JP2008200779A - Method of cutting axisymmetric diffraction curved surface - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a method of cutting an axisymmetric diffraction curved surface of a Fresnel lens made of a fragile material such as an optical crystal of a different type and a glass, easily, at a low machining cost, and with high precision. <P>SOLUTION: The method comprises a step in which while a minute sword-shaped cutting tool 11 having a pointed tip is driven on the workpiece 1 having a rotation axis along a constant locus, straight line enveloping is carried out by uniaxial (Z) straight line motion and triaxial simultaneous control of a B axis in addition to two axes (XZ), and a discontinuous diffraction curved surface is formed in a surface of the rotating workpiece 1 by transfer and enveloping of a cutting edge of the sword-shaped cutting tool 11. In this step, firstly, the straight line enveloping by the uniaxial (Z) straight motion and the triaxial simultaneous control of the B axis in addition to the two axes (XZ), is carried out for a first groove in the vicinity of the rotation axis of the workpiece 1, and the discontinuous diffraction curved surface is formed in the surface of the rotating workpiece 1, followed by successively machining grooves formed on the outside of the rotation axis. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

  TECHNICAL FIELD The present invention relates to an axially symmetric diffraction curved surface cutting method, and in particular, an axially symmetric diffraction curved surface cutting method suitable as a manufacturing method of a Fresnel lens used for dark-field infrared optical devices, in-vehicle night vision systems, infrared sensors, and the like. It is about.

  A Fresnel lens is obtained by dividing the surface of a spherical or aspherical lens into concentric fine widths and replacing only the inclination angle with a prism on a plane, and is composed of a discontinuous diffractive surface. As a lens base material of the Fresnel lens, there are glass, plastic, optical crystal and the like. Among these, plastic Fresnel lenses can be produced at a relatively low cost by injection molding technology. However, when a brittle material such as various optical crystals or a part of glass is used as a lens substrate, injection molding is impossible and it is necessary to rely on machining. However, the processing of the Fresnel lens shape of such a hard brittle material is technically difficult, and there is a problem that the processing cost is very high.

  As a prior art of the present invention, there has been proposed a processing method for manufacturing an optical element having a saw blade shape with a straight section (see Patent Document 1). However, this prior document has a problem that curved surface processing cannot be performed. A method of processing a pseudo curved surface by connecting short straight lines inside an ideal curve has also been proposed (see Patent Document 2). However, since this method does not control the cut thickness per rotation of the workpiece, it is effective in the manufacture of a metal mold for forming an optical element. There is a problem that it cannot be applied to processing.

  Also, a new processing method of a Fresnel shape of a hard brittle material has been proposed (see Non-Patent Document 1). To summarize this new processing method, a disc-shaped grinding wheel with a sharp tip is used, and the rotation axis of the grinding wheel and the rotation axis of the workpiece are arranged so as to be orthogonal to each other. This is a grinding method in which the rotation direction of the workpiece is parallel. However, with this processing method, it is impossible to accurately grasp the shape and distribution of a plurality of abrasive grains, and therefore high quality and high stability processing suitable for hard brittle materials is difficult. In addition, for optical parts such as Fresnel lenses to be processed, a large arc is left at the surface diffraction part, that is, the bottom of the groove due to the limitation of the grinding wheel forming technique, which affects the performance of the optical part. Furthermore, there is also a problem that the processing must be intermittently stopped during processing and the grindstone must be shaped and corrected.

  On the other hand, the present inventor has previously proposed a method of processing an axially symmetric diffraction curved surface suitable for a hard brittle material (see Patent Document 3). In this method, a fine sword-shaped cutting tool with a sharp tip is driven along a fixed trajectory on the plane where the axis of rotation of the axisymmetric workpiece is located, and swiveled around a straight line perpendicular to the plane. The workpiece to be rotated by performing linear envelope by simultaneous control of three axes of B axis in addition to one axis (Z) linear motion and two axes (XZ). A discontinuous diffraction curved surface is formed on the surface of an object.

JP-A-10-309601 JP-A-10-293205 JP 2005-96064 A Hirofumi Suzuki, Toshiro Higuchi, Nao Wazushima, Takayuki Kitajima, Shigeki Okuyama, Hiroshi Yamazaki, "Ultra-precision grinding of micro Fresnel lens molds-Verification of the possibility of grinding of cemented carbide-", Journal of Precision Engineering, 1999, 65 (8), p.1163-1168

  However, when a hard brittle material is processed by the processing method described in Patent Document 3, the processing is performed from the outermost groove in the direction of the rotation center. Therefore, in each Z-axis linear processing, the convex portion (Lens edge) of the workpiece is processed. ) Is likely to introduce defects such as brittle fracture and microchipping, and after processing, surface polishing is required to remove brittle fractures and microchipping, resulting in increased processing costs. Furthermore, there is a problem that the optical performance is deteriorated by processing. In particular, chipping tends to occur when the tip of the sword-shaped cutting tool is worn.

  The present invention has been made paying attention to such problems, and an object of the present invention is to provide a cutting method of an axially symmetric diffraction curved surface that can be performed with high precision at a low processing cost in a simple manner. There is to do.

  According to the present invention, there is provided an axisymmetric diffraction curved surface cutting method for rotating a workpiece around a predetermined rotation axis to form a diffraction curved surface that is symmetric with respect to the rotation axis. Linear movement along the Z axis parallel to the Z axis, linear movement along the X axis perpendicular to the Z axis, and rotational movement about the Y axis perpendicular to the Z axis and the X axis; By performing simultaneous control of three axes of a fine sword-shaped cutting tool with a sharp tip, the workpiece rotating around the rotation axis is cut by a linear envelope method, and the vicinity of the rotation axis An axially symmetric diffraction curved surface cutting method characterized in that the outer grooves are sequentially processed from the first groove to form a discontinuous diffraction curved surface on the surface of the workpiece.

  In addition, according to the present invention, chipping at the tip of the linearly processed portion of the workpiece, which occurs during linear movement along the Z-axis for processing a predetermined groove, is eliminated by the next outer groove processing. Thus, a cutting method for an axially symmetric diffractive curved surface characterized in that the sword-shaped cutting tool is simultaneously controlled by three axes and cutting is performed by a linear envelope method.

  According to the present invention, there is obtained an axisymmetric diffraction curved surface cutting method capable of easily and accurately cutting Fresnel lenses made of brittle materials such as various optical crystals and glass at a low processing cost. . With its practical application, it can be expected that optical devices will be dramatically improved in function, size and weight. Further, since the chipping can be eliminated in the machining process, the machining can be continued even if the sword-shaped cutting tool is worn, and the cost can be reduced.

  In the present invention, the main axis (workpiece axis) is the Z axis, the X axis is on the horizontal plane, the Y axis is vertically above, and the rotation axes around the XYZ axes are A, B, and C (the right screw direction is positive). A coordinate system in a processing machine is defined, and a single sword (Z) linear motion is driven while driving a fine sword-shaped cutting tool with a sharp tip on a workpiece having a rotation axis along a fixed trajectory, In addition to two axes (XZ), a linear envelope is obtained by simultaneous control of the B axis, and a discontinuous diffraction curved surface is formed on the surface of the rotating workpiece by transferring and enveloping the cutting blade of the sword-shaped cutting tool. In the forming step, first, the first groove in the vicinity of the rotation axis of the non-workpiece is subjected to the linear envelope by the uniaxial (Z) linear motion and the triaxial simultaneous control of the B axis in addition to the biaxial (XZ). To form a discontinuous diffraction curved surface on the surface of the rotating workpiece, and sequentially remove it from the rotation axis. This is a cutting method of an axially symmetric diffraction curved surface characterized by processing the groove in the same manner, and the brittle fracture of the tip of the linearly machined portion that occurs during the uniaxial (Z) linear motion of the first groove, An axisymmetric diffraction curved surface cutting method is obtained in which microchipping is eliminated by linear envelope processing by simultaneous three-axis control of the B-axis in addition to the next two-axis (XZ) of outer groove processing. It is done. Therefore, it is possible to carry out highly accurate cutting at a low processing cost in a simple manner.

  The present invention also provides an axisymmetric diffraction curved surface cutting method for rotating a workpiece around a predetermined rotation axis to form a diffraction curved surface that is symmetrical with respect to the rotation axis. Linear movement along the Z axis parallel to the Z axis, linear movement along the X axis perpendicular to the Z axis, and rotational movement about the Y axis perpendicular to the Z axis and the X axis; By performing simultaneous control of three axes of a fine sword-shaped cutting tool with a sharp tip, the workpiece rotating around the rotation axis is cut by a linear envelope method, and the vicinity of the rotation axis A cutting method for an axially symmetric diffraction curved surface characterized in that a discontinuous diffraction curved surface is formed on the surface of the workpiece by processing the outer grooves sequentially from the first groove. The added force that occurs during linear motion along the Z-axis to machine the groove Axisymmetric diffraction characterized in that cutting is performed by the linear envelope method with simultaneous control of the three axes of the sword-shaped cutting bite so that chipping at the tip of the linear processing part of the object is eliminated by the next outer groove processing. This is a curved surface cutting method.

  Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings. FIG. 1 is a sectional view showing a processing method of the present invention. First, in FIG. 1A, inner groove processing closest to the rotation center axis is performed in the order of A → B → C → D, and after processing, a minute sword shape is reached up to the next outer groove processing start position E. The cutting tool 11 is moved. Next, in FIG. 1B, the second inner groove machining from the rotation center axis is performed in the order of E → F → G → H, and after the machining, the sword-shaped cutting bit 11 is moved to the next outer groove machining position I. Moving. By repeating this procedure, grooving was performed from the inside to the outside.

  Here, a fine sword-shaped cutting tool (Diamond tool) 11 is used as a cutting tool. The minute sword-shaped cutting tool 11 can be made of diamond, CBN (cubic boron nitride), ceramics, or the like, which is a high hardness material. At this time, in order to prevent brittle fracture in processing of the brittle material, the rake face of the sword-shaped cutting tool 11 is chamfered. As a result, the tool rake angle of the sword-shaped cutting tool 11 is appropriately negative, and hydrostatic pressure stress is generated in the cutting region.

  As the Fresnel lens substrate of the work 1, a hard brittle material that is transmissive in all wavelength ranges from infrared to ultraviolet, specifically, optical glass, silicon, germanium, ZnSe, quartz, calcium fluoride, etc. Use. In particular, single crystal germanium (Ge) is an infrared optical material and can be used as a lens substrate material for an in-vehicle night vision system and an infrared sensor.

  FIG. 2 shows a process in which brittle fracture and microchipping are eliminated in the processing method of the present invention. In FIG. 2 (a), by performing a uniaxial (Z) linear motion of the sword-shaped cutting bite 11, brittle fracture is first caused at the end of the linearly machined portion (Lens edge) where the sword-shaped cutting bite 11 contacts the hard brittle material. Or microcrack 2 occurs. However, at the time of the next outer groove processing, in FIG. 2B, in addition to the two axes (XZ), a linear envelope is obtained by simultaneous control of the B axis, and the brittle fracture or micro- The chipping 2 can be removed by cutting.

FIG. 3 is a flowchart of a machining NC (numerical control) program.
(1) First, the cutting tool 11 is moved to the X coordinate of the cutting start point of the innermost groove (innermost groove) (n = 0).
(2) Next, the tool is brought into contact with the workpiece by interruption.
(3) The number of grooves (n = n + 1) is set.
(4) A vertical surface of the groove is formed (moved in the −Z direction).
(5) An arc surface of the groove is formed (perpendicular motion in the X and Z directions and B-axis rotation).
(6) It is checked whether or not the groove processing is completed.
(7) When the groove machining is completed, the sword-shaped cutting tool 11 is released (moved in the Z direction +) to complete the operation. If the groove machining is not completed, the next (+1) groove machining is continued.

  As a specific example, an aspherical surface processing machine ASP-15 equipped with XZB simultaneous three-axis controller manufactured by Fujikoshi Co., Ltd. was used for the experiment. FIG. 4 is an overview of the main part of the processing apparatus. The program of FIG. 3 was executed by mounting the sword-shaped cutting tool 11 on the tool table 13 provided on the B-axis rotary table 12 and fixing the workpiece of the workpiece 1 on the main spindle 14 rotating counterclockwise. .

  FIG. 5 is a schematic perspective view and a photomicrograph of the tip of the sword-shaped cutting tool 11. A sword-shaped cutting tool 11 made of single crystal diamond having a cutting edge angle of 60 °, a rake angle of -30 °, and a clearance angle of 6 ° was used as a cutting tool. A single crystal germanium wafer (110) surface having a diameter of 30 mm and a thickness of 1 mm was used as a workpiece of the workpiece 1. Since the lens curvature radius (R) of the Fresnel lens is 100 mm and the depth (d) of the fine groove is 50 μm, the number of grooves is 21, and the groove width (w) varies between 300 μm and 3 mm. . When machining the vertical surface of the groove, the feed amount in the Z-axis direction of the tool per rotation of the workpiece is 50 nm, and when machining the inclined surface, the angle between the tool main cutting edge and the groove inclined surface (cutting edge The angle k) was set to 0.2 °, the tool feed amount (f) in the XZ-axis direction per workpiece rotation was set to 10 μm / rev, and the substantially cut thickness (h) was set to less than 50 nm. The rotation speed of the main shaft is 1000 rpm, and the cutting fluid is kerosene spray.

  FIG. 6 is a comparative drawing of the front end portion of the linearly processed portion after processing according to the present invention and after processing by the conventional method. 6A is after the processing method of the present invention, and FIG. 6B is after the conventional processing method. By comparing the two, it is clear that brittle fracture and microchipping are eliminated by the production method of the present invention.

The sectional view at the time of inner side groove processing nearest to a rotation central axis which shows the cutting method of the axially symmetric diffraction curved surface of an embodiment of the invention, (b) The inner groove second from the rotation central axis It is sectional drawing at the time of a process. (A) Cross-sectional view showing a state in which brittle fracture or micro-chipping occurs at the tip of the linearly machined part due to a single axis (Z) linear motion in the cutting method of the axially symmetric diffraction curved surface shown in FIG. It is sectional drawing which shows the process of performing a linear envelope by 3 axis | shaft simultaneous control of B axis | shaft in addition to XZ), and removing a brittle fracture and microchipping. 2 is a flowchart of a machining NC (numerical control) program for executing the cutting method of the axially symmetric diffraction curved surface shown in FIG. 1. It is a perspective view which shows the general view of the main part of the processing apparatus for enforcing the cutting method of the axisymmetric diffraction curved surface shown in FIG. 2A is a schematic perspective view showing a sword-shaped cutting tool of the method of cutting an axially symmetric diffraction curved surface shown in FIG. 1, and FIG. 2B is a photomicrograph showing the tip of the sword-shaped cutting tool. (A) Enlarged cross-sectional view and micrograph showing the tip of the straight line processed part after machining by the cutting method of the axially symmetric diffraction curved surface shown in FIG. 1, (b) The tip of the straight line processed part after machining by the conventional machining method It is an expanded sectional view and a microscope picture which are shown.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Workpiece 2 Micro chipping 11 Sword-shaped cutting tool 12 B axis rotary table 13 Tool stand 14 Spindle

Claims (2)

A cutting method of an axially symmetric diffraction curved surface for rotating a workpiece around a predetermined rotational axis to form a symmetric diffraction curved surface with respect to the rotational axis,
Centered on a linear motion along the Z axis parallel to the rotation axis, a linear motion along the X axis perpendicular to the Z axis, and a Y axis perpendicular to the Z axis and the X axis By performing three-axis simultaneous control of a fine sword-shaped cutting tool with a sharp tip so as to perform a rotational motion, the workpiece that rotates about the rotational axis is cut by a linear envelope method, A cutting method for an axially symmetric diffraction curved surface, characterized in that a discontinuous diffraction curved surface is formed on the surface of the workpiece by sequentially processing outer grooves from the first groove in the vicinity of the rotation axis. The sword-shaped cutting so that chipping at the tip of the straight machining portion of the workpiece, which occurs during linear movement along the Z-axis for machining a predetermined groove, is eliminated by the next outer groove machining. 2. The cutting method for an axially symmetric diffraction curved surface according to claim 1, wherein the cutting is performed by a linear envelope method by simultaneously controlling three axes of the cutting tool.