Machining method of aspherical optical element

For the application of aspherical surface and the accuracy requirements are different, its processing methods are different. According to the process characteristics, the aspherical surface processing methods can be divided into three categories: injection molding, molding and cold processing.

 

Since the application of aspherical surface and the accuracy requirements are different, the processing methods are different. According to the process characteristics, aspherical processing methods can be divided into three categories: injection molding method, compression molding method, and cold working method.

Compression molding method

In the compression molding method, the aspherical blank can be made of glass, but it is specialized glass suitable for compression molding. The process diagram for this method is as shown below:

First, the glass to be compression molded is placed in the mold (note that the glass at this time is a preform that has been polished, often polished small spheres of equal quality to the aspherical surface). The mold is then heated (the mold is processed, made of high-temperature resistant steel with aspherical contours), and the upper and lower molds are closed. Utilizing the softening characteristics of the glass after heating, the heated glass is pressed and molded, followed by annealing, cooling, and separation, to obtain aspherical lenses. Depending on the different heating and annealing processes, the compression molding machine can be divided into single-station or multi-station equipment, with the processing diameter of the multi-station equipment typically ranging from 2 to 50mm. The mold is generally turned on a single-point diamond lathe, so the product precision of the compression molding method is limited by the mold's surface shape accuracy, with the PV value of the compression-molded lens typically in the range of 0.2-0.4μm, meeting the requirements for the use of conventional precision lenses.

 

Cold working method

Cold working method is the most classical optical element processing method, which belongs to the removal type processing. Cold working method is typically used for large aperture aspherical optical elements. The processing ofScreen Centrifuges generally requires a complex and lengthy process (the size of the processing surface determines the processing time), with a long cycle. The main process includes milling shaping, rough grinding, fine grinding, rough polishing, fine polishing, coating, etc. At the same time, each process must be equipped with corresponding one or more detection methods to ensure the efficient operation of the process. The processed materials can be glass, ceramics, or crystal materials, and can achieve the highest processing accuracy.

Rough grinding is the process of processing a blank into a certain geometric shape, dimensional accuracy, and surface roughness. In the rough grinding stage of aspherical surfaces, CNC machining equipment is usually used, equipped with grinding wheels of different diameters and granularities. The component is shaped by the targeted grinding of the blank by the grinding wheel. Fine grinding uses finer abrasives or smaller grain size grinding wheels to ensure that the workpiece meets the required surface shape accuracy, size tolerance, and surface roughness before polishing. In the polishing stage of aspherical surfaces, because the curvature of aspherical surfaces varies, CNC small grinding head methods are usually used to polish the surface and improve the surface shape error of the aspherical surface, such as single-point diamond turning (SPDT) or computer numerical control (CNC) machining. When high accuracy is required for aspherical surfaces, magnetic fluid grinding technology (MRF) and ion beam polishing technology (IBF) are used for precision surface finishing.

These methods offer high precision and accuracy in producing complex surface profiles that cannot be achieved through traditional grinding and polishing processes. Here is an overview of the machining methods commonly used for aspherical optical elements:

1.Single-point diamond turning (SPDT): SPDT is a precision machining process where a single-point diamond tool is used to cut and shape the surface of the optical element. The diamond tool follows a controlled path, guided by a computer program, to produce the desired aspherical surface profile. SPDT is capable of achieving sub-micron precision and can be used to create steep slopes and complex shapes with high accuracy.

2. Computer numerical control (CNC) machining: CNC machining is a versatile and automated process that uses computer-controlled cutting tools to shape the optical element. CNC machines can be programmed to follow precise tool paths and cutting parameters to produce the aspherical surface profile with high accuracy and repeatability. CNC machining is suitable for both small and large volume production of aspherical optical elements.

3. MRF (Magneto-Rheological Finishing): MRF is a polishing process that uses a magnetorheological fluid to polish and correct the surface profile of the optical element. MRF is often used as a finishing step after diamond turning or CNC machining to remove any surface irregularities and improve the surface quality of the optical element.

4.Ion Beam Polishing Technology (IBF) is a nano fabrication process used to polish and smooth surfaces with extreme precision. This technology involves bombarding a surface with a focused ion beam, which is made up of charged particles accelerated to high energies. The ions interact with the atoms on the surface, causing sputtering and material removal to occur, resulting in a polished and smooth surface. IBF is commonly used in semiconductor manufacturing, optics, and other high-precision industries to improve the surface finish of various materials.

Because the optical equipment is usually very expensive, manual polishing methods are still used in the finishing stage. By using handheld polishing tools, aspherical products are polished and shaped by hand to improve the surface shape accuracy of aspherical surfaces.

 

Injection molding method

The materials commonly used in injection molding are optical plastics. Common optical plastics include PMMA (polymethyl methacrylate), PS (polystyrene), PC (polycarbonate), etc. They have the characteristics of high light transmittance, light weight, and low cost, making them suitable for mass production. The coefficient of thermal expansion of optical plastics is much larger than that of glass, so injection molding method is mainly used to produce low-precision mass-produced aspherical components. Due to the cooling and solidification shrinkage during the processing, the surface shape accuracy of injection-molded optical components is typically at the level of PV value 1-2μm.

 

Overall, the machining method for aspherical optical elements depends on factors such as the desired surface profile, material, volume of production, and required precision. Advanced machining techniques such as diamond turning and CNC machining are commonly used for fabricating high-quality aspherical optical elements with complex surface profiles.

MG Optics has advanced processing equipment and rich processing experience, producing high-precision optical aspherical mirrors and lenses, and also CGH to test aspherical surfaces From 50 mm to 2000mm, materials including Fused silica, Zerodure, SiC and Al RSA6061.Make-to-print manufacturing, 1/100 RMS or better. 

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