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Traditional optical systems are limited by the technology of the times, and the structure and component shapes are relatively simple. For the processing of traditional optical components, the processing precision depends on the processing method, and the low-precision machining machine can still achieve high optical component processing precision effects. This type of machine is often referred to as a nondeterministic machine. Non-deterministic machining machines using traditional machining methods are only suitable for processing optical components such as balls, planes and other simple shapes and glass-based hard and brittle materials.
With the development of science and technology, especially the development of modern optoelectronic technology and computing technology, today's optical application systems have developed tremendously regardless of the complexity of the optical component's shape, the diversity of materials, and the geometric dimensions of both small and large. Variety. Traditional non-deterministic machining machines and methods have been unable to adapt to the processing needs of modern optical system components: either they are impossible to machine at all or the processing efficiency is extremely low. Modern ultra-precision machining machines came into being, especially for Deterministic ultra-precision machining machines.
Key technology of ultra-precision machining machine tools
The overall integrated design technology of the machine tool system. Conventional machine tool design and manufacturing, all aspects of the technical have a great tolerance. All aspects of ultra-precision machine tools are basically in a technical limit or critical application state. Which part is slightly considered or not handled well will lead to overall failure. Therefore, the design requires a very comprehensive and profound understanding of the overall and various parts of the machine tool system. According to the feasibility, from the overall optimization, the comprehensive design is carried out in an extremely detailed manner.
High rigidity, high stability machine body structure design and manufacturing technology. Especially for LODTM machine tools, due to the large size of the fuselage and the weight of the load bearing workpiece, any slight deformation will affect the machining accuracy. In addition to the requirements in terms of materials, structural forms and processes, the structural design must also take into account the operability of the machine when it is running.
Ultra-precision workpiece spindle technology. Medium and small machine tools often use an aerostatic spindle scheme. The air static pressure spindle has small damping and is suitable for high speed rotary machining applications, but its load carrying capacity is small. The aerostatic spindle has a rotation accuracy of 0.05μm.
The LODTM machine tool spindle carries the size and weight of the workpiece. Generally, the hydrostatic spindle should be used. The hydrostatic main shaft has large damping, good vibration resistance and large bearing capacity, but the hydrostatic main shaft has high-speed heating, and liquid cooling and constant temperature measures are required. The hydrostatic spindle has a rotation accuracy of 0.1 μm. In order to ensure the accuracy and stability of the spindle, constant pressure, filtration and pressure precision control are required regardless of the air pressure source or the hydraulic source.
Ultra-precision rail technology. Early ultra-precision machines used air-floating hydrostatic guide technology. The air-floating hydrostatic guide rail is easy to maintain, but has small damping and poor anti-vibration performance. It has been used less frequently. The closed hydrostatic guide rail has the advantages of high vibration damping resistance, high rigidity and large bearing capacity. The main super-precision machining in foreign countries mainly uses hydrostatic guide rails. The ultra-precision hydrostatic guide rail can reach a straightness of 0.1μm.
Nano-resolution dynamic ultra-precise coordinate measurement technology. Laser interferometry is a high-precision standard geometry measurement benchmark, but is susceptible to environmental factors (air pressure, humidity, temperature, airflow disturbances, etc.). To this end, the LLNL's LODTM coordinate laser measurement circuit in the United States uses vacuum isolation, and measures of the zero temperature coefficient of the Invar coordinate measurement framework. This is also the top application for laser coordinate measurement.
Most of today's ultra-precision machine tool coordinate measuring systems use diffraction gratings. The grating measurement system has high stability and resolution up to nm. In order to further obtain ultra-high position control characteristics and surface quality, DSP subdivision is used, and the resolution of the measurement system can reach nanometer level.
Nano-scale repeat positioning precision ultra-precision transmission, drive control technology. In order to achieve deterministic ultra-precision machining of the optical stage, the machine tool must have the tool motion control quality of nano-level repeat positioning accuracy. The servo drive and drive system need to eliminate all nonlinear factors, especially the friction of the motion mechanism with nonlinear characteristics. Therefore, the use of air-floating, liquid floating and other static friction-free bearings, guide rails, balance mechanism has become an inevitable choice. In addition to the high resolution and high real-time requirements of the servo motion controller, the control algorithm mode needs to be continuously improved.
Open high performance CNC CNC system technology. Starting from the processing precision and performance, in addition to meeting the requirements of ultra-precision machine tool control display resolution, accuracy, real-time and other requirements, the numerical control system also needs to expand many auxiliary functions such as on-machine measurement, tool setting and compensation. The general CNC system is difficult to meet the requirements. Therefore, ultra-precision machine tools are now basically using PC + motion controller to develop an open CNC CNC system mode.
High-precision gas, liquid, temperature, vibration and other working environment control technology. Machine vibration isolation and horizontal attitude control. The impact of vibration on ultra-precision machining is very obvious, and cars that drive far have an impact. The vibration isolation of the machine tool requires special ground treatment and airbag vibration isolation and vibration isolation measures. The machine body air-floating vibration isolation system also needs to have an automatic leveling function to prevent the influence of horizontal state changes on the machining during machining. For LODTM machines with high vibration isolation requirements, the natural frequency of the vibration isolation system is below 1HZ.
temperature control. Temperature has a great influence on the machining accuracy. Therefore, LODTM machine tools require extremely high temperature control.
Application prospect
The general development trend of ultra-precision machining machine tools and technologies: higher surface quality and surface accuracy; development in both large and small scales; improved complex shape of workpieces and processing adaptability of different materials.
Large-scale development applications such as ultra-large SLODTM machine tools for light-weight, high-rigidity metal-based primary mirrors for future space- and space-based laser weapons; ground-based super-large-diameter deep-space telescopes (eg Europe's Euro50 (Φ50m), OWL (Φ100m) )) Multi-axis ultra-precision grinding of spliced ​​off-axis aspherical mirrors (several meters).
In recent years, terahertz (THZ) has received extensive attention as an emerging technology, and it is an extremely large and important application field for ultra-precision machining technology and machine tools in the future. On a large scale, terahertz applications are no less than the big development needs of the forefront, such as terahertz antenna mirror processing requirements. On a small scale, the miniature corrugated horn antenna in the terahertz system (millimeter-scale complex shape cavity, micron-level machining accuracy) is one of the ultra-precision machining problems that need to be solved in the future. In terms of the complexity of the machined surface shape, due to the electromagnetic characteristics of the surface of the terahertz beam control element, the design of the component surface is more complicated, such as asymmetrically shaped freeform surfaces. In terms of processing materials, terahertz applications are more diverse.
To develop ultra-precision machining machine tools, the key technologies that need to be solved in China include: high-precision, high-resolution, high-stability, large-displacement coordinate measurement systems, advanced control algorithms (adaptive control, second-order dynamic no-difference control, etc.) High-performance multi-axis motion controller, workpiece in-machine ultra-precision measurement and compensation technology, ultra-precision environmental control technology.
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