What should be paid attention to when processing optical glass lenses with different shapes?

Matters needing attention in processing optical glass lenses with different shapes

The shape of optical glass lens directly determines the processing path and the focus of precision control. Common shapes include flat (flat/plano-convex/plano-concave), spherical mirror (biconvex/biconvex/meniscus), aspherical mirror and irregular mirror (prism/cylindrical mirror/free-form mirror). The processing needs to focus on four cores: material characteristics, shape accuracy, surface quality and stress control. Matters needing attention for different shapes are as follows:

I. Flat lenses (flat, plano-convex and plano-concave)

The core processing requirements of plain film are flatness, parallelism and surface smoothness, which are common in window film, beam splitter and plano-convex lens substrate.

Coarse grinding stage

Adopt diamond grinding wheel to grind, control the grinding pressure evenly, and avoid the lens edge collapse or deep scratches on the surface due to excessive pressure.

Plano-convex/plano-concave lenses need to process the plane side first, and then locate and process the curved side with the plane as the reference to ensure that the center of the curved surface is coaxial with the plane reference.

Fine grinding and polishing

When polishing, soft polishing pads (such as asphalt pads and polyurethane pads) are used, and cerium oxide polishing solution is used, and the process of "small pressure and slow rotation speed" is adopted to prevent concave or convex deformation in the plane.

Strictly control the parallelism error (usually ≤30″), which can be detected in real time by laser interferometer to avoid lens warping caused by uneven clamping force of fixture.

Edge processing

The edge of the flat sheet is prone to crack. When chamfering, diamond chamfering wheel is used for low-speed machining, and the chamfering size should be consistent to prevent stress concentration at the edge.

Cleaning and testing

After polishing, it is cleaned by an ultrasonic cleaner to avoid water stains caused by residual polishing solution; The inspection focuses on flatness, parallelism and surface scratches (≤Scratch-Dig 40-20).

Second, spherical lenses (biconvex, biconcave, meniscus)

The core of spherical mirror is curvature radius accuracy and surface accuracy, which are widely used in telescopes, microscopes and camera lenses.

Matching of mold and tooling

Before machining, the curvature radius of the spherical mold should be calibrated to ensure that it is consistent with the design value (the error is ≤±0.001mm), and the surface of the mold should be clean and free from damage to prevent it from being imprinted on the lens surface.

The lens is fixed by vacuum sucker or wax bonding method, and the stress balance between thick edge and thin edge of meniscus lens should be paid attention to to avoid surface distortion caused by clamping.

Grinding and polishing

In the rough grinding, a formed grinding wheel is used, and in the fine grinding stage, a spherical grinding disc is used, and the grinding trajectory needs to be "circular reciprocating motion" to ensure uniform grinding in all areas of the spherical surface and avoid "local high points".

When polishing, select the matching polishing die (such as annular polishing die and spherical polishing die) according to the curvature of the spherical surface, and the flow rate of polishing solution should be stable to prevent curvature deviation caused by excessive local polishing.

Surface type detection

Using Newton's ring interferometry or laser spherical interferometer to detect the surface accuracy, the surface error should be controlled within λ/10~λ/20(λ=632.8nm).

The biconvex/biconvex lens should ensure that the curvature centers of both surfaces coincide to avoid the imaging deviation of the optical system caused by eccentricity.

Special requirements for materials

High refractive index optical glasses (such as LaK series and ZF series) are brittle, so it is necessary to reduce the grinding wheel speed and increase the coolant flow to prevent microcracks on the surface.

Third, aspheric lenses

Aspheric mirror has no fixed radius of curvature, but its core is surface profile accuracy and surface roughness. It is used in high-end imaging system and laser collimation system, and its machining difficulty is much higher than that of spherical mirror.

Processing technology selection

Single-point diamond turning (SPDT) is used in small and medium-sized batch production, and ultra-precision lathe is needed to control spindle speed (5000~10000r/min) and tool feed (≤0.001mm/r), and the arc radius of tool tip should be adapted to aspheric curvature.

Mass production adopts compression molding, and it is necessary to strictly control the compression temperature (optical glass transition temperature TG 5℃) and pressure to prevent the surface shape from being incomplete due to insufficient glass softening or the glass from crystallizing due to excessive temperature.

Clamping and positioning

Vacuum adsorption+centering tooling is adopted to ensure that the lens has no radial runout during machining, and the datum plane of aspheric surface needs to be coaxial with the lathe spindle, and the eccentricity error is ≤ 5 μ m.

Polishing and correction

After turning, magnetorheological polishing (MRF) or ion beam polishing (IBF) is needed to remove the turning texture, and the surface roughness should reach Ra≤0.005μm m..

In the polishing process, aspheric interferometer should be used to detect the surface shape in real time, and the polishing parameters should be corrected in real time by numerical control system to avoid the surface shape deviation.

stress control

The edge thickness of aspheric mirror varies greatly, and it is easy to produce residual stress during polishing. It needs to be annealed (heated to 50~100℃ below Tg, and slowly cooled after 2~4 hours of heat preservation) to eliminate stress and prevent cracking in subsequent use.

Special-shaped lenses (prism, cylindrical mirror, free-form mirror)

The special-shaped mirror is complex in structure and has no uniform symmetry plane. Its core is geometric dimension accuracy, angle accuracy and surface consistency, which is used for beam splitting, deflection and imaging correction.

1. Prisms (right-angle prism, pentagonal prism, roof prism)

Angle control: It is necessary to ensure that the right angle error of prism is ≤3″ and the included angle error of roof surface is ≤1″ during processing, which can be realized by the angle positioning fixture of CNC grinding machine to avoid optical path deviation caused by angle deviation.

Chamfering and edge collapse prevention: The edges and corners of the prism are prone to edge collapse, so the soft grinding chamfering process should be adopted, and the chamfering width should be uniform. A special polishing fixture should be used when polishing the roof surface to prevent the edges and corners from being damaged.

Bonding treatment: some prisms need to be bonded and combined. Before bonding, the flatness of the bonding surface should be ensured to be ≤λ/20. When cleaning, use anhydrous ethanol to wipe it to avoid bubbles in the adhesive layer.

2. Cylindrical mirror (flat column, double column)

Molding: The straightness of the generatrix of the cylindrical mirror should be ≤0.001mm, and the cylindrical grinding wheel should be used for rough grinding, and the grinding track should be parallel to the generatrix to prevent "drum" or "saddle" distortion.

Anti-deformation during clamping: the curved surface of cylindrical mirror has one-dimensional curvature, so it is necessary to avoid squeezing the direction of the generatrix of the curved surface during clamping. V-shaped fixture or vacuum adsorption fixture can be used to ensure that the curved surface is not deformed during machining.

Key points of detection: The curvature and straightness of cylinder are detected by laser cylindrical interferometer, and the surface roughness should reach Ra≤0.01μm m..

3. Free-form mirror

Machining equipment: five-axis linkage ultra-precision machining center is needed, and the tool path needs to be accurately planned by CAM software to ensure the uniform cutting allowance at each point of the curved surface.

Polishing correction: robot polishing system or air bag polishing is used to locally correct the complex area of the curved surface, and the surface accuracy should be controlled within λ/50.

Detection difficulty: It is necessary to use a three-dimensional profilometer or laser scanning interferometer to detect the whole surface to ensure that the contour of the surface is consistent with the design model.

Five, all shapes of optical glass lens processing general precautions

Material pretreatment

Before processing, optical glass should be annealed to eliminate internal residual stress and prevent deformation or cracking caused by stress release during processing.

Different brands of optical glass (such as Guanpai glass K9 and flint glass ZF6) have different hardness and brittleness, so it is necessary to match the corresponding grinding wheel granularity and polishing solution formula.

environmental control

The processing environment should be kept at constant temperature and humidity (temperature 20 0.5℃, humidity 45%~60%), so as to avoid the influence of temperature change on the accuracy detection.

The processing area should be dust-free (Class 1000 or above) to prevent dust particles from embedding into the lens surface and forming scratches or pits.

Cleaning and protection

Special optical glass cooling liquid should be used during processing to avoid oily cooling liquid and prevent oil pollution on the lens surface.

The lens should be wrapped with dust-free cloth immediately after processing and stored in a special anti-static packaging box to avoid scratching or electrostatic adsorption of dust.

Detection standard

Surface quality: Scratch-Dig shall meet the requirements of customers, and the high-end lenses shall reach grade 20-10.

Surface accuracy: spherical mirror is detected by Newton's ring, aspheric mirror is detected by aspheric interferometer, and special-shaped mirror is detected by three-dimensional profilometer.

Dimensional accuracy: thickness tolerance ≤±0.002mm, angle tolerance ≤ 3 "and radius of curvature tolerance ≤ 0.001 mm..

summary

When processing optical glass lenses with different shapes, it is necessary to make clear the core accuracy index (flatness/curvature/angle) of the shape, then match the corresponding processing technology, fixture and testing equipment, and strictly control the environment, material stress and surface cleanliness, so as to ensure that the optical performance of the lenses meets the design requirements.