Chapter 2 constructional feature of cnc machine

Chapter 2
Constructional Feature of
CNC machine
Prepared by: Prof. Rahul Thaker (ACET)

CLASSIFICATION OF CNC
SYSTEMS
 According to the type of machine:
Point-to-point
Straight-cut
Continuous path
 According to the programming method:
Absolute
Incremental
 According to the type of control system:
Open-loop
Closed-loop

According to the type of machine: Point-to-Point
 Point-to-point machines move only in straight
lines. They are limited to drilling, reaming,
boring, etc. and straight milling cuts parallel
to a machine axis. When making an axis
move, all affected drive motors run at the
same speed. When one axis motor has moved
the instructed amount, it stops while the other
motor continues until its axis has reached its
programmed location. The point-to-point
positioning NC system is illustrated in Figure

 The simplest example of a point-to-point (PTP)
NC machine tool is a drilling machine. In a drilling
machine, the work piece is moved along the axes of
motion until the center of the hole to be drilled is
exactly beneath the drill. Then the drill is
automatically moved towards the work piece, the
hole is drilled and the drill moves out in a rapid
traverse feed. The work piece moves to a new point
and the above sequence of actions are repeated.Prepared by: Prof. Rahul Thaker (ACET)

 Point-to-Point systems
Also called position systems
System moves to a location and performs an operation at that
location (e.g., drilling)
Also applicable in robotics
PTP is simplest of all control mode
In this type movement of cutting tool moves from one
“Predefined position” to another Predefined Position , while path
along which this tool moves irevenant.
Specially used in local position base operation i.e: drilling
,punching or welding

According to the type of machine: Straight cut
NC machine
 It is extension of point to point control mode with
provision of straight line cut or machining
 This straight line cut is obtained by controlling the
movement of tool with controlled feed rate in one of
the axis direction at time
 Ex: face milling ,pocket milling , stepped turning

According to the type of machine: Continuous
path control CNC Machine
 Also called contouring(when continuous path control is used for
simultaneous control of two or more axes) systems in machining
 System performs an operation during movement (e.g., milling and
turning)
 Used for continuous simulation and coordinate motion of cutting
tool and workpiece along different AXES
 Such control enable for machining of different contorting profile
and curved surface
Due to simultaneously controlling relative position pass trough the
desired countering path of the final shape of require product

According to Programming
Method
 Why programming needed ?
• For preparing programme ,it is necessary to specify
the path tool movement .
• Therefore a geometric data of a component is
converted in to machine understandable
programme.
• Which gives specific path to tool on workpiece.
• Programing is the bridge between machine and
humans.
 Types of Programming method
a. Absolute programming NC machine system
b. Incremental Programming NC Machine system

According to Programming Method :
Absolute programming CNC machine system
 In Absolute Programming
system each point is
always start from same
specific point.
 In this method first point
(ZERO POINT or
REFRENCE POINT ) is
fixed.
 All Process Start from
same reference point

According to Programming Method :
Incremental programming CNC machine system
 In incremental Programming
system each point is
specified using the path
differential from the
preceding point position.
 In this method first point
(ZERO POINT or
REFRENCE POINT ) is last
point of previous path.
 Process Start from different
reference point

Absolute vs. Incremental Positioning
The workhead is presently at point (20, 20) and is to be moved to
point (40, 50)
In absolute positioning, the move is specified by x =40, y = 50
In incremental positioning, the move is specified by x =20, y = 30.

According to control loop feedback system :
Open loop type CNC Machine
• This type pf control have only motion control
without feed back mechanism
• The open loop system do not have feedback
mechanism
• Ex: drilling Machine

Open-Loop Motion Control System
 Operates without verifying that the actual
position achieved in the move is the desired
position

According to control loop feedback system :
Close loop type CNC Machine
This type of control have motion control with feed back
mechanism
The close loop system have feedback mechanism which can
accurately controlling the drive system.
It drive system by comparing with input information until
require position achieved
Feed back is measured by monitoring device to determine the
drive displacement.

Closed-Loop Motion Control System
 Uses feedback measurements to confirm
that the final position of the worktable is the
location specified in the program

Open Loop vs. Closed Loop controls

Open loop control of a Point-to-Point CNC drilling machine
NOTE: this machine uses stepper motor control

According to number of axes:
2& 3 axes CNC machines
 CNC lathes will be coming under 2 axes machines. There
will be two axes along which motion takes place. The saddle
will be moving longitudinally on the bed (Z-axis) and the
cross slide moves transversely on the saddle (along X-axis).
 In 3-axes machines, there will be one more axis,
perpendicular to the above two axes. By the simultaneous
control of all the 3 axes, complex surfaces can be machined.

According to number of axes:
4 & 5 axes CNC machines
 4 and 5 axes CNC machines
provide multi-axis machining
capabilities beyond the standard
3-axis CNC tool path
movements. A 5-axis milling
centre includes the three X, Y, Z
axes, the A axis which is rotary
tilting of the spindle and the B-
axis, which can be a rotary
index table..

Five Axis CNC

ADVANTAGES OF 5 AXIS CNC
 to machine complex shapes in a single setup
 reduces the machinist setup time and increases production rates
 By eliminating multiple set-ups, time and errors are reduced
 the feature-to-features accuracy is improved because the same
zero or datum reference frame is used throughout the
manufacturing process
 since simultaneous movement is allowed along the X andY axis,
shorter and more rigid tools may be used
 higher spindle/cutting tool speeds may be achieved while
reducing the load on the cutting tool
 Shorter and thicker cutters also reduce vibration when machining
deep pockets or contoured features with three-axis machines.
Prepared by: Prof. RahulThaker (ACET)

Application
 complex three dimensional profiles
 for impellers, turbine blades, and plastic mold tools

Specification of a 5 axis CNC Machine
 Manufacturers
HURCO
TARUS
CMS north America
OKUMA
CNC TAKANG Taiwan
maximart corporation
YCM (YEONG CHIN MACHINERY INDUSTRIES
CO., LTD )

Specification cont…
 TARUSTPGM5X1083 SPECIFICATIONS
Parameters
X Axis Travel 3048 mm
YAxis Travel 2540 mm
Z Axis Travel 1117 mm
Space between columns 3886 mm
Workable size,L*W 3568 mm*2235 mm
X , Y and Z axis maximum feed rate 30m/min
A axes tip +/-100 degrees
C axis tip Continuous
A and C axis positioning peak torque 4073 NM
A and C axis positioning Feed 30 degrees per second
Milling Spindle HSK 63A, 0-20,000 RPM
(3 axis and 5 axis milling)
4,000 RPM base speed
70 NM constant torque,160 NM peak torque
29 KW rated power
Linear Accuracy (X,Yand Z) ± .013 mm , ± 0.006 mm, ± 0.005 mm

Conventional milling machines
Vertical milling machine

Vertical Milling machine architecture

Horizontal Milling machine architecture
How does the table move along X- Y- and Z- axes ?

Axes configuration
 X axes moves from right to left as you face the machine
 Y axes move toward and away from you
 The Z axes is the spindle movement up and down spindle .
 A move toward work is Z(-Z)
 A move away from work is Z(+ Z)

CNC Machine Axes of Motion
• The coordinate system used for the tool path must be identical to the
coordinate system used by the CNC machine.
CNC machines milling machines can perform
simultaneous linear motion along the three axis and are
called three-axes machines.
Horizontal
milling machine
Right hand rule
Vertical
milling
machine

CNC Machine Axes of Motion
Five-axis
machine
configurations

Elements of a CNC System
 Input Device
 Central Processing Unit/ Machine Control Unit
 Machine Tool
 Driving System
 Feedback Devices
 Display Unit

Input Devices
 Floppy Disk Drive
 USB Flash Drive
 Serial Communication
 Ethernet communication
 Conversational Programming

Central Processing Unit/ Machine Control Unit
 The CPU is the heart of a CNC system.
 It accepts the information stored in the memory as part program.
 This data is decoded and transformed into specific position control and velocity
signals.
 It also oversees the movement of the control axis or spindle and whenever this
does not match with the programmed values, a corrective action as taken.

Machine Tool
 Most are made from high speed steel (HSS), tungsten carbide or ceramics.
 Tools are designed to direct waste away from the material.
 Some tools need coolant such as oil to protect the tool and work.

Driving System
 The requirement is that the driving system has to response accurately according
to the programmed instructions.
 The motor is coupled either directly or through a gear box to the machine lead
screw to moves the machine slide or the spindle.
Three types of electrical motors are commonly used:
1. DC Servo motor
2. AC Servo motor
3. Stepping motor as explained ahead.

1. DC Servo Motor
 The principle of operation is based on the rotation of an armature winding in a
permanently energized magnetic field.
 The armature winding is connected to a commutator, which is a cylinder of
insulated copper segments mounted on the shaft.
 DC current is passed to the commutator through carbon brushes, which are
connected to the machine terminals.

DC Servo Motor Continues
 The change of the motor speed is by varying the armature voltage and the
control of motor torque is achieved by controlling the motor's armature current.
 In order to achieve the necessary dynamic behavior it is operated in a closed
loop system equipped with sensors to obtain the velocity and position feedback
signals.

2. AC Servo Motor
 In an AC servomotor, the rotor is a permanent magnet while the stator is
equipped with 3-phase windings.

 The speed of the rotor is equal to the rotational frequency of the magnetic
field of the stator, which is regulated by the frequency converter.

3. Stepping Motor
 The stepper motor is known by its property to convert a train of input pulses
(typically square wave pulses) into a precisely defined increment in the shaft
position.
 Each pulse moves the shaft through a fixed angle.
 Multiple "toothed" electromagnets arranged around a central gear-shaped piece of
iron.
 The electromagnets are energized by an external driver circuit or a micro controller.
In that way, the motor can be turned by a precise angle.

 To make the motor shaft turn, first, one electromagnet is given power, which
magnetically attracts the gear's teeth.
 When the gear's teeth are aligned to the first electromagnet, they are slightly
offset from the next electromagnet.
 This means that when the next electromagnet is turned on and the first is turned
off, the gear rotates slightly to align with the next one.
 From there the process is repeated. Each of those rotations is called a "step",
with an integer number of steps making a full rotation.
What does Stepper means?

 However, stepping motors are not commonly used in machine tools due to the
following drawbacks:
 slow speed,
 low torque,
 low resolution
 easy to slip in case of overload.

Recirculating ball screw
• The recirculating ball steering mechanism contains a
worm gear inside a block with a threaded hole in it.
• This block has gear teeth cut into the outside to
engage the shaft.
• They convert sliding friction into rolling friction.
 Advantages
• wear will be reduced
• reliability of the system will be increased
• the efficiency of ball screws may reach up to 90%.

Roller screw
• This type of screw provide backlash-free
movement
• Their efficiency is same as that of ball screws.
• These are capable of providing more accurate
position control.
• Cost of the roller screws are more compared to
ball screws.
• The thread form is triangular with an included
angle of 90 degrees.Prepared by: Prof. Rahul Thaker (ACET)

Planetary roller screw
• These are capable of transmitting high load at
fast speed.
• There is no axial movement of the rollers
relative to the nut.
• The rollers are threaded with a single start
thread.

Recirculating roller screw
• The rollers in this case are not threaded.
• The rollers are provided with a circular groove
and are positioned circumferentially by a cage.
• Recirculating roller screws are slower in
operation.
• They are capable of transmitting high loads with
greater accuracy

Ball Lead Screws
 Ball lead screw is the heart of the drive system.
 Advantages of ball lead screw are:
 Precise position and repeatability
 High Speed capability
 Less Wear
 Longer life

Feed drive

Feedback Devices
Two types of feed back devices normally used are:
1. Positional Feed Back Devices
1.1 Linear Transducers - a device mounted on the machine table to measure the
actual displacement of the slide in such a way that backlash of screws; motors etc
would not cause any error in the feed back data.

1.2 Rotary Encoders: a device to measure the angular displacement. It cannot
measure linear displacement directly so that error may occur due to the backlash of
screw and motor etc.

Encoders
• An encoder is a device, circuit, transducer,
software program, algorithm that converts
information from one format or code to another,
for the purposes of standardization.

Components of Servo-motor controlled CNC
Motor speed control
Two types of encoder configurations
Motor lead screw rotation table moves
position sensed by encoderfeedback

Optical Encoder
• Device for measuring rotational position and
speed
• Common feedback sensor for closed-loop NC
control

Motion Control and feedback
Encoder outputs: electrical pulses (e.g. 500 pulses per revolution)
Rotation of the motor  linear motion of the table: by the leadscrew
The pitch of the leadscrew: horizontal distance between successive threads
One thread in a screw  single start screw: Dist moved in 1 rev = pitch
Two threads in screw  double start screw: Dist moved in 1 rev = 2* pitch

2. Velocity Feedback Device
 The actual speed of the motor can be measured in terms of voltage generated
from a tachometer mounted at the end of the motor shaft.
 The voltage generated by the DC tachometer is compared with the command
voltage corresponding to the desired speed.
 The difference of the voltages is used to actuate the motor to eliminate the error.

Component and Function of CNC
• Feed drive
• Measuring system  Direct / Indirect
• Work spindle  hydraulic
• Cooling system  reduce heat
• Tool turret

Measuring
System

Tool change facilities

3. Coordinate System
3.1 CNC Axes

Axes on a CNC lathe

Axes on a CNC milling machine

According to the power supply :
Electric, Hydraulic & Pneumatic
systems

Chapter 2 constructional feature of cnc machine

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