rus
Issue #1/2021
V. V. Lapshin, E. M. Zakharevich, M. S. Kuznetsov, K. S. Zaramenskikh, A. V. Osipov
Technology of Machining Optical Parts Made of KRS‑5 Crystals by Diamond Turning and Milling
Technology of Machining Optical Parts Made of KRS‑5 Crystals by Diamond Turning and Milling
DOI: 10.22184/1993-7296.FRos.2021.15.1.18.28
The article concerns the development of a technology for machining workpieces from KRS‑5 crystals. The optimal machining method, as well as the method for determining the optimal orientation of the workpiece, was experimentally determined. The developed technology was tested in the manufacture of a beam-splitting ring. The results of metrological control of the ring, as well as ways of improving the quality and accuracy of machining are presented.
The article concerns the development of a technology for machining workpieces from KRS‑5 crystals. The optimal machining method, as well as the method for determining the optimal orientation of the workpiece, was experimentally determined. The developed technology was tested in the manufacture of a beam-splitting ring. The results of metrological control of the ring, as well as ways of improving the quality and accuracy of machining are presented.
Теги: diamond cutter diamond milling diamond turning krs‑5 monocrystals optical components ultra-precision machine tools алмазное точение алмазное фрезерование алмазный резец монокристаллы крс‑5 оптические детали ультрапрецизионные станки
Technology of Machining Optical Parts Made of KRS‑5 Crystals by Diamond Turning and Milling
V. V. Lapshin1, E. M. Zakharevich1, M. S. Kuznetsov2, K. S. Zaramenskikh2, A. V. Osipov3
Research and Production Enterprise Machine-Tool Plant “Tulamash”, LLC
State Research and Design Institute of Rare Metal Industry “Giredmet”, JSC
Research and Production Company “MacroOptics”, LLC
The article concerns the development of a technology for machining workpieces from KRS‑5 crystals. The optimal machining method, as well as the method for determining the optimal orientation of the workpiece, was experimentally determined. The developed technology was tested in the manufacture of a beam-splitting ring. The results of metrological control of the ring, as well as ways of improving the quality and accuracy of machining are presented.
Keywords: diamond cutter, diamond milling, diamond turning, optical components, ultra-precision machine tools, KRS‑5 monocrystals
Received on: 05.01.2021
Accepted on: 18.02.2021
Introduction
Optical instrumentation of the infrared (IR) range is an important direction in the development of advanced branches of technology, including in connection with the increasing requirements for spectrometric research in the infrared range at enterprises of the oil and gas and pharmaceutical industries. The creation of optical devices is impossible without such elements as windows, lenses, prisms, mirrors, beam splitters, etc., for the manufacture of which optical materials with a complex of mechanical, chemical, optical properties that meet a set of specific requirements must be investigated and manufactured.
One of the most promising optical materials are thallium halide crystals with uniform transparency in a very wide wavelength range, covering the visible and mid-infrared spectral regions from 0.35 to 50 μm (depending on the composition), 0.54 to 50 μm for KRS‑5 (TlBr –TlI). The transmission is up to 70% in the absence of absorption bands. The magnitude of the optical loss is determined by the reflectance; accordingly, when applying an antireflection coating, absolute transmission can be achieved. Crystals have mechanical, vibration, chemical and moisture resistance, making them suitable for work in atmospheric conditions without special protection [1–3].
The combination of all the above characteristics makes it possible to improve the properties of equipment in comparison with existing analogues in the range up to 10 microns, as well as to create devices operating in the range from 10 to 50 microns, which have no analogues, using KRS‑5 crystals.
The main problem is the complexity of optical machining of thallium halide crystals due to the softness and plasticity of the material, high coefficient of thermal expansion, low thermal conductivity, and high anisotropy. Conventional machining technologies lead to the formation of a deep damaged layer of the crystal structure during grinding and a low yield to usable when finishing the optical surface by hand polishing.
In connection with the above disadvantages of conventional methods of machining crystals, it is proposed as an alternative way to use diamond machining with cutting tool on ultra-precision machines, which will avoid the formation of a significant damaged layer, charge of the treated surface with abrasive particles, as well as high machining temperatures. Also, diamond machining of products from KRS‑5, carried out without coolant in a sealed cabinet, will allow collecting chips that can be reused when growing crystals. Diamond machining will ensure the required quality and precision of the machined surface, as well as significantly increase productivity. As a rule, the surface roughness of optical products made from KRS‑5 is Ra 0.01 μm, and the accuracy depends on the shape and purpose of the products.
Thus, the following technological tasks for the machining of KRS‑5 crystals have been set: to work out the optimal modes of diamond machining; work out a way to determine the optimal crystal orientation; determination of the optimal way of basing the workpiece.
Technological unit description
Pilot machining of KRS‑5 crystals was carried out at the pilot production facility of Research and Production Enterprise Machine-Tool Plant “Tulamash”, LLC on an ultra-precision machine tool (Fig. 1), which has the following design features:
The machine tool offers the possibility of machining KRS crystals by fly cutting, as well as by turning. To carry out research, special equipment was designed and manufactured, which is necessary for fixing the workpiece. The equipment consists of a vacuum faceplate and a retaining ring.
Before conducting research on the machine tool, a number of preparatory operations were carried out:
We used diamond cutters with radius r 1 mm and 5 mm (Fig. 2) with a zero rake angle as a cutting tool in the research. A special holder has been developed for fastening diamond cutters, which is installed in the body of a single-cutter head.
The base surface of the vacuum faceplate was also machined with a diamond cutter after all the preparatory operations were completed.
Technology
of machining KRS‑5 products
Preliminary, the tests on machining a cylindrical KRS‑5 workpiece were carried out according to a turning scheme (the workpiece rotates around its axis, the feed motion is communicated to the tool). Machining of the end face of the KRS‑5 specimen showed that the crystallographic orientation affects the surface quality, since alternation of brittle fracture zones and zones where no defects were observed on the surface were visually observed on the workpiece (Fig. 3). Due to the fact that changing the machining modes did not lead to the elimination of defective zones, it was decided to abandon the turning scheme and proceed to milling, since milling will take into account the position of the workpiece relative to the tool path.
Taking into account the revealed peculiarities of machining KRS‑5 crystals, a technique for machining crystals by the method of diamond milling was developed and implemented, consisting of two stages:
The first stage, where the optimal orientation of the workpiece is determined, was implemented on an ultra-precision machine tool according to the scheme shown in Fig. 4. A single-cutter head with a diamond cutter with a radius of r = 1 mm was installed on the tool spindle. The faceplate with the workpiece was installed on the product spindle. During machining, the workpiece made rotational movements around its axis. The path of the diamond cutter passed through the axis of rotation of the workpiece.
Machining was carried out in the following modes: tool spindle rotation frequency of 500 rpm, workpiece rotation frequency of 0.5 rpm, cutting depth t = 10 μm. After machining, brittle fracture zones formed on the workpiece, which could be detected with the naked eye (Fig. 5).
From the analysis of the obtained picture of the location of brittle fracture zones and knowing the path of the diamond tool, it is possible to orient the part in an angular position so that only plastic removal of material is provided during machining, without brittle fracture.
The second stage consists in machining a correctly oriented part according to the scheme shown in Fig. 6. Unlike the scheme in which the location of fragile zones was determined, the workpiece does not rotate, but moves relative to the milling head with a feed motion.
Prior machining in the following modes eliminated traces of brittle fracture obtained on the workpiece during the test: tool spindle speed 1 000 rpm, workpiece feed rate 12 mm / min, cutting depth 50 μm. To eliminate all traces of brittle fracture, it was necessary to remove an allowance of more than 0.25 mm from the workpiece. Further, the machining was carried out in finishing modes with a diamond cutter with a radius of 5 mm. Finishing modes of machining, which obtained the best surface quality, are as follows: tool spindle speed 1 000 rpm, workpiece feed rate 12 mm / min, depth of cut t = 3 μm.
There were no traces of brittle fracture on the treated surface, which confirmed the correct orientation of the workpiece, determined at the first stage, as well as the correctness of the machining technique used. The surface roughness measured with a profilometer was Ra 0.01 μm.
According to the developed technique described above, the end surfaces of the beam splitting ring made of KRS‑5, which is a ring with an outer diameter of 60 mm, an inner diameter of 36 mm, and a thickness of 6 mm, were machined. The following accuracy and quality requirements are imposed on the product: surface roughness Ra 0.01 μm, shape accuracy N = 0.5, local error ΔN = 0.2, wedging of less than 2 arc seconds. Beam-splitting rings are used in Fourier-transform spectrometers and are currently made from zinc selenide, the disadvantage of which is water solubility. KRS‑5 crystals are moisture resistant, which will help improve the characteristics of Fourier spectrometers.
The sequence of machining the end surfaces of the ring consisted of the following stages:
Rough machining of the workpiece from one of the sides in order to eliminate defects in the preliminary machining.
Determination of the optimal orientation of the workpiece according to the scheme in Fig. 4
Orientation of the workpiece to the optimal position and roughing in order to eliminate traces of brittle fracture.
Machining the workpiece in finishing modes according to the scheme in Fig. 6.
Turning the workpiece over and machining the second end surface by repeating points 1–4.
Turning the workpiece over and finishing the first surface.
Depending on the required shape accuracy, the number of workpiece turns can be increased. This is done to eliminate technological heredity, which can lead to distortion of the product shape.
A number of test samples were machined by the method described above. A photo of the ring machining is shown in Fig. 7.
Metrological control of the shape of the treated surfaces was carried out on an OWI150 HP XT interferometer with a TF 6" λ / 20 Zygo Corp flat optics objective lens by Zygo Corp. The working wavelength of the interferometer is 632.8 nm.
Fig. 8 shows the measurement of the surface shape of the best sample. The shape accuracy was N = 4.5 (1.239 µm), ΔN = 2.1 (0.678 µm), the wedging was 6.9 arc seconds. As can be seen from the results of the interferogram, the machined workpiece has the shape of a saddle. The rest of the machined blanks had the same saddle shape.
In order to search for the reasons for the formation of a saddle-shaped surface, a copper ring with dimensions matching the dimensions of a ring made of KRS‑5 was machined on the machine tool. The modes and scheme of machining were the same as when machining crystals, with the exception of the need to identify the correct orientation of the workpiece. Research on the machining of copper rings showed no apparent saddle-like shape of the workpieces, as well as a significant improvement in shape accuracy compared to crystal machining, N = 1.47 versus N = 4.5.
The required accuracy of the ring shape cannot be ensured on the machine tool due to its design features: low rigidity, lack of a mechanism for precise alignment of the product spindle axis in the vertical plane.
Conclusions
The studies carried out made it possible to understand the features of machining and the physics of the cutting of KRS‑5 crystals, while it was possible to find out that the shape accuracy is associated with the anisotropy of the KRS‑5 crystals and the change in hardness in different directions, as well as the unequal cutting conditions, since the workpiece has a round shape.
The tests carried out on beam-splitting rings made of KRS‑5 made it possible to work out the machining technology, as well as select the optimal cutting conditions. The achieved parameters of accuracy and quality (N = 4.5, Ra = 0.01 μm) of the surface are sufficient for standard optical products (for example, lenses and windows). However, modern optical instruments place higher demands on the quality and accuracy of optical surfaces.
To improve the accuracy and quality of machining, one will need to:
The proposed method for machining crystals KRS‑5 will increase the productivity of manufacturing optical products while ensuring the required accuracy and quality. The manufacture of ultra-precision machine tools for similar tasks in the Russian Federation is carried out by Research and Production Enterprise Machine-Tool Plant “Tulamash”, LLC.
Conflict of interest
The authors declare no conflicts of interest.
Authors
Lapshin V. V., corresponding author, lapshin_v@cnc-tulamash.ru, Research and Production Enterprise Machine-Tool Plant “Tulamash”, LLC. Tula, Russia. https://cnc-tulamash.ru.
ORCID: 0000-0002-6971-8534
Zakharevich E. M., Research and Production Enterprise Machine-Tool Plant “Tulamash”, LLC; https://cnc-tulamash.ru; Tula, Russia.
ORCID: 0000-0001-6997-3335
Kuznetsov M. S., State Research and Design Institute of Rare Metal Industry “Giredmet”, JSC; https://giredmet.inni.info; Moscow, Russia.
ORCID: 0000-0002-8441-4424
Zaramenskikh K. S., State Research and Design Institute of Rare Metal Industry “Giredmet”, JSC; https://giredmet.inni.info; Moscow, Russia.
ORCID: 0000-0001-8573-3470
Osipov A. V., Research and Production Company “MacroOptics”, LLC; https://macrooptica.ru; Moscow, Russia.
ORCID: 0000-0002-8847-9428
V. V. Lapshin1, E. M. Zakharevich1, M. S. Kuznetsov2, K. S. Zaramenskikh2, A. V. Osipov3
Research and Production Enterprise Machine-Tool Plant “Tulamash”, LLC
State Research and Design Institute of Rare Metal Industry “Giredmet”, JSC
Research and Production Company “MacroOptics”, LLC
The article concerns the development of a technology for machining workpieces from KRS‑5 crystals. The optimal machining method, as well as the method for determining the optimal orientation of the workpiece, was experimentally determined. The developed technology was tested in the manufacture of a beam-splitting ring. The results of metrological control of the ring, as well as ways of improving the quality and accuracy of machining are presented.
Keywords: diamond cutter, diamond milling, diamond turning, optical components, ultra-precision machine tools, KRS‑5 monocrystals
Received on: 05.01.2021
Accepted on: 18.02.2021
Introduction
Optical instrumentation of the infrared (IR) range is an important direction in the development of advanced branches of technology, including in connection with the increasing requirements for spectrometric research in the infrared range at enterprises of the oil and gas and pharmaceutical industries. The creation of optical devices is impossible without such elements as windows, lenses, prisms, mirrors, beam splitters, etc., for the manufacture of which optical materials with a complex of mechanical, chemical, optical properties that meet a set of specific requirements must be investigated and manufactured.
One of the most promising optical materials are thallium halide crystals with uniform transparency in a very wide wavelength range, covering the visible and mid-infrared spectral regions from 0.35 to 50 μm (depending on the composition), 0.54 to 50 μm for KRS‑5 (TlBr –TlI). The transmission is up to 70% in the absence of absorption bands. The magnitude of the optical loss is determined by the reflectance; accordingly, when applying an antireflection coating, absolute transmission can be achieved. Crystals have mechanical, vibration, chemical and moisture resistance, making them suitable for work in atmospheric conditions without special protection [1–3].
The combination of all the above characteristics makes it possible to improve the properties of equipment in comparison with existing analogues in the range up to 10 microns, as well as to create devices operating in the range from 10 to 50 microns, which have no analogues, using KRS‑5 crystals.
The main problem is the complexity of optical machining of thallium halide crystals due to the softness and plasticity of the material, high coefficient of thermal expansion, low thermal conductivity, and high anisotropy. Conventional machining technologies lead to the formation of a deep damaged layer of the crystal structure during grinding and a low yield to usable when finishing the optical surface by hand polishing.
In connection with the above disadvantages of conventional methods of machining crystals, it is proposed as an alternative way to use diamond machining with cutting tool on ultra-precision machines, which will avoid the formation of a significant damaged layer, charge of the treated surface with abrasive particles, as well as high machining temperatures. Also, diamond machining of products from KRS‑5, carried out without coolant in a sealed cabinet, will allow collecting chips that can be reused when growing crystals. Diamond machining will ensure the required quality and precision of the machined surface, as well as significantly increase productivity. As a rule, the surface roughness of optical products made from KRS‑5 is Ra 0.01 μm, and the accuracy depends on the shape and purpose of the products.
Thus, the following technological tasks for the machining of KRS‑5 crystals have been set: to work out the optimal modes of diamond machining; work out a way to determine the optimal crystal orientation; determination of the optimal way of basing the workpiece.
Technological unit description
Pilot machining of KRS‑5 crystals was carried out at the pilot production facility of Research and Production Enterprise Machine-Tool Plant “Tulamash”, LLC on an ultra-precision machine tool (Fig. 1), which has the following design features:
- slot type aerostatic supports on longitudinal and transverse movements;
- aerostatic tool spindle and product spindle;
- machine bed is installed on vibration-insulating supports;
- pneumatic drive of the longitudinal and transverse support;
- resolution of the linear axis displacement system is 0.1 µm.
The machine tool offers the possibility of machining KRS crystals by fly cutting, as well as by turning. To carry out research, special equipment was designed and manufactured, which is necessary for fixing the workpiece. The equipment consists of a vacuum faceplate and a retaining ring.
Before conducting research on the machine tool, a number of preparatory operations were carried out:
- setting the spindle axis of the product relative to the X-axis guides in the horizontal and vertical planes;
- setting the parallelism of the axis of the tool spindle and the product spindle in the vertical and horizontal plane;
- balancing a single-cutter head.
We used diamond cutters with radius r 1 mm and 5 mm (Fig. 2) with a zero rake angle as a cutting tool in the research. A special holder has been developed for fastening diamond cutters, which is installed in the body of a single-cutter head.
The base surface of the vacuum faceplate was also machined with a diamond cutter after all the preparatory operations were completed.
Technology
of machining KRS‑5 products
Preliminary, the tests on machining a cylindrical KRS‑5 workpiece were carried out according to a turning scheme (the workpiece rotates around its axis, the feed motion is communicated to the tool). Machining of the end face of the KRS‑5 specimen showed that the crystallographic orientation affects the surface quality, since alternation of brittle fracture zones and zones where no defects were observed on the surface were visually observed on the workpiece (Fig. 3). Due to the fact that changing the machining modes did not lead to the elimination of defective zones, it was decided to abandon the turning scheme and proceed to milling, since milling will take into account the position of the workpiece relative to the tool path.
Taking into account the revealed peculiarities of machining KRS‑5 crystals, a technique for machining crystals by the method of diamond milling was developed and implemented, consisting of two stages:
- Determination of the optimal orientation of the workpiece during milling, in which brittle fracture is not observed;
- Machining the workpiece taking into account the correct orientation.
The first stage, where the optimal orientation of the workpiece is determined, was implemented on an ultra-precision machine tool according to the scheme shown in Fig. 4. A single-cutter head with a diamond cutter with a radius of r = 1 mm was installed on the tool spindle. The faceplate with the workpiece was installed on the product spindle. During machining, the workpiece made rotational movements around its axis. The path of the diamond cutter passed through the axis of rotation of the workpiece.
Machining was carried out in the following modes: tool spindle rotation frequency of 500 rpm, workpiece rotation frequency of 0.5 rpm, cutting depth t = 10 μm. After machining, brittle fracture zones formed on the workpiece, which could be detected with the naked eye (Fig. 5).
From the analysis of the obtained picture of the location of brittle fracture zones and knowing the path of the diamond tool, it is possible to orient the part in an angular position so that only plastic removal of material is provided during machining, without brittle fracture.
The second stage consists in machining a correctly oriented part according to the scheme shown in Fig. 6. Unlike the scheme in which the location of fragile zones was determined, the workpiece does not rotate, but moves relative to the milling head with a feed motion.
Prior machining in the following modes eliminated traces of brittle fracture obtained on the workpiece during the test: tool spindle speed 1 000 rpm, workpiece feed rate 12 mm / min, cutting depth 50 μm. To eliminate all traces of brittle fracture, it was necessary to remove an allowance of more than 0.25 mm from the workpiece. Further, the machining was carried out in finishing modes with a diamond cutter with a radius of 5 mm. Finishing modes of machining, which obtained the best surface quality, are as follows: tool spindle speed 1 000 rpm, workpiece feed rate 12 mm / min, depth of cut t = 3 μm.
There were no traces of brittle fracture on the treated surface, which confirmed the correct orientation of the workpiece, determined at the first stage, as well as the correctness of the machining technique used. The surface roughness measured with a profilometer was Ra 0.01 μm.
According to the developed technique described above, the end surfaces of the beam splitting ring made of KRS‑5, which is a ring with an outer diameter of 60 mm, an inner diameter of 36 mm, and a thickness of 6 mm, were machined. The following accuracy and quality requirements are imposed on the product: surface roughness Ra 0.01 μm, shape accuracy N = 0.5, local error ΔN = 0.2, wedging of less than 2 arc seconds. Beam-splitting rings are used in Fourier-transform spectrometers and are currently made from zinc selenide, the disadvantage of which is water solubility. KRS‑5 crystals are moisture resistant, which will help improve the characteristics of Fourier spectrometers.
The sequence of machining the end surfaces of the ring consisted of the following stages:
Rough machining of the workpiece from one of the sides in order to eliminate defects in the preliminary machining.
Determination of the optimal orientation of the workpiece according to the scheme in Fig. 4
Orientation of the workpiece to the optimal position and roughing in order to eliminate traces of brittle fracture.
Machining the workpiece in finishing modes according to the scheme in Fig. 6.
Turning the workpiece over and machining the second end surface by repeating points 1–4.
Turning the workpiece over and finishing the first surface.
Depending on the required shape accuracy, the number of workpiece turns can be increased. This is done to eliminate technological heredity, which can lead to distortion of the product shape.
A number of test samples were machined by the method described above. A photo of the ring machining is shown in Fig. 7.
Metrological control of the shape of the treated surfaces was carried out on an OWI150 HP XT interferometer with a TF 6" λ / 20 Zygo Corp flat optics objective lens by Zygo Corp. The working wavelength of the interferometer is 632.8 nm.
Fig. 8 shows the measurement of the surface shape of the best sample. The shape accuracy was N = 4.5 (1.239 µm), ΔN = 2.1 (0.678 µm), the wedging was 6.9 arc seconds. As can be seen from the results of the interferogram, the machined workpiece has the shape of a saddle. The rest of the machined blanks had the same saddle shape.
In order to search for the reasons for the formation of a saddle-shaped surface, a copper ring with dimensions matching the dimensions of a ring made of KRS‑5 was machined on the machine tool. The modes and scheme of machining were the same as when machining crystals, with the exception of the need to identify the correct orientation of the workpiece. Research on the machining of copper rings showed no apparent saddle-like shape of the workpieces, as well as a significant improvement in shape accuracy compared to crystal machining, N = 1.47 versus N = 4.5.
The required accuracy of the ring shape cannot be ensured on the machine tool due to its design features: low rigidity, lack of a mechanism for precise alignment of the product spindle axis in the vertical plane.
Conclusions
The studies carried out made it possible to understand the features of machining and the physics of the cutting of KRS‑5 crystals, while it was possible to find out that the shape accuracy is associated with the anisotropy of the KRS‑5 crystals and the change in hardness in different directions, as well as the unequal cutting conditions, since the workpiece has a round shape.
The tests carried out on beam-splitting rings made of KRS‑5 made it possible to work out the machining technology, as well as select the optimal cutting conditions. The achieved parameters of accuracy and quality (N = 4.5, Ra = 0.01 μm) of the surface are sufficient for standard optical products (for example, lenses and windows). However, modern optical instruments place higher demands on the quality and accuracy of optical surfaces.
To improve the accuracy and quality of machining, one will need to:
- increase the rigidity of the main units of the machine (will eliminate the effect of crystal anisotropy on the quality of the machined surfaces);
- use the built-in mechanisms for setting the machine units (they will eliminate the deviation from the perpendicularity of the spindle axis to the plane of the workpiece, as well as eliminate the reasons causing the wedging of a workpiece);
- control the shape of the workpiece without removing it from the machine, to correct the path of movement of the supports on the finishing pass using a special function embedded in the CNC (will improve the accuracy of the machined products).
The proposed method for machining crystals KRS‑5 will increase the productivity of manufacturing optical products while ensuring the required accuracy and quality. The manufacture of ultra-precision machine tools for similar tasks in the Russian Federation is carried out by Research and Production Enterprise Machine-Tool Plant “Tulamash”, LLC.
Conflict of interest
The authors declare no conflicts of interest.
Authors
Lapshin V. V., corresponding author, lapshin_v@cnc-tulamash.ru, Research and Production Enterprise Machine-Tool Plant “Tulamash”, LLC. Tula, Russia. https://cnc-tulamash.ru.
ORCID: 0000-0002-6971-8534
Zakharevich E. M., Research and Production Enterprise Machine-Tool Plant “Tulamash”, LLC; https://cnc-tulamash.ru; Tula, Russia.
ORCID: 0000-0001-6997-3335
Kuznetsov M. S., State Research and Design Institute of Rare Metal Industry “Giredmet”, JSC; https://giredmet.inni.info; Moscow, Russia.
ORCID: 0000-0002-8441-4424
Zaramenskikh K. S., State Research and Design Institute of Rare Metal Industry “Giredmet”, JSC; https://giredmet.inni.info; Moscow, Russia.
ORCID: 0000-0001-8573-3470
Osipov A. V., Research and Production Company “MacroOptics”, LLC; https://macrooptica.ru; Moscow, Russia.
ORCID: 0000-0002-8847-9428
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