Introduction to CNC milling machine
CNC milling machine is the earliest CNC machine tool that appeared and was used, and it has a pivotal position in the manufacturing industry. Now the increasingly widely used machining center is also developed on the basis of CNC milling machine. CNC milling machines have been widely used in automotive, aerospace, military, mold and other industries.
1. Classification and structural characteristics of CNC milling machines
(1) According to the layout of the spindle of the machine tool and the layout characteristics of the machine tool, the CNC milling machine can be divided into CNC vertical milling machine, CNC horizontal milling machine and CNC gantry milling machine.
1) Vertical CNC milling machine. The spindle of the vertical CNC milling machine is perpendicular to the working surface of the machine tool, and the workpiece is easy to install, which is convenient for observation during processing, but not convenient for chip removal. There are two types of vertical CNC milling machines: vertical bed type and vertical lifting table type. Figure 5-1a shows a vertical bed-type CNC milling machine, which generally adopts a fixed column structure, the worktable is not lifted, the spindle moves up and down, and the weight of the headstock is balanced by a weight in the column. In order to ensure the rigidity of the machine tool, the distance between the center line of the spindle and the guide rail surface of the column should not be too large, so this structure is mainly used for CNC milling machines of medium and small sizes. The lifting table structure can be used on the economical or simple CNC milling machine, as shown in Figure 5-1b, but its feed accuracy and speed are not high.
2) Horizontal CNC milling machine. As shown in Figure 5-2, the spindle of the horizontal CNC milling machine is parallel to the working surface of the machine tool, which is inconvenient to observe during processing, but the chip removal is smooth. Generally, it is equipped with a CNC rotary table, which is convenient for machining different sides of the part. There are fewer simple horizontal CNC milling machines, and most of them are equipped with automatic tool changers (ATC) to become horizontal machining centers. 3) Gantry CNC milling machine. For large-size CNC milling machines, symmetrical double-column structures are generally used to ensure the overall rigidity and strength of the machine tool, so the gantry CNC milling machine has two forms: workbench movement and gantry frame movement. It is suitable for processing aircraft integral structural parts, large box parts and large molds, as shown in Figure 5-3.
(2) According to the function of the CNC system, the CNC milling machine can be an economical CNC milling machine, a full-function CNC milling machine and a high-speed milling CNC milling machine. 1) Economical CNC milling machine. Generally, an economical CNC system, such as SINUMERIK 802S, etc., is adopted, which adopts open-loop control and can realize three-coordinate linkage. This kind of CNC milling machine has low cost, simple function, low machining accuracy, and is suitable for the processing of generally complex parts.
2) Full-featured CNC milling machine. Using semi-closed-loop control or closed-loop control, the numerical control system is rich in functions, and can generally realize the linkage of more than four coordinates, with strong processing adaptability and the most widely used.
3) High-speed milling CNC milling machine. High-speed milling is a development direction of CNC machining, and the technology has been relatively mature and has gradually been widely used. This CNC milling machine adopts a new machine tool structure, functional components and a powerful CNC system with superior processing performance, the spindle speed is generally 8000~40000r/min, and the cutting feed speed can reach 10~30m/min, which can carry out high-efficiency and high-quality processing on large-area curved surfaces. But at present, this kind of machine tool is expensive, and the cost of use is relatively high.
2. The main functions of CNC milling machinesThe functions of different grades of CNC milling machines are quite different, but they should all have the following main functions. (1) The CNC milling machine for milling processing should generally have the linkage function of more than three coordinates, which can carry out linear interpolation and arc interpolation, and automatically control the rotation of the milling cutter relative to the movement of the workpiece for milling, as shown in Figure 5-4. The higher the number of coordinate linkage axes, the lower the clamping requirements for the workpiece, and the larger the processing technology range. (2) Hole and thread processing can be drilled, expanded, reamed, countersinking, boring and other processing by using fixed-size hole processing cutters, and holes of different sizes can also be milled with milling cutters, as shown in Figure 5-5.On the CNC milling machine, the thread tap can be used to process the threaded hole, and the thread milling cutter can also be used to mill the internal and external threads, which is much more efficient than the traditional tap machining, and is being widely used. As shown in Figure 5-6, to carry out thread milling, the CNC system of the machine tool should have the function of spiral interpolation.
(3) Tool compensation function: generally includes tool radius compensation function and tool length compensation function. The tool radius compensation function can solve the position size relationship between the tool center motion trajectory and the part wheel during the plane contour machining, and the tool size can be directly programmed according to the part wheel size without considering the tool size, and the tool diameter size can be adapted to the change of the tool diameter size by changing the tool radius compensation value, so that the same program has greater flexibility when it is used. The tool length compensation function is mainly used to solve the coordination problem between the set position of the tool program in the length direction and the actual height position of the tool.
(4) Conversion of metric system and inch unit According to the marking of the drawing, the metric unit (mm) and the inch unit (in) can be selected for programming, so as to adapt to the specific situation of different enterprises.
(5) Absolute coordinates and incremental coordinates programming The coordinate data in the program can adopt absolute coordinates or incremental coordinates to make data calculation or program writing more convenient.
(6) Feed speed and spindle speed adjustment The control panel of the CNC milling machine is generally equipped with a rate switch for feed speed and spindle speed, which is used to adjust the actual feed speed and the actual spindle speed at any time according to the machining state and program setting value in the program execution, so as to achieve the best cutting effect. Generally, the feed speed adjustment range is between 0~150%, and the spindle speed adjustment range
The circumference is between 50%~120%. (7) Fixed CycleThe fixed cycle is a sub-program solidified into the G instruction, and adapts to different processing requirements through various parameters, mainly used to achieve some typical processing actions that need to be repeated many times, such as the processing of various holes, internal and external threads, grooves, etc. The use of fixed loops can effectively simplify the programming of the program. But different CNC systems have a fixed cycle
There is a big difference in righteousness, and attention should be paid to the difference when using it. (8) workpiece coordinate system setting workpiece coordinate system is used to determine the clamping position of the workpiece on the machine tool workbench, generally 4~6 workpiece coordinate systems can be used at the same time, and the workpiece coordinate system can be translated or rotated according to the change of the workpiece position, which is of great significance for the execution of the processing program.
(9) Data input and output and DNC function CNC milling machine generally through the RS232C interface for data input and output, including processing programs and machine tool parameters, etc. can be carried out between machine tools and machine tools, machine tools and computers. The program storage space provided by the CNC milling machine according to the standard configuration is generally relatively small, especially the low-end CNC milling machine is about between tens of kilobytes and hundreds of kilobytes. When the machining program exceeds the storage space, DNC machining should be used, i.e. an external computer directly controls the CNC milling machine for machining, which is often encountered when machining surfaces. Otherwise, the program can only be divided into several parts to be executed separately, which is cumbersome and affects productivity.
(10) Sub-program For the processing action or processing area that needs to be repeated many times, it can be programmed into a sub-program, in the main
Programs can call it when needed, and multi-level nesting of subroutines can be implemented to simplify program writing. (11) Data acquisition function After the CNC milling machine is equipped with a data acquisition system, it can measure and collect the required data on the workpiece or physical object (sample, sample, model, etc.) through the sensor (usually electromagnetic induction, infrared or laser scanning). For the profiling CNC system, the collected data can also be automatically processed and the CNC machining program can be generated, which provides an effective means for imitation and reverse design manufacturing engineering.
(12) Self-diagnosis function Self-diagnosis is the self-diagnosis of the CNC system in operation. When the CNC system fails, with the help of the system's self-diagnosis function, the cause can often be quickly and accurately identified and the fault location can be determined. It is an important function of the CNC system and plays an important role in the maintenance of CNC machine tools.
