Abstract:
There is provided an attachment for a spark erosion machine having a working head, the attachment including a top part fixedly attachable to the working head, a body suspended from, and moveable relative to, the top part with one degree of freedom in translation, a guide for guiding the body in the translational movement and a main shaft mounted in the body and rotatable about an axis parallel to the translational movement. There is also provided a driver for imparting the main shaft a rotational movement, a drive pin having an axis parallel to the axis of the main shaft and rotatably connected thereto by an adjustable connector adapted to connect the drive pin to the main shaft in a plurality of positions between a first limit position of coaxiality and a second limit position of maximal eccentricity relative to the main shaft. The attachment also includes an electrode holder associated with the drive pin, and coupler connected to the electrode holder for resolving the rotary movement of the drive pin, when in an axially eccentric position, into an electrode movement in a compound XY-direction only, producing an orbital movement of an electrode mounted in the electrode holder.

Description:
FIELD OF THE INVENTION  
       [0001]     The present invention relates to an attachment for conventional spark erosion machines, allowing the latter to produce, in particular, threads.  
       BACKGROUND OF THE INVENTION  
       [0002]     Spark-erosion machines work by producing electrical sparks between an electrode and a workpiece immersed in a dielectric fluid. The spark dislodges small particles from the workpiece which are subsequently flushed away by a jet of the dielectric. If such particles are not completely swept away, pre-settable electrical conditions will be altered, causing the electric controls to temporarily withdraw the electrode until the debris has been flushed away and the pre-set conditions have been re-established, after which the initial erosion process continues.  
         [0003]     Spark-erosion machines that are capable of producing threads do in fact exist in which the X-Y working table can be programmed to carry out a compound movement in the X-Y plane and the rotary electrode movement around the Z-axis can be coordinated with its longitudinal movement along that axis. Either one or the other of these features can be used to produce threads either by the direct method in which an electrode in the form of a thread tap (smaller than a standard tap by the required spark gap) is advanced into the workpiece, producing the thread turn by turn, or by the orbital method in which the electrode in the form of a tap of an outside diameter slightly smaller than the core diameter of the thread is introduced into the pre-existent bore, is fed sidewise into the workpiece to the full depth of the thread, and then moved orbitally, finishing the thread in one orbital motion of the electrode.  
         [0004]     The above-mentioned machines, known as CNC (Computerized Numerical Control) spark erosion machines are, however, far more complex and expensive than the well-known and widely used non-CNC spark erosion machines.  
       DISCLOSURE OF THE INVENTION  
       [0005]     It is thus one of the objects of the present invention to provide an attachment for non-CNC spark-erosion machines that will allow such machines to produce threads, undercut shapes and the like.  
         [0006]     In accordance with the present invention this is achieved by an attachment for a spark erosion machine having a working head, said attachment comprising a top part fixedly attachable to said working head; a body suspended from, and moveable relative to, said top part with one degree of freedom in translation; guide means for guiding said body in said translational movement; a main shaft mounted in said body and rotatable about an axis parallel to said translational movement; means for imparting said main shaft a rotational movement; a drive pin having an axis parallel to the axis of said main shaft and rotatably connected thereto by adjustable connection means adapted to connect said drive pin to said main shaft in a plurality of positions between a first limit position of coaxiality and a second limit position of maximal eccentricity relative to said main shaft; an electrode holder associated with said drive pin, and coupling means connected to said electrode holder for resolving the rotary movement of said drive pin when in an axially eccentric position into an electrode movement in a compound XY-direction only, producing an orbital movement of an electrode mounted in said electrode holder. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0007]     The invention will now be described in connection with certain preferred embodiments with reference to the following illustrative figures so that it may be more fully understood.  
         [0008]     With specific reference now to the figures in detail, it is stressed that the particulars shown are by way of example and for purpose of illustrative discussion of the preferred embodiments of the present invention only and are presented in the cause of providing what is believed to be the most useful and readily understood description of the principles and conceptual aspects of the invention. In this regard, no attempt is made to show structural details of the invention in more detail than is necessary for a fundamental understanding of the invention, the description taken with the drawings making apparent to those skilled in the art how the several forms of the invention may be embodied in practice.  
         [0009]     In the drawings:  
         [0010]      FIG. 1  is an exploded view of a preferred embodiment of the attachment according to the present invention;  
         [0011]      FIG. 2  is an enlarged view of Section I of  FIG. 1 ;  
         [0012]      FIG. 3  is an enlarged view of Section II of  FIG. 1 ;  
         [0013]      FIG. 4  is an enlarged view of Section III of  FIG. 1 ;  
         [0014]      FIG. 5  is a cross-sectional view in the plane of the crank block;  
         [0015]      FIG. 6  is a view in cross-section along plane VI-VI in  FIG. 5 ;  
         [0016]      FIG. 7  is a view in cross-section along plane VII-VII in  FIG. 5 ;  
         [0017]      FIG. 8  is a view in cross-section along plane VIII-VIII in  FIG. 5 ;  
         [0018]      FIG. 9  is a cross-sectional top view of the crank block as seen with the drive pin in its extreme position of eccentricity;  
         [0019]      FIG. 10  is a cross-sectional top view of the electrode holder with the drive pin in its extreme position of eccentricity;  
         [0020]      FIG. 11  is a greatly enlarged schematic view of the electrode and its path inside the workpiece;  
         [0021]      FIGS. 12   a  to  12   m  illustrate the step-by-step production of an internal thread in a workpiece;  
         [0022]      FIG. 13  depicts an electrode in the centre of an internal thread, with an enlarged insert showing the spark gap between the electrode and the workpiece;  
         [0023]      FIG. 14  represents the electrode and the workpiece as shown schematically in  FIG. 12   m;    
         [0024]      FIG. 15  shows an embodiment of the invention suitable for the preparation of recesses of a rotational symmetry;  
         [0025]      FIGS. 16   a - 16   d  illustrate the stages of preparation of a workpiece with such a recess;  
         [0026]      FIG. 17  is an elevational view of yet another embodiment of the invention in which the helix has been replaced by an electrical motor;  
         [0027]      FIG. 18  is a view in cross-section along plane XVIII-XVIIII in  FIG. 17 ;  
         [0028]      FIG. 19  is a view in direction of arrows A in  FIG. 18 ;  
         [0029]      FIG. 20  shows an embodiment of the invention equipped with a signalling device indicating the conclusion of the erosion process;  
         [0030]      FIGS. 21, 22  and  23  illustrate a variant of the electrode holder;  
         [0031]      FIG. 24  is atop view of a variant of the electrode holder of  FIG. 4 ;  
         [0032]      FIG. 25  is an elevation of  FIG. 24 ;  
         [0033]      FIG. 26  is a perspective view of the device depicted in  FIGS. 24 and 25 , and  
         [0034]      FIG. 27  represents an enlarged view of a portion of  FIG. 25 . 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0035]     Referring now to the drawings there is seen in  FIG. 1  an exploded view of the attachment according to the invention. For the sake of clarity, this view has been subdivided into three sections I, II and III, which are represented separately and to a larger scale in  FIGS. 2, 3  and  4 , respectively.  
         [0036]     The attachment ( FIG. 2 ) comprises a top plate  2 , carrying a protective skirt  3  and provided with a chucking shaft  4  by which the attachment is mounted on the head of the machine. To top plate  2  are affixed two posts  6 ,  6 ′ which slide in linear bearings  8 ,  8 ′ mounted in body  10 , so that the latter is attached to the head of the machine with one degree of freedom in translation in direction of the Z-axis. A compression spring  12 , the upper end of which is guided by a bar  14 , rests against the bottom  15  ( FIG. 5 ) of a tube  16  mounted in body  10 . Spring  12  tends to push body  10  away from top plate  2 . To the underside of body  10  is attached a retaining strip  17 , which, being attached to posts  6 ,  6 ′, prevents the upper and lower parts of the attachments from being pulled apart.  
         [0037]     Also shown in  FIGS. 2 and 4  are bearing housings  18 ,  18 ′, advantageously integral with body  10 , and linear bearings  19 ,  19 ′, the purpose of which will become apparent further below.  
         [0038]     Further seen is a helix  20 , advantageously made of a twisted metal strip, the upper end of which is fixedly attached by means of a pin  22  to a helix holder  24  which, in turn, is held by means of screw  26  in a bore  28  in top plate  2 . Helix  20  screws into an appropriately shaped hole  30  in a plate  32  fixedly attached to main shaft  34 , shown in the exploded view of  FIG. 3  and the cross-sectional assembly of  FIG. 5 . Main shaft  34  is rotatably mounted in ball bearings  36 ,  36 ′ held in position by retaining rings  38 ,  38 ′. These components, as well as a spiral spring  40  are seen to better advantage in  FIG. 5 . Spring  40 , the inner end of which is attached to main shaft  34  and the outer end of which is attached to body  10 , is intended to prevent lost motion.  
         [0039]     In the lower half of  FIG. 3  is seen a bottom plate  42  closing up body  10  from below. As plate  42  must be electrically insulated from body  10 , there is provided an insulating gasket  44 , which constitutes the interface between plate  42  and body  10 . An additional component is a nut  46  pressed into plate  42 , into which is threaded a metal stop  48  provided with locking disk  50 . As will be explained in detail further below, as soon as the settable stop  48 , descending with the entire attachment, hits the working table, helix  20  continues descending and sets into motion the entire erosion process. Sleeves  52  made of an insulating material, serve to accommodate screws (not shown) to attach bottom plate  42  to body  10 .  
         [0040]      FIG. 4  comprises the end effectors of the so far described attachment components. There is seen a crank housing  54  attached to main shaft  34  by means of screws  56 ,  56 ′. Housing  54  has two coaxial bores  58  of which only the top one is visible, into which bores fit two ball bearings  60  accommodating the ends of a shaft  62  that, seated in a bore  63  of crank block  64 , serves as a hinge for the latter. Crank block  64  has a second bore,  66 , into which is press-fitted a drive pin  68  which slide-fits a bore  70  in electrode holder  72 .  
         [0041]     Further seen in  FIG. 4  is a scale-bearing setting screw  74  seated in a collar in a recessed part  75  of crank housing  54 , accessible through an opening  76  in body  10 , by means of which screw the depth of penetration of the electrode on its orbital path can be set between a position of rest in which pin  68  is concentric with the axis of main shaft  34  ( FIGS. 5, 6 ,  7  and  8 ) and a position of maximal penetration in which the eccentricity of pin  68  relative to main shaft  34  is maximal, as is penetration ( FIGS. 9 and 10 ). As can be seen in  FIG. 6 , the position of rest is defined by a counter-screw  78  seated in crank housing  54 .  
         [0042]     Also shown is a yoke-shaped coupling member  80 , which is in fact a variant to the well-known Oldham coupling, and consists of a fork-like central portion  82 , in which is mounted a linear bearing  84  and in the tines of which are fixedly seated bars or rods  86 ,  86 ′. These bars are held and guided in linear bearings  19 ,  19 ′ shown in  FIGS. 2 and 4 , which guide coupling member  80  in the X-direction.  
         [0043]     A third rod,  86 ″, is mounted in electrode holder  72 , bridging recess  88  (see  FIG. 5 ). It is along this rod that linear bearing  84  guides coupling member  80  in the Y-direction.  
         [0044]     While linear bearings  84  contribute to smooth working of coupling  80 , its efficiency could be further improved by adding another bar coaxial with bar  86 ″.  
         [0045]     The electrode intended to perform the erosion process is clamped into V-notch  90  of the electrode holder  72  by means of clamp  92  attached to electrode holder  72  with screws  94 ,  94 ′.  
         [0046]     Coupling member  80  is one of the major components of the attachment according to the invention: it resolves the rotary motion of drive pin  68  in its eccentric position into a succession of XY-steps that add up to the electrode moving along an orbital, circular path, while not rotating about its own axis, as will be explained in detail further below.  
         [0047]     A further component important to the proper performance of the attachment is a friction pad  96  pressed by a spring  98  against pin  68 . Spring  98  is compressed and retained by a screw  100 , all seen to best advantage in  FIGS. 4 and 7 . This friction arrangement is necessary to ensure immediate reaction to the frequent stoppages, withdrawals and re-engagements inherent in the spark erosion process, that are required to flush away erosion debris.  
         [0048]      FIG. 11  illustrates the path of the centre of electrode  102  inside the preliminary bore  104  after the first penetration of electrode  102  to the full depth T of the thread  106 . The initial position of the centre of electrode  102  is denoted  0 , which is the position in which electrode  102  is still in the centre of bore  104 . Having penetrated to the depth T, the orbital motion begins, with the triangle marking an arbitrary point at the electrode circumference that, as can be seen in the following  FIGS. 12   a  to  12   m , remains stationary, indicating the absence of rotational movement. Several virtual stations are marked along this path, as well as some stoppages, withdrawals and re-engagements that have already been mentioned as inherent to the spark erosion process. Thus, it can be seen that e.g., at station  3  a stoppage occurs, followed by a withdrawal right to the centre and a re-engagement at a point that has been already passed. It will be appreciated that these stoppages, causing reversal of the head movement, are effected by the control unit of the spark erosion machine itself, while the prompt withdrawals and re-engagements are facilitated by the above-mentioned friction pad  96  that, immediately after a stoppage and head reversal, causes crank block  64  to be thrown against counter-screw  78 , thereby moving electrode  102  towards centre station  0 . Conversely, when the blockage has been flushed away, the head of the machine resumes its descent, causing electrode  102  to return, due to the inevitable delay, to point  5 , i.e., a point already passed by electrode  102 . Minor stoppages, such as that between stations  7 ,  8 ,  9 , are more transient, inasmuch as electrode  102  doesn&#39;t even return fully to centre  0 .  
         [0049]      FIGS. 12   a  to  12   m  schematically illustrate an erosion cycle from the introduction of electrode  102  ( FIG. 12   a ) into preliminary bore  104  to the finished thread  106  ( FIG. 12   m ).  
         [0050]     From its central position in  FIG. 12   a , electrode  102  has been brought into contact with the wall of bore  104  ( FIG. 12   b ), at which instant the spark erosion process begins, with full penetration concluded in  FIG. 12   c . Now the orbital movement sets in, with the finished thread portion ( FIG. 12   d ) indicated by dashed lines. Inside electrode  102  the movement of the electrode centre is also indicated by dashed lines, straight or curved.  
         [0051]     Erosion continues in  FIG. 12   e , when the control unit of the machine senses an interference with the proper erosion process, causing a withdrawal of electrode  102  towards the central position, attained in  FIG. 12   g , via  FIG. 12   f.    
         [0052]     The interfering factor having been eliminated (e.g., by flushing), electrode  102  ( FIG. 12   h ) returns to the erosion position, as will be noted, to a point that has already been passed (comp.  FIG. 11 , stations  3 ,  4 ,  5 ), with the electrode position of  FIG. 12   e  being re-attained in  FIG. 12   i . From that point the erosion process is seen to go on without hindrance except for a short, transient withdrawal in  FIG. 12   k . The process is concluded in  FIG. 12   m . It will be appreciated that the movement of electrode  102  throughout the entire process was merely orbital, not rotational, as can be seen by the non-changing position of the triangular mark.  
         [0053]     The electrodes used for the attachment according to the invention are advantageously made of graphite or copper and have the same pitch and tooth shape as the internal thread to be formed, except that its outside diameter d ( FIG. 13 ) must be smaller than the preliminary hole  104  prepared in the workpiece. 
 
 The relevant dimensions are given by the expression:  
       B   =         D   -   d     2     -   G         
 
 where, for a tooth shape of 60°, G=2g, with g, being the spark gap, typically varying between 0.05 and 0.15 mm. 
 
         [0054]     The smaller the G, the better the surface quality to the thread produced, but the slower the erosion process and the higher electrode wear.  
         [0055]     Electrodes with a left-handed thread will obviously produce a left-handed internal thread.  
         [0056]     The attachment can also be used to produce external threads by using electrodes in the form of nuts.  
         [0057]      FIG. 14  represents the electrode and the workpiece as shown schematically in  FIG. 12   m.    
         [0058]     While for the production of internal threads the purely orbital, non-rotational method was used, the attachment according to the invention also allows the application of orbital-rotational methods.  
         [0059]      FIG. 15  shows an embodiment of the invention as used for the preparation, by electro-erosion, of workpieces having recesses of a rotational symmetry which, moreover, have an undercut, as seen in  FIG. 15 .  
         [0060]     In this application, the device uses an electrode in the form of an appropriately shaped blade  108  attached to a block-like body  109 , which in its turn, is mounted on a shaft  110  connected to pin  68  via a sleeve  112 . With shaft  110  in the central position (i.e., coaxial with the main shaft  34  ( FIG. 5 ), electrode blade  108  is introduced into the preliminary bore ( FIG. 16   a ). In  FIG. 16   b , the erosion process has started, with blade  108  penetrating to the required depth. In  FIG. 16   c  blade  108  has begun to rotate, producing the above-mentioned undercut. In  FIG. 16   d  the erosion process has been concluded and blade  108  withdrawn to the central position, to facilitate extraction. In this embodiment, the same stoppage, withdrawal and re-engagement episodes occur as with the above-discussed thread production.  
         [0061]     FIGS.  17  to  19  represent an additional embodiment of the invention, in which helix  20  ( FIG. 2 ) is replaced by an electric servo-motor  114  to the rotor of which is connected a worm  116  engaging a worm wheel  118  mounted on a shaft  120  that drives the main shaft  34 . The other components are identical to those of the preferred embodiment, except for stop  48  and locking disk  50  which here, are superfluous. This embodiment is intended to be the central item in a kit including per-se known components for converting a simple milling machine having a vertical head into a spark erosion machine.  
         [0062]      FIG. 20  illustrates an embodiment of the invention provided with a signalling device indicating the conclusion of the erosion process.  
         [0063]     As spark-erosion machining is mostly an automatic, but lengthy process, operators of the equipment cannot be expected to be continuously present and require some alarm means drawing their attention to the fact that the erosion process has come to an end.  
         [0064]     Conventional spark-erosion machines have safety devices in the form of a limit switch that is set to the required travel along the Z axis, at the end of which the machine is switched off. If the limit switch is not properly set, there exists the danger in ordinary erosion work that the electrode will go on eroding whatever is in its way, including the working table. With the device for producing threads, an additional difficulty may be encountered: a full orbital movement of the electrode requires a Z-movement of 50 mm. If, due to a setting error of the limit switch this distance is exceeded, the machine head, including top plate  2 , will continue to descend until retaining strip  17  will encounter bottom plate  42 . This will produce an overloading of the machine&#39;s servomotor, the overload protection of which will eventually stop the machine.  
         [0065]     To avoid this rather rough method of stopping the machine, there is provided a small incandescent bulb  122 , one terminal of which is connected to ground (−) via bottom plate  42  and stop  48 . The other terminal is a contact spring  124  insulated from ground. Bulb  122  lights up when contact spring  124  is connected to the head of the machine (+). This connection is effected by a metal strip  126  attached to skirt  3 . When stop  48  hits the work piece and the upper part of the attachment descends while activating main shaft  34 , metal strip  126  descends until it touches contact spring  124 , closing the bulb circuit. At this moment, bulb  122  lights up, producing enough light to trigger a light sensor with which today, most spark erosion machines are equipped and which causes the mechanism to be stopped and a visible or audible alarm to be activated.  
         [0066]     It has been found that the accuracy of the work performed considerably improves when the electrode is attached in coaxiality with pin  68  rather than axially offset in V-block  90  ( FIG. 4 ). While  FIG. 15  discloses such a coaxiality, this arrangement is only suitable were rotational, as well as orbital, movement required, but cannot be used for purely orbital movement.  
         [0067]     The variant of electrode holder  72 ′ shown in  FIG. 21  permits the use of the previously mentioned off-axis mounting of the electrode (which has the advantage of better visibility of the working spot) and the coaxial mounting of the electrode (which provides greater accuracy). There is seen in  FIG. 21 a  cylindrical projection  128  integral with electrode holder  72 ′, with pin  68  projecting beyond projection  128 .  
         [0068]      FIG. 22  shows the electrode holder of  FIG. 21 , together with sleeve  112  of  FIG. 15 , mounting the electrode  108  with the aid of two screws  130 . This enables the electrode to perform an orbital, as well as a rotational, movement such as that explained in conjunction with  FIGS. 15 and 16   a  to  d.    
         [0069]     In  FIG. 23 , electrode  132  is connected to electrode holder  72 ′ by means of sleeve  134 , which is fixedly attached to cylindrical projection  128  that, as already mentioned, is an integral part of electrode holder  72 ′. Clearly, electrodes attached to this variant, while coaxial with pin  68 , can only perform an orbital movement.  
         [0070]     A variant of electrode holder  72  seen to best advantage in  FIGS. 4 and 5  is illustrated in FIGS.  24  to  27 . The main different between the original holder  72  and the variant resides in the way the electrode  136  is pressed into V-notch  90  of holder  72 . While in the original holder  72  this is accomplished by means of a clamp  92  ( FIG. 4 ), in the present variant this is effected by an arm  138  pivotably mounted on a block  140  itself fixedly attached to holder  72 . On the electrode side, arm  138  is provided with a curved knife edge  142 , the angle α of which corresponds to the angle of the thread of electrode  136 . The other edge of arm  138  is provided with a slanting surface  144 . A thumbscrew  146  has a rounded tip  148  which can be brought to bear on the slanting surface  144 . When thumbscrew  146  is rotated, the force produced by tip  148  resolves into a torque swivelling arm  138  and causes knife edge  142  to press electrode  136  into V-notch  90 , and, due to slanting surface  144 , to force the pivoted end of arm  138  against its supporting surface on block  140 , thereby eliminating any play that would allow arm  138  to move in the Z-direction.  
         [0071]     The advantage of the above variant consists in the fact that the threaded electrode  136  can be moved in the Z-direction only by multiples of its pitch. This facilitates the removal of an electrode, the end portion of which has been badly eroded, cutting away that end portion, and returning the thus repaired electrode, while being sure that the returned electrode will properly re-engage the already existing internal threads of the workpiece. Another useful application of the above variant is the possibility at any time, to increase the length of an internal thread without danger that already existing threads will be damaged by possible misengagement of the electrode.  
         [0072]     It should also be noted that the initial movement from point  0  to point  1  is linear ( FIG. 11 ) and that this linear movement can be utilized, e.g., for engraving or undercutting.  
         [0073]     It will be evident to those skilled in the art that the invention is not limited to the details of the foregoing illustrated embodiments and that the present invention may be embodied in other specific forms without departing from the spirit or essential attributes thereof. The present embodiments are therefore to be considered in all respects as illustrative and not restrictive, the scope of the invention being indicated by the appended claims rather than by the foregoing description, and all changes which come within the meaning and range of equivalency of the claims are therefore intended to be embraced therein.