Introduction of Kunshan Nuoli Optics: What should be paid attention to when learning to process special-shaped parts with glass?

What matters should be paid attention to when processing special-shaped parts with optical glass?

Optical glass shaped parts are different from conventional spherical and planar optical elements, including aspheric surfaces, free-form surfaces, oblique prisms, shaped windows, irregular polyhedrons and other structures. The processing flow is complex, and the requirements for tooling, cutting, polishing, inspection and environmental control are strict. Improper control of any link will cause problems such as surface deviation, edge collapse, stress and light transmission defects. The following matters need to be paid special attention to in the whole processing process.

Material selection and pre-evaluation are the first steps in machining special-shaped parts. Special-shaped parts are complicated in structure, and stress concentration is easy to occur at corners, thin walls and grooves. Before processing, it is necessary to confirm the material characteristics of optical glass, and distinguish the brittleness, thermal expansion coefficient and hardness of different materials such as Shi Ying glass, Guanpai glass and flint glass. For materials with high brittleness and easy edge collapse, it is necessary to evaluate in advance whether the wall thickness, sharp corners, deep grooves and other structures of the parts have machining feasibility, optimize the drawings when necessary, increase the process transition fillet, and avoid sharp internal right angles. At the same time, check the defects such as bubbles, stripes, stones, etc. in the material. The machining and removal of special-shaped parts are large, and the internal defects of the material will be scrapped directly after exposure. The blank quality inspection must be completed before blanking.

Fixture clamping scheme directly determines the machining accuracy of special-shaped parts, and it is also the most prone to deformation. Conventional optical parts are mostly adsorbed by suction cups, and the profile of special-shaped parts is irregular, so it is impossible to directly vacuum adsorption, so it is necessary to design special fixtures. When clamping, it is necessary to avoid single-point hard clamping, and the clamping force should not be too large to prevent the glass from producing extrusion stress, stress birefringence and cracks in subsequent polishing or use. Flexible support, paraffin bonding and low-stress glue fixation are preferred for thin-walled special-shaped parts, and the thickness of the bonding layer is uniform to ensure the force dispersion. The positioning datum of clamping should be unified, and the optical design datum should be used as the machining datum to avoid the positioning error caused by repeated clamping, and the shape and position tolerance should be rechecked after each clamping.

In the cutting and rough grinding stage, edge collapse and surface cracks are mainly controlled. Special-shaped contours are mostly milled and shaped by diamond grinding wheel, and the grinding wheel granularity, rotation speed and feed rate need to match the glass material. The feed speed should not be too fast, especially in the corner and arc transition area, to reduce the cutting impact and prevent the edge from collapsing. Cooling liquid must be continuously supplied during the machining process to take away the cutting heat in time. The uneven heat and cold of glass will produce microcracks, which will continue to expand in the subsequent polishing and coating process, resulting in the failure of components. After rough grinding, a reasonable finishing allowance is reserved. Too little allowance can't eliminate rough grinding lines, while too much allowance increases the polishing burden. Generally, the allowance is controlled according to the part size and material characteristics, and all edges are chamfered to avoid bumping in subsequent processes.

Precision grinding and polishing control surface shape accuracy and surface quality. Special-shaped free-form surfaces can't adopt traditional spherical polishing process, but need to rely on CNC polishing, magnetorheological polishing and other processes. When machining, the polishing pressure should be controlled evenly, and the inconsistent pressure at the ups and downs of the special-shaped surface will cause local edge collapse and surface distortion. Pay close attention to surface defects, scratches, pits and pits are more difficult to clean in special-shaped concave areas. Optical stress should also be monitored during polishing. The residual stress of special-shaped parts will change the refractive index and affect the imaging effect. After processing, annealing can be carried out to release the internal residual stress.

Detection and environmental control can not be ignored. Special-shaped parts can't be detected by ordinary spherical template, so it is necessary to use three-coordinate system, interferometer, profilometer and other equipment to make a multi-point detection scheme to check key positions such as curved surface, inclined surface and chamfer point by point. The processing environment is kept at constant temperature and humidity, and the temperature fluctuation will cause the glass to expand when heated and contract when cooled, which will destroy the processing surface shape; Keep the workshop clean to prevent dust particles from scratching the optical surface during polishing. Special foam fixture is used in the transfer process, and direct contact with the optical working surface is strictly prohibited to avoid bumping damage.

Subsequent coating and cleaning processes also need to adapt to the special-shaped structure. Polishing powder is easy to remain in grooves and dead corners of special-shaped parts, and ultrasonic cleaning parameters should be optimized to prevent impurities from remaining. When coating, the film thickness of each area of the special-shaped surface is easy to be uneven, so it is necessary to adjust the rotation mode of the tooling to ensure the consistency of the film.

In a word, the key difficulties in machining optical glass special-shaped parts are clamping deformation, corner collapse and surface shape control caused by complex contours. From drawing process review, blank preparation, tooling design, grinding and polishing to inspection and cleaning, stress and surface defects should be controlled in every step, and geometric dimensions and optical properties should be taken into account, so as to stably produce qualified special-shaped optical elements.