High-speed cutting (HSC) requires the ability to machine complex 3D curves and surfaces at high speeds, with efficiency, precision, and quality. These complex shapes, often referred to as free-form curves and surfaces, cannot be described by simple quadratic equations. For example, forging dies used in urban air tram systems are now directly machined from hardened steel (52HRC) using high-speed milling. This method replaces the traditional approach of manufacturing graphite electrodes for EDM, significantly reducing time but requiring specialized tools and advanced milling strategies. The required precision is ±0.02mm, with a surface roughness of Ra < 0.7μm, which demands precise trajectory control and high adjustment accuracy from CNC machines.
During new product development, CAD systems are first used to create the initial model sketch. Then, CAM systems calculate the exact coordinates and motion paths for roughing and finishing using HSC strategies. The complex contours of the part are approximated using straight lines, arcs, or higher-order curves like parabolas. NC programs divide the path into segments based on intersection points. The larger the approximation interval, the fewer the nodes and program blocks, improving efficiency while maintaining accuracy.
The core task of the CNC system is to generate feed commands for each axis according to the programmed instructions. Interpolation is essential to determine intermediate coordinate values between start and end points. Linear interpolation, commonly used in CNCs, calculates straight-line segments, but when dealing with complex contours, it may require more intermediate points, increasing program size and execution time. This can lead to issues such as acceleration jumps, reduced productivity, and surface imperfections, especially in high-speed applications.
Spline interpolation, particularly NURBS (Non-Uniform Rational B-Spline), offers a more accurate representation of curved surfaces, replacing multiple linear segments with a single smooth curve. This reduces data volume and improves machining quality. NURBS also allows for consistent mathematical descriptions across CAD/CAM systems, enhancing data exchange and enabling smoother tool paths.
Modern CNC systems use digital bus technology and integrated electronics to improve resolution, reduce noise, and enable precise control of feed drives and ball screws. Digital drive adjustments allow for high-resolution speed and position detection, supporting advanced control algorithms. They also help compensate for mechanical errors, ensuring smooth operation and high accuracy.
Speed pre-control (look-ahead) is critical for high-speed cutting, identifying abrupt changes in speed and acceleration caused by curvature. A sufficient buffer is needed to handle short NC blocks, especially at high feed rates. Multi-axis transformations and geometric corrections ensure accurate tool positioning, even during complex 5-axis operations. Tool compensation and error correction, including thermal and frictional errors, further enhance accuracy.
Safety is also a priority. CNC systems must monitor and restrict movements to protect operators, machines, and workpieces. Enclosures and dual-channel monitoring systems prevent accidents and ensure safe operation, especially on large-scale equipment.
In conclusion, NURBS interpolation, advanced speed control, multi-axis transformations, and error compensation significantly improve the performance of high-speed CNC systems. By leveraging digital drive technology and bus communication, modern CNCs meet the demanding requirements of high-speed cutting, offering greater efficiency, precision, and safety.
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