rus
Issue #1/2014
V.Gorokhov, E.Zakharevich, M.Skvortsova
Accuracy Improvement of Metal-Optical Parts during Diamond Turning on Ultra-Precision Equipment
Accuracy Improvement of Metal-Optical Parts during Diamond Turning on Ultra-Precision Equipment
The results of experimental activities on the development of control methods of metal-optical part surface shapes formed on the basis of diamond turning method on 3-axial ultra-precision turning machine are presented.
Теги: metal optics smoothness measurement surface profile control измерение шероховатости контроль профиля поверхности металлооптика
Diamond turning method for the production of metal-optical components (mirrors, reflector prisms) has a number of advantages in comparison with the traditional methods of optical surfaces making by the lapping methods. These advantages include the following:
higher productivity (it increases by 5 to 50 times);
absence of impregnated layer on the processing surface;
capability to obtain parts with different shapes (polynomial up to the 33rd order);
higher accuracy especially for the parts with irregular shape (PV (Peak to Valley) 0.1 µm per ∅ 100 mm);
capability of separate control of shape accuracy and roughness value of the part surface.
In general, diamond turning method is more advanced than the lapping and it is actively implemented in leading industrially developed countries. This technology is restrained in the Russian Federation due to the absence of the special ultra-precision equipment and absence of the software for quality assurance during the diamond turning. In order to achieve high quality of images obtained with the help of metal-optical elements, it is necessary to obtain the set shape of optical surface of spherical and aspherical metal-optical elements with the effective roughness within the range Ra of 0.2 µm to 1 µm depending on the operating wavelength. Traditional technologies of the deep grinding and polishing ensure roughness Ra of more than 1.5 µm. During the experimental research activities restrictions of the traditional methods of roughness and surface shape measurements are found, developed sensor ensures the solution of technical tasks at the quality level specified for the metal-optical elements for imaging.
The method of monitoring and control of shape accuracy and roughness of metal-optical parts was developed using the contactless optical sensor. It allowed measuring the shape precision and surface roughness compliance with the set parameters. Optical sensor is fixed on the machine which makes it possible to control the processing surface not changing the operating position of the component. NC data of the machine is constantly corrected by adjustments which follow the analysis of measurement results of the processing surface parameters. Iterations chain will not stop until the required results are obtained. This procedure is intended for the improvement of part accuracy.
Operation of the measuring sensor is illustrated in Fig. 1. Lighting unit with red light-emitting diode lamp forms the measuring spot on the controlled surface. It exposes to light the major part of processing area. Therefore, the light scattered upon the reflection and focused with the help of objective lens on the photodiode rule carries the integral characteristic of the surface quality. Then, the signal arrives to microprocessor for the further processing and formation of control signal. Undoubtedly, such measurement principle has several advantages: first of all, evaluation of the total surface area is integral; secondly, vibrations do not influence on the measurement results; thirdly, the method is reliable when operating under the strict production conditions.
Physical basis of the measurement method is widely known: indicatrix parameters of the light reflected from the surface – scattering angle and intensity spatial distribution – depend on the surface waviness (Fig. 2). These two parameters are analyzed when the scattered radiation gets on photodetector matrix. Deviations of the geometric shape and waviness are determined on the basis of macro-shape angle deviation (Fig.3). Measurement procedure of the surface roughness includes the following: parallel light beam forms the light spot with the diameter of 0.01 to 0.9 mm (depending on the sensor configuration) on the controlled surface.
When summing up the angles on the total measured surface(Fig.4), the surface geometric shape is obtained (Fig.5). Deviations of the set surface shape can be measured on the surfaces well-reflecting the visible radiation with Rz of 1.6 and higher (Fig.6).
The quality enhancement of the metal-optical elements production performed on aluminum cylindrical samples was confirmed experimentally; non-roundness of the produced parts did not exceed 0.08 µm. The results also were verified using other methods based on different physical principles.
higher productivity (it increases by 5 to 50 times);
absence of impregnated layer on the processing surface;
capability to obtain parts with different shapes (polynomial up to the 33rd order);
higher accuracy especially for the parts with irregular shape (PV (Peak to Valley) 0.1 µm per ∅ 100 mm);
capability of separate control of shape accuracy and roughness value of the part surface.
In general, diamond turning method is more advanced than the lapping and it is actively implemented in leading industrially developed countries. This technology is restrained in the Russian Federation due to the absence of the special ultra-precision equipment and absence of the software for quality assurance during the diamond turning. In order to achieve high quality of images obtained with the help of metal-optical elements, it is necessary to obtain the set shape of optical surface of spherical and aspherical metal-optical elements with the effective roughness within the range Ra of 0.2 µm to 1 µm depending on the operating wavelength. Traditional technologies of the deep grinding and polishing ensure roughness Ra of more than 1.5 µm. During the experimental research activities restrictions of the traditional methods of roughness and surface shape measurements are found, developed sensor ensures the solution of technical tasks at the quality level specified for the metal-optical elements for imaging.
The method of monitoring and control of shape accuracy and roughness of metal-optical parts was developed using the contactless optical sensor. It allowed measuring the shape precision and surface roughness compliance with the set parameters. Optical sensor is fixed on the machine which makes it possible to control the processing surface not changing the operating position of the component. NC data of the machine is constantly corrected by adjustments which follow the analysis of measurement results of the processing surface parameters. Iterations chain will not stop until the required results are obtained. This procedure is intended for the improvement of part accuracy.
Operation of the measuring sensor is illustrated in Fig. 1. Lighting unit with red light-emitting diode lamp forms the measuring spot on the controlled surface. It exposes to light the major part of processing area. Therefore, the light scattered upon the reflection and focused with the help of objective lens on the photodiode rule carries the integral characteristic of the surface quality. Then, the signal arrives to microprocessor for the further processing and formation of control signal. Undoubtedly, such measurement principle has several advantages: first of all, evaluation of the total surface area is integral; secondly, vibrations do not influence on the measurement results; thirdly, the method is reliable when operating under the strict production conditions.
Physical basis of the measurement method is widely known: indicatrix parameters of the light reflected from the surface – scattering angle and intensity spatial distribution – depend on the surface waviness (Fig. 2). These two parameters are analyzed when the scattered radiation gets on photodetector matrix. Deviations of the geometric shape and waviness are determined on the basis of macro-shape angle deviation (Fig.3). Measurement procedure of the surface roughness includes the following: parallel light beam forms the light spot with the diameter of 0.01 to 0.9 mm (depending on the sensor configuration) on the controlled surface.
When summing up the angles on the total measured surface(Fig.4), the surface geometric shape is obtained (Fig.5). Deviations of the set surface shape can be measured on the surfaces well-reflecting the visible radiation with Rz of 1.6 and higher (Fig.6).
The quality enhancement of the metal-optical elements production performed on aluminum cylindrical samples was confirmed experimentally; non-roundness of the produced parts did not exceed 0.08 µm. The results also were verified using other methods based on different physical principles.
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