Computer Aided Manufacturing
Department of Automobile Engineering
SRM University, kattankulathur campus
US Air Force commissioned MIT to develop the
first "numerically controlled" machine in 1949. It
was demonstrated in 1952.
At 1970-1972 first Computer Numeric Control
machines were developed.
Today, computer numerical control (CNC)
machines are found almost everywhere, from
small job shops in rural communities to
companies in large urban areas.
Basics of CNC
A numerical control, or “NC”, system controls
many machine functions and movements which
were traditionally performed by skilled
Numerical control developed out of the need to
meet the requirements of high production rates,
uniformity and consistent part quality.
Programmed instructions are converted into
output signals which in turn control machine
operations such as spindle speeds, tool
selection, tool movement, and cutting fluid flow.
Coordinates of CNC
All computer controlled machines are able to
accurately and repeatedly control motion in
various directions. Each of these directions of
motion is called an axis. Depending on the
machine type there are commonly two to five
Additionally, a CNC axis may be either a linear
axis in which movement is in a straight line, or a
rotary axis with motion following a circular path.
Basic principle of CNC
Each axis consists of a mechanical
component, such as a slide that moves, a
servo drive motor that powers the
mechanical movement, and a ball screw to
transfer the power from the servo drive
motor to the mechanical component.
These components, along with the
computer controls that govern them, are
referred to as an axis drive system.
CNC SYSTEM ELEMENTS
A typical CNC system consists of the
following six elements
1. Part program
2. Program input device
3. Machine control unit
4. Drive system
5. Machine tool
6. Feedback system
point to point (PTP)
continuous (contouring) path
2. Control loops:
3. Power drives:
hydraulic, electric or pneumatic
4. Positioning systems:
5. Hardwired NC and soft wired Computer
Numerical Control (CNC)
Classification of CNC system
Point-to-Point . each axis driven
separately and at different velocities
Moves at max velocity initially then slows
as gets to desired position
After operation complete tool is retracted
and moved rapidly to next position and
Eg:Mainly used in drilling, punching, and
straight milling operations
Classification of CNC system
Continuous Path . positioning and
cutting operations both controlled along
paths but at different velocities
Tool is cutting as it travels
Requires synchronization of velocities and
OPEN LOOP SYSTEM
In open-loop control system step motors
Step motors are driven by electric pulses
Every pulse rotates the motor spindle
through a certain amount
By counting the pulses, the amount of
motion can be controlled
No feedback signal for error correction
Lower positioning accuracy
CLOSED LOOP SYSTEM
In closed-loop control systems DC or AC
motors are used
Position transducers are used to generate
position feedback signals for error
Better accuracy can be achieved
Suitable for large size machine tools
Types of Interpolation
Linear . tool moves in straight line from
start to end in 2 or 3 axes
Circular . needs start and end points,
coordinates of center of circle, radius, and
direction of tool along the arc
Parabolic and Cubic . paths approximated
by curves using higher order mathematical
Aircraft Turbine Machined by
5-Axis CNC Milling Machine
Advantages of CNC
Machine utilisation is increased because
more time is spent cutting and less time is
taken by positioning.
Reduced setup time increases utilisation
Parts are more accurate.
Parts are more repeatable.
Less waste due to scrap.
Advantages of CNC
Reduced setup time permits smaller economic
Lower lead time allows lower stock levels.
Lower stock levels reduce interest charges and
working capital requirements.
Machining Complex shapes
Slide movements under computer control.
Computer controller can calculate steps.
First NC machine built 1951 at MIT for aircraft
Advantages of CNC
CNC leads to CAD
Draw backs of CNC
High capital cost
Machine tools cost $30,000 - $1,500,000
Retraining and recruitment of staff
New support facilities
High maintenance requirements
Not cost-effective for low-level production on
As geometric complexity or volume increases
CNC becomes more economical
Maintenance personnel must have both
mechanical and electronics expertise
CNC Machine Components
Cutting Tool / Collet Caddy
Very common to have 100 or
more diverse cutting tools
loaded into a collet caddy
They are numbered for ease
The operator must know the
common tools by simple
Boxford Vertical Mill Cutting
Collet Caddy # Tool in Collet (different lengths)
5 2mm end mill, ball end
6 2mm end mill, slot drill
3 4mm end mill, slot drill
4 6mm end mill, slot drill
1 8mm end mill, slot drill
2 25 mm end mill, facing cutter
CNC Bench Vise
Notice the smooth machined surfaces; be careful not to damage surfaces
CAM & X-axis Table Alignment
To machine a part
with CNC, the end of
the tool must be
known relative to the
bench vise, x-axis
table, or some other
Keep the lands clean
A slight tilt will cause
Align the protrusions
with slots in the cutting
tool quick change
Keeping cutting area
free of excessive
When supplied with
cutting tool cooling
End Mill / Facing Cutter
Hogs off large amounts
Radius of cut very large
Provides a known
surface from which to
Requires workpiece to be
robustly supported due
to high torque created
Quick Change End Mill Adapter
Helps provide quick
changing of cutting
tools without having to
use complex setup
Lockable On-Off Switch
1. FEED HOLD
Acts as an interrupt control to the program when pressed, preventing
any further tool movement until canceled. During an Automatic cycle,
after canceling CYCLE START must be pressed to resume the cycle.
2. SINGLE BLOCK
With machine running in automatic mode controlled from the PC, this
allows the operation of only a single block (line) of the program at a
time to be toggled on/off. Press CYCLE START to proceed.
3. CYCLE START
This sets the machine in automatic mode and starts the machining
from the PC program. Also used to resume an Automatic cycle after
a FEED HOLD is canceled and to action the next line if Single Block
4. FEED SPEED CONTROL
Provides stepless control of tool feed speed in the X, Y and Z axes from 0 to 100%
in 10% increments.
5. JOG TABLE AXIS CONTROLS
These four buttons control manual movement of the table in the X and Y axes.
Move the table in the X axis while held pressed.
Move the table in the Y-axis while held pressed.
6. JOG HEAD AXIS CONTROLS
Move the head in the Z axis while held pressed.
Pressing this button together with any of the other six axis control buttons provides
rapid movement of the axes in the indicated direction.
When lit, machine is operated from control panel (Manual mode). Pressing MAN
puts light out and machine is controlled by computer (Automatic mode). Press
MAN again to revert to manual mode operation.
8. AIR MIST CONTROL
Switches on or off the optional air mist coolant when fitted.
9. DATUM M/C - PROGRAM ABORT
After initially powering up the machine and pressing the Power Reset button (23),
with the machine in Manual mode (Manual mode button lit) press to datum the
• With the machine in Manual mode, the machine can be re-datumed at any
point. This should always be done if a physical stall of the axes has
•During an Automatic Cycle with a Feed Hold (1) active, pressing will Abort
the current job.
10. TOOL CHANGE
With auto tool changer fitted, press to change to next tool automatically.
11. TOOL RELEASE
Releases the tool holder from the spindle if fitted with the auto tool
changer. Guard door must be open and tool holder held.
12. WORK HOLDING UN-CLAMP
Controls un-clamping of workpiece in pneumatic work holder vice, when
13. WORK HOLDING CLAMP
Controls clamping of workpiece in pneumatic work holder vice, when fitted.
14. DOOR OPEN
Controls opening of the automatic doors when fitted.
15. DOOR CLOSE
Controls closing of the automatic doors when fitted.
16. SPINDLE CONTROLS
These control the speed and direction of rotation of the spindle.
Spindle Forward - Pressing this button starts the spindle rotating
forward. Holding it down increases forward speed. Pressing and holding the
'Spindle Reverse' button reduces speed in the forward direction.
Spindle Reverse - Pressing this button starts the spindle rotating in
reverse direction. Holding it down increases spindle speed in reverse
direction. Pressing and holding the 'Spindle Forward' button reduces speed in
the reverse direction.
Spindle Stop - Pressing this button stops spindle rotation.
17. EMERGENCY STOP
Pressing this pushbutton stops all machine movement and
switches off the electronics, and the button locks in the
If the machine is in production, an appropriate message is
displayed on the PC screen.
The button must be turned to release it and the Hard Reset
button (23) pressed to enable the machine to be started. The
machine must be datumed using the Datum M/C key (9).
If the machine was performing a production cycle, the cycle
must be repeated from the beginning.