rus
Issue #2/2025
E. M. Zakharevich, V. V. Lapshin, M. A. Shavva, R. A. Poshekhono, P. V. Volkov
Measuring System for Inspection of the Germanium Aspherical Lenses Using the UTM‑250 Ultra-Precision Machines
Measuring System for Inspection of the Germanium Aspherical Lenses Using the UTM‑250 Ultra-Precision Machines
DOI: 10.22184/1993-7296.FRos.2025.19.2.116.129
The article describes a homegrown embedded measuring system for the shape inspection of optical parts that can be used in the ultra-precision machines designed for diamond turning. The control system includes the contact and contactless sensors, a low-coherence interferometer, a signal converter and software that allows calculating any shape deviations of the spherical and aspherical surfaces and adjusting the CNC code to eliminate systematic shape deviations. The examples of using a contact sensor to increase the P-V shape accuracy of aspherical germanium lenses with a diameter of 90 mm from 1.4 μm to 100 nm after the systematic shape deviation compensation are provided. The possible application of a contactless sensor included in the measuring system to adjust an ultra-precision machine, as well as to control flat and cylindrical surfaces are described.
The article describes a homegrown embedded measuring system for the shape inspection of optical parts that can be used in the ultra-precision machines designed for diamond turning. The control system includes the contact and contactless sensors, a low-coherence interferometer, a signal converter and software that allows calculating any shape deviations of the spherical and aspherical surfaces and adjusting the CNC code to eliminate systematic shape deviations. The examples of using a contact sensor to increase the P-V shape accuracy of aspherical germanium lenses with a diameter of 90 mm from 1.4 μm to 100 nm after the systematic shape deviation compensation are provided. The possible application of a contactless sensor included in the measuring system to adjust an ultra-precision machine, as well as to control flat and cylindrical surfaces are described.
Теги: air pressure bearings diamond turning germanium aspherical lenses inspection of aspherical lens integrated measuring system optical parts processing ultra-precision machines алмазное точение асферические линзы из германия аэростатические подшипники встроенная измерительная система контроль асферических линз. обработка оптических деталей ультрапрецизионные станки
Measuring System for Inspection of the Germanium Aspherical Lenses Using the UTM‑250 Ultra-Precision Machines
E. M. Zakharevich 1, V. V. Lapshin 1, M. A. Shavva 1, R. A. Poshekhono 2, P. V. Volkov 3
Scientific and Production Association “Asferika” LLC, Moscow, Russia
Bauman Moscow State Technical University, Moscow, Russia
Research and Production Enterprise “TEOS” LLC, Nizhny Novgorod, Russia
The article describes a homegrown embedded measuring system for the shape inspection of optical parts that can be used in the ultra-precision machines designed for diamond turning. The control system includes the contact and contactless sensors, a low-coherence interferometer, a signal converter and software that allows calculating any shape deviations of the spherical and aspherical surfaces and adjusting the CNC code to eliminate systematic shape deviations. The examples of using a contact sensor to increase the P-V shape accuracy of aspherical germanium lenses with a diameter of 90 mm from 1.4 μm to 100 nm after the systematic shape deviation compensation are provided. The possible application of a contactless sensor included in the measuring system to adjust an ultra-precision machine, as well as to control flat and cylindrical surfaces are described.
Keywords: integrated measuring system, ultra-precision machines, air pressure bearings, diamond turning, optical parts processing, germanium aspherical lenses, inspection of aspherical lens.
Article received: January 21, 2025
Article accepted: February 20, 2025
Introduction
The shape accuracy of optical parts is characterized by the fractions of micrometers that determines a number of requirements for the design of ultra-precision diamond turning machines used for processing. Such machines shall be manufactured using the aero- or hydrostatic supports, ironless synchronous motors for spindles and linear movement supports, thermal stabilization and vibration isolation systems that reduce the influence of random factors on the shape of parts. However, there are factors leading to a systematic error in the part shape, the influence of which can be reduced by inspection of the shape of the processed parts and the CNC code adjustment.
For serial production, the part measurement and shape adjustment procedure shall be performed on the machine by the embedded part shape inspection facilities. The article [1] provides a review of machine measurement systems based on the atomic force microscopes, interferometers, confocal microscopy, holography, and contact pens. For example, the ultra-precision Nanoform machines for diamond turning of the optical parts manufactured by Precitech [2] have a built-in contact sensor with a differential displacement transducer (LVDT) and a rod on aerostatic supports that eliminate any friction when the rod is displaced and allow for measurement with a constant pressing force with a resolution of 0.1 nm. The pressing force of the measuring probe is maintained constant and can be set at a minimum of 0.1 grams.
The contact sensor developed by Lion Precision [3] has the similar design. It has a measurement range of 0.5 mm and a resolution of 1 nm and provides a contact force of 0.2 to 100 grams. To reduce wear and the friction coefficient when sliding along a part, the contact sensor has a diamond probe tip.
The Scientific and Production Association “Asferika” LLC processes the critical optical parts for various purposes, and also develops and manufactures the UTM ultra-precision machines for such processing by diamond turning, milling and grinding [4–5]. At present, the company has gained significant experience in the field of diamond turning of non-ferrous metals and alloys, various crystals (silicon, germanium, KPC, KDP, etc.), plastics, etc. The surface shape can vary from flat and cylindrical to spherical and aspherical.
To equip the UTM machines, the Scientific and Production Association “Asferika” LLC, together with the Research and Production Enterprise “TEOS” LLC, has developed a embedded measuring system based on a low-coherence interferometer to control the parts with a contact (Fig. 1a) and contactless (Fig. 1b) sensor that allows the shape verification procedure immediately after processing, without removing the workpiece. This measuring system is used to control the part shape, as well as to adjust the CNC program if it is necessary to perform repeated processing to improve the shape accuracy.
The contact sensor is designed to control the spherical and aspherical surfaces. The contactless sensor can be used to control cylindrical and flat surfaces with the optical surface quality, without damaging the treated surface.
The main specifications of the measuring system based on a low-coherence interferometer are given in Table 1.
1. Description of the contact sensor design
The contact sensor design includes a housing where two aerostatic supports with an independent air supply with the pressures P1 and P2 are built in, and a rod to which a measuring probe with a ruby tip is attached. When air is supplied to the aerostatic supports, the rod can slide freely with almost no friction, wear, play or frictional vibrations. The A and B sensors (Fig. 2) are used to control the axial and radial displacements of the rod. During measurement, the ruby ball rests against the part due to the supply of adjustable pressure PSUP into the cavity to be clamped. To regulate the radial rigidity at the point of contact with the part, the air supply pressure to one of the supports can be changed.
The rod displacement is measured using the tandem low-coherence interferometry (TLCI). The TLCI procedure implements a comparison scheme in which two interferometers connected in series are illuminated with light having a short coherence length. In this case, one of the interferometers is the measured object, and the second has the ability to controllably adjust the difference in arm lengths. The short coherence length of the light source (in our case, a superluminescent diode with a wavelength of 1310 nm and a spectrum width of about 60 nm) leads to the generation of a narrow interference peak when the optical delays in the reference and sensor interferometers coincide, the position of which can be determined with an accuracy of up to a few nanometers. As a result, TLCI allows for highly accurate remote measurement of the thickness of objects being two reflective quasi-parallel surfaces located at a certain distance from each other. In our case, the measured object is the gap between the polished end of a single-mode optical fiber and the rod, and the reference interferometer is implemented as a fully fiber device that ensures its stability and ability to quickly change the velocity and modulation range.
Thus, information from the A and B sensors is supplied to the converter, is digitized, transmitted to the computer. Information about the rod displacement in the axial and radial directions is displayed in the window of a special program.
2. Features of using a contact sensor to inspect the lenses
2.1. Bridging the Contact Sensor to the Machine Coordinate System
The contact sensor is installed on the X-axis travel carriage only for the measurement and is not located on the machine during the product processing. A rigid support has been developed for precise installation of the contact sensor, ensuring quick mounting with a locating error of no more than 1–2 μm. The support is installed on the X-axis carriage, next to the tool holder. If the machine is reconfigured for another type of parts, and the support is shifted on the carriage, then it is necessary to perform a sensor bridging that aligns the position of the ruby ball center with the spindle rotation axis. A digital microscope is used for preliminary bridging with an accuracy of 1 μm.
To bridge the sensor with an accuracy of about 100 nm, the scheme shown in Fig. 3 is applied. For this purpose, a ruby ball is brought to the treated surface from both sides at symmetrical points, and the contact sensor readings are registered. Based on the difference in readings of the contact sensor ΔUZ, it is possible to determine the bridging errors:
ΔX = ,
where β is the inclination angle of the surface normal to the Z axis. By changing the ball’s X reference in the machine’s CNC system, it is possible to achieve a difference in the readings ΔUZ of no more than 100 nm.
The sensor is bridged to the Z axis by touching the surface being processed.
2.2. Calibration of the contact sensor
Calibration of the contact sensor by measuring the reference sphere allows eliminating errors caused by the following factors:
deviations in shape (waviness) and size of the ruby ball;
elastic deformations of the probe and rod;
flexibility of the aerostatic supports of the rod.
When calibrating and measuring the parts on UTM‑250 ultra-precision machines, the errors related to the deviations from straightness and deviations from perpendicularity of the linear axes of the machine are not considered due to the fact that they have the values of about 20–30 nm over a stroke length of 100 mm.
Calibration is performed using a reference sphere with a shape accuracy of no worse than 50 nm the radius of which is specified in the technical data sheet. The reference sphere is mounted on the spindle of an ultra-precision machine, after which the reference sphere is measured. To do this, the machine reproduces the movement of the ruby ball center along an equidistant to the nominal surface of the reference item (Fig. 4 a, b), and the readings of the contact sensor UZ are demonstrated in the window of a special program.
Ideally, if the factors listed at the beginning of the section are absent, the sensor graph UZ(X) would show zero values. Based on the appearance of the graph UZ(X), it is possible to draw conclusions about the sources of error and adjust the measurement program. For example, the radius deviation of a ruby ball from the nominal value leads to a symmetrical graph UZ(X) in the form of an arc curved upward or downward.
The ball radius r can be determined by its nominal value rnom and the arrow of the arc S based on the graph UZ(X):
r = rnom − . (1)
By changing the ruby ball radius in the measurement program, it is possible to select the actual radius with an accuracy of up to 100–200 nm in several iterations. In this case, the sensor readings UZ(X) are reduced by eliminating the component responsible for the error in the measuring ball radius. Fig. 5 shows the calibration results of the contact sensor using a reference sphere with a radius of 20 mm. The abscissa axis of the graph demonstrates the movement of carriage with the sensor along the X axis, and the zero value coincides with the axis of the reference sphere. On the basis of Fig. 5 it is clear that the value of the systematic compensated error can be about 100 nm.
The calibration results of contact sensor are used later when measuring the machined surfaces and adjusting the CNC program.
2.3. Control and Correction of the Treated Surface Shape
2.3.1. Shape Errors That Can Be Compensated For
Processing of the parts with subsequent shape adjustment allows to eliminate any systematic errors that are caused by the following factors:
waviness of the cutting edge of the tool;
discrepancy between the actual value of the tool radius and the value specified in the CNC program;
bridging error of the tool to the machine coordinate system along the X axis;
availability of a negative rake angle at the cutter;
elastic deformations of the workpiece due to the impact of centrifugal forces, pinching forces and cutting forces;
elastic springing-away of the cutting tool.
Some types of errors have their own typical shape and can be found by the data graph appearance of the sensor UZ(X).
Thus, if during processing the actual radius of the cutting tool does not coincide with the radius specified in the processing program, then the graph UZ(X) shall have the shape of an arc curved either upward or downward that can be adjusted using the equation above (1).
If there is an error in bridging the tool radius center to the machine coordinate system along the X axis, the UZ(X) graph shall obtain either a V-shaped (Fig. 7.b) or an ω-shaped (Fig. 7a) form.
The cutting edge waviness also results in the profile of the machined parts being distorted. In addition, when turning the brittle materials (crystals, germanium, etc.), the best surface quality is achieved when using the diamond cutters with a negative rake angle. Thus, the cutting edge is the intersection of the rear surface (cone) and the inclined plane, and its projection onto the horizontal plane is an ellipse. Moreover, when generating the CNC program code, the cutting edge is described by a circle. The difference between an ellipse and a circle results in the machined surface distortion from the nominal profile that shall be repeated for each part.
Elastic deformations of the parts due to the impact of cutting forces and springing-away of the cutting tool can be excluded after the measurement process if the contact sensor pressing force is 1–2 orders of magnitude less than the cutting forces. For this purpose, the rod pressing force is adjusted from 0.5 to 20 grams by the pressing pressure PSUP (Fig. 2).
The elastic deformations of the workpiece due to the centrifugal forces are excluded, since the part measurement is performed without any rotation.
The elastic deformations due to the workpiece clamping can be reduced if a vacuum clamp is used and the workpiece is measured at the lowest possible vacuum.
Thus, due to the contact sensor and adjustment procedure, influence of the listed factors is prevented. However, when reinstalling or changing the tool, when changing the processing modes (the feed rate per revolution, the cutting depth, the rotation speed) or when changing the part clamping vacuum value, systematic errors are added up differently, and the adjustment procedure shall be repeated.
2.3.2. Processing of Aspherical Germanium Lenses Using the UTM‑250 Machine With Adjustment
The measurement of processed aspherical lens is performed using a contact sensor along an equidistant to the processed surface. The measurement circuit and the measurement process are shown in Fig. 6 a, b. The measurement result is a graph of deviation of the actual profile from the set one. The examples of measuring aspherical lenses processed with various cutters are shown in Fig. 7 a, b.
Figure 7 demonstrates the readings of contact sensor UZ depending on the movement of carriage with the sensor along the X axis. The zero value of X coincides with the rotation axis of the workpiece. After the first turning procedure, the profile deviation from the set one is more than 1 µm.
In order to eliminate the shape error, repeated turning of the parts was performed on the machine with the adjusted tool movement trajectory, performed using the GCode_correction software (SW) specially developed by Asferika. For this purpose, the software was loaded with calibration data, shape measurement data of the machined surface and the source code of the CNC program, after which the software generated a new code, according to which the re-machining was performed.
Main functions of G.Code_correction software:
elimination of the installation error influence of the contact sensor on the measurement results;
elimination of systematic errors of the measuring system determined during the sensor calibration procedure;
filtration of the noise and outlier data values of the sensor;
selection of the curvature parameter R0 and the conical constant k for the aspherical profile;
determination of the P-V parameters relative to the nominal profile or profile with the selected values of R0, k.
The examples of surface shapes obtained after the adjusted processing are shown in Fig. 7 a, b. It is evident that the accuracy of the P-V shape is improved from 1 μm to 0.23 μm (Fig. 7 a) and from 1.4 μm to 0.1 μm (Fig. 7 b).
During calibration and measurement, the measuring system settings shall match, namely the clamping pressure of the sensor to the part and the supply pressure to the aerostatic supports of the rod P1 and P2.
3. Application of a contactless sensor
The measuring system based on a low-coherence interferometer includes a sensor that allows measurements to be taken without touching the surface being processed. The sensor can operate within a precise range with a measurement frequency of 10 Hz at a distance of 1 mm ± 0.5 mm and 11 mm ± 0.5 mm from the controlled surface.
The contactless sensor can be used on a machine to control any flat and cylindrical surfaces of the workpieces, as well as to perform a procedure for fine adjustment of the mutual arrangement of machine units. Thus, while using a contactless sensor, it is possible to adjust the perpendicularity of linear axes of the machine, as well as to adjust the spindle axis parallel or perpendicular to the movement axis of the carriage on which it is installed.
The adjustment procedure consists of the following main stages:
Processing of the test part on a machine along the cylindrical or end surface.
Measurement of the treated surface with a contactless sensor.
Adjustment of the selected axis using the embedded devices.
Re-processing and measurement of the test part.
If necessary, repetition of steps 3–4 until the required accuracy level is achieved.
The contactless sensor can also be used for the following measuring operations on an ultra-precision machine (Fig. 8 a– c):
measurement of deviation from straightness of the flat or cylindrical surfaces;
measurement of deviation from the roundness and flatness values.
Information provided by the contactless sensor is also displayed on the screen using special software that is able to analyze the obtained shape deviation graphs. An example of measuring a cylindrical surface is shown in Fig. 9. More detailed examples of application of the contact and contactless sensors are given in the presentation [6].
Conclusions
The result of work performed is the development of a domestic measuring system for control and adjustment of the part shape with accuracy parameters at the level of foreign analogues.
The application of a contactless sensor on the UTM‑250 machine has made it possible to adjust the perpendicularity of linear axes to a value of 20–30 nm on a 100 mm base.
The developed measuring system for the parts inspection can be manufactured and supplied as an independent product for the ultra-precision and precision machines.
At present, Scientific and Production Association “Asferika” LLC is developing the measuring system with an additional radial B sensor (Fig. 2) that shall allow for the control of linear dimensions of the external and internal cylindrical surfaces. This sensor configuration shall allow for the development of a domestic analogue of contact sensors manufactured by such companies as Renishaw, Hexagon, Blum.
The application of a contact sensor on the UTM‑250 machine when processing germanium aspherical lenses has made it possible to achieve the best processing accuracy according to the P-V parameter of 100 nm.
AUTHORS
Zakharevich Evgeniy Mefodievich, chief technologist, “The Scientific and Production Association Aspherica” LLC, Moscow, Russia;
e-mail:.zaharev@gmail.com
ORCID: 0000-0001-6997-3335
Lapshin Vasilii Vladimirovich, senior engineer researcher, “The Scientific and Production Association Aspherica” LLC, Moscow, Russia; e-mail: lapshin_v@aspherica.ru.
ORCID: 0000-0002-6971-8534
Shavva Mariia Aleksandrovna, Cand.of Tech. Sciences, chief designer, “The Scientific and Production Association Aspherica” LLC, Moscow, Russia; e-mail: shavva_m@aspherica.ru
ORCID: 0000-0002-4676-2567
Poshekhonov Roman Aleksandrovich, Cand. of Tech. Sciences, Associate Professor, Bauman Moscow State Technical University (National Research University)”, Moscow, Russia; e-mail: poshekhonov_r@aspherica.ru
ORCID: 0000-0003-2188-7854
Volkov Petr Vitalevich, Cand. of Phys. and Math. Sciences, director, “Technological Electronic Optical Systems” LLC, N. Novgorod, Russia; e-mail: p.volkov.79@yandex.ru
ORCID: 0000-0003-2479-0716
E. M. Zakharevich 1, V. V. Lapshin 1, M. A. Shavva 1, R. A. Poshekhono 2, P. V. Volkov 3
Scientific and Production Association “Asferika” LLC, Moscow, Russia
Bauman Moscow State Technical University, Moscow, Russia
Research and Production Enterprise “TEOS” LLC, Nizhny Novgorod, Russia
The article describes a homegrown embedded measuring system for the shape inspection of optical parts that can be used in the ultra-precision machines designed for diamond turning. The control system includes the contact and contactless sensors, a low-coherence interferometer, a signal converter and software that allows calculating any shape deviations of the spherical and aspherical surfaces and adjusting the CNC code to eliminate systematic shape deviations. The examples of using a contact sensor to increase the P-V shape accuracy of aspherical germanium lenses with a diameter of 90 mm from 1.4 μm to 100 nm after the systematic shape deviation compensation are provided. The possible application of a contactless sensor included in the measuring system to adjust an ultra-precision machine, as well as to control flat and cylindrical surfaces are described.
Keywords: integrated measuring system, ultra-precision machines, air pressure bearings, diamond turning, optical parts processing, germanium aspherical lenses, inspection of aspherical lens.
Article received: January 21, 2025
Article accepted: February 20, 2025
Introduction
The shape accuracy of optical parts is characterized by the fractions of micrometers that determines a number of requirements for the design of ultra-precision diamond turning machines used for processing. Such machines shall be manufactured using the aero- or hydrostatic supports, ironless synchronous motors for spindles and linear movement supports, thermal stabilization and vibration isolation systems that reduce the influence of random factors on the shape of parts. However, there are factors leading to a systematic error in the part shape, the influence of which can be reduced by inspection of the shape of the processed parts and the CNC code adjustment.
For serial production, the part measurement and shape adjustment procedure shall be performed on the machine by the embedded part shape inspection facilities. The article [1] provides a review of machine measurement systems based on the atomic force microscopes, interferometers, confocal microscopy, holography, and contact pens. For example, the ultra-precision Nanoform machines for diamond turning of the optical parts manufactured by Precitech [2] have a built-in contact sensor with a differential displacement transducer (LVDT) and a rod on aerostatic supports that eliminate any friction when the rod is displaced and allow for measurement with a constant pressing force with a resolution of 0.1 nm. The pressing force of the measuring probe is maintained constant and can be set at a minimum of 0.1 grams.
The contact sensor developed by Lion Precision [3] has the similar design. It has a measurement range of 0.5 mm and a resolution of 1 nm and provides a contact force of 0.2 to 100 grams. To reduce wear and the friction coefficient when sliding along a part, the contact sensor has a diamond probe tip.
The Scientific and Production Association “Asferika” LLC processes the critical optical parts for various purposes, and also develops and manufactures the UTM ultra-precision machines for such processing by diamond turning, milling and grinding [4–5]. At present, the company has gained significant experience in the field of diamond turning of non-ferrous metals and alloys, various crystals (silicon, germanium, KPC, KDP, etc.), plastics, etc. The surface shape can vary from flat and cylindrical to spherical and aspherical.
To equip the UTM machines, the Scientific and Production Association “Asferika” LLC, together with the Research and Production Enterprise “TEOS” LLC, has developed a embedded measuring system based on a low-coherence interferometer to control the parts with a contact (Fig. 1a) and contactless (Fig. 1b) sensor that allows the shape verification procedure immediately after processing, without removing the workpiece. This measuring system is used to control the part shape, as well as to adjust the CNC program if it is necessary to perform repeated processing to improve the shape accuracy.
The contact sensor is designed to control the spherical and aspherical surfaces. The contactless sensor can be used to control cylindrical and flat surfaces with the optical surface quality, without damaging the treated surface.
The main specifications of the measuring system based on a low-coherence interferometer are given in Table 1.
1. Description of the contact sensor design
The contact sensor design includes a housing where two aerostatic supports with an independent air supply with the pressures P1 and P2 are built in, and a rod to which a measuring probe with a ruby tip is attached. When air is supplied to the aerostatic supports, the rod can slide freely with almost no friction, wear, play or frictional vibrations. The A and B sensors (Fig. 2) are used to control the axial and radial displacements of the rod. During measurement, the ruby ball rests against the part due to the supply of adjustable pressure PSUP into the cavity to be clamped. To regulate the radial rigidity at the point of contact with the part, the air supply pressure to one of the supports can be changed.
The rod displacement is measured using the tandem low-coherence interferometry (TLCI). The TLCI procedure implements a comparison scheme in which two interferometers connected in series are illuminated with light having a short coherence length. In this case, one of the interferometers is the measured object, and the second has the ability to controllably adjust the difference in arm lengths. The short coherence length of the light source (in our case, a superluminescent diode with a wavelength of 1310 nm and a spectrum width of about 60 nm) leads to the generation of a narrow interference peak when the optical delays in the reference and sensor interferometers coincide, the position of which can be determined with an accuracy of up to a few nanometers. As a result, TLCI allows for highly accurate remote measurement of the thickness of objects being two reflective quasi-parallel surfaces located at a certain distance from each other. In our case, the measured object is the gap between the polished end of a single-mode optical fiber and the rod, and the reference interferometer is implemented as a fully fiber device that ensures its stability and ability to quickly change the velocity and modulation range.
Thus, information from the A and B sensors is supplied to the converter, is digitized, transmitted to the computer. Information about the rod displacement in the axial and radial directions is displayed in the window of a special program.
2. Features of using a contact sensor to inspect the lenses
2.1. Bridging the Contact Sensor to the Machine Coordinate System
The contact sensor is installed on the X-axis travel carriage only for the measurement and is not located on the machine during the product processing. A rigid support has been developed for precise installation of the contact sensor, ensuring quick mounting with a locating error of no more than 1–2 μm. The support is installed on the X-axis carriage, next to the tool holder. If the machine is reconfigured for another type of parts, and the support is shifted on the carriage, then it is necessary to perform a sensor bridging that aligns the position of the ruby ball center with the spindle rotation axis. A digital microscope is used for preliminary bridging with an accuracy of 1 μm.
To bridge the sensor with an accuracy of about 100 nm, the scheme shown in Fig. 3 is applied. For this purpose, a ruby ball is brought to the treated surface from both sides at symmetrical points, and the contact sensor readings are registered. Based on the difference in readings of the contact sensor ΔUZ, it is possible to determine the bridging errors:
ΔX = ,
where β is the inclination angle of the surface normal to the Z axis. By changing the ball’s X reference in the machine’s CNC system, it is possible to achieve a difference in the readings ΔUZ of no more than 100 nm.
The sensor is bridged to the Z axis by touching the surface being processed.
2.2. Calibration of the contact sensor
Calibration of the contact sensor by measuring the reference sphere allows eliminating errors caused by the following factors:
deviations in shape (waviness) and size of the ruby ball;
elastic deformations of the probe and rod;
flexibility of the aerostatic supports of the rod.
When calibrating and measuring the parts on UTM‑250 ultra-precision machines, the errors related to the deviations from straightness and deviations from perpendicularity of the linear axes of the machine are not considered due to the fact that they have the values of about 20–30 nm over a stroke length of 100 mm.
Calibration is performed using a reference sphere with a shape accuracy of no worse than 50 nm the radius of which is specified in the technical data sheet. The reference sphere is mounted on the spindle of an ultra-precision machine, after which the reference sphere is measured. To do this, the machine reproduces the movement of the ruby ball center along an equidistant to the nominal surface of the reference item (Fig. 4 a, b), and the readings of the contact sensor UZ are demonstrated in the window of a special program.
Ideally, if the factors listed at the beginning of the section are absent, the sensor graph UZ(X) would show zero values. Based on the appearance of the graph UZ(X), it is possible to draw conclusions about the sources of error and adjust the measurement program. For example, the radius deviation of a ruby ball from the nominal value leads to a symmetrical graph UZ(X) in the form of an arc curved upward or downward.
The ball radius r can be determined by its nominal value rnom and the arrow of the arc S based on the graph UZ(X):
r = rnom − . (1)
By changing the ruby ball radius in the measurement program, it is possible to select the actual radius with an accuracy of up to 100–200 nm in several iterations. In this case, the sensor readings UZ(X) are reduced by eliminating the component responsible for the error in the measuring ball radius. Fig. 5 shows the calibration results of the contact sensor using a reference sphere with a radius of 20 mm. The abscissa axis of the graph demonstrates the movement of carriage with the sensor along the X axis, and the zero value coincides with the axis of the reference sphere. On the basis of Fig. 5 it is clear that the value of the systematic compensated error can be about 100 nm.
The calibration results of contact sensor are used later when measuring the machined surfaces and adjusting the CNC program.
2.3. Control and Correction of the Treated Surface Shape
2.3.1. Shape Errors That Can Be Compensated For
Processing of the parts with subsequent shape adjustment allows to eliminate any systematic errors that are caused by the following factors:
waviness of the cutting edge of the tool;
discrepancy between the actual value of the tool radius and the value specified in the CNC program;
bridging error of the tool to the machine coordinate system along the X axis;
availability of a negative rake angle at the cutter;
elastic deformations of the workpiece due to the impact of centrifugal forces, pinching forces and cutting forces;
elastic springing-away of the cutting tool.
Some types of errors have their own typical shape and can be found by the data graph appearance of the sensor UZ(X).
Thus, if during processing the actual radius of the cutting tool does not coincide with the radius specified in the processing program, then the graph UZ(X) shall have the shape of an arc curved either upward or downward that can be adjusted using the equation above (1).
If there is an error in bridging the tool radius center to the machine coordinate system along the X axis, the UZ(X) graph shall obtain either a V-shaped (Fig. 7.b) or an ω-shaped (Fig. 7a) form.
The cutting edge waviness also results in the profile of the machined parts being distorted. In addition, when turning the brittle materials (crystals, germanium, etc.), the best surface quality is achieved when using the diamond cutters with a negative rake angle. Thus, the cutting edge is the intersection of the rear surface (cone) and the inclined plane, and its projection onto the horizontal plane is an ellipse. Moreover, when generating the CNC program code, the cutting edge is described by a circle. The difference between an ellipse and a circle results in the machined surface distortion from the nominal profile that shall be repeated for each part.
Elastic deformations of the parts due to the impact of cutting forces and springing-away of the cutting tool can be excluded after the measurement process if the contact sensor pressing force is 1–2 orders of magnitude less than the cutting forces. For this purpose, the rod pressing force is adjusted from 0.5 to 20 grams by the pressing pressure PSUP (Fig. 2).
The elastic deformations of the workpiece due to the centrifugal forces are excluded, since the part measurement is performed without any rotation.
The elastic deformations due to the workpiece clamping can be reduced if a vacuum clamp is used and the workpiece is measured at the lowest possible vacuum.
Thus, due to the contact sensor and adjustment procedure, influence of the listed factors is prevented. However, when reinstalling or changing the tool, when changing the processing modes (the feed rate per revolution, the cutting depth, the rotation speed) or when changing the part clamping vacuum value, systematic errors are added up differently, and the adjustment procedure shall be repeated.
2.3.2. Processing of Aspherical Germanium Lenses Using the UTM‑250 Machine With Adjustment
The measurement of processed aspherical lens is performed using a contact sensor along an equidistant to the processed surface. The measurement circuit and the measurement process are shown in Fig. 6 a, b. The measurement result is a graph of deviation of the actual profile from the set one. The examples of measuring aspherical lenses processed with various cutters are shown in Fig. 7 a, b.
Figure 7 demonstrates the readings of contact sensor UZ depending on the movement of carriage with the sensor along the X axis. The zero value of X coincides with the rotation axis of the workpiece. After the first turning procedure, the profile deviation from the set one is more than 1 µm.
In order to eliminate the shape error, repeated turning of the parts was performed on the machine with the adjusted tool movement trajectory, performed using the GCode_correction software (SW) specially developed by Asferika. For this purpose, the software was loaded with calibration data, shape measurement data of the machined surface and the source code of the CNC program, after which the software generated a new code, according to which the re-machining was performed.
Main functions of G.Code_correction software:
elimination of the installation error influence of the contact sensor on the measurement results;
elimination of systematic errors of the measuring system determined during the sensor calibration procedure;
filtration of the noise and outlier data values of the sensor;
selection of the curvature parameter R0 and the conical constant k for the aspherical profile;
determination of the P-V parameters relative to the nominal profile or profile with the selected values of R0, k.
The examples of surface shapes obtained after the adjusted processing are shown in Fig. 7 a, b. It is evident that the accuracy of the P-V shape is improved from 1 μm to 0.23 μm (Fig. 7 a) and from 1.4 μm to 0.1 μm (Fig. 7 b).
During calibration and measurement, the measuring system settings shall match, namely the clamping pressure of the sensor to the part and the supply pressure to the aerostatic supports of the rod P1 and P2.
3. Application of a contactless sensor
The measuring system based on a low-coherence interferometer includes a sensor that allows measurements to be taken without touching the surface being processed. The sensor can operate within a precise range with a measurement frequency of 10 Hz at a distance of 1 mm ± 0.5 mm and 11 mm ± 0.5 mm from the controlled surface.
The contactless sensor can be used on a machine to control any flat and cylindrical surfaces of the workpieces, as well as to perform a procedure for fine adjustment of the mutual arrangement of machine units. Thus, while using a contactless sensor, it is possible to adjust the perpendicularity of linear axes of the machine, as well as to adjust the spindle axis parallel or perpendicular to the movement axis of the carriage on which it is installed.
The adjustment procedure consists of the following main stages:
Processing of the test part on a machine along the cylindrical or end surface.
Measurement of the treated surface with a contactless sensor.
Adjustment of the selected axis using the embedded devices.
Re-processing and measurement of the test part.
If necessary, repetition of steps 3–4 until the required accuracy level is achieved.
The contactless sensor can also be used for the following measuring operations on an ultra-precision machine (Fig. 8 a– c):
measurement of deviation from straightness of the flat or cylindrical surfaces;
measurement of deviation from the roundness and flatness values.
Information provided by the contactless sensor is also displayed on the screen using special software that is able to analyze the obtained shape deviation graphs. An example of measuring a cylindrical surface is shown in Fig. 9. More detailed examples of application of the contact and contactless sensors are given in the presentation [6].
Conclusions
The result of work performed is the development of a domestic measuring system for control and adjustment of the part shape with accuracy parameters at the level of foreign analogues.
The application of a contactless sensor on the UTM‑250 machine has made it possible to adjust the perpendicularity of linear axes to a value of 20–30 nm on a 100 mm base.
The developed measuring system for the parts inspection can be manufactured and supplied as an independent product for the ultra-precision and precision machines.
At present, Scientific and Production Association “Asferika” LLC is developing the measuring system with an additional radial B sensor (Fig. 2) that shall allow for the control of linear dimensions of the external and internal cylindrical surfaces. This sensor configuration shall allow for the development of a domestic analogue of contact sensors manufactured by such companies as Renishaw, Hexagon, Blum.
The application of a contact sensor on the UTM‑250 machine when processing germanium aspherical lenses has made it possible to achieve the best processing accuracy according to the P-V parameter of 100 nm.
AUTHORS
Zakharevich Evgeniy Mefodievich, chief technologist, “The Scientific and Production Association Aspherica” LLC, Moscow, Russia;
e-mail:.zaharev@gmail.com
ORCID: 0000-0001-6997-3335
Lapshin Vasilii Vladimirovich, senior engineer researcher, “The Scientific and Production Association Aspherica” LLC, Moscow, Russia; e-mail: lapshin_v@aspherica.ru.
ORCID: 0000-0002-6971-8534
Shavva Mariia Aleksandrovna, Cand.of Tech. Sciences, chief designer, “The Scientific and Production Association Aspherica” LLC, Moscow, Russia; e-mail: shavva_m@aspherica.ru
ORCID: 0000-0002-4676-2567
Poshekhonov Roman Aleksandrovich, Cand. of Tech. Sciences, Associate Professor, Bauman Moscow State Technical University (National Research University)”, Moscow, Russia; e-mail: poshekhonov_r@aspherica.ru
ORCID: 0000-0003-2188-7854
Volkov Petr Vitalevich, Cand. of Phys. and Math. Sciences, director, “Technological Electronic Optical Systems” LLC, N. Novgorod, Russia; e-mail: p.volkov.79@yandex.ru
ORCID: 0000-0003-2479-0716
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