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In the early stages of CNC machine tool development, the mechanical structure was largely similar to conventional machine tools, with only minor changes in areas such as automatic gear shifting, tool holders, table indexing, and manual operations. However, as numerical control technology advanced, the demands on machine tool performance increased significantly. The control mode and operational characteristics of CNC systems required higher levels of productivity, precision, and durability. As a result, the main mechanical components of CNC machines developed several key features:
First, the use of high-performance, stepless speed control spindles and servo drive systems greatly simplified the transmission structure, shortening the transmission chain. Second, to support continuous automated processing and enhance productivity, the mechanical structure must possess high static and dynamic stiffness, good damping, and resistance to wear, along with minimal thermal deformation. Third, to reduce friction, eliminate backlash, and improve accuracy, efficient transmission components like ball screw pairs, rolling guides, and anti-backlash gears are commonly used. Lastly, to improve working conditions, reduce auxiliary time, and increase operability, CNC machines incorporate automatic tooling systems, including tool magazines, automatic tool changers, and chip removal devices.
CNC machine tools must meet specific structural requirements based on their application and functional characteristics. First, they need high static and dynamic stiffness. Since CNC machines operate automatically based on programmed instructions, any geometric inaccuracies or deformations in the mechanical structure—such as the bed, guide rails, table, or spindle—cannot be manually adjusted during machining. Therefore, it is essential to minimize elastic deformation to ensure accurate and high-quality results.
To enhance spindle rigidity, three-support structures are often used, combined with bearings like double-row short cylindrical roller bearings and angular contact radial thrust bearings, which help reduce both radial and axial deformation. For larger components, closed-loop beds and hydraulic balancing systems are employed to counteract deformation caused by moving parts. Improving contact stiffness between components involves scraping surfaces and applying sufficient pre-load to increase the contact area, thus enhancing load capacity.
To fully utilize the high-efficiency cutting capabilities of CNC machines, dynamic stiffness must be improved alongside static stiffness. This can be achieved through increased system stiffness, enhanced damping, and frequency adjustment. Increasing damping has proven effective in reducing vibrations. Steel plate welded structures not only boost static stiffness but also reduce weight and improve inherent damping. As a result, many modern CNC machines now use welded steel plates for beds, columns, beams, and tables. Additionally, sealed sand castings contribute to vibration damping and improve overall stability.
Second, minimizing thermal deformation is crucial. Heat from internal and external sources causes different parts of the machine to expand or contract unevenly, disrupting the precise motion relationship between the workpiece and the cutting tool. This thermal distortion is especially problematic for CNC machines, where all processes are computer-controlled. To mitigate this, CNC machine designs often incorporate cooling systems, heat-insulating materials, and optimized layouts to maintain consistent temperatures and reduce thermal effects.