What is precision polishing in optic processing？

Polishing is a technique used to reduce the surface roughness of optical components using mechanical, chemical, or electrochemical methods to achieve a smooth and flat surface. Producing optical components typically involves steps like rough grinding, fine grinding, polishing, and coating. After fine grinding, a crack layer of about 2-3 microns may remain on the surface.Precision surface finishing, as the final step in ultra-precision manufacturing, aims to remove this surface damage layer, improve surface quality, and achieve surface roughness at the nanometer or sub-nanometer level.

7 Common Precision Polishing Technologies

The polishing process’s effectiveness in removing defects and controlling errors mainly determines the final surface quality of optical components. Meanwhile, the efficiency of the fine grinding process affects surface accuracy and the suppression of surface and sub-surface damage. Common precision polish methods include conventional asphalt polishing, CNC polishing, MRF polishing, IBF polishing, and CCOS polishing. Each method has unique advantages and applications, and 7 methods will be explored in detail in this article by Yudi Optics.

01 Conventional Asphalt Polishing

Asphalt polishing is a traditional optical polishing technique that uses the physical properties of asphalt materials for grinding. Asphalt becomes soft at high temperatures and can be cast into the desired shape. It hardens after cooling and is suitable for polishing. During the polishing process, the incompressibility and high viscosity of asphalt forms a stable polishing surface, while the groove structure stores the polishing liquid and helps dissipate heat.

Asphalt polishing technology is widely used in the polishing of spherical and flat optical components made of materials such as optical crystals, optical glass and fused quartz. During polishing, abrasive materials are added to the asphalt surface to achieve surface grinding when in contact with the workpiece. By adjusting the viscosity of the asphalt and the groove design, the workpiece can obtain a precise shape and surface finish during polishing.

Advantages of asphalt polishing:

• Ability to polish to extremely high surface quality and smoothness.
• Applicable to most optical glass materials.
• Relatively low cost.

Limitations of asphalt polishing:

• Time-consuming process and low production efficiency.
• Difficult to control shape errors.
• Not accurate enough for complex shapes and aspherical processing.

Yudi Optics currently has a large number of continuous single-sided polishing circles for asphalt polishing. They are available in different sizes to accommodate optical polishing of substrates from 3mm to 1000mm. Each circle can be configured to specific requirements and can currently polish any material listed on our materials page. In addition to traditional asphalt polishing, we have other ultra-precision polishing technologies to optimize the surface finish. At the same time, our 30-year-experienced masters can complete the most difficult polishing projects by hand.

02 CNC Polishing

CNC polishing is a process that uses computer numerical control technology to finely polish the surface of a part, which can meticulously remove defects, scratches or irregularities on the surface of the part without significantly changing its shape. This enhances the surface smoothness and appearance of the machined part without affecting the shape of the part. CNC polishingcan significantly reduce human errors and provide more efficient and precision surface finishing.

Its working principle is:

Program control: First, write a CNC program based on the geometry and surface requirements of the part. The program controls the path, speed and pressure of the polishing tool.

Automated operation: The polishing machine automatically moves the polishing tool according to the path set by the program to gradually polish the surface of the part. This ensures consistent polishing quality.

Precise polishing: By controlling the parameters, high-precision surface smoothness can be achieved, and tiny surface defects can be removed while maintaining the shape and dimensional accuracy of the part.

Scope of application: CNC polishing is suitable for a variety of materials, including metals and plastics, especially in optical and high-precision applications, to improve the surface quality and appearance of parts.

Yudi’s optical processing centers are equipped with advanced CNC systems, including self-developed grinding and polishing equipment, capable of processing a variety of materials and workpiece geometries. We provide a variety of high-performance solutions for your optical applications. Whether you need precision polishing services or CNC polishing machines, we can provide the best-customized solutions.

03 Laser Polishing

Laser polishing is a technology that uses a laser of a certain wavelength and energy density to irradiate the surface of a workpiece, causing the material to melt, vaporize, and peel off within a very thin range, thereby obtaining a smooth surface. It has been widely used for its non-contact processing, no mechanical stress, comprehensive control of multiple parameters, high polishing accuracy, and small single action area. It is particularly suitable for surface processing of brittle and hard materials. (High-hard and brittle materials mainly include sapphire, optical glass, crystalline silicon, cemented carbide, etc.)

1) The processing accuracy is high, and the surface roughness after laser processing is generally lower than that of traditional polishing methods, which can reach the nanometer level.

2) The various parameters of the laser (pulse frequency, energy density, etc.) are adjustable and controllable, which can more finely control the entire processing process, and it is also easier to integrate into an automated platform to provide a one-stop solution for different material processing.

3) The area of ​​laser single action is very small, and a part of the material surface can be selectively processed.

4) The non-contact processing method can process various complex-shaped surfaces, even flexible material surfaces, without damaging the workpiece due to mechanical stress.

5) Laser-induced photochemical action can be used for indirect etching or polishing with different gases or polishing liquids according to the different materials being processed, and suitable processing solutions can be flexibly formulated for different materials.

These characteristics make laser polishing one of the candidate technologies for high-precision polishing of hard and brittle materials.

04 Robotic Polishing

Robot precision polishing is a technology that uses a robot arm for automated polishing. Its working principle includes the following aspects:

Path planning: The polishing path is designed through computer software to ensure that the polishing tool can accurately cover the entire workpiece surface.

Force control system: The sensor is used to monitor the force during the polishing process in real time, and the action of the robot arm is adjusted to maintain constant pressure, thereby achieving uniform material removal.

Flexibility: The robot arm can move freely and can polish workpieces with complex shapes.

The advantage of robot precision polish inglies in its automation ability, which can perform long-term, highly repetitive operations and polish workpieces with complex shapes. It is widely used in automobiles, aerospace, electronic equipment and other industrial fields. Compared with traditional manual polishing, robot polishing improves production efficiency, reduces human errors, and can better control the force during polishing to obtain more uniform surface quality.

Yudi Optics pioneered robot precision polishing technology based on traditional polishing technology, CNC polishing and ion beam polishing. Our intelligent robot polishing machine uses the ABB IRB 2600 six-axis linkage robot arm and cutting-edge algorithms to flexibly adjust the tool’s contact geometry, dwell time, and pressure, so we can achieve automated and high-precision polishing for high-performance lenses and mirrors. Yudi’s robot polishing technologyexcels in deterministic polishing, with a maximum diameter of 1800 mm.

05 Ion Beam Figuring

Ion beam polishing technology was first proposed by Wilson and Reicher in 1988. It is an innovative polishing process that is completely different from traditional optical processing. It uses the fourth state of matter, plasma, to bombard the surface of the workpiece through inert gas ions (such as argon) in a vacuum, and uses the physical sputtering effect to remove materials. IBF polishing is a stress-free, non-contact process that can avoid surface and subsurface damage caused by mechanical pressure in traditional polishing.

IBF’s carried out in a high-cleanliness vacuum environment, which prevents the introduction of impurities and can produce ultra-smooth surfaces. Its characteristics are that it can achieve sub-nanometer material removal, quickly adjust the surface shape, meet strict surface shape accuracy requirements, and is very suitable for the manufacture of high-precision optical components.

Advantages：

• Stress-free and non-contact: Unlike traditional polishing, ion beam polishing does not contact the workpiece surface, so it does not generate mechanical pressure and avoids surface or sub-surface damage.
• High precision: The removal rate and distribution are highly controllable, with high convergence efficiency, and it is a deterministic processing method. It is carried out in a vacuum environment to prevent the introduction of impurities, achieve sub-nanometer precision, and is suitable for the manufacture of ultra-smooth surfaces.
• Gaussian removal function: It has stability and controllability, which is convenient for precise control of removal rate and residence time.
• Material removal: Through the physical sputtering effect, materials can be removed at the atomic level, which is suitable for optical components with high precision requirements.
• Strong adaptability: It is suitable for the processing of planes, spherical surfaces and high-steepness aspherical surfaces, and is suitable for a variety of optical materials.
• Wide application: It can not only remove materials, but also improve surface quality, which is suitable for complex processing tasks.

The process is particularly suitable for the manufacture of optical components that require high precision and no damage, but ion beam figuring also has a high application threshold, such as:

1. The technical threshold is high and the production equipment is expensive. Although new application areas are constantly being developed, it is still high-end equipment.
2. Ion beam polishing machine, divided into caliber size, the larger the caliber, the higher the requirements, and surface processing capability is also a very important indicator.

Therefore, IBF polishing is usually the last step in the precision processing of optical components. Combined with other polishing processes, it can effectively reduce the surface roughness of semi-finished components and obtain high-precision optical components and ultra-smooth surfaces. At the same time, it can effectively improve mass production efficiency and control production costs.

06 Computer Controlled Optical Surfacing

Computer Controlled Optical Surfacing (CCOS ) is a polishing process that uses computer control technology to precisely process optical surfaces. It combines computer-aided design (CAD), computer-aided manufacturing (CAM) and precision machining technology to produce optical components with complex geometries and high-quality surfaces. The core of CCOS is to control the movement trajectory and pressure of the polishing tool on the workpiece surface through computer algorithms to accurately remove materials to achieve the desired surface shape and roughness. Its working principles include:

• Computer control: Use a computer to control the path, speed and pressure of the tool to achieve precise polishing of the optical surface.
• Small tool local polishing: Use small polishing tools to polish local areas, which can better control the removal amount and surface shape.
• Error compensation: By detecting the surface shape error in real time and adjusting the polishing parameters, the surface deviation is gradually corrected to achieve the desired optical surface shape.
• High precision and consistency: Suitable for optical component processing with high precision requirements, it can achieve nanometer-level surface roughness and surface shape accuracy.

Among them, the residence time algorithm is a key component, which calculates the residence time of the polishing tool at each position on the workpiece surface to ensure accurate control of the amount of material removed. This technology is particularly suitable for the producing of aspheric mirrors, free-form mirrors and complex optical system components.

Since computer-controlled optical surface shaping technology (CCOS) is the most advanced polishing technology at present, it has high requirements for equipment and control systems, and the processing speed is relatively slow. Therefore, in YuDi CCOS polishing is mainly used in the manufacture of high-precision optical systems, such as astronomical telescopes, lasers, and high-end lenses. The processing of large or complex curved optical components. It can effectively improve the surface quality and performance of optical components while ensuring the consistency and accuracy of production.

At present, the precision surface finishing methods equipped by Yudi with our CCOS system mainly include the following 4 types:

• Small Grinding Head Polishing

Computer-controlled small-head polishing technology (CCOS) combines traditional grinding and polishing experience with modern CNC technology. During the machining process, the target morphology data of the workpiece surface is pre-entered into the control system, and the polishing effect is optimized by adjusting the dwell time, speed, trajectory, pressure of the small-head, as well as the pH value and temperature of the polishing liquid. It is widely used in the manufacture of high-end optical instruments and complex optical systems.

Compared with traditional technology, CCOS small-head technology achieves high-precision machining of aspheric and free-form optical components through precise control and optimization, but further improvements are needed in the machining of large-diameter components to improve efficiency. For this reason, large-size grinding heads are usually used to increase the removal amount. However, the accuracy of larger grinding heads is limited. To solve this problem, scientists optimized the grinding head (polishing disc) and developed stress disc polishing technology to improve efficiency and accuracy.

• Stress Disk Polishing

Stress disk polishing technology is a technology that dynamically adjusts the shape of the polishing disk to match the surface shape of the workpiece. This technology controls the radial translation and rotation of the stress disk by computer, realizes dynamic deformation to adapt to the aspheric surface, and thus removes material stably. The flexibility of the polishing disk is used to apply uniform pressure on the surface of the aspheric or complex-shaped workpiece to ensure uniform material removal.

Compared with the traditional computer-controlled optical surface (CCOS) technology, stress disk polishing technology is more efficient in processing large-aperture optical aspheric surfaces and can better correct medium- and high-frequency errors and surface irregularities. It can effectively remove surface high points first, making the mirror surface naturally smooth within a large spatial frequency range. It is particularly suitable for the processing of large-aperture and high-precision optical components, which can significantly improve processing efficiency and surface quality. It is widely used in the manufacture of high-precision optical instruments such as astronomical telescopes and laser systems. However, this technology also requires a complex control system to adjust the bending moment and torque of the drive to ensure that the stress disk always fits the workpiece surface.

• Airbag Polishing

Air bag precision polish technology is a precision optical polishing method that uses the CCOS modification theory and an inflatable airbag as a flexible polishing tool. The principle is that by adjusting the air pressure inside the airbag, the airbag can change its surface profile with deformation during the polishing process to adapt to complex or irregular optical surfaces, so that the polishing head and the workpiece surface are highly fitted, thereby ensuring consistent material removal functions, improving surface roughness and controlling surface shape accuracy. By controlling the pressure and movement of the airbag, uniform material removal and high-precision surface processing can be achieved. This technology is widely used in the manufacture of high-precision optical components such as lenses, mirrors and reflectors.

• Wheel Polishing

It is often considered a part of Computer Controlled Optical Surface Forming (CCOS). Wheel polishing uses a rotating polishing wheel to remove material from the surface of an optical component to achieve the desired finish and shape. This technology is controlled by a computer to adjust the contact area, pressure and speed of the polishing wheel in a deterministic manner to ensure the accuracy and consistency of the removal process. It is widely used in the precision machining of optical lenses and other optical components.

In order to meet the needs of modern astronomy, military and other fields for large-caliber, high-precision optical components, computer-controlled surface shaping technology (CCOS) has been widely used and developed in the processing of precision optical components. From small-sized optical lenses to large astronomical telescopes, CCOS polishing provides a deterministic material removal technology for the manufacture of optical devices. In addition to some series of polishing processes based on CCOS theory, there are other ultra-precision polishing technologies on the market, such as magnetorheological polishing.

07 Magnetorheological Polishing（MRF）

Magnetorheological polishing (MRF) is a high-precision optical surface processing technology. It uses a polishing fluid containing magnetic particles to form a flexible and fluid polishing tool under the action of a magnetic field to achieve material removal and complete the precision polish and correction of the optical surface. Magnetorheological fluid contains micron-sized magnetic particles. Through computer control of the viscosity and pressure of the liquid, MRF can accurately polish the surface of optical components to achieve nano-level smoothness. It is suitable for optical components with complex shapes and realizes high-precision and flexible optical surface processing. It is widely used in the field of high-end optical manufacturing, mainly for the final polishing of optical glass and high-hardness materials.

Working principle: Magnetic particles, surfactants, and some other additives are dispersed in a certain proportion in a base carrier liquid to form a suspension. When a magnetic field is applied, the magnetic particles are arranged in a chain or fiber shape, resulting in an increase in the viscosity of the entire fluid, showing solid-like properties; when the magnetic field disappears, the magnetic particles return to the original dispersed free state, and the entire system returns to the state of fluid. The process of “liquid-solid transition” can be completed in milliseconds. When a certain amount of abrasive particles are added to the fluid, the magnetorheological fluid has the effect of grinding and polishing. Different base carrier fluids and abrasive particles can be selected according to the different material properties of the polishing workpiece.

Features:

• It can achieve extremely low roughness, nanometer-level surface finish and shape accuracy, and is generally used for polishing precision optical lenses;
• Since the entire workpiece is in the liquid, the polishing heat is easy to dissipate, and no thermal stress is introduced during the polishing process, which is suitable for the processing of high-precision optical components.
• All parts of the entire workpiece are in uniform contact with the fluid, which can ensure uniform polishing effect, and is not limited by the shape of the workpiece, and can accurately adapt to complex surface shapes.

However, there is no absolutely perfect precision surface finishing technology. In practical applications, magnetorheological polishing may encounter problems such as magnetic field control accuracy, magnetorheological fluid stability, and magnetic field design for polishing workpieces with complex shapes.

Conclusion

Precision polishing is a vital technology in optical processing, with 7-10 distinct processes offering unique technical advantages and application areas. Each method varies in cost and efficiency, making it essential to choose an optical manufacturer capable of employing multiple polishing services to achieve optimal quality, cost, and efficiency.

Polishing Methods	Cost	Applications
Conventional Asphalt polishing	$$	General optical applications
CNC Polishing	$$$	Optics of complex shapes and high precision.
Laser Polishing	$$$$	Precision machining of hard materials.
Robotic Polishing	$$$$	Components with large or complex geometries
IBF Polishing	$$$$$	High-precision and ultra-smooth surfaces
CCOS Polishing	$$$$	High-precision optical systems
MRF Polishing	$$$$$	Ultra-precision lenses and aspherical optics

Yudi Optics, a leading Chinese optical manufacturer, is renowned for its ultra-precision polishing service of aspheric and large-diameter optical components. We offer a comprehensive range of polishing technologies, allowing us to refine optical components from various materials, enhance performance by increasing reflectivity, reduce scattering losses, and achieve nanometer-level surface roughness.