Abstract:
A pipe electrode holder for holding an elongated pipe electrode ( 6 ) having a relatively small diameter, the pipe electrode holder being coaxially attached to a rotating spindle ( 7 ) of a small hole electric discharge machine. The pipe electrode holder has a cylinder capable of fitting into the rotating spindle, the cylinder having a threaded upper end and a through hole ( 1   a ), and a collet ( 2 ) for gripping the pipe electrode having a through hole ( 2   a ) through which the pipe electrode may pass, the collet being fit into the through hole of the cylinder. A holder head ( 5 ) capable of adjusting the gripping force of the collet is not screwed to the lower end of the cylinder from which the pipe electrode extends, but to the threaded upper end of the cylinder.

Description:
FIELD OF THE INVENTION 
     The present invention relates to a small hole electric discharge machine and more particularly to a small hole electric discharge machine for forming ultrafine holes in a workpiece by generating electric discharge between a rotating long thin pipe electrode and the workpiece, and more particularly still to a pipe electrode holder for holding the pipe electrode, which can be attached to a rotating spindle of the small hole electric discharge machine so that the pipe electrode is arranged coaxially with the rotating spindle. 
     BACKGROUND OF THE INVENTION 
     When forming a start hole in a workpiece in order to pass a wire electrode through, for example, a small hole electric discharge machine is utilized that uses a copper or copper alloy pipe electrode having an external diameter of 0.30 to 0.05 mm. The pipe electrode is arranged vertically coaxial with a rotating spindle holder of the small hole electric discharge machine by a pipe electrode holder. By causing electric discharge between the pipe electrode and the workpiece, and feeding the pipe electrode downwards, an ultrafine hole is formed in the workpiece. During machining, dielectric fluid is supplied to the hole being formed in the workpiece by being passed through the pipe electrode. The inner diameter of the pipe electrode is normally 0.12 mm when the external diameter is 0.30 mm, 0.07 mm when the external diameter is 0.15 mm, and 0.04 mm when the external diameter is 0.1 mm. It is known that these types of small diameter pipe electrodes are severely consumed during machining. The length to which the pipe electrode is consumed may be the same or even greater than the extent to which the pipe electrode is moved downwards. In order to reduce the frequency of replacing the pipe electrode, the pipe electrode has a length a few hundred times larger than the outer diameter, namely 100-500 mm. A small hole electric discharge machine has a power supply for supplying current to the pipe electrode, and a fluid supply device for supplying dielectric fluid into the pipe electrode. 
     Japanese Patent Laid-open No. 8-290332 discloses a pipe electrode holder including a collet for gripping a pipe electrode. The collet is fitted into a through hole of a cylinder, and an upper end of the cylinder is capable of being attached to a lower end of a rotating spindle of the small hole electric discharge machine, while a lower end of the cylinder is screwed into a cap nut. An operator first inserts the pipe electrode into a through hole of the collet from the lower end of the cylinder. The force with which the collet holds the pipe electrode can be adjusted by tightening the cap nut. If the cap nut is tightened, the collet moves upward in the axial direction. In this way, a vertical joint in the collet is closed up to hold the pipe electrode tightly. During this operation, it is quite common for accidents to occur where the operator touches the pipe electrode causing it to become bent. 
     DISCLOSURE OF THE INVENTION 
     An object of the present invention is to provide a pipe electrode holder capable of moving a collet in an axial direction at an upper side of a cylinder to close a joint in the collet, so that an operator does not bend the pipe electrode. 
     Another object of the present invention is to provide a pipe electrode holder having a pipe electrode reliably arranged coaxial with a rotating spindle of a small hole electric discharge machine when the pipe electrode holder is attached to the rotating spindle. 
     Other objects of the present invention will become clear to a person skilled in the art upon reference to the following description and practicing the invention. 
     In order to achieve the above described objects, a pipe electrode holder for holding a pipe electrode, according to the present invention, is capable of being coaxially attached to a rotating spindle of a small hole electric discharge machine, and comprises a cylinder ( 1 ) capable of being fitted into a rotating spindle ( 7 ), having a threaded upper end and a through hole ( 1   a ), a collet ( 2 ), having a hole ( 2   a ) through which the pipe electrode ( 6 ) passes, fitted into the through hole of the cylinder, for gripping a pipe electrode, and a holder head ( 5 ) for screwing into the threaded upper end of the cylinder so as to make the gripping force of the collet adjustable. 
     Preferably, the pipe electrode holder further comprises an elastic seal block ( 4 ), having a through hole ( 4   a ) through which the pipe electrode passes, fitted into the through hole of the cylinder between the collet and the holder head. 
     More preferably, the pipe electrode holder further comprises a washer ( 3 ), having a through hole ( 3   a ) through which the pipe electrode passes, fitted into the through hole of the cylinder between the collet and the seal block. 
     The collet preferably has a first tapered surface ( 2   d ) with a joint, and the washer preferably has a tapered surface ( 3   d ) for engaging with the first tapered surface. 
     The collet also preferably has a second tapered surface ( 2   c,    2   e ) having a joint, and a tapered surface ( 1   c,    1   h ) for engaging with the second tapered surface is formed in the through hole of the cylinder. 
     In order to reliably arrange the pipe electrode coaxially with the rotating spindle, a flange ( 1   f ) extending radially and capable of contacting a bottom surface of the rotating spindle is preferably formed in the cylinder. 
     A flange ( 5   d ) extending radially and capable of contacting a top face of the cylinder is preferably formed on the holder head. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a vertical cross sectional drawing illustrating a pipe electrode holder of the present invention; 
     FIG. 2 is an exploded view illustrating the pipe electrode holder of FIG. 1; 
     FIG. 3 is a vertical cross sectional drawing illustrating a rotating spindle of an electric discharge machine, to which the pipe electrode holder of FIG. 1 can be attached. 
     FIG. 4 is a vertical cross sectional drawing illustrating a rotating spindle of an electric discharge machine, to which the pipe electrode holder of FIG. 1 is attached. 
     FIG. 5 is a vertical cross sectional drawing illustrating another pipe electrode holder of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     A preferred embodiment of a pipe electrode holder according to the present invention will now be described with reference to FIG.  1  and FIG.  2 . 
     In the illustrated embodiment, a pipe electrode  6  comprises a section  6   a  having an external diameter of about 0.30-0.05 mm and a length of about 180 mm, and a shank part  6   b  having an external diameter larger than that of the section  6   a  and a length of about 70 mm. The shank part  6   b  for inserting into a pipe electrode holder has an internal diameter of about 1.0 mm so that the pipe electrode is not smashed. The pipe electrode holder is comprised of a cylinder  1 , a collet  2 , a washer  3 , a seal block  4 , and a holder head  5 . 
     The cylinder  1  has a through hole  1   a  passing through it. A female screw  1   b  is formed in an upper end of the through hole  1   a.  The internal diameter of the through hole  1   a  is reduced stepwise from an upper end to a lower end at shoulders  1   c  and  1   d.  The shoulder  1   c  has an about 450 tapered surface. The internal diameter of the through hole  1   a  further down than the shoulder  1   d  is slightly larger than the shank part  6   b,  for example, about 1.010 mm. An annular groove  1   e  is formed in an outer surface of the cylinder  1 , and a flange  1   f  extending in the radial direction is formed further down than the annular groove  1   e.  A side surface  1   g  above the annular groove  1   e  is a tapered surface converging downwards. As shown in FIG. 2, the collet  2 , washer  3  and seal block  4  are sequentially fitted into the through hole  1   a  from an upper end of the cylinder  1 . After that, the holder head  5  capable of adjusting the gripping force of the collet  2  is screwed to the upper end of the cylinder  1 . Assembly of the pipe electrode holder is thus completed. 
     The collet  2  is made from a material having good electrical conductivity, such as beryllium alloy, and has a through hole  2   a  for insertion of the pipe electrode  6  in the center. The collet  2  is made up of a slide joint section  2 S having four joints  2   b  formed at equal intervals extending in the radial direction, and a tubular section  2 T not containing the joints  2   b.  An about 45° tapered surface  2   c  for engaging with the tapered surface  1   c  of the cylinder  1  is formed on the slide joint section  2 S. An about 45° tapered surface  2   d  is also formed on an upper end of the slide joint section 
     The washer  3  has a through hole  3   a  for insertion of the pipe electrode  6  in its center, and a tapered surface  3   b  for engaging with the tapered surface  2   d  of the slide joint section  2 S. 
     The seal block  4  is generally cylindrical and is made of an elastic material such as rubber, and has a through hole  4   a  for insertion of the pipe electrode  6  in its center. A bottom surface of the seal block  4  comes into contact with the top face of the washer  3 . A bottom surface of the holder head  5  comes into contact with a top face of the seal block  4 . 
     The holder head  5  has a through hole  5   a  for insertion of the pipe electrode  6  in its center. A male screw  5   b,  adopted to be for being screwed into the female screw  1   b  of the cylinder  1  is formed on a lower end of the holder head  5 . A protruding section  5   c  of about 3 mm extending in the axial direction is formed on the upper end of the holder head  5 . Also, a flange  5   d  having, for example, a hexagonal or rectangular cross section and extending in the radial direction is formed between the male screw  5   b  and the protruding section  5   c.    
     The procedure for fixing the pipe electrode  6  to the pipe electrode holder will now be described with reference to FIG.  1 . 
     First, the male screw  5   b  of the holder head  5  is loosened so that the flange  5   d  is separated from the top face of the cylinder  1  by a distance of about 1-2 mm. The shank part  6   b  of the pipe electrode  6  is inserted into the through hole  1   a  of the cylinder  1  from a lower end of the cylinder  1 . A tip end of the shank part  6   b  of the pipe electrode  6  passes through the through hole  2   a  of the collet  2 , the through hole  3   a  of the washer  3 , the through hole  4   a  of the seal block  4  and the through hole  5   a  of the holder head  5 , and projects, e.g., a few mm, from the top face of the holder head  5 . Finally, the male screw  5   b  of the holder head  5  is tightened up until the flange  5   d  comes into contact with the top face of the cylinder  1 . Inclination of the holder head  5  is corrected by the contact of the top face of the cylinder  1  and the flange  5   d,  and the pipe electrode  6  and the cylinder  1  are arranged coaxially. Lowering of the holder head  5  pressurizes the seal block  4 , and the suppression force of the seal block  4  causes the washer  3  and the collet  2  to be moved downwards. As a result, the tapered surface  3   b  of the washer  3  slides on the tapered surface  2   d  of the slide joint section  2 S, and the tapered surface  2   c  of the slide joint section  2 S slides on the tapered surface  1   c  of the cylinder  1 . The joints  2   b  of the collet  2  are closed up so that gripping claws of the collet  2  grip the pipe electrode  6 . The dimensions of the tapered surface  1   c,  the collet  2 , the washer  3 , the seal block  4 , the male screw  5   b  and the flange  5   d  are designed so that the compressed seal block  4  reliably seals the collet  2  and it is possible for the collet  2  to generate a suitable gripping force. Accordingly, contact between the top face of the cylinder  1  and the flange  5   d  can always ensure a suitable gripping force. Since the washer  3  is positioned between the seal block  4  and the collet  2 , the compressed seal block  4  is prevented from getting into the joints of the slide joint section  2 S. 
     A rotating spindle of a small hole electric discharge machine to which the pipe electrode holder of FIG. 1 can be attached will now be described with reference to FIG.  3  and FIG.  4 . 
     A hollow spindle  7  of the small hole electric discharge machine is rotatably vertically supported by a bearing  9 . The diameter of a through hole  7   a  of the spindle  7  becomes larger at a lower end side of the spindle  7  so as to accommodate the pipe electrode holder. A supply pipe  26  is fitted to an upper end of the spindle  7  using an O-ring  27  in order to supply dielectric fluid to the through hole  7   a.  A timing belt  25  is suspended across a pulley  23   b  fitted to the spindle  7  and a pulley  23   a  fitted to an output shaft of a motor  24 . A beryllium alloy coil spring holder  10  provided with a heat sink fin  10   a  is fastened to an outer surface of the spindle  7 . A current feeding brush  22  is pressure welded to the outer surface of the coil spring holder  10 . A sleeve  11  is provided capable of sliding on the outer surface of the spindle  7 . A coil spring  12  is held between the coil spring holder  10  and the sleeve  11 , and urges the sleeve  11  downwards. A plurality of holes  13  formed in the spindle  7  are arrayed circumferentially at equal intervals around the axis of the spindle  7 . A plurality of balls  14  are respectively arranged inside the holes  13 . A tapered surface  11   a  of the sleeve  11  presses the balls  14  radially against an inner side. A lower end of the sleeve  11  comes into contact with a U-shaped arm  20 , and the arm  20  is attached to one end of a lever  18  by a support bar  19 . The lever  18  is pivotably supported on a shaft  18   a,  and an indent  18   b  is formed in the other end of the lever  18 . A cylindrical member  17   a  fitted to a tip end of a piston rod  17  capable of reciprocating inside an air cylinder  16  is fitted into the indent  18   b.  An O-ring case  15  is inserted into the through hole  7   a  of the spindle  7 , and has a hole  15   a  for accommodating the protruding section  5   c  of the holder head  5 . The O-ring  15   b  is provided to seal up a gap between the O-ring case  15  and the protruding section  5   c.    
     Attachment of the pipe electrode holder to the spindle  7  will now be described. 
     First, by moving the piston rod  17  downwards, the arm  20  attached to the lever  18  acts against the spring force of the coil spring  12  to raise the sleeve  11  to an unclamped position. At this time, the lever  18 , arm  20  and sleeve  11  are as shown by the imaginary line in FIG.  3 . Next, the protruding section  5   c  of the holder head  5  is made to confront the spindle  7 , and the pipe electrode holder is fitted into the through hole  7   a  so as to be arranged coaxial with the spindle  7 . The protruding section  5   c  of the holder head  5  is inserted into the hole  15   a.  Finally, by moving the piston rod  17  upwards, the sleeve  11  is released from the arm  20  and is lowered to a clamped position under the spring force of the coil spring  12 . The pipe electrode holder when the sleeve  11  is in the clamped position is shown in FIG.  4 . The balls  14  are pressed radially against an inner side by the tapered surface  11   a  and engage with an annular groove  1   e  in the cylinder  1 . In this way, the pipe electrode holder is restrained by the balls  14 . At this time, the balls  14  are in contact with the tapered surface  1   g  of the cylinder  1 , the tapered surface  11   a  of the sleeve  11  and a lower surface of the hole  13 . As the balls  14  are supported at these three contact points, a component of force acting on the balls  14  presses the pipe electrode holder upward. As the flange  1   f  of the cylinder  1  is pressured to stick fast to the bottom surface of the spindle  7 , the cylinder  1  is reliably arranged coaxially with the spindle  7 . During machining, current is supplied to the pipe electrode  6  through the brush  22 , the coil spring holder  10 , the spindle  7 , the balls  14 , the cylinder  1  and the collet  2 , and dielectric fluid is supplied at a high pressure of, for example, about 60-80 kg/cm 2  from the supply pipe  26  through the through hole  7   a  of the spindle  7  and the hole  15   a  of the O-ring case  15  to the inside of the pipe electrode  6 . If the sleeve  11  is returned again to the unclamped position, the pipe electrode holder can be lowered down and pulled out. 
     Another example of pipe electrode holder according to the present invention will now be described with reference to FIG.  5 . 
     Reference numbers that are the same as those used in FIG. 1 will be attached to the same elements, and detailed description of those elements will be omitted. 
     The collet  2  in FIG. 5, similarly to the collet  2  in FIG. 1, comprises a slide joint section  2 S having four joints  2   b  formed at equal intervals extending in the radial direction, and a tube section  2 T. An about 45° tapered surface  2 e formed on a lower end of the slide joint section  2 S is engaged with a tapered surface  1   h  converging downwards formed in the through hole  1   a.  A bottom surface of the seal block  4  is in contact with a top face of the tube section  2 T not containing the joints  2   b,  which means that the washer  3  in FIG. 1 is not necessary. Similarly to the pipe electrode holder in FIG. 1, the gripping force of the collet  2  is adjusted by the holder head  5 . The seal block  4  is compressed by tightening the male screw  5   b  of the holder head  5 , and the elastic force of the seal block  4  causes the collet  2  to move downwards. As a result, the tapered surface  2   e  of the collet  2  slides on the tapered surface  1   h  of the cylinder  1 . The joints  2   b  of the collet  2  are then closed so as to grip the pipe electrode  6  with the gripping claws of the collet  2 . 
     The present invention is not intended to be limited to the forms disclosed. Various improvements and variations are clearly possible upon reference to the above description. For example, it is possible to form at least one joint  2   b  in the collet  2 , and it is possible to use a suitable well known collet  2  for gripping an elongated electrode. The illustrated embodiments have been selected simply to describe the essence and practical application of the invention. The scope of the invention is defined in the attached claims.