Classification of CNC control systems

            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.