What should be paid attention to when processing optical glass squares?

The processing of optical glass square is a precise process, which needs to pay attention to the following aspects:

I. Selection and inspection of raw materials

Glass quality

The material of optical glass should meet the required optical properties, such as refractive index, dispersion coefficient and other parameters should be accurate. For example, for optical glass used in telescope lenses, the refractive index error is usually required to be in a very small range to ensure the clarity and accuracy of imaging.

Check the uniformity of glass to avoid impurities, bubbles or stress concentration areas inside. Because these defects will affect the propagation of light, resulting in distortion or blurring of imaging.

dimensional accuracy

The initial size of raw glass square should be as close as possible to the size requirements of the final product, so as to reduce the subsequent processing capacity. At the same time, the dimensional accuracy should be strictly tested, and the side length error should generally be controlled within ±0.1mm and the thickness error should be controlled within ±0.05mm, so as to provide a good foundation for subsequent precision machining.

Second, the cutting process

Accuracy of cutting equipment

Use high-precision cutting equipment, such as CNC cutting machine. The cutting tool of the equipment should be sharp and high precision, and the runout of the tool should be controlled within a very small range, generally not exceeding 0.01mm, to ensure the flatness of the cutting surface.

The motion control system of the equipment should be accurate, which can achieve micron-level positioning accuracy, so as to ensure the accurate size of the cut square.

Cutting parameters

The cutting speed should be adjusted reasonably according to the thickness and hardness of glass. For example, for thick (more than 10mm) optical glass squares, the cutting speed may be controlled between a few millimeters and a dozen millimeters per minute to prevent the glass from collapsing or cracking.

The cutting pressure should be appropriate, too much pressure will easily break the glass, and too little pressure will not effectively cut it. At the same time, it is necessary to use appropriate cutting fluid, which can not only play a cooling role, but also reduce the friction in the cutting process and improve the cutting quality.

Third, grinding and polishing

Grinding equipment and abrasives

The working table of grinding equipment should be flat, and its flatness error should be controlled within 0.005 mm. The rotating speed of the grinding disc should be stable, and accurate speed regulation can be realized to adapt to different grinding stages.

Choosing a suitable abrasive, the particle size and hardness of abrasive should be determined according to the requirements of grinding accuracy and the hardness of glass. In the rough grinding stage, silicon carbide abrasive with large particles (such as 10-50μm) can be used; In the fine grinding stage, alumina abrasive with smaller particles (such as 1-10μm) should be used.

Grinding process parameters

Pressure control in grinding process is very important. Too much pressure may lead to scratches or excessive local wear on the glass surface, while too little pressure will lead to low grinding efficiency. Generally, the pressure during rough grinding can be appropriately larger, and the pressure during fine grinding should be reduced. For example, the rough grinding pressure can be between 0.1 MPa and 0.3 MPa, and the fine grinding pressure can be controlled between 0.05 MPa and 0.1 MPa.

The grinding time should be arranged reasonably according to the grinding quantity and precision requirements. At each grinding stage, testing tools (such as surface roughness meter) are needed to determine whether the expected grinding effect is achieved.

Polishing process

The polishing agent used for polishing is usually cerium oxide and other materials. When polishing, the rotating speed, polishing pressure and polishing time of the polishing wheel should be accurately controlled. Generally, the rotating speed of the polishing wheel is between several hundred revolutions per minute and several thousand revolutions, and the polishing pressure is between 0.02 MPa and 0.08 MPa. The polishing time depends on the area and precision requirements of the glass, usually ranging from several minutes to several tens of minutes.

In the process of polishing, it is necessary to ensure the uniform supply of polishing agent, so as to avoid local under-polishing or over-polishing. At the same time, attention should be paid to the cleanliness of the polishing environment to prevent impurities from mixing into the polishing agent and affecting the polishing quality.

Fourth, cleaning and coating

Cleaning link

Use ultra-pure water and a suitable cleaning agent for cleaning. The cleaning process should be thorough, not only to remove impurities such as abrasives and polishing agents remaining in the grinding and polishing process, but also to remove grease and dust on the glass surface.

The cleaned glass square should be dried to avoid water stains. Natural air drying, hot air drying or vacuum drying can be used to ensure that the glass surface is free of water stains and stains.

Coating process (if necessary)

Make sure that the cleanliness and roughness of the glass surface meet the requirements of coating before coating. The vacuum degree of coating equipment should reach the specified standard, generally between 10 and 10 Pa, to ensure that the coating material can be uniformly deposited on the glass surface.

The evaporation rate, deposition thickness and other parameters of coating materials should be accurately controlled. For example, for antireflection films, the deposition thickness is usually controlled at about a quarter of the wavelength (for visible light band) to achieve the best antireflection effect.

V. Quality inspection

Optical performance detection

Optical performance parameters such as refractive index and transmittance of glass square are detected by optical interferometer and other equipment. The transmittance detection should cover the required spectral range. For example, for optical glass used in photographic lenses, the transmittance in the visible light band (400-700nm) should be detected to ensure that the transmittance meets the design requirements.

Detect the aberration of optical glass, including spherical aberration, coma and astigmatism. By using the optical aberration tester, the imaging quality of the glass square is evaluated to ensure that its imaging accuracy meets the requirements.

Geometric accuracy detection

The dimensional accuracy of glass square is detected by three-coordinate measuring instrument, including side length, diagonal length and thickness. The accuracy of side length and diagonal length should be within ±0.01mm, and the accuracy of thickness should be controlled within 0.005 mm.

Check the flatness and parallelism of the glass square. The flatness error is generally required to be within ±0.002mm, and the parallelism error is controlled within ±0.003mm to ensure the installation accuracy and optical performance of glass in the optical system.

Surface quality inspection

Observe the surface of the glass square with a microscope to check whether there are scratches, pits and other defects. For high-precision optical glass squares, the depth of surface scratches should be controlled below 0.1μm, and the diameter and density of pits should also meet the corresponding quality standards.

To detect the surface roughness, use a surface roughness meter to measure the roughness parameters (such as Ra value) of the glass surface. Generally, the Ra value after fine grinding should reach 0.01-0.05μm and the Ra value after polishing should reach 0.001-0.005 μ m..