Basic of CNC CONTROL SYSTEM

1. CNC Control
Systems

2. Mode of the CNC Control System
• Point-to-point path
The point-to-point
control system is often
referred to as the
positioning system.
1. Axis path.
2. 45° line path.
3. Linear path.

3. In the linear path motion
mode, the controller has
the capability to
synchronize the motion
in X and Y directions to
generate a linear path.
The most common
applications of the PTP
control system are
drilling, boring, reaming,
tapping and sheet metal
punching.

4. The cutting of arcs and
angles other
not possible
PTP system,
than 45° is
with the
but it can
be approximated by a
series of straight-line
cuts.
The tolerance, or
deviation of the actual
path from the true path,
can be specified in
three ways.

5. Continuous-path
The continuous-path control
system is also known as the
contouring system. It involves
simultaneous motion control of
two or more axes. The contouring
system is more complex because
each axis of motion requires
separate position and velocity
loops. The contouring along a
predefined tool path is
implemented by means of
interpolation, in which the system
generates a set of intermediate
data points between given
coordinate positions

6. CNC Interpolation
An interpolator provides two functions:
It computes individual axis velocities to drive the
tool along the programmed path at the given feed
rate.
It generates intermediate coordinate positions
along the programmed path. There are five types of
interpolation: linear, circular,helical, parabolic, and
cubic.

7. Linear Interpolation
Linear interpolation requires three parameters:
start point coordinate, end point coordinate and the speed
command for each axis. In two-axis linear interpolation, the
interpolator calculates the speed commands, in pulses per
second, for the X and Y axes in such a way that it maintains
the speed ratio between the X and Y axes equal to the ratio of
the required incremental distance (dx / dy).

8. Theoretically, linear interpolation can be used to cut
all types of tool paths, including straight lines, circles,
arcs, curves, and helical contours, etc.. However, it
takes much more data to cut contours other than
straight lines.

9. The use of other interpolation method results in a
substantial reduction of data to be processed. Curves
that can not be defined mathematically can only be
approximated by using linear interpolation.

12. Circular Interpolation
The interpolator computes the axial velocity
components and produces a sequence of reference
pulses for each control axis of motion. The
advantage of circular interpolation is its ability to
generate an arc in a single program block. In some
NC controls, circular interpolation is limited to a 90°
arc in a single block.

13. The information required for programming a circular
interpolation includes:
(1)
(2)
(3)
coordinates of the start point and end point,
radius of
direction
the arc or coordinates of the arc centre, and
in which the tool is to proceed (CW or CCW).
interpolation is limited to the two-axis plane.The circular

16. Helical Interpolation
It combines the two-axis circular interpolation and a
linear interpolation along the third axis.

17. Helical interpolation:-
Producing a large-diameter hole is a common
application for many shops, and there are numerous
methods that can be used to achieve the end result.
However, there are often numerous obstacles to
completing the process cost effectively. Horsepower
consumption is frequently a concern in these types of
applications, especially on the more common 20
horsepower and below machine tools. These machines
are capable of high speeds and feeds, but rigidity is
sacrificed to the extent necessary to accomplish the
quick movements. Using conventional means, making
large diameter holes is hard on the machine...
Function and purpose:-
Command G02 or G03 with a designation for the third
axis allows synchronous circular interpolation on the
plane specified by plane-selection command G17, G18
or G19 with the linear interpolation on the axis.

18. Description:-
For helical interpolation, movement designation
is additionally required for one to two linear axes not
forming the plane for circular interpolation.
The velocity in the tangential direction must be
designated as the feed rate F.
Programming Format:-
G17 G02 (or G03) X___ Y___ I__ J__ P__ F__ ;
Or
G17 G02 (or G03) X__ Y__ R__ P__ F__ ;
X = Arc ending point coordinates X axis
Y = Arc ending Point coordinates Y axis
I & J = Arc center Coordinates
P = Number of pitches
F = Feed rate
R = Arc Radius
Example:-
G28 U0. W0. Y0. ;
G50 X0. Z0. Y0. ;
G17 G03 X100. Y50. Z-50.0 R50. F1000.
Notes:-
Plane selection:-
As with circular interpolation, the circular
interpolation plane for helical interpolation is
determined by the plane selection code and axis
addresses. The basic programming procedure for
helical interpolation is selecting a circular –
interpolation plane using a plane selection command
(G17, G18 or G19) and then designating the two axis
addresses for circular interpolation and the address of
one axis for linear interpolation.

19. Parabolic Interpolation
Parabolic interpolation uses three noncollinear points to
approximate free-form curves. It can be used to cut either
planar or spatial curves. It is primarily used in mold and die
making, where free-form designs are preferred over
precisely defined shapes.

20. Cubic Interpolation
It is of the third order and can be used to generate complex
tool path for machining complicated shapes such as
automobile sheet metal dies with a relatively small number of
programmed points.
However, it is very complex and requires considerable
computing power and a large memory.

21. Open-loop Control Systems
Open-loop systems normally use stepping motors as the
drive devices to move the machine slide.
Due to the advent of precision ball screw and stepping
motor control technology, open-loop control can be
refined to 0.001 in. resolution, which is accurate enough to
be used in many precision positioning and light-load
contouring applications.

23. Closed-loop Control Systems
A feedback loop is implemented to monitor the actual
output and correct any discrepancy from desired output.
Both analog-type and digital-type can be applied.
Most modern closed-loop NC systems are able to provide
very fine resolution of 0.0001 in.