With the development of processing technology, the functions of numerical control systems are constantly developing. These functions are mainly reflected in the following aspects: high-precision high-speed, 5-axis linkage, error compensation, networking, and security.
1. High-speed, high-precision machining function <br> High-precision, high-speed machining technology is the development of traditional CNC machining technology, and it has no essential difference with traditional CNC machining. For high-precision, high-speed CNC machining, the goal of CNC machine tools is to produce high-precision parts at high speed. In order to perform high-speed machining on the basis of accuracy, there are three important factors: mechanical systems, CNC controls and drives.
Regarding the requirements for high-speed machining on machinery, I will not go into details here. It is important to note that high speed and high precision machining requires high rigidity and light moving parts of the machine, especially the feed and spindle sections.
This is followed by the CNC system, which is the unit that issues speed and position commands. First, the command is required to be transmitted accurately and quickly. After processing, a position command is issued for each coordinate axis, and the servo system must drive the tool to move accurately according to the command.
The CNC system converts the input part program into shape trajectory to be machined, feedrate and other command information, and continuously sends position commands to each servo axis. In order to achieve high speed and high precision, the CNC must select the optimum feed rate according to the shape trajectory of the part machining, and generate the position command with the highest possible feed rate within the allowable precision. Especially at the corners and small radii, the CNC should be able to determine how much the machining speed will affect the accuracy, and automatically reduce the tangential speed of the tool before the tool reaches such a point. For mold processing, the general program segment is small, but the program is very long, so special control methods must be used to achieve high precision and high speed machining. The servo system requires accurate and fast driving to machine high-precision mechanical parts at high speed. For this reason, the servo system must have the ability to respond quickly, suppress the disturbance, and require the servo system to not generate vibration and eliminate resonance with the machine.
High-speed machining requires a high-speed spindle unit and a high-speed machine feed drive unit. High feed rates also require high accelerations. For example, the stroke of a high-speed machine tool is usually between 500 and 1 000 mm. When the feed rate of the machine tool is increased from zero to 40 m/min within such a short distance, the feed acceleration of the machine tool should exceed 1 g (9.8 m/s2). ). Feed acceleration is more important when machining curved surfaces. Its acceleration is proportional to the square of the feed rate. If a servo motor does not produce a sufficiently high acceleration, it cannot be processed at high speed and with high precision. At present, the spindle unit mainly adopts the vector-controlled AC asynchronous motor. Due to the heat generation of the asynchronous motor rotor, the internal cooling high-speed spindle motor is also used; the structure of the synchronous motor is also studied. In order to achieve a large feed addition (decrease) speed, linear motors have been increasingly used. Safety issues are important when machining at high speeds. Because the chips in high-speed machining are shot like bullets, the safety requirements for the system are very high.
The requirements of CNC for high-precision and high-speed machining can be summarized as follows:
(1) Ability to process blocks at high speed.
(2) The information flow can be processed and controlled quickly and accurately, so that the machining error is controlled to a minimum.
(3) It is possible to minimize the impact of the machine and make the machine move smoothly.
(4) To have sufficient capacity to allow high-volume machining programs to run at high speeds or to transfer large amounts of data over a network.
(5) Servo motors, spindle motors, and sensors with high-resolution, high-speed operation.
(6) Reliability and safety are important due to processing at high speeds.
The high-speed, high-precision functions mainly include the following aspects:
Feed rate control and addition (decrease) speed processing functions (including corner deceleration processing): The error during high-speed machining is mainly caused by the hysteresis of the control system plus (decrease) speed and the hysteresis of the servo system. Therefore, the control system should try to reduce the error in these two aspects. For example, feedforward control is used to reduce the error caused by servo lag. Improve servo control with digital servo technology. Thanks to the digital servo technology, the speed gain and position gain of the servo system can be increased, thus reducing the error caused by the servo lag. Reduce the error caused by the addition (decrement) speed lag. In high-speed machining, the addition (deceleration) speed and feed rate are the most important parameters. Only when the machining shape is strictly controlled, the addition (deceleration) speed and feed rate can be used to achieve high-speed machining. Large feed rates can cause large errors during system transitions, such as corners. In order to achieve high speed machining, the feed rate must be controlled. In addition, the use of the addition (decrease) speed before interpolation can also reduce the error caused by the addition (decrement) speed lag.
Look-ahead control, if the feed rate and acceleration and deceleration are pre-calculated in different machining shapes, the CNC system pre-calculates the motion trajectory and motion speed of each block after programming and before execution. That is, pre-processing the program to be run, according to the control feed rate and the acceleration and deceleration methods mentioned above, pre-calculate the feed rate and acceleration and deceleration of some blocks, and then calculate the geometric trajectory of the motion. And then sent to the multi-segment buffer, when the tool moves at a high speed at a high speed while running, the error of the processed shape is still small. This is the principle of "forward-looking control", sometimes referred to as "advance control" and "forward control."
With high-speed allocation of remote buffers and DNC operations, it is necessary to quickly transfer programs from the input to the CNC system for machining a large number of program components. After the CNC reads a program, it calculates the data of the program, generates a distribution pulse for each axis, and transmits it to the servo system to operate the servo motor. The time at which the dispense pulse is generated (the time the block is processed) is an important factor in the performance of the CNC. For a block, the operation of the high speed DNC allows (using the remote buffer) to generate a large amount of time required to distribute the pulses. This function shortens the distribution pulse that generates a block, thus ensuring that a program consisting of a small block does not pause between blocks. For example, when performing a DNC operation, a program consisting of a series of 1 mm blocks (3-axis linear interpolation) can operate at a speed of 60 m/min without interrupting the execution of the assignment. Thanks to the remote buffer function, high-speed input of data is realized, which also ensures high-speed machining.
Improve system resolution, for example, nano-interpolation, which uses a high-speed RISC processor, and machining in nanometers allows the machine to match the processing performance at the optimum feed rate.
The control of jerk, when moving in the shape of the curve, the change of acceleration may cause mechanical vibration. The control of jerk is to automatically measure such movement to reduce the speed and reduce the mechanical impact to reduce the roughness of the machined surface.
NURBS Interpolation: When designing molds using CAD, NURBS is widely used to represent free curves. Compared to general CNC, NURBS has higher transfer rates and shorter programs. The mechanical parts that are machined at the same time are closer to the geometry of the CAD design.
For high-speed, high-precision machining functions, when selecting, it is also necessary to select the function based on the machining speed or the machining accuracy.
2. 5-axis machining function
The shape of a general mechanical part is essentially a three-dimensional surface. However, the three-dimensional linkage processing of the three-dimensional surface is not optimal from the processing effect, and the efficiency is low and the surface roughness value is high. The 5-axis linkage is not only efficient but also greatly improved in roughness. With 5-axis linkage, you can cut with the best shape. Off-line programming is typically used for the 5-axis machining of rotating coordinate systems. The type, radius and length of the tool are required to be constant during CNC programming, which makes it difficult to modify the program and the tool. In order to solve this difficulty, a coordinate transformation method is provided in the CNC system, so that some programming and tool correction are performed directly in the machine without repeating the post processing. This can be achieved by defining a new workpiece coordinate system, which is transformed by the CNC system to the coordinates of the corresponding axis. The positioning of the tool can be programmed by the position of the rotary axis, the direction vector of the tool, and the PRY angle (PRY=Roll/Pitch/Yaw, or Euler angle). This function is also important for the manual mode; for example, when the tool breaks, the tool needs to be moved.
When a 5-axis machine tool uses a ball cutter for 3-axis simultaneous machining, only a part of the potential of milling can be utilized; high productivity can only be achieved when machining with a cylindrical or spiral cutter. However, in order to ensure that the cylindrical or spiral cutter tool moves along the required path, usually the 5-axis programming of the tool requires the insertion of many intermediate cutting points in the middle. The 5-axis transformation uses the handheld control unit to dynamically adjust the rake angle to ensure that the tool tip remains stable. Since the CNC can correct the length of the tool, in order to compensate for the event of tool breakage and to compensate for tool wear during machining, the tool can be measured directly on the machine when needed. These features allow the machine to run unattended at night. The CNC is linked to the laser measurement system and automatically provides the corresponding measurement cycle for tool set and breakage monitoring. Since different tool geometries can be corrected, such as cylindrical, spiral and conical spiral tools, the same procedure can use different tools. Currently, high-end CNC systems can perform 5-axis machining. The main functions are as follows: (1) It is suitable for different machine configuration 5-axis machining functions: it can be adapted to different machine configurations, including tool tilt type, table tilt type and composite type. The offset between the first and second rotating shafts due to the machine or the offset between the tool shaft and the rotating shaft can also be considered in the system. It can compensate for tool length in the direction of the tool axis. Even if the direction of the tool axis rotates with the rotary axis, it can be compensated in the direction of the tool axis. Tool center point control: Even if the tool axis changes direction, the tool center can still be controlled to follow the determined line. 5-axis machining tool radius compensation: Tool radius compensation can be performed on a plane perpendicular to a tilting tool, or a leading edge offest. 5-axis machining circular interpolation, which can define the arc on the inclined plane. Inclined plane machining instructions: It is convenient to make a part program in the case of oblique plane machining, and the rotary axis can be controlled so that the tool is perpendicular to the inclined plane. 5-axis machining manual feed: Move the tool along the inclined workpiece plane or manually along the axis of the diagonal tool. (2) Complex machining function: 5-axis machining tool center point control, cylindrical cutting interpolation machining cutting point compensation, AI high-precision contour control / AI nano high-precision contour control, 5-axis machining tool radius compensation, 5-axis machining manual Give the function. It can be turned on, CNC and 5-axis on one CNC.
3. Error compensation function <br> In order to ensure that the machining error of the high speed system is small, the system needs to have an error compensation device. These compensations include: full-stroke linear compensation and nonlinear bending compensation, pitch compensation, clearance compensation, over-quadrant compensation, tool offset and thermal expansion, static friction, dynamic friction compensation, etc.
4. Network functions <br> With a wealth of network functions and software packages, you can build the best system for your machine. (1) Centralized management, one computer can be used to control multiple machine tools, which is convenient for monitoring, operation and processing and NC program transmission and management. (2) Remote support and services.
5. Security features
In the future, CNC is at a high speed, so the reliability requirements are very high. The double check function is an important measure to ensure the safe operation of the CNC system.
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