There are many types of CNC machine tools, which can be classified from different perspectives. Today, we will introduce two classification methods, namely classification by motion mode and by control mode. The details are as follows.
CNC machine tools classified by motion mode can be divided into:
(1) Point-to-point control system
This system only controls the position of the tool moving from one point to another, without cutting during the movement, such as in jig boring machines, drilling machines, and punching machines. High positioning accuracy of the coordinate position is required. To improve production efficiency, the positioning movement is carried out at the upper limit feed speed set by the machine tool. Before approaching the positioning point, stepwise or continuous deceleration is applied to approach the end point at low speed, thereby reducing the inertial overshoot of moving components and the resulting positioning errors. Since no cutting is performed during the positioning movement, there are no requirements for the motion trajectory.
(2) Linear control system
The linear control system controls the tool or the basic worktable to move accurately from one position to another at a certain speed along a direction parallel to a certain coordinate axis. It is also called the point-to-point linear motion control system.
(3) Contouring control system
This system controls two or more coordinate axes simultaneously (two-axis, two-and-a-half-axis, three-axis, four-axis, five-axis linkage). It not only controls the start and end coordinates of the machine tool's moving components but also controls the speed, direction, and displacement at every point during the entire machining process, that is, it controls the machining trajectory to produce the required contour. The motion trajectory can be straight lines of arbitrary slope, circular arcs, helical lines, etc. The CNC devices of such machine tools have very complete functions, capable of two-axis or even multi-axis linkage control, as well as point-to-point and linear control.
CNC machine tools classified by control mode can be divided into:
(1) Open-loop CNC machine tools
This refers to a control system without feedback. There are no position feedback components in the system, and stepper motors are usually used as the actuating mechanism. The input data is processed by the CNC system, which issues command pulses. Through the ring distributor and drive circuit, the stepper motor rotates by a step angle, and then through the transmission mechanism, the worktable moves a distance equivalent to one pulse equivalent. The moving speed and displacement of the moving components are determined by the frequency and number of input pulses.
(2) Semi-closed-loop CNC machine tools
An angular displacement detection device (such as an optical encoder or an inductosyn) is installed at the end of the drive motor or the lead screw. By detecting the rotation angle of the motor or lead screw, the actual position or displacement of the actuating component is indirectly measured and then fed back to the CNC system. This achieves higher accuracy than an open-loop system, but its displacement accuracy is lower than that of a closed-loop system. Compared with a closed-loop system, it is easier to achieve system stability. Nowadays, most CNC machine tools widely adopt this semi-closed-loop feed servo system, and the machine tool moving components with large inertia are not included in the detection range.
(3) Closed-loop CNC machine tools
A detection device is directly installed on the machine tool's moving components, and the measured results are fed back directly to the CNC system. In essence, the displacement command value is compared in real time with the actual position feedback signal measured by the position detection device, and control is carried out based on the difference, enabling the moving components to move according to the actual requirements and achieve accurate positioning.
