JPH10166202A - Machining method for die for forming fresnel lens - Google Patents

Abstract

PROBLEM TO BE SOLVED: To accurately machine an original die for forming Fresnel lenses having an ideal blaze-grating. SOLUTION: The path and the feed quantity of a diamond bit 6 in relation to a blank are controlled so that an edge 6b of the diamond bit 6 is moved toward the center 0 of rotation the blank while it traces a target two dimensional contour line 30. Besides, the target two dimensional contour line is referred to a contour line of the crosssectional shape of an ideal blaze-grating. Since the edge 6b of the diamond bit 6 has roundness having the diameter (3μm or more and 10μm or less) determined by the feed rate, the spherical shape of the edge 6b of the diamond bit 6 is transferred to the surface of the blank as the blank A is rotated. Finally, a locus formed by rotating the target two dimensional contour lone 30 around the center O of rotation, namely an inverted shape of the ideal blaze-grating is created.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a precision processing method for a mold for molding a Fresnel lens.

[0002]

2. Description of the Related Art Fresnel lenses, which are thin but exhibit excellent optical performance, have been widely used in many optical devices. Ideally, a blaze grating having a cross-sectional shape in which a plurality of curves a are connected in a stepwise manner as shown in FIG. 7 is formed on the surface of the Fresnel lens, but if the pitch of the blaze grating is small, Since the curve a can be approximated to the line segment b, the surface of a Fresnel lens having a pitch of a blaze grating of about 30 μm, which is used in general optical equipment, usually has a plurality of surfaces as shown in FIG. A blazed grating having a cross-sectional shape in which the line segment b is connected in a stepwise manner is formed.

[0003] A mold master used as a top of a mold for molding such a Fresnel lens (hereinafter referred to as a Fresnel lens molding mold) generally has a sharp cutting edge (diameter of cutting edge of diamond bite) of a diamond bite. (About 20 nm to 50 nm)
It is created by ultra-precision turning (hereinafter referred to as conventional ultra-precision turning) using. That is, specifically, the path and the feed amount of the diamond tool with respect to the blank are controlled by using a CNC super-precision lathe of a simultaneous two-axis control method (while rotating the blank, as shown in FIG. 7). A three-dimensional shape obtained by inverting the blaze grating to be formed on the Fresnel lens (by cutting a blade of a diamond tool into the blank in accordance with the cross-sectional shape of the blaze grating to be formed on the surface of the Fresnel lens). This is called a blazed grating inverted shape). In order to prevent the cutting edge of the diamond cutting tool from being chipped, the corner of the cutting edge of the diamond cutting tool (a fine cutting edge region connecting two cutting edges) is usually chamfered to a width of about 0.5 μm to 5 μm. .

Some diamond cutting tools have a corner of about 10 μm in order to improve the surface accuracy of the finished surface.
Some are rounded.

[0005]

The optical performance of a Fresnel lens depends on the processing accuracy of a mold master used for molding the Fresnel lens.

[0006] However, ultra-precision turning using a diamond tool having a sharp cutting edge requires a fine uneven shape (hereinafter, referred to as a "tool") along the locus of the cutting edge of the diamond tool.
(Referred to as tool marks) on the finished surface. Therefore, it has been difficult to produce a good mold master (in terms of surface accuracy, a mold master having a surface roughness of about 0.01 μm). In particular, since the mold master used for molding the Fresnel lens having the ideal blaze grating has a complicated curved surface shape, it has been almost impossible to finish the surface with high precision.

On the other hand, if the cutting edge of the diamond tool is rounded with emphasis on improving the surface accuracy of the finished surface, the valley of the reversal shape of the blazed grating has a curvature, and the corner of the Fresnel lens which is a molded product is also formed. It is not appropriate to easily round the cutting edge of the diamond bite, because of the additional problem of curvature. The problem that the curvature of the corner of the Fresnel lens is problematic is that this causes a difference between the ray refracting power in the sagittal plane and the ray refracting power in the meridional plane, resulting in astigmatism. This is because the optical performance of the Fresnel lens deteriorates, such as occurrence of aberration. As a specific example of the disadvantage in practical use, if such a Fresnel lens is incorporated as a reticle of a camera or the like, there occurs a disadvantage that a good view cannot be formed in a finder.
Therefore, when rounding the corner of the diamond bite, it is not enough to focus on improving the surface accuracy of the finished surface as in the past, and also considering the optical performance of the molded Fresnel lens, Thorough consideration should be given.

In view of the above, it is an object of the present invention to provide a method of manufacturing a high-precision Fresnel lens molding die capable of molding a Fresnel lens having better optical performance. Then, by introducing the Fresnel lens molding die manufactured by the present manufacturing method into the Fresnel lens molding process, mass production of Fresnel lenses having more excellent optical performance is achieved.

[0009]

In order to solve the above-mentioned problems, the present invention provides a processing method for forming a target fine three-dimensional shape in which a plurality of curved surfaces are connected in a stepwise manner on a surface of a mold material.
While using a diamond tool having a corner having a diameter of 3 μm or more and 10 μm or less, the corner of the diamond tool cuts into the shape according to the undulation of a target three-dimensional shape in which a plurality of curved surfaces are connected in a stepwise manner. A machining method is provided, wherein a relative movement of the tool with respect to the mold is controlled so as to move at a feed speed on a predetermined path set on a surface.

According to this processing method, that is, a precision processing method of a contour control method using a diamond tool having a corner having a diameter of 3 μm or more and 10 μm or less, a fine surface in which a plurality of curved surfaces are connected in a step-like manner on the surface of a mold material. A three-dimensional shape can be created with higher accuracy. It should be noted that such an effect can be easily recognized only by visually observing the three-dimensional shape surface state created by the present processing method and the three-dimensional shape surface state created by the conventional ultraprecision turning described above. Can be. In addition, according to the present processing method, the fact that the three-dimensional valley does not have a curvature that affects the quality of the molded product does not occur. If it is used as a focusing screen, everyone can easily guess from the high optical performance.

That is, according to the present working method having such features, a Fresnel lens forming die for forming a Fresnel lens having an ideal blaze grating can be processed with high precision. Therefore, if the Fresnel lens molding die prepared by the present processing method is introduced into the Fresnel lens molding process, Fresnel lenses having excellent optical performance can be mass-produced.

[0012]

Embodiments of the present invention will be described below with reference to the accompanying drawings.

First, a basic configuration of a processing apparatus according to the present embodiment will be described with reference to FIG.

This machining apparatus is an ultra-precision CNC lathe of a simultaneous two-axis control system. That is, the present processing apparatus includes a Z-axis 4 for feeding a tool rest on which a tool 6 (in this embodiment, a diamond bite) is mounted in the z-direction, and an X-axis 5 for sending a main shaft 7 for rotating the blank A in the x-direction. , A length measuring laser device 3 for sequentially detecting the feed amount of the X axis 5 and the feed amount of the Z axis 4, and each of the axes 4, 5 based on the feed amounts of the respective axes 4, 5 sequentially detected by the length measuring laser device 3. An NC device (not shown) for numerically controlling the rotation angle of each servo motor (not shown) for driving the servo motor 5 can control the path and the feed amount of the tool 6 with respect to the blank A. I have. Also, a motor for rotating the main shaft 7 so that the fluctuation of the rotation speed of the main shaft 7 is suppressed.
(Not shown) is speed-controlled by a speed controller (not shown).

In this embodiment, a processing apparatus is installed on the vibration isolating table 9 in order to prevent a reduction in processing accuracy due to external vibration.

Next, the tool 6 according to the present embodiment will be described with reference to FIGS.

The tool 6 is a diamond cutting tool having a cutting portion 6a made of single crystal diamond, and has a cutting edge 6b having a diameter determined based on the following experimental results as shown in FIG. It is rounded.

It is generally known that as the feed speed f of the tool is reduced, the surface roughness Ra of the finished surface is suppressed, and the reflectance n of the finished surface is improved (FIG. 3).
However, if the feed rate f of the tool is excessively reduced, the efficiency of the cutting efficiency is reduced. Therefore, the feed rate f of the tool is usually a value that is appropriate from two viewpoints of the cutting efficiency and the finished surface accuracy (see FIG. For example, a feed speed at which the surface roughness Ra of the finished surface starts to converge and the reflectance n of the finished surface sharply increases is set. Therefore, also in the present embodiment, first, from the same viewpoint, the feed speed f of the tool 6 is determined.
It was set. Then, using a tool having corners having different diameters from each other, a master having an ideal blazed grating inverted shape described below was prototyped, and the difference in the diameter of the corner of the tool 6 was caused by the surface roughness of the finished surface and the Fresnel lens being a molded product. The effect on performance was examined. As a result, in a range of feed speed f which is commonly set in this kind of ultraprecision turning, by using a tool having a corner having a diameter of about 3 μm or more and 10 μm or less, the surface roughness of the finished surface is reduced. Was found to improve rapidly. In addition, it was also found that the astigmatism of the Fresnel lens was also rapidly reduced. And among them, it was confirmed that the tool 6 having a cutting edge diameter of about 5 μm exerts the best effect.

Next, a description will be given of a method of processing a master of a mold for molding a Fresnel lens using the processing apparatus. However,
Here, a description will be given of a processing method from a state where rough cutting has already been completed, that is, a state where only a margin required for finishing is left.

The NC device numerically controls the rotation angle of each servo motor for driving the Z axis 4 and the X axis 5, and the cutting edge 6b of the tool 6
At a predetermined feed speed f along a predetermined tool path. That is, the cutting edge 6b of the tool 6 moves toward the rotation center O of the blank A while tracing on the target two-dimensional contour line 30. Here, the target two-dimensional contour 30 is a contour of the cross-sectional shape of the blaze grating to be formed on the surface of an ideal Fresnel lens (see FIG. 7).

As a result, with the rotation of the blank A, the spherical shape of the cutting edge 6b of the tool 6 is transferred to the surface of the blank A, and finally, the target two-dimensional contour line around the rotation center O of the blank A is obtained. FIG.
3D shape with inverted blaze grating to be formed on ideal Fresnel lens surface as shown in
(Hereafter referred to as an ideal blaze grating inverted shape)
50 is created.

In this machining method, since a finished surface is created by transferring the spherical shape of the cutting edge 6b of the tool 6, a fine three-dimensional shape including a curved surface shape such as the above-mentioned ideal blazed grating inverted shape is sharpened. Blade tip (Blade diameter 20nm ~ 5
The surface can be finished with higher surface accuracy than conventional ultra-precision turning using a diamond bite having a diameter of about 0 nm). That is, according to the present processing method, it is possible to accurately process a mold master for forming a Fresnel lens having an ideal blaze grating.

In this embodiment, since the corner 6b of the cutting edge of the tool 6 is rounded to about 5 μm, the ideal blaze grating inverted shape 50 formed on the surface of the blank A is formed. It is inevitable that the valley 50a has a very small curvature. However, this degree of curvature has almost no effect on the optical performance of the molded Fresnel lens. The fact that a Fresnel lens created using this mold master was used as a reticle for a camera in the circumferential direction And the fact that the radial defocus has been significantly improved. In addition, since it has almost no effect on the optical performance of the Fresnel lens, the curvature of the valley 50a, which is extremely small, is rather useful considering that this derives an effect of improving the releasability. I can even say.

Finally, by comparing and observing the surface condition of the master created by the turning according to the present working method and the surface of the master created by the conventional ultra-precision turning under the same machining conditions, The effect obtained by using this processing method will be proved.

Although the surface of the master created by the turning process according to the present processing method is mirror-finished, the surface of the master created by the conventional ultra-precision turning is not mirror-finished. It was confirmed that the surface condition of the master created by both turnings was superior to the extent that it could be seen and clearly recognized.

A master is prepared by changing the feed speed f of the tool in several ways by double turning, and its surface roughness Ra
(See FIG. 6), it was quantitatively confirmed that the turning by the present machining method always yielded a better finished surface than the conventional ultra-precision turning.

Based on the above results, the effect obtained by using the present processing method is proved.

The present processing method can be sufficiently applied not only to processing of a mold for molding a Fresnel lens, but also to processing of other molds and processing of parts having a fine shape. As a result, it goes without saying that the effect of improving the quality of the finished product can be obtained.

[0029]

According to the precision processing method of the present invention, a fine three-dimensional shape in which an ideal blazed grating to be formed on the surface of a Fresnel lens is inverted can be created with high precision. That is, a master for forming a Fresnel lens having an ideal blaze grating can be processed with higher accuracy.

Therefore, if the master prepared by the precision processing method is used as a top of a mold for molding a Fresnel lens, Fresnel lenses having excellent optical performance can be mass-produced.

[Brief description of the drawings]

FIG. 1 is a diagram showing a basic configuration of a processing apparatus according to an embodiment of the present invention.

FIG. 2 is a view for explaining the shape of a cutting edge 6b of the tool 6 of FIG.

FIG. 3 shows the feed rate f of the tool and the surface roughness Ra of the finished surface.
FIG. 6 is a diagram showing a relationship between the reflectance and the reflectance n of the finished surface.

FIG. 4 is a view for explaining a path of a tool 6;

FIG. 5 is a diagram showing a surface shape of a master created by a processing method according to an embodiment of the present invention.

FIG. 6 shows the feed rate f of the tool and the surface roughness Ra of the finished surface.
FIG.

FIG. 7 is a diagram showing a cross-sectional shape of a blaze grating of a normal Fresnel lens.

FIG. 8 is a diagram showing a cross-sectional shape of a blaze grating of an ideal Fresnel lens.

[Explanation of symbols]

 3 ... Length measuring laser device 4 ... Z axis 5 ... X axis 6 ... Tool (diamond tool) 6a ... Tool blade 6a ... Tool blade 7 ... Spindle 9 ... Vibration isolation table

Claims (1)

[Claims] 1. A processing method for forming a target fine three-dimensional shape in which a plurality of curved surfaces are connected in a stepwise manner on a surface of a mold material, wherein a diamond tool having a corner having a diameter of 3 μm or more and 10 μm or less is formed, A corner of the diamond tool cuts into the mold according to the undulation of the target fine three-dimensional shape in which the plurality of curved surfaces are connected in a stepwise manner, and a predetermined feed speed along a predetermined path set on the surface of the mold. Controlling the relative movement of the tool with respect to the mold so that the tool moves.