Experiment No.13
Objective:
To study the coordinate systems and constructional features of CNC systems.
Coordinate Systems:
The Cartesian coordinate system is the fundamental system used to describe the motion
of the tool and workpiece within a three-dimensional space. CNC machines use
numbers to locate a particular point along the X-, Y-, and Z-axes. They perform a series
of instructions, one after another, to machine the workpiece and create incredibly
accurate
dimensions.
CNC machines use either incremental or absolute coordinates to
move from one location to the next. With incremental coordinates, the current position
acts as the origin for the next position. With absolute coordinates, the origin stays in a
fixed location, and each new location is calculated from that fixed position. Most CNC
machines can move along multiple axes at once to perform contour operations.Most
people are familiar with the rectangular (or Cartesian) coordinate system. This system
of coordinate planes uses real number lines arranged at right angles to allow any point
in space to be clearly and unambiguously determined. In mathematics, the order of
the coordinates (separated by a comma) designates a point on the graph. For instance
(2,3) would designate a point with an X value of 2 and a Y value of 3. The syntax for
this point is very similar in G-Code programming. This will be discussed later in this
chapter.
Fig 13.1 Coordinates of a Point
This system of planes can be overlaid on top of a machine table or workpiece to enable
the machine to be directed to any point on the workpiece or machine table. In the figure
below, the programmer has oriented the coordinate system so it is aligned with a
particular workpiece to be cut. Since all the cutting will take place in the First Quadrant
(I), the program will have all positive X and Y values when the tool is over the part.
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Fig 13.2 Coordinate System Overlaid on a Workpiece
It is the ease at which a set of Cartesian coordinates can be converted into machinereadable numbers that allows CNC to be really quite easy to program.On a lathe it may
be a little less intuitive, but the rule still holds. On a lathe the workpiece is mounted in
the spindle by means of a chuck or other workholding device. The axis that the spindle
rotates around is the Z axis. A typical two-axis lathe would have X and Z axes. The X
axis typically corresponds to the diameter of the workpiece, and the Z axis corresponds
to where the tool is located along the length of the workpiece.
Fig 13.3 Basic Axes of Motion on a Lathe
After finding the Z axis, the X axis is usually the easiest to find. Most of the time, the
X axis is parallel to the floor. The X axis will be orthogonal (at a right angle) to the Z
axis. Finally the Y axis will be at a right angle to both the X and Z axis. The orientation
of these three axes follows what is known as the right hand rule. If you orient your
right thumb with the Z axis, the X and Y axes can be aligned with your index and middle
finger respectively. The tips of your fingers will be show the positive direction of each
axis.
Fig 13.4 Right Hand Rule
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Constructional Features of CNC Machines
A conventional machine tool has an intelligent source for error compensation (the
operator). During machining a skilled operator can vary the cutting conditions to
compensate for deflection, vibration, etc. to generate the desired shape, size and finish.
The CNC machine can only compensate for an error that is detected and communicated
to the control. Deflection, vibration etc. cannot, as yet, be easily monitored. For this
reason NC machines are made stronger and stiffer to perform to a more accurate
standard than their conventional counterparts The capacity for varying the conditions
while machining is therefore limited. As far as possible the conditions have to be
established as the program is produced. In addition to this, CNC machines are spending
more time per shift cutting than conventional machines did in the past. This higher
percentage of cutting time results in faster wear rates on the slides and transmission
systems. Conventional machine tools are also designed with the view to having the
skilled operator standing directly in front controlling the machine. This is no longer
required for CNC since the machine is operating under program control. Optimum
cutting speeds and feeds, continuous path machining, rapid slide movement to bring the
tool close to the work and then sudden stopping, all subject the machine to forces which
are not encountered on conventional machines.
Features of CNC Machine:
The feature of CNC machines are as follows:
 Part program input may be through the keyboard.
 The part program is entered into the computer and stored in the memory. Then it is
used again and again.
 The entered part program can be edited for any errors or design changes.
 A graphical display of the cutter path and shape of the finished work is possible
before actually running the program (simulation).
 Tool wear compensation is possible.
 Able to get machine utilization information’s like the number of components
produced, time per component, time for setting the job etc.,
 The sub-program facility is also possible for repetitive machining sequences.
Machine Structure :
Since rigidity plays a major part in the accuracy of a machine tool, modern CNC
machines tend to have over-proportionated slide ways, guides and spindles. Thicker
cast sections than conventional machines are also used. The use of symmetrical castings
assists in reduction of thermal stresses within the machine. These structural design
features are employed to cope with the torsional forces and heavy duty cutting imposed
on these machines.
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The main parts of the CNC machine are:
Fig 13.5 Basic Features of CNC Machine
Basic Features of CNC Machine:
 Input devices
 Machine control unit (MCU)
 Machine tool
 Driving system
 Feedback system
 Display uni
Input Devices:
These are devices that are used to input the part program into a CNC machine. There
are three generally used input devices and these are punch tape reader, magnetic tape
reader and computer via RS-232-C communication.
Machine Control Unit (MCU):
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Fig 13.6 Machine Control Unit
The machine control unit is called the heart of the CNC machine. It performs all the
control functions of the CNC machine, there are various tasks performed by MCU that
are
 It reads the coded instructions given in it.
 Machine control unit decodes the coded instruction.
 This axis implements interpolation (linear, circular and helical) to generate motion
commands.
 Machine control unit feeds the axis motion command to the amplifier circuit to
drive the axis mechanism.
 It takes a feedback signal of position and speed for each drive axis.
 It implements the auxiliary control functions such as coolant or spindle on/off and
tool change.
Machine Tool:
Fig 13.7 Machine Tool
A CNC machine tool always has a sliding table and a spindle to control the position
and speed. The table of the machine is controlled in the X and Y-axis direction and the
spindle is controlled in the Z-axis direction.
Driving System:
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The driving system of a CNC machine include of an amplifier circuit, drive motors, and
ball lead screws. The MCU supplies the signals (ie, of position and speed) of each axis
to the amplifier circuits. The control signals are then augmented (increased) to actuate
the drive motors. And the actuated drive motors rotate the ball lead screw to put in
position the machine table.
Feedback System:
The feedback system has transducers that act as sensors. It is also called a measuring
system. It consists of position and speed transducers that continuously monitor the
position and speed of the cutting tool located at any given moment.
The MCU receives signals from transducers and it uses the difference in reference
signals and feedback signals to generate control signals to correct position and speed
errors.
Display Unit:
The monitor is employed to display programs, commands and other useful data of the
CNC machine.
Read also:


6 Different types of Shaper Machine Operations
22 Different Types of Lathe Machine Operations
First, the part program is entered into the MCU of the CNC.
The MCU processes all the data and according to the program prepared, it prepares all
the motion commands and gives them to the driving system.
The drive system acts as motion commands sent by the MCU. The drive system
manages the motion and velocity of the machine tool.
The feedback system records the position and velocity measurements of the machine
tool and gives a feedback signal to the MCU.
Conclusion:
CNC machine can produce jobs with highest accuracy and precision than any other
manual machine. It eliminates human errors.It can be operated for 24 hours of a day.
Higher flexibility also.
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Experiment No.14
Objective:
To study and overview of CNC programming, work holding devices of CNC
machines.
Computer Numerical Control (CNC) is a specialized and versatile form of Soft
Automation and its applications cover many kinds, although it was initially developed
to control the motion and operation of machine tools. The definition of CNC given by
Electronic Industry Association (EIA) is as follows: “A system in which actions are
controlled by the direct insertion of numerical data at some point. The system must
automatically interpret at least some portion of this data”. In a simple word, a CNC
system receives numerical data, interpret the data and then control the action
accordingly.
Control Systems:
 Open loop system
 Closed loop system
Open Loop Control System:
In open loop control system, the output does not affect the control action of the system.
In other words, the system whose working depends on time is known as the open loop
control system. The open loop system is free from the feedback.
Fig 14.1 Open Loop Control System
Closed Loop Control System:
The closed-loop control system means the output of the system depends on their input.
The system has one or more feedback loops between its output and input. The closedloop system design in such a way that they automatically provide the desired output by
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comparing it with the actual input. The closed-loop system generates the error signal
which is the difference between the input and output.
Fig 14.2 Closed Loop Control System
Types of CNC Programming:
Three main types of CNC programming manual, computer-aided manufacturing
(CAM) and conversational. Each has unique pros and cons. Beginner CNC
programmers should learn what distinguishes each type of programming from the
others and why all three methods are essential to know.
Manual CNC Programming:
Manual CNC programming is the oldest and most challenging variety. This type of
programming requires the programmer to know how the machine will respond. They
need to visualize the program’s outcome. Therefore, this type of programming is best
for the simplest tasks or when an expert must create a highly specific design.
CAM CNC Programming:
CAM CNC programming is ideal for those who may lack advanced math skills. The
software converts CAD design into the CNC programming language and overcomes
many of the mathematical hurdles required when using a manual programming method.
This approach presents a reasonable middle ground between the level of expertise
necessary for manual programming and the extreme ease of conversational
programming. However, by using CAM for programming, you have more options
compared to the latter and can automate much of the process with a CAD design.
Conversational or Instant CNC Programming:
The easiest type of programming for beginners is conversational or instant
programming. With this technique, users don’t need to know G-code to create the
intended cuts. Conversational programming allows the user to enter the essential details
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in simple language. The operator also can verify tool movements before executing the
program to ensure the design’s accuracy. The downside to this method is its inability to
accommodate complex paths.

A CNC part programme to cut the part as shown in Fig.14.3 from an aluminium
slab of 19 mm thickness and size 90 mm x 90 mm.
Fig 14.3 CAD Model
Programming:
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CNC Work Holding Devices:
Workholding is the generic term for any device used to firmly hold your
workpiece while machining it. Closely related terms are “jigs” and “fixtures.” A fixture
holds your workpiece while it is being cut. A jig holds the workpiece and also guides
the cutter.

T-Slot Tables. ...

Work Pallets. ...

Wedge Clamping Elements. ...

Compact Centric Clamps. ...

Pneumatic Clamps: Short stroke, positional. ...

Pneumatic Vertical Clamps. ...

Vacuum Chuck, Vacuum Table. ...

Precision Rotary Axis.
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T-Slot Tables:
T-Slot table CNC work holding mounts to the machine bed and can be equipped with
wedge clamping or short stoke pneumatic clamps among other solutions.
Fig 14.1 T-Slot Table
Work Pallets:
Our work pallets or Quick Pallets are one of our CNC work holding solutions that are
keyed using a beveled boss-in-cavity system to insure location repeatability.
Fig 14.2 Work Pallets
Wedge Clamping Elements:
The wedge clamping elements consist of a guide housing with a one-piece clamping
bolt. It has a spring force and hydraulic unclamping, retention force 25 up to 120 kN,
with and without position monitoring.
Compact Centric Clamps:
Compact Machine Table Clamp For Machine Tool Tables, Milling Tables, T-Slot
Press
Tables.
These Clamps Are
Also
Known
As
Milling Clamp,
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Vertical Clamp, Compact Clamp, Mill Clamps. Quick Clamp. Mill Clamp. Quick
Machine Table Clamp. Milling Clamp.
14.3.4 Compact Centric Clamps
Pneumatic Clamps:
A pneumatic clamp is a clamp that combines a pneumatic cylinder with a toggle
mechanism. Clamps are fixtures that hold down and fix work pieces in place, and can
reliably hold work pieces without requiring the use of hands. Pneumatic clamps are
available in forms including downward pushing, vertical and horizontal.
Vertical pneumatic clamps:
They are characterized by higher holding and clamping forces (heavy-duty version).
Both clamps are particularly suitable for installation into specialized machines and
handling plants. Maintenance-free operation is ensured by tempered and ground
bearing pins in Teflon bearings.
Comments:
CNC Computer Numerical Control machines are widely used in
manufacturing industry. Traditional machines such as vertical millers, centre
lathes, shaping machines, routers.
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