For numerical control machine tools, we can classify them from different perspectives.
(I) Classification based on the characteristics of the control system
1. Point-to-point control numerical control machine tools
For some numerical control machine tools used for hole machining, only the precise coordinates of the holes need to be obtained, regardless of the trajectory from one hole to another. Examples include coordinate drilling machines, coordinate boring machines, and punching machines. These machines can adopt a simple and inexpensive point-to-point control system. In order to ensure accurate positioning, this type of control system generally uses three-level deceleration after high-speed operation to reduce positioning errors. However, due to the inertia of the moving parts and the possibility of frictional force changes at low speeds, even after the drive device stops moving, the worktable does not stop immediately, resulting in a positioning error Δd, and this value has a certain dispersion.
2. Linear control numerical control machine tools
Some numerical control machine tools not only require precise positioning but also require linear movement from one point to another, with control over the displacement speed. This type of numerical control machine tool performs cutting operations while moving between two points. Therefore, different cutting amounts and feed speeds need to be selected for different tools and workpieces.
This category of numerical control machine tools includes numerical control boring and milling machines, numerical control lathes, machining centers, etc. Generally, these numerical control machine tools have two to three controllable axes. In order to obtain qualified parts after tool wear or tool replacement, the numerical control system of this type of machine tool often has functions such as tool radius compensation, tool length compensation, and spindle speed control.
3. Contour control numerical control machine tools
More advanced numerical control machine tools have contour control capabilities, which means they can process parts with curves or surfaces. This category of machine tools includes two-coordinate and multi-coordinate numerical control milling machines, numerical control lathes capable of machining surfaces, machining centers, etc.
These types of numerical control machine tools should be able to simultaneously control two or more axes and have interpolation capabilities to strictly control displacement and speed without interruption. Most modern numerical control machine tools have the function of two-coordinate or multi-coordinate linkage, which includes not only tool radius compensation and tool length compensation but also a series of compensation functions such as machine tool axial motion error compensation, screw and gear backlash compensation.
According to the number of axes that can be linked (simultaneously controlled) and the number of independent axes, there can be 2-axis control, 2.5-axis control, 3-axis control, 4-axis control, 5-axis control, etc. The principle of 2.5-axis control (two axes are continuous control, and the third axis is point-to-point or linear control) achieves two-dimensional control within the three main axes x, y, and z.
3-axis control involves simultaneous interpolation of the three coordinate axes X, Y, and Z, enabling three-dimensional continuous control. 5-axis continuous control is an important machining form, where the three linear coordinate axes X, Y, and Z are linked with the rotation of the turntable and the swinging of the tool (it can also be linked with the two-axis numerical control turntable or the swinging of the tool in two directions). Because the tool tip can be guided according to mathematical rules, making it perpendicular to any double curve plane, it is particularly suitable for machining blades, wings, etc.
(II) Classification based on the type of servo system of the actuating mechanism
1. Open-loop servo system numerical control machine tools
This is a relatively primitive type of numerical control machine tool. The numerical control system of this type of machine tool processes the part program and outputs digital instructions to the servo system to drive the machine tool's movement. There is no feedback signal from a position sensor. The most typical system uses a stepper motor servo system. These types of machine tools are more economical but have lower speed and accuracy. Therefore, they are still used as economical numerical control machine tools in China, often for retrofitting old machine tools.
2. Closed-loop servo system numerical control machine tools
These machine tools can accept instructions from interpolators and constantly compare them with the actual position feedback signals obtained at the worktable, continuously correcting the errors based on the difference. This type of numerical control machine tool can eliminate the influence of accuracy errors in the transmission components on the machining of workpieces.
Numerical control machine tools that use closed-loop servo systems can achieve high machining accuracy. However, due to various mechanical transmission components, such as screw pairs and worktables, being included in the feedback loop, and the friction characteristics of various mechanical transmission components, including the screw and nut pair and the worktable and guide rail, as well as the rigidity of each component and the transmission chain clearance of the installed displacement measurement element, are all variable and directly affect the adjustment parameters of the servo system. Some of these parameters are nonlinear, so the design and adjustment of closed-loop systems are challenging. If not designed and adjusted properly, it can easily lead to system instability. Therefore, closed-loop servo system numerical control machine tools are mainly used for high-precision applications such as precision boring and milling machines, ultra-precision lathes, and ultra-precision milling machines.
3. Semi-closed-loop servo system numerical control machine tools
Most numerical control machine tools use semi-closed-loop servo systems, where the measurement elements are moved from the worktable to the motor end or the screw end. The closed-loop loop of this system does not include the screw, nut pair, and worktable, allowing for stable control characteristics. Additionally, by using high-resolution measurement elements, satisfactory accuracy and speed can be achieved.
(III) Classification based on the type of numerical control device
1. Hardware-based numerical control machine tools (NC machine tools)
These were early types of numerical control machine tools where the input, operation, interpolation calculations, and control functions of the numerical control device were all composed of integrated circuits or transistor devices. Generally, different numerical control machine tools required specially designed logic circuits. The universality of this type of numerical control device is poor because it is entirely composed of hardware, resulting in limited functionality and flexibility. Therefore, this type of machine tool is no longer commonly used.
2. Software-based numerical control machine tools (CNC machine tools)
In the mid-1970s, with the development of microelectronics technology, the integration level of chips increased. It became possible to use medium, large-scale, and very large-scale integrated circuits to form CNC devices. The main functions of numerical control machine tools using these devices are almost entirely implemented by software. Different software programs can be developed to achieve different types of numerical control machine tools, while the hardware can be almost universal. This provides conditions for mass production of hardware. Mass production helps ensure product quality, shorten production cycles, and reduce production costs. Therefore, modern numerical control machine tools use CNC devices. This software-based numerical control provides high flexibility, which greatly benefits machine tool manufacturers and users. They can develop different user programs according to their needs, making the application of numerical control machine tools more extensive and reaching various areas of the mechanical machining industry.
(IV) Classification based on the processing method
1. Metal cutting numerical control machine tools: such as CNC lathes, machining centers, CNC drilling machines, CNC grinding machines, CNC boring machines, etc.
2. Metal forming numerical control machine tools: such as CNC bending machines, CNC pipe bending machines, CNC turret punch presses, etc.
3. Numerical control special processing machine tools: such as CNC wire (electrode) cutting machines, CNC electrical discharge machining machines, CNC laser (plasma) cutting machines, etc.
4. Other types of numerical control machine tools: such as flame cutting machines, CNC coordinate measuring machines, etc.
(V) Classification based on functional levels
Numerical control machine tools can be classified into three categories: high-end, mid-range, and low-end (economical):
This classification method is widely used in China, but due to the lack of a precise definition, the meaning is not very clear. We habitually classify them based on the following functional levels.
1. Resolution and feed rate: Low-end machines have a resolution of 10 μm and a feed rate of 8-15 m/min. Mid-range machines have a resolution of 1 μm and a feed rate of 15-24 m/min. High-end machines have a resolution of 0.1 μm and a feed rate of 15-100 m/min.
2. Servo feed type: Low-end machines use open-loop and stepper motor feed systems. Mid- to high-end machines use semi-closed-loop DC servo systems and AC servo systems (including closed-loop servo systems).
3. Axis linkage: Low-end numerical control machine tools have a maximum of 2-3 linked axes, while mid- to high-end machines have 2-4 axes or 3-5 axes or more.
4. Communication function: Low-end numerical control machines generally have no communication function. Mid-range machines may have RS-232 or DNC (direct numerical control) interfaces. High-end machines may also have MAP (manufacturing automation protocol) communication interfaces, enabling networking capabilities. Recent CNC devices also have storage card and USB interfaces.
5. Display function: Low-end numerical control machines typically have simple digital display or basic CRT/LCD information display. Mid-range machines have more comprehensive CRT/LCD information displays, including characters, graphics, human-machine dialogue, self-diagnosis, and other functions. High-end numerical control machines can also have 3D graphics display.
6. Built-in PC (programmable controller): Low-end numerical control machines generally do not have a built-in PC, while mid- to high-end machines have one. High-end machines have powerful built-in PCs with extended functions for axis control.
7. Main CPU (central processing unit): Low-end numerical control machines usually use 8-bit CPUs, while mid-range and high-end machines are gradually transitioning from 16-bit CPUs to 32-bit CPUs. Some new foreign numerical control systems now use 64-bit CPUs with reduced instruction set computing (RISC) central processing units to improve processing speed. Based on these functions and indicators, various types of products can be classified into low-end, mid-range, and high-end categories of numerical control systems.
