3. HISTORY
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.
4. Basics of CNC
A numerical control, or “NC”, system controls
many machine functions and movements which
were traditionally performed by skilled
machinists.
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.
5.
6. 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
axes.
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.
7.
8. 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.
9. 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
10.
11.
12. CNC CLASSIFICATION
1.Motion control:
point to point (PTP)
continuous (contouring) path
2. Control loops:
open loop
closed loop
3. Power drives:
hydraulic, electric or pneumatic
4. Positioning systems:
incremental
absolute positioning
5. Hardwired NC and soft wired Computer
Numerical Control (CNC)
13. 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
repeats operation
Eg:Mainly used in drilling, punching, and
straight milling operations
15. 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
movements
18. OPEN LOOP SYSTEM
In open-loop control system step motors
are used
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
19. 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
correction
Better accuracy can be achieved
More expensive
Suitable for large size machine tools
20. INTERPOLATION
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
equations
28. Advantages of CNC
Productivity
Machine utilisation is increased because
more time is spent cutting and less time is
taken by positioning.
Reduced setup time increases utilisation
too.
Quality
Parts are more accurate.
Parts are more repeatable.
Less waste due to scrap.
29. Advantages of CNC
Reduced inventory
Reduced setup time permits smaller economic
batch quantities.
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
skin milling.
30. Advantages of CNC
Management Control
CNC leads to CAD
Process planning
Production planning
31. 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
simple parts
As geometric complexity or volume increases
CNC becomes more economical
Maintenance personnel must have both
mechanical and electronics expertise
33. 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
of identification
The operator must know the
common tools by simple
visual observation
34. Boxford Vertical Mill Cutting
Tools
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
35. CNC Bench Vise
Notice the smooth machined surfaces; be careful not to damage surfaces
36. 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
fixed distance
37. Z-axis Arbor
Keep the lands clean
and oiled
A slight tilt will cause
noticeable cut
deformations
Align the protrusions
with slots in the cutting
tool quick change
adapter body
38. Air Nozzle
Used for:
Keeping cutting area
free of excessive
cutting debris
Quick cleaning
When supplied with
misting, provides
cutting tool cooling
39. End Mill / Facing Cutter
Hogs off large amounts
of material
Radius of cut very large
Provides a known
surface from which to
measure from
Requires workpiece to be
robustly supported due
to high torque created
40. Quick Change End Mill Adapter
Helps provide quick
changing of cutting
tools without having to
use complex setup
procedures
43. Control Panel
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
is active.
44. Control Panel
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.
45. Control Panel
7. MANUAL
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
machine axes.
• 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
occurred.
•During an Automatic Cycle with a Feed Hold (1) active, pressing will Abort
the current job.
46. Control Panel
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
fitted.
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.
47. Control Panel
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.
48. Control Panel
17. EMERGENCY STOP
Pressing this pushbutton stops all machine movement and
switches off the electronics, and the button locks in the
depressed position.
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.
