Abstract:
There is provided a vertical lathe which, when machining gradually varying lens grooves in a mold for molding of a Fresnel lens, can change the angle of a cutting tool without positional displacement of the cutting tip, enabling high accuracy machining of the gradually varying lens grooves. The vertical lathe includes: a column; a rotary table for rotating a workpiece; a cantilevered cross rail supported by the column; a saddle movable in a horizontal direction along a guideway of the cross rail; a ram suspended from the saddle and movable vertically along a guideway of the saddle; and an inclined head mounted to the lower end of the ram and having an inclined swivel shaft for holding a cutting tool, wherein said swivel shaft is configured to swivel the cutting tool around a nose of the cutting tool as the center of swiveling.

Description:
BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates to a vertical lathe useful for ultra-precision machining of, for example, a mold for molding a Fresnel lens, and more particularly to a vertical lathe suited for machining of lens grooves having gradually varying inclination angles, such as the lens grooves of a Fresnel lens. 
   2. Background Art 
     FIG. 9  shows a Fresnel lens which is utilized as a condenser lens in a precision optical instrument. The Fresnel lens  2  has concentric or spiral lens grooves  4  each having a V-shaped cross-section. As shown in  FIG. 10 , one side surface of each lens groove  4  is called Fresnel surface f, while the other side surface is called rise surface r. The Fresnel lens  2  is characterized in that the inclination angle (Fresnel lens angle Ø) of Fresnel surface f slightly differs between adjacent grooves and gradually varies. 
   Fresnel lenses have recently been produced by injection molding processes. In machining of a mold for molding a Fresnel lens, it is necessary to machine with precision lens grooves whose inclination angles of Fresnel surfaces f vary gradually. 
   Vertical lathes have been employed as a machine tool for carrying out machining of such gradually varying grooves (see Japanese Laid-Open Publication No. 1998-142405). 
   Conventional vertical lathes for carrying out machining of such gradually varying grooves include a tool post swivel mechanism for changing the nose angle of a cutting tool.  FIGS. 12 and 13  show tool post swivel mechanisms of conventional vertical lathes. In vertical lathes that employ the respective tool post swivel mechanism, a saddle moves linearly and horizontally in the X-axis direction and a ram moves linearly and vertically in the Z-axis direction by means of a mechanism common to the lathes. For swiveling of the respective tool posts in the B-axis direction, however, the lathes employ different mechanisms. 
   Referring first to the tool post swivel mechanism of  FIG. 12 , the tool post  110  is of a type provided with four cutting tools  100 . A swivel head  112 , which supports a swivel shaft  114  by an air bearing, is mounted to the lower end portion of a ram  111 . The swivel shaft  114  is secured in the center of the tool post  110 , and the four cutting tools  100  are held symmetrically about the swivel shaft  114  at an angle of 90° with each other. In  FIG. 12 , reference numeral  115  denotes a servo motor for driving the swivel shaft  114 . 
   In the tool post swivel mechanism shown in  FIG. 13 , on the other hand, an R guide  118  having an arc-shaped guide surface is provided at the lower end of a ram  117 . A swivel base  119  swings or tilts by the guide of the R guide  118  so that a cutting tool  100 , which is held by a tool post  120  fixed on the swivel base  119 , is allowed to swivel with its nose as the center of swiveling. Thus, only one cutting tool  100  is held by the tool post  120 . As a drive mechanism for the B-axis swiveling is employed a mechanism (not shown) which converts a linear movement, for example by means of a ball screw mechanism, into the tilting movement of the swivel base  20 . The tool post swivel mechanism of  FIG. 13  differs in this respect from the tool post swivel mechanism of  FIG. 12  which simply swivels the tool post  110  by means of the B-axis swivel shaft  114 . 
   Machining (cutting) of lens grooves in a mold for molding of a Fresnel lens, using the conventional vertical lathe having the tool post swivel mechanism shown in  FIG. 12 , is carried out in the following manner. First, while a cutting edge  101  of the cutting tool  100  is inclined in conformity with a Fresnel lens angle Ø, as shown in  FIG. 11 , the cutting tool  100  is lowered while a turning table is rotated so that by synthesized feed of the tool by X-axis and Z-axis movements, cutting of a rise surface r is effected to a certain cut-in depth in the rise surface r of a workpiece W. When the cutting tool  100  has reached a predetermined depth (in the Z-axis direction), the cutting edge  101  of the cutting tool  100  is transferred to the workpiece W to create a Fresnel surface f. The rise surface r cannot be cut into a mirror surface by the above cutting operation because of the synthetic feed by X-axis and Z-axis movements. 
   Next, following the machining of the Fresnel surface f, finish machining of the rise surface r with a cutting edge  102  is carried out by the following operation. 
   The cutting tool  100  is released from the workpiece W, and the tool post  110  is swiveled so that the cutting edge  102  of the cutting tool  100  matches the inclination of the rise surface r. By the swiveling of the tool post  110 , the X-axis position of the cutting tip of the cutting tool  100  is displaced widely from the lowermost point at the bottom of the lens groove. Accordingly, the cutting tool  100  must be moved by X-axis movement to align the cutting tip of the cutting tool  100  with the lowermost point at the bottom of the lens groove. Thereafter, the cutting tool  100  is lowered by Z-axis movement to carry out machining of the rise surface r to a certain cut-in depth. 
   The angular change of the cutting tool  100  thus involves X-axis movement and Z-axis movement, which may entail errors, such as a positioning error and a displacement of the position of the cutting edge due to an error in the X-axis linear feed motion. Such errors lead to a deviation of the position of the nose of the cutting tool  100  from the intended lowermost point at the bottom of each lens groove, resulting in errors in the pitch and the depth of lens grooves. 
   In the case of the tool post swivel mechanism shown in  FIG. 13 , on the other hand, the cutting tool  100  swivels with its nose as the center of swiveling, as described above. It is, therefore, theoretically possible that after machining of a Fresnel surface f, the cutting edge  102  of the cutting tool  100  can be matched to a rise surface r only by swiveling the swivel base  119  while keeping the nose of the cutting tool  100  at the lowermost point at the bottom of the lens groove. 
   In actual machining, however, it is very difficult to position the nose of the cutting tool  100  at the center of swiveling and keep the position during the swiveling operation. It is noted in this regard that the accuracy of movement (swiveling) is greatly influenced by the configurational accuracy (roundness) of the guide surface of the R guide  118  which guides the swivel base  119 , and it is very difficult to provide the guide surface of the R guide  118  with high roundness. Thus, depending upon how the swivel base  119  makes contact with the guide surface of the R guide  118 , it is possible that the position of the nose of the cutting tool  100  deviates from the intended position. Further, according to the tool post swivel mechanism shown in  FIG. 13 , because of the large radius of swiveling of the cutting tool  100 , the swivel mechanism of a considerable weight is disposed on the front side of a cross rail. This tends to cause a torsion in the cross rail, which may significantly affect the position of the cutting tip of the cutting tool  100 , leading to displacement of the actual position of the cutting tip from the intended position. 
   SUMMARY OF THE INVENTION 
   It is therefore an object of the present invention to solve the above problems in the prior art and provide a vertical lathe which, when machining gradually varying lens grooves in a mold for molding of a Fresnel lens, can change the angle of a cutting tool without positional displacement of the cutting tip, enabling high accuracy machining of the gradually varying lens grooves. 
   In order to achieve the above object, the present invention provides a vertical lathe comprising: a column; 
   a rotary table for rotating a workpiece; a cantilevered cross rail supported by the column; a saddle movable in a horizontal direction along a guideway of the cross rail; a ram suspended from the saddle and movable vertically along a guideway of the saddle; and an inclined head mounted to the lower end of the ram and having an inclined swivel shaft for holding a cutting tool, wherein said swivel shaft is configured to swivel the cutting tool around a nose of the cutting tool as the center of swiveling. 
   In a preferred embodiment of the present invention, the inclined head includes the swivel shaft rotatably supported by a bearing and inclined with respect to a horizontal plane, an eccentric arm eccentric to the axis of the swivel shaft and extending in the axial direction, a tool holder mounted to the front end of the eccentric arm and holding the cutting tool, and a swivel index means for rotating the swivel shaft and indexing the swivel the cutting tool. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a perspective view of a vertical lathe according to an embodiment of the present invention; 
       FIG. 2  is a side view of the vertical lathe according to the embodiment; 
       FIG. 3  is a cross-sectional view showing the constructions of the saddle and the ram of the vertical lathe according to the embodiment; 
       FIG. 4  is a side view of an inclined head provided in the vertical lathe according to the embodiment; 
       FIG. 5  is a diagram illustrating the lens grooves of a mold for molding a Fresnel lens; 
       FIGS. 6A and 6B  are diagrams illustrating the movement of a cutting tool in machining of gradually varying grooves; 
       FIG. 7  is a side view of the vertical lathe with a planer table mounted on the turning table; 
       FIG. 8  is a diagram illustrating the movement of a cutting tool in planing of gradually varying lens grooves; 
       FIG. 9  is a plan view showing a Fresnel lens; 
       FIG. 10  is a diagram illustrating lens grooves of a Fresnel lens; 
       FIG. 11  is a diagram illustrating a conventional machining of the lens grooves of a mold for molding a Fresnel lens; 
       FIG. 12  is a diagram illustrating a tool post swivel mechanism provided in a conventional vertical lathe; and 
       FIG. 13  is a diagram illustrating another tool post swivel mechanism provided in a conventional vertical lathe. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
   A Preferred embodiment of the present invention will now be described with reference to the drawings. 
     FIG. 1  is a perspective view showing the overall construction of a vertical lathe according to an embodiment of the present invention, and  FIG. 2  is a side view of the vertical lathe. In  FIGS. 1 and 2 , reference numeral  10  denotes a bed. A column  12  is installed on the upper surface of the bed  10 . According to this embodiment, a cantilevered cross rail  14 , integrally with the column  12 , constitutes an integral upper structure. On the cross rail  14  is provided a saddle  16  which is movable horizontally on the cross rail  14 . A vertically movable ram  18  is suspended from the saddle  16 . To the lower end of the ram  18  is mounted an inclined head  20 . A cutting tool  100  is held at the front end of the inclined head  20 . 
   Reference numeral  22  denotes a rotary table. The rotary table  22  is comprised of a turning table  24  provided with a vacuum chuck  23  for fixing a workpiece to be machined, and a table body  25  rotatably supporting the turning table  24  in a horizontal attitude. The table body  25  is connected to a table drive unit  26 . 
   According to the vertical lathe of this embodiment, the saddle  16  is moved horizontally in the X-axis direction, and a ball screw feed mechanism for the X-axis movement of the saddle  16  is driven by an X-axis servo motor  27 . The ram  18  is moved vertically in the Z-axis direction, and a ball screw feed mechanism for the Z-axis movement of the ram  18  is driven by a Z-axis servo motor  28 . 
     FIG. 3  shows the details of the constructions of the saddle  16  and the ram  18 . In  FIG. 3 , two X-axis guide surfaces  30   a ,  30   b , parallel to each other, for guiding the movement of the saddle  16  are provided in the upper surface of the cross rail  14 . A hollow portion  32  housing the ram  18  is formed in the center of the cross rail  14 . The saddle  16  bridges the hollow portion  32  and is supported on either side by the X-axis guide surfaces  30   a ,  30   b . The X-axis guide surfaces  30   a ,  30   b  are comprised of a limited-type rolling guide. Reference numeral  33  denotes an X-axis ball screw constituting the X-axis ball screw feed mechanism. 
   A ram guide member  34  in the shape of a square cylinder is suspended from the bottom of the saddle  16 . Z-axis linear guide sections  35   a ,  35   b , each comprised of a vertically-extending rolling guide, are fixed to the left and right inner side surfaces of the ram guide member  34 . Vertically-extending Z-axis guide rails  36   a ,  36   b  are mounted to the left and right side surfaces of the ram  18 , and the Z-axis guide rails  36   a ,  36   b  engage the Z-axis linear guide sections  35   a ,  35   b.    
   An X-axis servo motor  28  is installed on the upper surface of the saddle  16 . A vertically-extending Z-axis ball screw  38  is rotatably supported by a bearing member  37  at the bottom of the saddle  16 . A ball nut  39  is fixed to the upper end of the ram  18 , and the ball nut  39  makes screw engagement with the Z-axis ball screw  38 . A pair of balanced cylinders  40  is mounted on the upper surface of the saddle  16 . The weight of the ram  18  is supported by the balanced cylinders  40 , thereby reducing the load on the Z-axis servo motor  28 . 
   To the lower end of the ram  18  is mounted, instead of the conventional tool post with a swivel head shown in  FIG. 12 , the below-described inclined head  20  having an inclined swivel shaft  44 . The use of the inclined head  20  makes it possible to set the center of swiveling at the cutting tip of the cutting tool  100  and change the cutting edge angle of the cutting tool  100  without changing the position of the cutting tip. 
     FIG. 4  shows the inclined head  20 . In  FIG. 4 , reference numeral  50  denotes the axis of the swivel shaft  44 , and reference numeral  52  denotes a horizontal plane (table surface). The “a” indicates the angle of inclination formed between the axis  50  of the swivel shaft  44  and the table surface  52 . 
   The inclined head  20  is held by a bracket  42  such that the axis  50  of the swivel shaft  44  forms an inclination angle a with the table surface  52 . Those portions of the inclined head  20  which interfere with the table surface  52  are chamfered as much as possible so as to make the inclination angle a small. According to this embodiment, the inclination angle a is set at 15°. The inclination angle a of the swivel shaft  44  is preferably within 30°. 
   The inclined head  20  is comprised of a head body  45 , an eccentric arm  46 , a tool holder  47  for holding the cutting tool  100 , and a swivel index section  48  for indexing the swivel angle of the cutting tool  100 . 
   A base portion  46   a  of the eccentric arm  46 , extending at a right angle with the axis  50 , is fixed to the front end of the swivel shaft  44 . The eccentric arm  46 , which is eccentric to the axis  50  with a predetermined radius of eccentricity, further extends from the base portion  46   a  at a right angle therewith and parallel to the axis  50 . The tool holder  47  is mounted to the front end of the eccentric arm  46 . The tool holder  47  holds the cutting tool  100  at a right angle with the eccentric arm  46 . The use of such eccentric arm  46  enables the nose A of the cutting tool  100  to be positioned on the axis  50 , as shown in  FIG. 4 , that is, the nose A can be set as the center of swiveling. 
   The swivel index section  48  is driven by a servo motor  54 . The swivel index section  48  takes positional feedback by means of a rotary encoder  56  and controls the rotation of the servo motor  54  to thereby index the swivel angle of the swivel shaft  44  with an accuracy of {fraction (1/100,000)} degree. The B-axis denotes the control axis of such swivel index section  48 . 
   In response to the B-axis indexing accuracy, an air bearing  58  for rotatably supporting the swivel shaft  44  is incorporated into the head body  45 . 
   A description will now be given of machining of the gradually varying lens grooves of a mold for molding a Fresnel lens by way of an machining example as carried out with the vertical lathe of this embodiment having the above-described construction. 
     FIG. 5  is a diagram illustrating the configuration of the lens grooves of a mold for molding a Fresnel lens. In the lens grooves arranged concentrically, the inclination angle of Fresnel surface f of a groove gradually increases as the groove becomes closer to the circumference. Depending upon the type of Fresnel lens, the inclination angle of rise surface r either vary gradually or is constant. According to this embodiment, the lens grooves having such Fresnel surfaces are machined in the follow manner. 
   In preparation for machining, positioning is made in the inclined head  20  so that the nose A of the cutting tool  100  held by the tool holder  47  lies on the axis  50  of the swivel shaft  44 , as shown in FIG.  4 . 
   Referring to  FIG. 2 , the workpiece W whose lens grooves are to be machined by turning, is fixed by means of the vacuum chuck  23  while the center of the workpiece W is aligned with the center of rotation of the turning table  24 . The turning table  24  is then rotated. 
   By moving the saddle  16  in the X-axis direction, the inclined heal  20  is moved to a machining position.  FIG. 6  shows the positional relationship between the workpiece W and the nose A of the cutting tool  100 . First, as shown by the broken lines in  FIG. 6A , the X-axis position of the nose A is aligned with the intended position of the lowermost point at the bottom of a lens groove with a Fresnel surface f 1 . The swivel index section  48  is then driven by the B-axis servo motor  54  to swivel the swivel shaft  44 , so that the angle of a cutting edge  101  of the cutting tool  100  becomes identical with the inclination angle Ø 1  of the Fresnel surface f 1 . 
   Thereafter, the cutting tool  100  is lowered in the Z-axis direction, and the cutting edge  101  of the cutting tool  100  is transferred to the workpiece W to create a Fresnel surface f 1 . 
   At the time of the creating of Fresnel surface f 1 , the nose A of the cutting tool  100  is positioned just on the lowermost point at the bottom of the lens groove and also on the axis  50  of the swivel shaft  44  of the inclined head. Accordingly, the cutting tool  100  can swivel with the lowermost point at the bottom of the lens groove as the center of swiveling. 
   Therefore, in order to machine a rise surface r 1  subsequently to the machining of the Fresnel surface f 1 , the cutting tool  100  is swiveled by B-axis swiveling with the nose A as the center of swiveling, without a Z-axis movement, whereby a cutting edge  102  of the cutting tool  100  can be matched to the inclination of the rise surface r 1  (FIG.  6 B). The cutting edge  102  of the cutting tool  100  can thus be transferred to the workpiece W to create a rise surface r 1  without changing the position of the nose A of the cutting tool  100 . 
   After machining of one lens groove is thus completed, the cutting tool  100  is released from the lens groove by Z-axis movement, and is then moved by X-axis movement to a position above the next lens groove having a Fresnel surface f 2 . The X-axis position of the nose A of the cutting tool  100  is aligned with the lowermost point at the bottom of the lens groove, and the cutting tool  100  is then swiveled by B-axis swiveling so as to make the angle of the cutting edge  101  identical with the inclination angle Ø 2  of the Fresnel surface f 2 . Thereafter, the same operation as in the machining of the first lens groove is repeated, whereby the Fresnel surface f 2  and the rise surface r 2  can be machined successively without changing the position of the nose A of the cutting tool  100 . 
   As described above, according to the inclined head  20  in which the nose A of the cutting tool  100  is positioned on the axis of the swivel shaft  44 , the angles of the cutting edges of the cutting tool  100  can be changed to be matched to a Fresnel surface f and a rise surface r only by the B-axis swiveling operation without an X-axis or Z-axis movement. This eliminates a displacement which may be caused by an X-axis movement of the saddle  16  or a Y-axis movement of the ram  18  and which would cause a nose position error of the cutting tool. 
   Further, the inclined swivel shaft  44  is supported by the air bearing  58  and, in addition, the swivel movement of the cutting tool  100  is made under a numeral control with the index mechanism  48 , which is driven by the servo motor  54 , as the B-axis. Accordingly, as compared with the conventional swivel mechanism which likewise can set the cutting tip as the center of swiveling but which relies on the roundness of the R guide for the movement accuracy, the inclined head  20  according to the present invention can obtain an incomparably high index accuracy. Further, since neither an X-axis movement nor a Y-axis movement is involved in the operation of changing the cutting edge angle of the cutting tool  100  from the inclination angle of a Fresnel surface f to the inclination angle of a rise surface r, the operation time can be reduced. Because of the generally large number of lens grooves, this leads to a considerable reduction in the total machining time. 
   Having described above machining (turning) of a mold for molding a Fresnel lens, the vertical lathe of the present invention, in combination with a linear movement-type planer table, can also be applied to machining (planing) of a mold for molding an optical waveguide. 
     FIG. 7  is a side view showing the vertical lathe adapted for setting a planer table  60  on the turning table  24  to perform planing. 
   The planer table  60  is basically comprised of a table base  62  fixed on the upper surface of the turning table  24 , and a movable table  64  which is movable on the table base  62 . V-shaped grooves, constituting a linear guide, are formed in the upper surface of the table base  62 , so that by the guide of the grooves, the movable table  64  can move linearly by means of a not-show ball screw feed mechanism. 
   According to the vertical lathe, the Y-axis indicates a control axis for the feed of the movable table  64 . The axial direction of the ball screw can change with the rotation of the turning table  24 . When carrying out planing, therefore, the position of the turning table  24  is indexed so that the axial direction of the ball screw coincides with the direction orthogonal to both the X-axis and the Z-axis (direction perpendicular to the paper sheet of FIG.  7 ), and the turning table  24  is then fixed by means of a not-shown clamp means, thereby making the axial direction of the ball screw coincident with the direction of the Y-axis which is the positional control axis on the machine. 
   According to the vertical lathe of this embodiment thus provided with the planer table  60  on the rotary table  22 , machining (planing) of gradually varying lens grooves can be carried out in the following manner. 
     FIG. 8  shows the configuration of the lens grooves of a mold for molding an optical waveguide. The lens grooves extend parallel to each other, and the angle formed between the inclined surfaces of a lens groove gradually increases as the groove is closer to the left side end of the mold. In each lens groove, the inclination angles of the both inclined surfaces are equal. 
   First, the inclined head  20  is moved by X-axis movement of the saddle  16  to a machining position, and the X-axis position of the nose A of the cutting tool  100  is aligned with the lowermost point at the bottom of a lens groove L 1 . The B-axis servo motor  54  is then driven to swivel the swivel shaft  44  so as to make the angle of the cutting edge  101  of the cutting tool  100  identical with the inclination angle of an inclined surface Kr 1  on the right side. 
   Next, the cutting tool  100  is lowered by Z-axis movement, and the cutting edge  101  of the cutting tool  100  is transferred to the workpiece W while cutting the workpiece W into a predetermined depth and moving the movable  64  by Y-axis movement, thereby creating a Fresnel surface Kr 1 . 
   Thereafter, the cutting tool  100  is swiveled by B-axis swiveling with the nose A as the center of swiveling so as to make the angle of the cutting edge  102  of the cutting tool  100  identical with the inclination angle of an inclined surface Kl 1  on the left side. The cutting edge  102  of the cutting tool  100  is then transferred to the workpiece W while moving the movable table  64  by Y-axis movement, thereby creating a Fresnel surface Kl 1 . The right inclined surface Kr 1  and the left inclined surface Kl 1  can thus be processed successively without changing the position of the nose A of the cutting tool  100 . The other lens grooves can also be machined in the same manner. 
   As described hereinabove, the vertical lathe of the present invention, when machining gradually changing lens grooves in a mold for molding a Fresnel lens, can change the angle of a cutting tool without positional displacement of the cutting tip and effect high accuracy machining of the gradually varying lens grooves.