The CNC control systems can be
classified based on below types:
1.
Motion
type CNC
1.1.
Contouring
systems
1.2.
Point
to point systems
2.
Control loop CNC
2.1.
Closed
loop system
2.2.
Open
loop system
3.
Number
of axis type CNC
3.1.
2-axis
machines
3.2.
2.5
axis machines
3.3.
3
axis machines
3.4.
4
axis machines
3.5.
5
axis machines and above.
1.
Motion
type CNC Control:
Based on the basic difference of the
machine to be controlled there are two types of machine tools and control
system. “Contouring systems” and “Point to point control system”.
1.1 Contouring system:
This type of machine tools works in a continuous
path by cutting the material and following a contour of the part. These machine tools are known as contouring
machines which includes Milling, lathe and routing machines. These contouring
machines are also capable of doing the work same as point to point system.
These contouring system machines require simulations movement of the tool and
the work piece i.e., the positions of the work piece and the tool are simultaneously
controlled by a control system. These machines are uneconomical if it’s used as
only point to control without continuous motions. The contouring motion works as below shown in
figure.
1.2 Point to point control
system:
The
work piece and the tool are placed in the position and the tool does its work.
These types of machines are called point to point systems. The drilling, tapping and boring machines are
characterized under these types. The work piece is not moved until the tool
finishes the job and retracts to the safety.
The point to point control system works as shown in the below figure.
2.
Loop
control CNC systems:
Based on the looping system the CNC
systems are classified into two categories, “Open loop system” and “closed loop
system”.
2.1 Closed Loop system:
Since this type of system has
feedback from the control system to the actual and the programmed input we call
it as closed loop system. The CNC systems works on servo mechanism and it’s a
closed loop principle. The feedback can be measured by analog or the digital
systems. The analog systems measure the physical variables such as position and
velocity in terms of voltage levels and digital systems monitor by electrical
pulses. These closed loop systems are
accurate and powerful because of their capability of monitoring operational
condition through feedback systems and automatically adjusting the variations
in real time.
2.2 Open Loop system:
Open
loop system are the systems in which instructions of program are sent into the
controller through the input device and then these instructions are converted
to the signals by the controller and sent to the servo amplifiers to energize
the servo motors. The open loop systems are usually used in the point to point
control systems where the accuracy doesn’t matter much and in few continuous
path control system since there is no feedback from the system the result may
deviate from the actual.
3.
Number
of axis type CNC Control:
3.1 Two axis control system:
Two axis control system is a
machine which give access to only two axis. The best example is the lathe
machine. The machine driven with the
servo motor allows you only 2 axis, i.e., X and the z Axis. Here in lathe the job will be rotating and
the tool will be moving in 2 directions, indicating the depth Z and the cut in
X direction. So the program is done with
x and the Z direction, where as in lathe universally use U and V which is
implies same as X and Y.
3.2 Two and half axis control
system:
Two and half axis control system is also a three axis machine but the movement
will not be 3 dimensional. These are the best example for drilling and tapping
machines. First the X and the Y axis are moved to the position and then the
third axis comes to effect. In some of the
machines the first X movement is made and the Y movement to reach the XY
destination when it is in rapid, But while in feed or machining mode it does
its work with 2 dimensional moving along x and y simultaneously, These type of
machines may also called as two and half axis machines or 2.5 axis machine.
3.3 Three axis control system:
Three axis control systems are the
machines which moves in three dimensional i.e., X, Y and Z axis move
simultaneously. These are the most popular machines which can produce high
accurate precision parts with three axis machines. The servo motor control the
movements as per the instructions are given.
The three axis has verity of machines based on their bed lengths. As the bed length increases the cost of the
machine increases. The number of setups increases
and the cost also increases, three axis machines are limited for simple jobs
and straight forward 3 axis jobs.
3.4 Four axis control system:
In Four axis control system is a three
axis machine with an extra rotation on B-axis, four axis can be a vertical machine
or an horizontal machine.
In vertical CNC machine
the rotary head is added on the side of the machine bed. The machine works as
the three axis machine but it has a rotary head for example if the holes have
be machined on the tube, the tube can be mounted on the rotary B-axis and the
drilling in the required angle. But vertical 4-axis machines are limited for
the small jobs.
Horizontal 4-axis
machines are used worldwide for successful machining of big parts in a single
setup, where it takes 2-3 setups in a 3-axis machine. The part is mounted on
the bed which has 360 rotations around Y direction i.e., B-Axis giving chance
for the tool to cut on the angled faces in a single setup.
3.5 Five axis control system:
Five axis control system are the three
axis machines with an extra rotation along Y and Z directions which are called
B-axis and A-axis. As in Four axis machines B rotation is given by the bed and
the A-axis rotation is given by the spindle movement called PIVOT point. Using a five axis machine reduces the cycle
time by machining the complex parts in a single setup and improves the accuracy
on the positional errors by eliminating the setups.
Five
axis machining gives improved access to the under cuts and deep pockets by
tilting the tool and also gives good surface finish and the tool life by
tilting the tool to maintain optimum tool to part contact all the times.
There are more than five axis
control systems in the world making aerospace and automobile industries to achieve
their accuracy and meet the requirements.