to provide
features of the variable reluctance and permanent
magnet motors together.
The number of rotor pole pair is equal to the number
of teeth on one of the rotor's parts. The hybrid motor
stator has teeth creating more poles than the main
poles windings.

Advantages of stepper motors
• Low cost
• Ruggedness
• Simplicity of construction
• Low maintenance
• Less likely to stall or slip
• Will work in any environment
• Excellent start-stop and reversing responses

Disadvantages of stepper motors
• Low torque capacity compared to DC motors
• Limited speed
• During over loading, the synchronization will
be broken. Vibration and noise occur when
running at high speed.

SERVO MOTORS
• Servomotors are special electromechanical
devices that produce precise degrees of rotation.
• A servo motor is a DC or AC or brushless DC
motor combined with a position sensing device.
• Servomotors are also called control motors as
they are involved in controlling a mechanical
system.
• The servomotors are used in a closed-loop servo
system as shown in Figure A reference input is
sent to the servo amplifier, which controls the
speed of the servomotor.

• A feed back device is mounted on the
machine, which is either an encoder or
resolver.
• This device changes mechanical motion in
to electrical signals and is used as a
feedback.
• This feed back is sent to the error detector,
which compares the actual operation with
that of the reference input.
• If there is an error, that error is fed directly
to the amplifier, which will be used to make
necessary corrections in control action.

DC Servo Motors
DC operated servo motors are usually
respond to error signal abruptly and
accelerate the load quickly.
Components of DC Servo Motor:
• DC motor
• Gear assembly
• Position-sensing device
• Control circuit

AC Servo Motor
• Magnetic force is generated by a permanent
magnet and current which further produce
the torque.
• It has no brushes so there is little
noise/vibration. This motor provides high
precision control with the help of high
resolution encoder.
• The stator is composed of a core and a
winding. The rotor part comprises of shaft,
rotor core and a permanent magnet.
• Digital encoder can be of optical or magnetic
type. It gives digital signals, which are in
proportion of rotation of the shaft.

Advantages of servo motors
• Provides high intermittent torque, high torque to
inertia ratio and high speeds
• Work well for velocity control
• Available in all sizes
• Quiet in operation
• Smoother rotation at lower speeds
Disadvantages of servo motors
• More expensive than stepper motors
• Require tuning of control loop parameters
• Not suitable for hazardous environments or in
vacuum
• Excessive current can result in partial
demagnetization of DC type servomotor

Types of CNC Machine
• Drilling machine
• Turning machine
• Boring machine
• Milling machine
• Grinding machine
• Pipe bending machine
• Coil winding machine
• Flame cutting machine
• Welding, wire cut EDM and several other areas

Types of Machining Centre
Machining Centre:
It is a Computer numerically controlled machine
tool which capable of carrying out a range of
machining function (Operations) normally
performed by a number of different types of
conventional machine tools.
Types:
• Vertical machining centre
• Horizontal machining centre
• Universal machining centre

Advantages of Machining Centers
• High productivity
• It reduces idle time
• Improves part accuracy
• Higher spindle speeds
• Higher cutting speed
• It facilitates faster tool change

Automatic Tool Changer (ATC)
ATC is an equipment that reduces cycle times by
automatically changing tools between cuts
through programmed instructions. The tools
are fitted on a tool magazine or drum.
Types:
• Tool change system with gripper arm
• Tool change system with chain magazine
• Tool change system with disc magazine

Tool change system with gripper arm

Tool change system with chain magazine

Tool change system with disc magazine

DIRECT NUMERICAL CONTROL (DNC)
MACHINES
Direct Numerical Control can be defined as a type
of manufacturing system in which several NC or
CNC machines are controlled remotely from a
Host/Main frame computer or direct numerical
control (DNC) – Control of multiple machine
tools by a single (main frame) computer through
direct connection.

PART PROGRAMMING
The part program is a set of instructions
proposed to get the machined part starting
with the desired blank and NC machine tool.
Part programming contains geometrical data
about the part and motion information to
move the cutting tool with respect to the
workpiece.
• Manual part programming
• Computer assisted part programming
(CAD/CAM)

Manual Part Programming
To prepare a part program using a manual method,
the programmer writes the machining
instructions on a special format called Part
programming manuscript. The programmer first
prepares the program manuscript in a standard
format. The manuscript is the relative tool and
workpiece locations.
It includes the preparatory functions,
miscellaneous instructions and speed/feed
specifications which are needed to operate the
machine under the tape control.

It consist of blocks, words and addresses. Each line
of instruction is capable of specifying dimensional
and non dimensional data and it is written in a
specific format.
• Word:
A block is composed of one or more words. A word
is composed of an identification letter and a series
of numerals, for example, the command for a feed
rate of 100mm/min is F100.
• Block:
A block is equivalent to a line of codes in a part
program. A group of such NC words constitutes a
complete NC instruction known as block.

• Address:
The identification letter at the beginning of each word is
called address. The meaning of the address is in
accordance with EIA (Electronic Industries
Association) standard RS-274-D.
Function Address
Sequence Number N
Preparatory function G
Coordinate word X,Y,Z
Parameters for circular Interpolation I,J,K
Feed Function F
Spindle Function S
Tool Function T
Miscellaneous Function M

Important Terms used in Part Programming
• Sequence Number (N Address):
Its used to identify the block. Its always placed at the
beginning of the block and it can be regarded as the name
of the block.
• Preparatory Function (G Address):
A preparatory function determines how the tool is to move
to the programmed target. Various preparatory functions
used in programming are given in further topics.
• Coordinate word (X/Y/Z Address):
The target point of the tool movement (Absolute dimension
system) or the distance to be moved (Incremental
dimension).
Example: X100 Y200 represents the movements to (100,20)

• Parameters for circular Interpolation (I/J/K address):
These parameters specify the distance measured from the
start point of the arc to the centre.
• Spindle Function (S Address):
The spindle speed is commanded under an S address and it
is always in revolution per minute.
Spindle Speed = Surface cutting speed in mm/min X100
Π x Cutter Diameter in mm
• Feed Function (F Address):
Feed is used to measure the distance travelled by the tool.
Unit : mm/Rev (Turning )
mm/Minute (Milling)

• Tool Function (T Address):
The selection of the tool is commanded under a T
address. The cutting tool will be coded using 1
to 5 digit numbers. The respective tool and its
position can automatically be identified and
changed by means of a tool changer.
Ex: T121 means that the tool from station 1 is
used having the second cutting edge.
• Miscellaneous Function (M Address):
It is programmed to control the machine
operation other than for coordinate movement.

Preparatory Function (G Codes) in Part
Programming
Preparatory commands which prepare the machine
or tool for different modes of movement,
positioning, contouring, thread cutting and also
precede the dimension word. They are grouped.
The group can not affect each other. Only one
function from the same group can be performed
at a time.

• Absolute and Incremental Dimensioning
(G90 /G91)
G90 and G91 are used to control the
dimensioning system that will be used in the
data input.
G90 – Absolute Dimensions
G91 – Incremental Dimensions
• Rapid Positioning (G00)
This is to command the cutter to move from the
existing pointer to the target point at the
fastest speed of the machine. (G00 X Y Z)

• Linear Interpolation (G01)
This is to command the cutter to move from
the existing point to the target point along a
straight line at the speed designated by the
F address. (G01 X Y Z F).
• Circular Interpolation (G02/G03)
This is to command the cutter to move from
the existing point to the target pint along a
circular arc in clockwise direction (G02) or
counter clockwise direction (G03).

Clockwise (G02) - Counter clockwise (G03)

G-Codes (Preparatory Functions)
• G00 Rapid positioning
• G01 Linear interpolation
• G02 Circular interpolation clockwise (CW)
• G03 Circular interpolation counters clockwise
(CCW)
• G20 Inch input (in.)
• G21 Metric input (mm)
• G24 Radius programming
• G28 Return to reference point

• G29 Return from reference point
• G32 Thread cutting
• G40 Cutter compensation cancel
• G41 Cutter compensation left
• G42 Cutter compensation right
• G43 Tool length compensation positive
(+) direction
• G44 Tool length compensation minus (-)
direction
• G49 Tool length compensation cancels
• G 53 Zero offset or M/c reference

• G54 Settable zero offset
• G70 Finishing turning cycle
• G71 Canned cycle for turning
• G84 canned turn cycle
• G81 Canned cycle for grooving, drilling in
turning centre
• G90 Absolute programming
• G91 Incremental programming
• G92 Canned cycle for thread cutting

M-Codes (Miscellaneous Functions)
• M00 Program stop
• M02 End of program
• M03 Spindle start (forward CW)
• M04 Spindle start (reverse CCW)
• M05 Spindle stop
• M06 Tool change
• M08 Coolant on
• M10 Chuck - clamping

• M11 Chuck - unclamping
• M12 Tailstock spindle out
• M13 Tailstock spindle in
• M17 Tool post rotation normal
• M18 Tool post rotation reverse
• M30 Main program end
• M98 Transfer to subprogram
• M99 End of subprogram
• M09 Coolant off

MICROMACHINING
Micromachining is used to fabricate three-
dimensional microstructures and it is the
foundation of a technology called Micro-
Electro-Mechanical-Systems (MEMS) in the
range of 1 to 500micrometers.
Its metal working process for producing very
fine surface finishes.
Types of Micromachining process:
• Bulk Micromachining
• Surface Micromachining

Bulk micromachining
In bulk micromachining, the structures are
carved out of a substrate by chemical or
physical etching methods. The structures
selectively etching inside a substrate.

Surface Micromachining or Wafer
Machining
The structures are created on the top of a
substrate. In this case a silicon substrate
(wafer) is selectively etched to produce
structures.

Lithography Based Micromachining
• Photolithography process
• Etching
• LIGA
• Thin film Deposition

1. Photolithography Process
A photo polymer called photo resist (PR) is the
basis for photolithography. If a layer need to
be patterned.
Photolithography is the process of transferring
geometric shapes on a mask to the surface of a
silicon wafer.

2. Etching
Etching is traditionally the process of using
strong acid or mordant to cut into the
unprotected parts of a metal surface to create
a design in the metal.

3. LIGA
The LIGA-process is used to manufacture micro
structures by deep X-ray lithography. LIGA is the
German acronym. It refers Lithography,
Galvanoformung, Abformung (LIGA). It involves
lithography, electroplating and molding processes.
Five steps of LIGA:
1. Resist Development
2. X Ray radiation and masking
3. Electroforming
4. Resist Removal
5. Plastic Molding

4. Thin film Deposition
There are a number of techniques for creating
a thin layer of deposit over the wafer.
• Physical Vapor Deposition (PVD)
• Chemical Vapor Deposition (CVD)
• Sputtering
• Electroplating

Non-Lithography based Micromachining
• Abrasive Jet Machining (AJM) /Abrasive
water jet Machining (AWJM)
• Electrical Discharge Machining (EDM)
• Laser Beam Machining (LBM)
• Electron Beam Machining (EBM)

Electrical Discharge Machining

Unit V CNC Machining

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