History and Current Status of Ultra-precision CNC Machine Tools in the United States

In the late 1950s, the United States first began to conduct research on ultra-precision machining machine tools. At that time, the development of laser nuclear fusion experimental devices and infrared experimental devices required large metal reflectors, and ultra-precision machining technology and ultra-precision machining machine tools for reflectors were urgently needed. People used the high-pr

History and Current Status of Ultra-precision CNC Machine Tools in the United States
In the late 1950s, the United States first began to conduct research on ultra-precision machining machine tools. At that time, the development of laser nuclear fusion experimental devices and infrared experimental devices required large metal reflectors, and ultra-precision machining technology and ultra-precision machining machine tools for reflectors were urgently needed. People used the high-precision precision machine tools at that time and used single-point diamond turning tools to mirror-cut aluminum alloys and oxide-free copper. Starting from this, ultra-precision machining has developed rapidly as a new mechanical processing technology.

In 1962, UnionCarbide first developed an ultra-precision hemispherical lathe using porous graphite air bearings, which successfully achieved ultra-precision mirror turning, with a dimensional accuracy of ~0.6btm and a surface roughness of Rd0.025gm, thus taking the first step in sub-micron machining. However, diamond ultra-precision turning is more suitable for some softer metal materials, while in satellite cameras in aerospace, astronomy, military and other application fields, non-metallic devices such as glass and ceramics are the most commonly used brittle materials. Cutting these materials with diamond tools will cause cracks on the machined surface. Ultra-precision grinding is more conducive to the processing of brittle materials.

Another representative product of UnionCarbide is the R-e type aspheric surface creation machine tool developed successfully in 1972. This is a dual-coordinate CNC lathe with position feedback, which can change the corner and radius of the tool holder guide rail in real time to achieve mirror processing of aspheric surfaces. The processing diameter is 380mm, the shape accuracy of the workpiece is plus or minus 0.63um, and the surface roughness is Ra0.023um.

In 1968, Moore Company (MoorlSpecialT001) developed the Moori type ultra-precision mirror lathe with air spindle. However, in order to achieve ultra-precision processing of brittle materials, the company developed the world's first three-coordinate controlled M-18AG type ultra-precision aspheric diamond tool turning and diamond wheel grinding machine tool in 1980. The machine tool uses an air spindle with a radial rotation accuracy of 0.075 μm. It uses an Allen-Braley 7320 CNC system. The X and Z axis travels are 410 mm and 230 mm respectively. The straightness of the guide rails is within 0.5 μm within the full travel range. The positioning accuracy of the B axis is 0.38 μm within 360'. The diamond grinding wheel can process various non-spherical metal reflectors with a maximum diameter of 356 mm.

In 1980, RankPneumo launched the MSG type ultra-precision CNC lathe with a dual-axis linkage that uses a laser interferometer to complete position closed-loop control. The lathe can process non-spherical metal reflectors with a diameter of 350 mm, and the surface roughness of the processed surface is R~0.05 μm. In 1988, the company successfully developed ASG2500, ASG2500T, and Nanoform300 machine tools: these machine tools can not only perform ultra-precision cutting, but also process aspheric reflectors with a diameter of 300mm. Based on the above machine tools, the company developed Nanoform600 in 1990, which can process aspheric reflectors with a diameter of 600mm, with a workpiece shape accuracy better than 0.14 and a surface roughness better than Ra0.01um. In 1996, the company launched the Nanoform250 ultra-precision composite machining machine tool, which can not only perform diamond cutting, grinding and polishing, but also directly grind out hard and brittle optical parts that meet the optical surface quality and profile accuracy.

At present, there are at least 30 well-known companies and enterprises engaged in ultra-precision machining in the United States. Among them, the most representative and significant achievements are the LLNL (Lawrence Livermore National Laboratory) of Union Carbide mentioned above. Since the 1960s, LLNL has developed DTM-1 and DTM-2 ultra-precision machine tools, and in September 1983, it successfully developed the DTM-3 horizontal large optical diamond ultra-precision lathe, which represents the highest level of contemporary ultra-precision machine tools. The machine can process workpieces with a diameter of 2100rain and a weight of 4497k2; the transmission device of its tool holder adopts friction drive and uses laser interferometer for position measurement; it adopts liquid hydrostatic bearings and liquid hydrostatic guide rails, and the position accuracy can reach 0.013~m: the surface roughness of brass parts can reach R~0.0076Bra.

In September 1984, LLNL cooperated with the US Air Force WRIGHT Aeronautical Research Institute and other units to successfully develop the LODTM (Large Optics Diaramond Turning Machine) large vertical ultra-precision CNC lathe. The lathe can process workpieces with a diameter of 1625mm; a specially developed 7-way dual-frequency laser interferometer is used to measure various position information, and then through data processing, accurate feedback information is provided to the servo system to drive the tool holder to ensure the position of the tool relative to the workpiece, with a measurement resolution of 0.635nm. In order to ensure the accuracy of position servo control, a precise digital servo control method is adopted, and the control part is a built-in CNC device. In order to achieve micro-feeding of the tool, the machine tool adopts a piezoelectric micro-displacement mechanism, which can achieve nanometer-level micro-displacement, and the processing accuracy can reach 0.025~m. LODTM is recognized as one of the world's highest-level ultra-precision lathes.