JP2005254516A - Cutting method for brittle material - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a cutting method for a brittle material which can obtain a prescribed mirror surface without polishing. <P>SOLUTION: The brittle material of a single crystal is cut by an elliptic vibration cutting method cutting the material while elliptic vibration is applied to the blade edge of a tool. By this method, even when the brittle material is calcium fluoride, since it is processed while scraps are pulled up, the thickness of the scraps and cutting force are reduced sharply, and the material can be cut efficiently in a wide ductility mode region. More specifically, the processing by a ductility mode is possible even when the thickness of cutting exceeds 85 nm. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

  The present invention relates to a method for cutting a brittle material. In particular, the present invention relates to a method for cutting an optical material such as calcium fluoride with high accuracy.

Conventionally, calcium fluoride (CaF ₂ ), which has very low light dispersion, has been used as a lens for optical products. Among them, the aspherical lens has a high aberration correction capability, and the optical system can be made compact and the optical performance can be improved. Therefore, there is a great demand for a highly accurate aspherical lens.

There are two main methods for producing a calcium fluoride lens.
(1) First, a calcium fluoride base material is cut or ground, and then the processed surface is polished to obtain a predetermined mirror surface.

  (2) A calcium fluoride base material is rotated, and this base material is cut by pressing a single crystal diamond tool moved along a predetermined aspherical locus by numerical control. At that time, a predetermined mirror surface is obtained only by cutting without performing polishing under specific cutting conditions with a very small cut (for example, Non-Patent Document 1).

"Development of processing technology for aspherical lenses made of single crystal calcium fluoride by a new ductile mode cutting method" Noriaki Tsuji Hiroki Shimizu Katsuo Shoji

  However, if both cutting (grinding) and polishing are performed, the number of processes increases, which is disadvantageous in terms of cost. In particular, in the processing of aspherical lenses, it takes time to perform highly accurate polishing.

  In addition, mirror processing can be performed only by grinding without performing polishing in ultraprecision processing using a single crystal diamond tool. However, this processing method has very low cutting efficiency. In order to obtain a mirror surface necessary as a lens by cutting, ductile mode processing is required in which cutting is performed while discharging thin tape-like chips without causing brittle fracture. In order to perform this ductile mode processing, the cut amount must be very small. For example, the cut thickness shown in Non-Patent Document 1 is only 85 nm. Practical processing is impossible by cutting with such a small cutting thickness.

  In particular, when processing a small-diameter lens having a diameter of 30 mm or less, it is difficult to process an aspherical surface by controlling the blade edge position with high accuracy for a small lens. In particular, it is very difficult to process not a convex aspherical lens but a concave aspherical lens having a concave surface or a combination of a convex surface and a concave surface. This is because the angle between the clearance angle of the cutting edge and the object to be machined is smaller in the case of a concave surface than in the case of a convex surface. This is because it cannot be processed.

  Therefore, a main object of the present invention is to provide a method for cutting a brittle material that can obtain a predetermined mirror surface without polishing.

  As a result of studying a technique for efficiently processing a brittle material in view of the above circumstances, the present inventors have found that the brittle material can be mirror-finished using the elliptical vibration cutting method, and the present invention has been completed. It was.

  That is, the cutting method of the present invention is characterized in that a single crystal brittle material is cut by an elliptical vibration cutting method in which cutting is performed while applying elliptical vibration to the cutting edge of the tool.

  In the elliptical vibration cutting method, as shown in FIG. 1, vibrations in the cutting direction and the chip outflow direction are superimposed on the cutting edge 1A of the cutting tool 1 by a piezoelectric element or the like to give the cutting edge 1A an elliptical vibration trajectory. In this state, the cutting edge 1A is pressed against the work material 3 to perform cutting. Since the machining is performed while pulling up the chips 4 by the elliptical vibration of the cutting edge 1A, the cutting force is reduced to enable high-precision machining (for example, JP 2000-52101 A).

  Conventionally, this elliptical vibration cutting method has been mainly performed on metal materials, particularly steel materials. When an iron-based material such as steel is normally cut without vibrating the tool with a diamond tool for ultra-precision machining, the diamond tool and the iron-based material are always in contact with each other. At that time, the diamond tool (carbon) has a high chemical affinity with the iron-based material, and the diamond tool is very easily worn, so that ultra-precision cutting with high shape accuracy and mirror surface cannot be performed.

  On the other hand, in the elliptical vibration cutting method, not only horizontal (cutting direction) vibration but also vertical (chip discharge direction) vibration is applied to the cutting edge to cut the work material, and frictional force against the chip The wear of the diamond tool can be suppressed by reducing the cutting force, reducing the cutting resistance, and improving the penetration effect of the cutting fluid.

  However, even if the elliptical vibration cutting method is used, in order to mirror-finish hard and brittle materials such as calcium fluoride in a ductile mode with high accuracy, the condition of a cut thickness of 85 nm or less shown in Non-Patent Document 1 is required It was thought that if this condition was not satisfied, it could not be processed.

  The present inventors have found that, when this elliptical vibration cutting method is applied to the cutting of brittle materials, mirror finishing can be performed even with a larger cut.

Examples of the single crystal brittle material to be cut by the cutting method of the present invention include those having strong ionic bonding properties, particularly fluorides and chlorides. More specifically, it is CaF ₂ , LiF, MgF ₂ , BaF ₂ , LiAlCaF ₆ or SrF ₂ . In particular, calcium fluoride (CaF ₂ ) has a low dispersion and is suitable as a lens material. In addition, it is considered that cutting by the method of the present invention is suitable for crystals such as GaN, Si, GaAs, Ge, ZnSe, and ZnS.

  The cutting according to the present invention is preferably performed in a ductile mode. By performing the processing in the ductile mode in which brittle fracture does not occur, the workpiece can be mirror-finished only by cutting.

  The cutting thickness in cutting is preferably more than 85 nm. The cutting thickness is the thickness of the chips. Conventionally, in order to process a brittle material in a ductile mode, it was possible to process only with a very small cut thickness, and the processing efficiency was very poor. According to the method of the present invention, it is possible to perform processing in the ductile mode even with a cutting thickness exceeding 85 nm. A more preferable cutting thickness is 1 μm or more. In the case of cutting calcium fluoride, the upper limit of the cut thickness at which ductile mode processing by the method of the present invention is possible is about 5 μm.

  Further, the cutting depth is preferably 20 μm or less. By using such a cutting amount, more practical processing can be performed.

  The frequency of vibration applied to the tool is preferably about 20 to 40 kHz. By making this frequency higher, efficient cutting becomes possible.

  When processing a lens by the method of the present invention, it is preferable to use a single crystal diamond tool. At that time, the rake angle of the blade edge is preferably set to 0 to −40 °. By using such a rake angle, a mirror surface with a smaller surface roughness can be obtained. Further, by setting the rake angle as a negative, the strength of the blade edge can be increased, chipping of the blade edge can be suppressed, and the tool life can be extended.

  The clearance angle is preferably a positive value. For example, the clearance angle is preferably about 5 to 10 °. Furthermore, it is desirable that the nose radius of the blade edge be 1.0 mm or less. A mirror surface with a small surface roughness can be obtained by satisfying the specified conditions of the clearance angle and the nose radius.

  By using the method of the present invention, an aspheric lens made of a single crystal brittle material can be obtained without polishing the lens surface. In particular, it is possible to obtain an aspheric lens having a concave surface, which has been impossible in the past, from a brittle material only by cutting. The shape of the aspheric lens is preferably a shape in which a convex surface is combined with a concave surface. Although it has been thought that it is impossible to obtain a lens having a shape in which a convex surface is combined with a concave surface by a conventional method, a complex aspherical surface can also be formed by cutting according to the method of the present invention. As a shape combining the concave surface and the convex surface, there is a shape of the aspherical lens 5 having an annular convex portion 51 as shown in FIG. 5 and the concave portion 52 on the inner peripheral side and the outer peripheral side of the convex portion 51. It is done.

  Moreover, if the method of the present invention is used, a highly accurate lens having the following characteristics alone or in combination can be obtained industrially efficiently.

(1) The aspheric amount (see FIG. 5) of the aspheric lens is 50 μm or more.
(2) The shape accuracy of the aspherical lens is λ / 10 or less, more preferably λ / 20 or less, and further preferably λ / 100 or less with respect to the wavelength λ for which the lens is used.
(3) The diameter of the aspheric lens is 50 mm or less.
(4) The surface roughness Ra of the aspherical lens is 0.01 μm or less.

  The above aspherical lens is formed only by cutting, but skin polishing may be further applied to the cutting surface. By applying skin polish that removes only ripples (unevenness caused by cutting marks) formed on the cut surface, a lens with a smaller surface roughness can be obtained. In addition, skin polishing can be easily performed in a very short time.

Embodiments of the present invention will be described below.
Machining was performed by an elliptical vibration cutting method using a single crystal calcium fluoride base material. The elliptical vibration cutting method is a processing method in which cutting is performed while pulling up chips by applying elliptical vibration in the ultrasonic region to the cutting edge. Here, cutting was performed using an ultra-precision processing machine “Nagase Integrex NIC-300” equipped with an ultrasonic elliptical vibration control system. A vibration tool is attached to the index table mounted on the Z axis, and the calcium fluoride protruding from the (111) surface is fixed to the X axis index table located below it. Planar type groove processing and planar processing in which the amount of cut was changed. At that time, the transition from ductile mode to processing accompanied by brittle fracture was examined. Moreover, the cutting by the conventional method which cuts without giving a vibration to a tool was also performed similarly. Cutting conditions are as follows.

Cutting depth: 0-3μm (grooving) 0-15μm (planar machining) Cutting speed: 0.37m / min
Feed: 10-40μm Cutting direction: <-12-1>, <1-21>
Vibration: Circular locus Amplitude: 4μm Frequency: 19.5kHz
Tool: Single crystal diamond tool Nose radius: 1mm Rake angle: 0 °

  FIGS. 2 and 3 show differential interference micrographs of the machined surfaces grooved with a cutting depth of 2 μm in the <-12-1> and <1-21> directions. (A) of each figure shows the machined surface by the elliptical vibration cutting method, and (B) shows the machined surface by the conventional method. As is apparent from this photograph, a surface that has been brittlely fractured appears on the processed surface by the conventional method, whereas it can be seen that mirror processing is possible by the processing method of the present invention. Therefore, it has been found that the region that can be processed by the ductility mode is greatly expanded as compared with the conventional method.

  Next, from the grooving test, a differential interference micrograph of the machined surface when flattening was performed by changing the depth of cut and feed in the <-12-1> direction, where ductile mode machining was the most difficult. Is shown in FIG. 4A also shows a processed surface by the elliptical vibration cutting method, and FIG. 4B shows a processed surface by the conventional method. Here, the machined surface is shown when the cut depth is 5 μm and the feed amounts are 10 and 20 μm.

  As a result, the machined surface properties do not change much even if the feed amount changes. However, it can be seen that a brittle fracture surface is observed in normal cutting, and that the brittle fracture surface increases as the feed rate increases. Further, when the depth of cut was further increased, the machined surface properties slightly deteriorated, but it was confirmed that a remarkable brittle fracture surface was hardly generated by the elliptical vibration cutting method. From the above results, it was found that the region that can be processed by the ductile mode is greatly expanded compared to the conventional method.

  According to the method of the present invention, a highly accurate aspheric lens can be obtained only by cutting. Therefore, it is expected that the method of the present invention is used in the field of processing optical materials. In addition, the aspherical lens obtained by the method of the present invention can be used for various optical products.

It is explanatory drawing which shows the processing state of elliptical vibration cutting. It is a differential interference micrograph of the processed surface when grooved in the <-12-1> direction. (A) shows the processed surface by the method of the present invention, and (B) shows the processed surface by the conventional method. It is a differential interference micrograph of the processed surface when grooved in the <1-21> direction, (A) shows the processed surface by the method of the present invention, and (B) shows the processed surface by the conventional method. It is a differential interference micrograph of the processed surface when it is planarized in the <-12-1> direction, (A) shows the processed surface by the method of the present invention and (B) by the conventional method. It is a shape block diagram of an aspherical lens.

Explanation of symbols

1 Cutting tool 1A Cutting edge 2 Elliptical track 3 Work material 4 Chip
5 Aspheric lens 51 Convex 52 Concave

Claims (5)

  A method for cutting a brittle material, characterized in that a single crystal brittle material is cut by an elliptical vibration cutting method in which an elliptical vibration is applied to a cutting edge of a tool. _{2. The} method for cutting a brittle material according to claim 1, wherein the single crystal brittle material is CaF ₂ , LiF, MgF ₂ , BaF ₂ , LiAlCaF ₆ or SrF ₂ .   The brittle material cutting method according to claim 1, wherein the cutting is performed in a ductile mode.   4. The method for cutting a brittle material according to claim 1, wherein a cutting thickness in the cutting is greater than 85 nm.   The brittle material cutting method according to any one of claims 1 to 4, wherein a rake angle of the blade edge is 0 to -40 °.