1. Classification by the Control Path of Machine Tool Motion

⑴ Point-Control CNC Machine Tools

Point-control only requires accurate positioning of the machine tool's moving parts from one point to another. The motion trajectory between points is not strictly required. No machining is performed during the movement, and the motion between coordinate axes is unrelated. To achieve both fast and accurate positioning, displacement between two points is generally performed initially with a rapid movement followed by a slow approach to the positioning point to ensure accurate positioning. The figure below shows the motion trajectory for point-control.

Machine tools with point-to-point control capabilities primarily include CNC drilling machines, CNC milling machines, and CNC punching machines. With the advancement of CNC technology and the reduction in the price of CNC systems, CNC systems solely used for point-to-point control are becoming rare.

(2) Linear Control CNC Machine Tools

Linear control CNC machine tools, also known as parallel control CNC machine tools, are characterized by not only controlling the precise positioning between points but also controlling the speed and path (trajectory) of movement between two related points. However, their movement path is parallel to the machine's coordinate axes, meaning only one axis is controlled simultaneously (meaning interpolation is not required within the CNC system). During this movement, the tool can cut at a specified feed rate and can generally only process rectangular or stepped parts.

Machine tools with linear control capabilities primarily include relatively simple CNC lathes, CNC milling machines, and CNC grinders. The CNC systems for these machine tools are also called linear control CNC systems. Similarly, CNC machine tools solely used for linear control are also rare. (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 simultaneously control the displacement and velocity of two or more moving coordinates.

In order to ensure that the tool's relative motion trajectory along the workpiece contour conforms to the workpiece machining profile, the displacement and velocity control of each coordinate motion must be precisely coordinated according to a specified proportional relationship.

This type of control method therefore requires the CNC device to have interpolation capabilities. Interpolation involves mathematically processing the shape of a line or arc based on basic program input data (such as the end point coordinates of a line or arc, and the center coordinates or radius). This calculation is then performed while pulses are distributed to the controllers of each coordinate axis based on the results. This controls the coordinated displacement of each axis to conform to the desired contour. During this motion, the tool continuously cuts the workpiece surface, enabling machining of various straight lines, arcs, and curves.

Contour control machining trajectories. This type of machine tool primarily includes CNC lathes, CNC milling machines, CNC wire-cut machines, and machining centers. Their corresponding CNC devices are called contour control CNC systems. Depending on the number of coordinated axes they control, they can be categorized into the following types:

① Two-axis linkage: Mainly used for machining rotating surfaces on CNC lathes or for machining curved cylindrical surfaces on CNC milling machines.

② Two-axis semi-linkage: Mainly used for controlling machine tools with three or more axes, where two axes can be linked, while the third axis can perform cyclic feed.

③ Three-axis linkage: Generally, there are two types. One type involves the linkage of the three linear coordinate axes (X/Y/Z), which is commonly used in CNC milling machines and machining centers. The other type, in addition to simultaneously controlling two linear axes (X/Y/Z), also controls a rotary axis that rotates around one of these axes.

For example, in a turning machining center, in addition to the linkage of the longitudinal (Z-axis) and transverse (X-axis) linear axes, it also requires simultaneous control of the spindle (C-axis) that rotates around the Z-axis.

④ Four-Axis Motion: Simultaneously controls the three linear coordinate axes (X, Y, and Z) in conjunction with a rotational axis.

⑤ Five-Axis Motion: In addition to simultaneously controlling the three linear coordinate axes (X, Y, and Z), it also simultaneously controls two of the A, B, and C axes that rotate around these linear axes, resulting in simultaneous control of five axes. This allows the tool to be positioned in any spatial orientation.

For example, the tool can be controlled to oscillate simultaneously around both the X and Y axes, ensuring that the tool always maintains a normal orientation to the contour being machined at its cutting point. This ensures smoothness of the machined surface, improves machining accuracy and efficiency, and reduces surface roughness.

II. Classification by Servo Control Method

⑴ Open-Loop Control CNC Machine Tools

The feed servo drive of this type of machine tool is open-loop, meaning it lacks a detection and feedback device. It is typically driven by a stepper motor. The key feature of a stepper motor is that the motor rotates one step angle for each command pulse signal from the control circuit, and the motor itself has a self-locking capability. The feed command signal output by the CNC system controls the drive circuit through a pulse distributor. This control method controls the coordinate displacement by varying the number of pulses, the displacement speed by varying the pulse frequency, and the direction of displacement by varying the pulse distribution order.

Thus, the key advantages of this control method are ease of control, simple structure, and low cost. The command signal flow from the CNC system is unidirectional, eliminating control system stability issues. However, since mechanical transmission errors are not corrected through feedback, displacement accuracy is low.

Early CNC machine tools all employed this control method, albeit with a relatively high failure rate. However, due to improvements in drive circuits, it is still widely used. In my country, in particular, this control method is often used in cost-effective CNC systems and CNC retrofits for older equipment. Furthermore, this control method can be configured with a single-chip microcontroller or single-board computer as the CNC device, reducing the overall system cost. (2) Closed-Loop Control Machine Tools

The feed servo drive of this type of CNC machine tool operates using closed-loop feedback control. The drive motor can be either a DC or AC servo motor and requires position and velocity feedback. During machining, the actual displacement of the moving component is detected at all times and promptly fed back to a comparator in the CNC system. This comparator then compares the actual displacement with the command signal generated through interpolation. The difference serves as the control signal for the servo drive, which in turn drives the moving component to eliminate displacement errors.

Depending on the installation location of the position feedback sensor and the feedback device used, this control method can be categorized as either full-closed-loop or semi-closed-loop.

① Fully Closed-Loop Control

As shown in the figure, the position feedback device uses a linear displacement sensor (currently typically a grating scale) installed on the machine tool's saddle. This directly detects the linear displacement of the machine tool coordinates. This feedback eliminates transmission errors in the entire mechanical transmission chain from the motor to the saddle, thereby achieving very high static positioning accuracy.

However, due to the nonlinear friction, stiffness, and backlash characteristics of many mechanical transmission links within the entire control loop, and the significantly longer dynamic response time of the entire mechanical transmission chain compared to the electrical response time, this poses significant challenges in calibrating the stability of the entire closed-loop system and complicates system design and adjustment. Therefore, this fully closed-loop control method is primarily used for CNC coordinate machines and CNC precision grinders, which require very high precision.

② Semi-Closed-Loop Control

As shown in the figure, the position feedback uses an angle sensor (currently primarily an encoder) directly installed on the servo motor or lead screw end. Since most mechanical transmission links are not included in the system's closed-loop circuit, relatively stable control characteristics are achieved. Mechanical transmission errors, such as those in lead screws, cannot be constantly corrected through feedback, but software-based compensation methods can be used to improve accuracy. Currently, most CNC machine tools utilize semi-closed-loop control.

⑶ Hybrid Control CNC Machine Tools

Selectively combining the characteristics of the aforementioned control methods can create a hybrid control scheme. As mentioned earlier, open-loop control offers excellent stability, low cost, and poor precision, while full-closed-loop control suffers from poor stability. Therefore, hybrid control is preferable to compensate for these differences and meet the control requirements of certain machine tools. The two most commonly used methods are open-loop compensation and semi-closed-loop compensation.

III. Classification by Functionality of CNC Systems

Based on their functional level, CNC systems are generally categorized into three categories: low, medium, and high. This classification is widely used in my country. The boundaries between these three categories are relative, and the criteria for classification vary over time. Based on the current level of development, various types of CNC systems can be categorized into low, medium, and high-end based on certain functions and indicators. The medium and high-end categories are generally referred to as full-function CNC systems or standard CNC systems. ⑴ Metal Cutting

These machines employ various cutting processes, including turning, milling, ramming, reaming, drilling, grinding, and planing. They can be further divided into the following two categories:

① General CNC Machine Tools: Examples include CNC lathes, CNC milling machines, and CNC grinders.

② Machining Centers: These machines primarily feature a tool library with an automatic tool changer. After a workpiece is clamped once, various tools are automatically changed, allowing the workpiece's various surfaces to be milled (turned), keyed, reamed, drilled, and tapped continuously on the same machine. Examples include (milling/stacking) machining centers, turning centers, and drilling centers.

⑵ Metal Forming

These machines employ forming processes such as extrusion, punching, pressing, and drawing. Commonly used machines include CNC presses, CNC bending machines, CNC pipe bending machines, and CNC spinning machines.

⑶ Specialty Machining

Mainly include CNC wire EDM machines, CNC Sinking EDM machines, CNC flame cutting machines, and CNC laser processing machines. ⑷ Measurement and Plotting

Mainly includes three-dimensional coordinate measuring machines, CNC tool setters, CNC plotters, etc.