How Are CNC Machine Tools Classified?

Author: Evelyn

Mar. 22, 2024

There are many varieties and specifications of CNC machine tools, and the classification methods are also different. Generally, they can be classified according to the following 4 principles based on function and structure.


1. Classification according to the control trajectory of machine tool motion

⑴ Point control CNC machine tools

Point control only requires the accurate positioning of the moving parts of the machine tool from one point to another. The requirements for the motion trajectory between points are not strict. No processing is performed during the movement. The movement between each coordinate axis is irrelevant. In order to achieve fast and accurate positioning, the displacement between two points generally moves quickly first, and then approaches the positioning point slowly to ensure positioning accuracy. As shown in the figure below, it is the motion trajectory of point control.

Machine tools with point control functions mainly include CNC drilling machines, CNC milling machines, CNC punching machines, etc. With the development of CNC technology and the decrease in the price of CNC systems, CNC systems purely used for point control are no longer common.


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⑵ Linear control CNC machine tools

Linear control CNC machine tools are also called parallel control CNC machine tools. Its characteristic is that in addition to controlling the accurate positioning between points, it also controls the movement speed and route (trajectory) between two related points. However, its movement route is only related to The machine tool coordinate axes move in parallel, which means that there is only one coordinate axis controlled at the same time (that is, there is no need for interpolation operation function in the CNC system). During the shifting process, the tool can cut at the specified feed speed. Generally, it can only process Rectangular and stepped parts.

Machine tools with linear control functions mainly include relatively simple CNC lathes, CNC milling machines, CNC grinders, etc. The CNC system of this machine tool is also called a linear control CNC system. Similarly, CNC machine tools used solely for linear control are rare.


⑶Contour control CNC machine tools

Contour control CNC machine tools are also called continuous control CNC machine tools. Its control feature is that it can control the displacement and speed of two or more motion coordinates simultaneously.

In order to meet the requirements that the relative motion trajectory of the tool along the workpiece contour conforms to the workpiece machining contour, the displacement control and speed control of each coordinate motion must be accurately coordinated according to the prescribed proportional relationship.

Therefore, in this type of control method, the CNC device is required to have an interpolation operation function. The so-called interpolation is to describe the shape of the straight line or arc through the mathematical processing of the interpolation operator in the CNC system based on the basic data input by the program (such as the end point coordinates of the straight line, the end point coordinates of the arc, and the center coordinate or radius). , that is, while calculating, allocate pulses to each coordinate axis controller according to the calculation results, thereby controlling the linkage displacement of each coordinate axis to comply with the required contour. During the movement, the tool continuously cuts the workpiece surface, which can perform various Processing of straight lines, arcs and curves. Contour controlled machining trajectory.


This type of machine tools mainly include CNC lathes, CNC milling machines, CNC wire cutting machines, machining centers, etc. Their corresponding CNC devices are called contour control CNC systems. According to the number of linkage coordinate axes it controls, it can be divided into the following types: 


① Two-axis linkage: mainly used for CNC lathes to process rotating surfaces or CNC milling machines to process curved cylindrical surfaces.


② Two-axis semi-linkage: It is mainly used for the control of machine tools with more than three axes. Two of the axes can be linked, and the other axis can perform periodic feed.


③ Three-axis linkage: Generally divided into two categories, one is the linkage of three linear coordinate axes of X / Y / Z, which is mostly used in CNC milling machines, machining centers, etc. The other type is to simultaneously control the two linear coordinates in X/Y/Z and also control the rotation coordinate axis that rotates around one of the linear coordinate axes.


For example, a turning machining center, in addition to the linkage of two linear coordinate axes in the longitudinal direction (Z-axis) and transverse direction (X-axis), it also needs to simultaneously control the linkage of the spindle (C-axis) rotating around the Z-axis.


④ Four-axis linkage: Simultaneously control the three linear coordinate axes X / Y / Z to be linked with a certain rotation coordinate axis.


⑤ Five-axis linkage: In addition to controlling the linkage of the three coordinate axes of X / Y / Z at the same time. It also simultaneously controls two of the A, B, and C coordinate axes that rotate around these linear coordinate axes, forming a simultaneous control of five axes. At this time, the tool can be positioned in any direction in space.


For example, the tool is controlled to swing around the x-axis and the Y-axis at the same time, so that the tool always maintains a normal direction to the contour surface being processed at its cutting point, so as to ensure the smoothness of the surface being processed and improve its processing accuracy and processing. efficiency, reducing the roughness of the machined surface.


2. Classification according to servo control method

⑴ Open-loop control CNC machine tools

The feed servo drive of this type of machine tool is open-loop, that is, there is no detection feedback device. Generally, its drive motor is a stepper motor. The main feature of a stepper motor is that every time the control circuit changes the command pulse signal, the motor rotates one step. distance angle, and the motor itself has self-locking ability.

The feed command signal output by the CNC system controls the drive circuit through the pulse distributor. It controls the coordinate displacement amount by changing the number of pulses, controls the displacement speed by changing the pulse frequency, and controls the displacement by changing the pulse distribution sequence. direction.

Therefore, the biggest features of this control method are convenient control, simple structure and low price. The command signal flow issued by the CNC system is one-way, so there is no stability problem of the control system. However, since the error of the mechanical transmission is not corrected by feedback, the displacement accuracy is not high.

Early CNC machine tools all used this control method, but the failure rate was relatively high. At present, due to the improvement of the drive circuit, it is still widely used. Especially in our country, this control method is often used in general economical CNC systems and CNC transformation of old equipment. In addition, this control method can configure a single-chip computer or single-board computer as a CNC device, reducing the price of the entire system.


⑵ Closed-loop control machine tools

The feed servo drive of this type of CNC machine tool works according to a closed-loop feedback control method. The drive motor can use two types of DC or AC servo motors, and needs to be configured with position feedback and speed feedback to detect the actual displacement of the moving parts at any time during processing. The amount is fed back to the comparator in the CNC system in time. It is compared with the command signal obtained by the interpolation operation. The difference is used as the control signal of the servo drive, which in turn drives the displacement component to eliminate the displacement error.

According to the installation location of the position feedback detection element and the feedback device used, it is divided into two control modes: full closed loop and semi closed loop.


① Fully closed loop control

As shown in the figure, the position feedback device uses a linear displacement detection element (currently a grating ruler is generally used), which is installed on the saddle of the machine tool. That is, it directly detects the linear displacement of the machine tool coordinates. Through feedback, the signal from the motor to the machine tool saddle can be eliminated. The transmission error in the entire mechanical transmission chain is reduced, thereby obtaining high static positioning accuracy of the machine tool.

However, in the entire control loop, the friction characteristics, rigidity and clearance of many mechanical transmission links are nonlinear, and the dynamic response time of the entire mechanical transmission chain is very large compared with the electrical response time. This brings great difficulties to the stability correction of the entire closed-loop system, and the design and adjustment of the system are also quite complicated. Therefore, this fully closed-loop control method is mainly used for CNC coordinate machine machines, CNC precision grinders, etc. that require high precision.


② Semi-closed loop control

As shown in the figure, its position feedback uses angle detection components (currently mainly encoders, etc.), which are directly installed on the end of the servo motor or screw. Since most of the mechanical transmission links are not included in the closed loop of the system, more stable control characteristics are obtained. Mechanical transmission errors such as screws cannot be corrected at any time through feedback, but software fixed value compensation methods can be used to appropriately improve their accuracy. At present, most CNC machine tools adopt semi-closed loop control mode


⑶ Hybrid control CNC machine tools

The characteristics of the above control methods can be selectively concentrated to form a hybrid control scheme. As mentioned before, since the open-loop control method has good stability, low cost, and poor accuracy, while the full-closed-loop control method has poor stability, in order to compensate for each other and meet the control requirements of some machine tools, a hybrid control method should be used. There are two commonly used methods: open-loop compensation type and semi-closed-loop compensation type.


3. Classification according to the functional level of CNC systems

According to the functional level of the CNC system, CNC systems are usually divided into three categories: low, medium and high. This classification method is commonly used in our country. The boundaries of low, medium and high grades are relative, and the classification standards will be different in different periods. Judging from the current level of development, various types of CNC systems can be divided into three categories: low, medium and high-end based on some functions and indicators. Among them, medium and high-end are generally called full-function CNC or standard CNC.


⑴ Metal cutting

Refers to CNC machine tools that use various cutting processes such as turning, milling, impacting, reaming, drilling, grinding, and planing. It can be divided into the following two categories.


① Ordinary CNC machine tools such as CNC lathes, CNC milling machines, CNC grinders, etc.


②The main feature of the machining center is the tool library with an automatic tool changing mechanism, and the workpiece passes through it once. After clamping, various cutting tools are automatically replaced, and various processes such as milling (turning) keys, reaming, drilling, and thread tapping are continuously performed on each processing surface of the workpiece on the same machine tool, such as (building/milling) processing. Center, turning center, drilling center, etc.


⑵ Metal forming

Refers to CNC machine tools that use extrusion, punching, pressing, drawing and other forming processes. Commonly used ones include CNC presses, CNC bending machines, CNC pipe bending machines, CNC spinning machines, etc.


⑶ Special processing category

There are mainly CNC wire EDM machines, CNC EDM forming machines, CNC flame cutting machines, CNC laser processing machines, etc.


⑷ Measurement and drawing

Mainly include three-dimensional coordinate measuring instrument, CNC tool setter, CNC plotter, etc.


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