Machine tool processing is classified into ordinary processing, precision processing, high-precision processing, and ultra-precision processing based on processing accuracy. The accuracy of ordinary processing is generally 10~100um, the accuracy of precision processing is generally 3~10um, the accuracy of high-precision processing is generally 0.1~3um, and the accuracy requirement is higher than 0.1um, which is classified as ultra-precision processing.
The most significant difference between ultra-precision machine tools and ordinary machine tools is the extremely high precision requirements. Therefore, factors that are often overlooked or ignored in ordinary machine tools can become critical, such as thermal deformation, force deformation, vibration, positioning, and materials, all of which affect the machining accuracy of ultra-precision machine tools. These factors impose very high precision requirements on the machine tool, including its materials, spindle rotation accuracy, spindle stress and thermal deformation, spindle drive, workpiece clamping, guideway linearity, and positioning.
Machine tool structural materials
The structural materials of ultra-precision machine tools have a significant impact on their accuracy. Cast iron and steel were the primary structural materials for machine tools in the past, and will continue to play a significant role as materials for ordinary machine tools for a considerable period. However, the structural materials for ultra-precision machine tools must meet requirements such as stable performance, minimal thermal deformation, and good vibration damping. Cast iron and steel can no longer meet these requirements, especially regarding thermal deformation. Therefore, non-metallic materials, such as natural granite, have long been used abroad as substitutes. Natural granite has many advantages; its coefficient of thermal expansion is only 2/3 that of cast iron, and it is already used in many ultra-precision machine tools. Japan has also developed precision ceramic materials, with a coefficient of thermal expansion approximately 1/20 to 1/240 that of cast iron.
machine tool spindle
The spindle of an ultra-precision machine tool directly supports the movement of the workpiece or cutting tool during machining. Therefore, the spindle's rotational accuracy directly affects the machining accuracy of the workpiece. Thus, the spindle can be considered the most important component of an ultra-precision machine tool, and the accuracy and characteristics of the machine tool spindle can be used to evaluate the overall precision of the machine tool. Currently, the ultra-precision machine tool spindle with the highest spindle roughness is the air hydrostatic bearing spindle. High-precision air bearing spindles can achieve a rotational accuracy of 0.05µm, with a maximum of 0.03µm.
linear guide
Linear guides, serving as the relative positioning mechanism between the tool and the workpiece, are the second most important component after the spindle. The basic requirements for linear guides in ultra-precision machine tools are: flexible movement without crawling or other discontinuous actions; good linear accuracy; rigidity suitable for practical use; low heat generation during high-speed operation; and ease of maintenance. Commonly used guides in ultra-precision machine tools include VV-type sliding and rolling guides, hydrostatic guides, and air hydrostatic guides. Air hydrostatic guides are currently...
The best guide rails available now have a straightness of 0.1~0.2um/250mm.
Feed and micro-feed systems
The most commonly used feed systems are various feed screws. The ball screws in ultra-precision machine tools typically have a precision grade of C0. Ball screws do not require the auxiliary devices necessary for hydrostatic screws, making them extremely convenient to use. However, as feed screws for sub-micron level ultra-precision machine tools, the slight vibrations caused by the rotation of the balls and the contact sliding between them, as well as their poor vibration damping characteristics compared to sliding screws, must be considered. Micro-feed mechanisms are also widely used in ultra-precision machine tools to meet the requirements for higher positioning accuracy and feed resolution. Common methods include: using a combination of rolling screw feed and elastic feed, and combining coarse and fine adjustment piezoelectric elements.
Environmental conditions
There are three environmental conditions required for ultra-precision machining. First, cleanliness: the working environment for ultra-precision machine tools should be below Class 2000-3000. Second, vibration: ultra-precision machine tools must be equipped with high-performance vibration isolation devices. Currently, most ultra-precision machine tools, both domestically and internationally, use air spring vibration isolation systems. Third, temperature: ultra-precision machining must be performed under constant temperature conditions. This is usually addressed from two aspects: first, selecting suitable component materials; materials used and considered for ultra-precision machine tools include alumina ceramics, cast iron, steel, Invar steel, granite, resin concrete, and zero-expansion glass. Second, ensuring constant temperature control.
