How are CNC machine tools classified?
CNC machine tools have evolved dramatically since their invention. By now, its varieties and specifications are many and classified in different ways. The primary purpose of classifying CNC machine tools is to understand, manage, and apply them to improve productivity and optimize the machining process. Generally, according to the function and structure, according to the following four principles of classification.
First, according to the machine movement control track classification
(1) point control of CNC machine tools
Point control only requires the power of the machine tool’s moving parts to move from one point to another point of accurate positioning. The movement between the point and the point of the trajectory of the requirements is not strict. In the process of moving without machining, the movement between the coordinate axes is not relevant.
To achieve both fast and accurate positioning, the displacement movement between the two points is generally the first rapid movement and then slow convergence of the positioning point to ensure positioning accuracy, as shown in the figure below for the point position control movement trajectory.
Machine tools with point control functions are mainly CNC drilling machines, CNC milling machines, CNC punching machines, and so on. With the development of CNC technology and the reduction in the price of CNC systems, CNC systems are not shared purely for point control.
(2) linear control CNC machine tools
Linear control CNC machine tools, also known as parallel control CNC machine tools, are characterized by their ability to control positioning between points accurately.
In addition to positioning, these machines regulate the moving speed and route (trajectory) between two relevant points.
However, the movement route parallels the machine tool’s coordinate axes.
Only one coordinate axis is controlled at a time, meaning that interpolation operations are not required within the CNC system.
During operation, the tool can shift to the specified processing position and cut at a specified feed rate.
Generally, these machines are limited to processing rectangular or step-shaped parts.
The main machine tools with linear control functions are simple CNC lathes, milling machines, grinding machines, etc. This machine tool CNC system is also known as a linear control CNC system. Similarly, the linear control of CNC machine tools is not shared.
(3) contour control CNC machine tools
Contour-control CNC machine tools, also known as continuous-control CNC machine tools, are characterized by the ability to control two or more of the displacement and speed movement coordinates simultaneously.
To meet the relative motion of the tool along the contour of the workpiece trajectory in line with the requirements of the workpiece machining contour, it is necessary to coordinate the displacement control and speed control of the movement coordinates under the prescribed proportionality.
Therefore, the numerical control device must have an interpolation function in this type of control mode.
Interpolation is a process based on the basic data entered into the program, such as the end coordinates of a line, the end coordinates of an arc, and the center coordinates or radius of a circle.
Within the CNC system, the interpolation operator mathematically processes this data to describe the shape of a straight line or an arc.
As the calculations are performed, the system distributes pulses to the controller, enabling coordinated movement between axes.
This ensures the required contour is followed accurately.
During this movement, the tool continuously cuts along the surface of the workpiece.
As a result, the machine can process various straight lines, arcs, and curves. Contour-controlled machining trajectory.
These machines are mainly CNC lathes, CNC milling machines, CNC wire cutting machines, ice machining centers, etc., and their corresponding CNC devices are called contour control CNC systems according to the number of linkage axes they control differently. They can be divided into the following forms
① Two-axis linkage:
Mainly used for CNC lathe machining rotating surfaces or CNC milling machine machining curve column surface.
② Two-axis semi-linkage:
Mainly used for more than three-axis machine tool control, in which two axes can be linked, while the other axis can be a cycle of victory feed.
③ Three-axis linkage:
generally divided into two categories: X / Y / Z, which has three linear coordinate axis linkage, more for CNC milling machines, machining centers, etc. Another category is in addition to the simultaneous control of X / Y / Z in the two linear coordinates, but also at the same time to control around one of the linear coordinate axis rotation of the rotary axis.
Such as turning machining centers, it is in addition to longitudinal (Z-axis), transverse (X-axis) two linear coordinate axis linkage, but also need to simultaneously control around the Z-axis rotation of the spindle (C-axis) linkage.
④ Four-axis linkage:
Simultaneous control of X /Y/Z three linear coordinate axes and a rotary coordinate axis linkage.
⑤ Five-axis linkage:
In addition to the simultaneous control of the X / Y / Z three linear coordinate axes linkage, the rotation around these linear coordinate axes of the A, B, and C axes of the two coordinate axes can also be controlled. Thus, the simultaneous control of the tool’s five-axis linkage can be formed in any direction in space.
For example, controlling the tool to swing around the x-axis and Y-axis in two directions at the same time so that the tool is always kept regular to the machined contour surface at its cutting point ensures the smoothness of the machined surface, improves its machining accuracy and efficiency, and reduces the roughness of the machined surface.
Second, according to the servo control mode classification
(1) Open-loop control CNC machine tools
This type of machine tool feed servo drive is open-loop, that is, there is no detection of feedback devices, generally its drive motor for the stepper motor, stepper motor is the main feature of the control circuit for each change in the command pulse signal, the motor will rotate a step angle, and the motor itself can self-locking.
The CNC system output feed commands a signal through the pulse distributor to control the drive circuit, which changes the number of pulses to control the coordinate displacement, changes the frequency of the pulse to control the displacement speed, and changes the order of the distribution of the pulse to control the direction of displacement.
Therefore, the most significant characteristics of this control method are convenience, simple structure, and low cost. The CNC system issued by the command signal flow is unidirectional, so there are no control system stability problems. However, due to the error of mechanical transmission, which is not corrected by feedback, the displacement accuracy is not high.
Early CNC machine tools used this control method, but the failure rate was relatively high. Due to the improvement of the drive circuit, it is still more widely used today.
The general economic CNC system and the old equipment CNC transformation are more than this control mode. In addition, this control method can also be configured with a single chip or board as a numerical control device, reducing the whole system price.
(2) Closed-loop control machine tools
This CNC machine tool feed servo drive type operates based on a closed-loop feedback control system. The drive motor can use either a DC or AC servo motor.
Position and speed feedback systems are required for precise control. During processing, the system continuously detects the actual displacement of the moving parts.
This displacement data is fed back to the numerical control system’s comparator, which compares the feedback data with the command signal generated by the interpolation operation.
The difference between these values is used as a control signal for the servo drive. This control signal drives the displacement parts, correcting any displacement errors.
The installation part of the position feedback detecting element and the different feedback devices used are divided into fully closed-loop and semi-closed-loop control.
① Full closed-loop control
As shown in the figure, the position feedback device using linear displacement detection element (currently generally used scale), installed in the machine tool saddle parts, that is, direct detection of linear displacement of the machine tool coordinates, through the feedback can be eliminated from the motor to the machine tool saddle of the entire mechanical transmission chain in the transmission error, to obtain a very high machine tool static positioning accuracy.
However, in the entire control loop, many mechanical transmission links’ friction characteristics, rigidity, and clearance are nonlinear, and the dynamic response time of the whole mechanical transmission chain is considerably longer than the electrical response time.
This makes stabilizing the whole closed-loop system very difficult, and system design and adjustment are also quite complex. Therefore, this complete closed-loop control is mainly used for the high-precision requirements of CNC coordinate block beds, CNC precision grinding machines, and so on.
② semi-closed-loop control
As shown in the figure, the position feedback using the corner detection element (currently used mainly encoders, etc.), installed directly in the servo motor or screw end. Because most of the mechanical transmission links are not included in the system closed loop loop, so called to obtain more stable control characteristics.
Screw and other mechanical transmission errors can not be readily corrected through feedback, but can be used to improve the accuracy of the software fixed-value compensation method. At present, most of the CNC machine tools use a semi-closed-loop control mode
(3) Mixed control CNC machine tools
The characteristics of the above control methods are selectively focused and can be composed of a mixed control scheme.
As mentioned above, due to the open-loop control mode’s stability, low cost, and poor accuracy, while the complete closed-loop stability is inadequate, so to compensate for each other to meet the control requirements of specific machine tools, it is appropriate to use a hybrid control mode. The use of more open-loop compensation and semi-closed-loop compensation type of two types of methods
Third, according to the functional level of the CNC system classification
According to the functional level of the CNC system, the CNC system is usually divided into three categories: low, medium, and high.
The low, medium, and high three-class boundaries are relative, and they correspond to different periods; the division criteria will be different.
Based on various functions and indicators, the current level of development of CNC systems can be divided into low, medium, and high-grade categories. Medium and high-grade systems are generally called full-featured CNC or standard CNC.
(1) Metal cutting category
It refers to CNC machine tools that use cutting processes such as turning, milling, bumping, reaming, drilling, grinding, and planing. It can be further divided into the following two categories.
① general type CNC machine tools such as CNC lathes, CNC milling machines, CNC grinding machines, etc..
② Machining centers These machines have a tool magazine with an automatic tool changer, which works on one workpiece at a time.
Clamping, through the automatic replacement of a variety of tools, on the same machine tool for the workpiece machining surface continuous milling (turning) key, reaming, drilling, tapping threads and other processes, such as (block/milling class) machining centers, turning centers, drilling centers and so on.
(2) metal molding class
This refers to using extrusion, punching, pressing, pulling, and other molding processes, as well as CNC machine tools, commonly used CNC presses, CNC bending machines, CNC spinning machines, and so on.
(3) special processing class
Mainly CNC EDM wire cutting machine, CNC EDM molding machine, CNC flame cutting machine, CNC laser processing machine.
(4) measurement, drawing class
Mainly three-coordinate measuring instruments, CNC tool setting instruments, CNC plotters, etc..
Conclusion
In conclusion, the classification of CNC machine tools is an essential means of improving production management, optimizing processes, promoting technical exchanges, and advancing industry development.