Abstract:
A levelling apparatus which comprises a lower part and an upper part which is mounted to the lower part and which may be adjusted with respect to the lower part. The lower part is provided with reference elements for its definite positioning, and the upper part is provided with mechanisms for mounting a workpieceholder for a workpiece which is to be worked on by a machine tool especially a wire erosion machine. For ensuring an improved positioning of the workpiece relative to the erosion wire at elevated pressures of the rinsing fluid, the lower side of the upper part has a first beam structure and the upper side of the lower part has a second beam structure which is complementary to the first beam structure such that the beams of the first beam structure extend between the beams of the second beam structure.

Description:
BACKGROUND OF THE INVENTION 
     (1) Field of the Invention 
     The invention relates to a levelling apparatus having a lower part and an upper part which is mounted to the lower part and which is adapted to be adjusted relative to the lower part. The lower part has reference elements for its definite positioning. The upper parts equipped with means for mounting a workpieceholder for a workpiece to be operated upon by a machine tool especially a wire erosion machine. 
     (2) Description of Related Art 
     Such a levelling apparatus is disclosed in the document U.S. Pat. No. 5,769,561. The workpieceholder is mainly a vise which is adapted to hold a workpiece to be cut by an erosion wire. An example of such a vise is disclosed in EP-B-196,544 (=U.S. Pat. No. 4,925,168). 
     Modern wire cut erosion machines cut more precisely and more efficiently than older machines of such type. This improvement is achieved by a higher electric current flow and an improved control of the current flow and an improved removal of waste by an elevated pressure of the rinsing fluid. The nozzle through which the rinsing fluid emerges under high pressure, is positioned in most of said machines in close proximity to the workpiece to be worked upon. The pressure of the rinsing fluid is not constant but varies in short intervals of time. Such variations do not impair the rapid removal of waste but have an impact on the definite position of the workpiece. The workpiece is induced to vibrate and vibrations of the workpiece caused thereby can hardly be suppressed by the known levelling device. Thus, the workpiece suffers from deviations from its definite position and an accurate cutting of the workpiece can hardly be achieved. 
     BRIEF SUMMARY OF THE INVENTION 
     It is therefore an object of the invention to devise an improved levelling apparatus which ensures rigidity and sturdiness of the workpiece positioning close to the cutting wire even if the pressure of the rinsing fluid varies. 
     To this end the invention provides for a levelling apparatus which has a lower part and an upper part mounted to the lower part and adapted to be adjusted with respect to the lower part, wherein the lower part is provided with reference elements for its definite positioning, and wherein the upper part is provided with means for mounting a workpieceholder for a workpiece which is to be worked on by a machine tool especially a wire erosion machine, and wherein the lower side of the upper part has a first beam structure and the upper side of the lower part has a second beam structure which is complementary to the first beam structure such that the beams of the first beam structure extend between the beams of the second beam structure. The invention allows a substantial improvement of the rigidity and sturdiness of the levelling apparatus without the necessity to enhance the thickness of the upper part and of the lower part. If the thickness of the upper part and of the lower part would be augmented it would no longer be possible to position the workpiece very close to the exit of the cutting wire of a wire erosion machine in view of the restricted space left free for the workpiece. On the other hand it is mandatory to place the workpiece as close to the cutting wire exit as possible for obtaining high precision cuts. 
     According to a preferred embodiment of the invention the beams of the first beam structure and the beams of the second beam structure are parallel and straight. Preferably, the beams of the first beam structure are directed towards the means for mounting the workpieceholder. The rigidity is further improved if according to an embodiment of the invention the height of each beam of the first beam structure measures at least one third of the width of such beam. It is also advantageous with respect to rigidity if at least some of the beams of the first beam structure are connected by transverse beams. Thereby a type of a beam lattice work is obtained. 
     Furthermore, the means for mounting the workpieceholder may include an extension form from a front edge of the upper part wherein the extension may be connected to the lower side of the upper part by reinforcing bars. Such reinforcing bars may extend into spaces between beams of the first beam structure. Moreover, the reinforcing bars may be inclined downwardly and forwardly in view of an improvement according to which the extension extends downwardly towards a lateral side of the lower part, specifically if the upper part is mounted on the top of the lower part. 
     According to an additional improvement of the invention a space is formed between neighboring beams of the first beam structure, the space having a width which is substantially equal to the width of one of the laterally adjacent beams. The second beam structure may comprise two opposite lateral beams extending outside and adjacent to the upper part and, preferably, transversely to a front edge of the upper part. It is useful to make the lateral beams run parallel to each other. In adaption to the inclination of the reinforcing bars the second beam structure may comprise intermediate beams each having a front ramp. 
     The exact positioning of the workpiece clamped to the workpieceholder is made easier if according to another prefered improvement of the invention a first adjustment screw is arranged with great distance to the front edge of the upper part for an up or down adjustment of the upper part relative to the lower part, an axis about which the upper part may be adjusted running closely and parallel to said front edge. Specifically, said axis may be defined by an imaginary connecting line drawn through a bolt which is surrounded by a spring assembly producing a bias on the upper part and the lower part, and through a second adjustment screw for a tilting adjustment of the upper part relative to the lower part. Structurally, the axis may be formed by a first rib which extends parallel to said front edge and projects out of the upper side of the lower part or out of the lower side of the upper part. Moreover, a second rib may be provided which extends transversely to the front edge and projects out of the lower side of the upper part or out of the upper side of the lower part, for tilting the upper part relative to the lower part to the right or to the left. A composite adjusting movement of the upper part therefore will take place about a pivot point which is situated in the crossing of both ribs. 
     Last not least a specifically preferred embodiment of the invention resides in the provision of a leaf spring of T form which extends between the upper part and the lower part and is mounted either to the upper part or to the lower part, a transverse leg of the leaf spring being spaced to the front edge and being fastened to the upper part or to the lower part, and a longitudinal leg of the leaf spring being directed to the front edge and being fastened to a middle beam of the first beam structure or of the second beam structure. Moreover, the leaf spring has a front end which is close to the front edge and extends between the first rib and the second rib. Thus, the rib of the upper part will rest upon the front end and the front end will rest upon the rib of the lower part. 
     According to a further aspect of the invention free surfaces of the apparatus may be covered by vibrations damping material. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Presently prefered embodiments of the invention are described hereinafter in detail with reference to the accompanying drawings of which show: 
     FIG. 1 a perspective view from above of a levelling apparatus; 
     FIG. 2 a plan view of the apparatus according to FIG. 1; 
     FIG. 3 a vertical section through the apparatus according to FIG. 1 along the line A—A in FIG. 2; 
     FIG. 4 a vertical section in enlarged scale through the apparatus according to FIG. 1 along the line B—B in FIG. 2, a portion of the apparatus being broken away; 
     FIG. 5 a plan view of a leaf spring of the apparatus according to FIG. 1; 
     FIG. 6 a perspective view from below of the upper part of the apparatus according to FIG. 1; 
     FIG. 7 a plan view of the upper part according to FIG. 6 from below; 
     FIG. 8 a vertical section through the upper part along the line C—C of FIG. 7; 
     FIG. 9 a perspective view from above of the lower part of the apparatus; 
     FIG. 10 a perspective view from below of the lower part of the apparatus; 
     FIG. 11 a vertical section through the apparatus similar to FIG. 4; and 
     FIG. 12 a detail out of FIG. 11 in an enlarged scale. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The levelling apparatus  1  comprises a lower part  2  and an upper part  5  screwed to the lower part  2 , and a leaf spring  9  fitted between the lower part and the upper part as may be seen from FIGS. 1,  4  and  5 . An extension  8  is formed from the front edge  55  of the upper part  5  being a one piece steel part. The extension  8  extends forwardly and next to the lower part  2  and has a surface  82  which is stepped forwardly and downwardly. The extension  8  is provided with two rows of through holes  85  which rows are spaced and parallel to the front edge  55 . Thus, a workpieceholder like a vise disclosed in EP-B-196,544 may be fastened to the plane lower surface  84  of the extension  8 . 
     The leaf spring has the form of a T having a transverse leg  91  and a middle longitudinal leg  93 . The longitudinal leg  93  is broadened in its transition to the transverse leg  91  symmetrically to his longitudinal axis  92 . Through holes  94 ,  96  are located symmetrically to the longitudinal axis  92  at the ends of the transverse leg  91 . Not shown screws which engage threaded bores  34 ,  36  in the upper side  20  of the lower part  2  (FIG.  9 ), extend through the through holes  94 ,  96  and fix the transverse leg  91  of the leaf spring  9  to the lower part  2 . Two further holes  97 ,  99  are provided on either sides of a big central hole  95  in the longitudinal leg  93  along the longitudinal axis  92 . Screws  67 ,  69  engage threaded blind holes  57 ,  59  which extend from the lower side  50  into the upper part  5 . The screws  67 ,  69  extend through the holes  97 ,  99  and fix the longitudinal leg  93  to the upper part  5 . 
     Another hole  98  is situated in the middle of the transverse leg  91  upon the longitudinal axis  92 . A first adjustment screw  48  extends through the hole  98  with some clearance. The outwardly threaded shank  47  of the first adjustment screw  48  engages a threaded blind hole  37  extending into the lower part  2  from the upper side  20  thereof. The upper part  5  has a through hole  58  for receiving the first adjustment screw  48 , the through hole  58  having a radial ring, shoulder  56  which widens radially the through hole  58 . A ring  46  from hard metal rests upon the ring shoulder  56  and forms a seat for the broadened head  49  of the adjustment screw  48 . 
     A further hole  90  is provided in the transversal leg  93  between the holes  97 ,  98 , a counter screw  35  extending through hole  90 . The counter screw  35  engages an internal thread of a through bore  52  of the upper part  5  and abuts the upper side  20  of the lower part  2 . 
     A bolt  70  is provided close to the front edge  55  of the upper part  5  in a distance to the longitudinal axis  92  and a second adjustment screw  78  is provided close to the front edge  55  in the same distance to the longitudinal axis  92  but opposite to the bolt  70  on the other side of the longitudinal axis  92 , an imaginary line which is perpendicular to the axis  92 , being drawn through the bolt  70  and the second adjustment screw  78 . Similar to the first adjustment screw  48  a shank having an outer thread, of the second adjustment screw  78  extends through a through hole through the upper part  5  and engages a threaded blind hole in the lower part  2 . The radially broadened head of the second adjustment screw  78  abuts a hard metal ring resting on an inner radial shoulder in the through hole. 
     A threaded shank  72  of bolt  70  engages similarly a threaded blind hole  42  in the lower part  2 . A section which is close to the shank  72  extends with play through a through hole  54  in the upper part  5 . The through hole  54  is radially broadened in the vicinity of the upper side such that a disk spring assembly  77  is received within the broadening. The head of bolt  70  rests in a broad ring collar  70  which covers the disk spring assembly. The section of the threaded shank  72  which extends through the hole  54  is surrounded by a metal sleeve. Thus, the disk spring assembly  75  is supported on one end by the sleeve and on the other end abuts the inner wall of the broadening of the hole  54 . Thereby a bias is created acting on the upper part  5  relative to the lower part  2 . 
     Therefore, the extension  8  may be adjusted vertically or downwardly relative to the lower part  2  by turning the first adjustment screw  48  and may be tilted to the left or to the right relative to the lower part  2  by turning the second adjustment screw  78 . 
     The lower side of the lower part  2  is equipped with reference elements which may cooperate with counter reference elements provided on the end surface e.g. of a not shown chuck for a definite positioning of the lower part relative to an orthogonal x-y-z-system (or a polar coordinate system). An example of such cooperation of reference elements with counter reference elements may be found in the document EP-B-255,042 the disclosure of which is incorporated herein. Thus, the lowerside of the lower part  2  is provided with four crosswise arranged pairs  25 ,  26 ,  27 ,  28  of opposing axially elastic lips such as lips  22 ,  24  of pair  25 . The lower surface of the lower part  2  has also four plane corner surface portions  17 ,  19 ,  21 ,  23  each of which serves as a reference element for a z direction. The chuck includes a clamping device for a not shown draw bar which may extend into a central hole  53  of the upper part  5  and through the opening  95  in the leaf spring  9  and through a central opening  11  in the lower part  2  and penetrates the clamping device. 
     As explained above the upper part  5  and the lower part  2  should have substantial rigidity and sturdiness. On the other hand, when using the levelling apparatus for holding a workpiece in the vicinity of a cutting wire only very restricted space is available. The desired rigidity and sturdiness is obtained by a strong first beam structure  60  on the lower side  50  of the upper part  5  and by a strong second beam structure  10  on the upper side  20  of the lower part  2  which is complementary to the first beam structure  60  such that after mounting the upper part  5  to the lower part  2  beams of the first beam structure  60  are situated between beams of the second beam structure  10 . 
     For the illustrated embodiment it may be seen from FIG. 6 that the first beam structure  60  comprises two parallel lateral beams  62 ,  64  and a middle beam  66  which is parallel to the lateral beams  62 ,  64 . The ends of said beams are connected by a rearward transverse beam  63  and by a front transverse beam  61 . The height of each beam measures at least one third of the width of a beam. The lateral beams  62 ,  64  and the middle beam  66  extend transversely to the front edge  55  and are directed towards the extension  8 . Two parallel reinforcing bars  68 ,  69  are provided for transmitting the rigidity and sturdiness of the upper part  5  obtained by the beam structure  60  to the extension  8 . The reinforcing bars  68 ,  69  extend downwardly from the front end of the interspaces between the middle beam  66  and each of the lateral beams  62 ,  64  into the body of the extension  8  thereby forming an angle of approximately 45°. 
     The second beam structure  10  of the lower part  2  comprises (FIG. 9) four parallel beams  12 ,  14 ,  16 ,  18  which project from the upper side  20  of the lower part  2 . The opposing lateral beams  12 ,  18  extend next to the lower portion of the upper part  5 , see FIGS. 1 and 2, and along the total length of the lower part  2 . The intermediate beams  14 ,  16  extend from the front to the rear, each having a maximal width which is only a little smaller than the width of each of the interspaces between the lateral beams  62 ,  64  and the middle beam  66  of the upper part  5 . Moreover, the maximal height of the intermediate beams  14 ,  16  above the upper side  20  of the lower part  2  is only a little smaller than the depth of said interspaces. Both intermediate beams  14 ,  16  have at the front thereof a ramp  13 ,  15  for preventing interference with the reinforcing bars  68 ,  69 . Additionally, the intermediate beams  14 ,  16  taper to the rear and terminate at a position ahead a rearward edge  29  of the lower part  2  which allows mounting of the transverse leg  91  of the leaf spring  9  to the upper side of the lower part  2  as mentioned above, and a collision with the transverse beam  63  is avoided. The longitudinal leg  93  finds sufficient freedom between the intermediate beams  14 ,  16  and the tapers thereof. 
     Thus, if the upper part  5  is fastened to the lower part  2  as disclosed above the intermediate beams  14 ,  16  and the lateral beams  12 ,  18  fit like a comb between the beams  62 ,  64 ,  66 ,  61  of the upper part  5 . 
     The embodiment of the invention according to FIGS. 11 and 12 is almost the same as the above explained embodiment. The levelling apparatus again comprises a lower part  102  and an upper part  105  and a leaf spring  109  interposed between upper part  105  and the lower part  102 . A deviating feature may be seen in the blind holes  157 ,  159  each having an internal thread and each extending into the lower part  102 . The blind holes  157 ,  159  are aligned to the holes  97 ,  99  in the leaf spring  109 . Screws  167 ,  169  extend through the holes  97 ,  99  and engage the threads of the blind holes  157 ,  159 , thereby fastening the leaf spring  109  to the lower part  102 . However, the features which are essential for the invention are not influenced by the modified mounting of the leaf spring  109 . 
     On the other hand, FIGS. 11 and 12 show more clearly the structure of the pivot point about which the upper part may be adjusted upwardly or downwardly by turning the first adjustment screw  48  and about which the upper part may be tilted to the left or to the right by turning the second adjustment screw  78 . In the vicinity of the front edge  155  of the lower part  102  a first rib  110  having approximately a halfcylindric contour projects from the upper surface thereof which extends substantially parallel to the front edge  155 . The end  115  of the leaf spring  109  which is proximate to the front edge  155  lies upon the first rib  110 . A second rib  120  having also approximately a halfcylindric contour projects from the lower side of the upper part  105  opposite to the first rib  110  which extends substantially transverse to the front edge  155  and lies upon the end  115 . The pivot point is therefore situated within the crossing area of both ribs  110  and  120 , between which the end  115  of the lead spring  109  extends. 
     Of course it is within the scope of the invention to let project the first rib out of the lower side of the upper part an to let project the second rib out of the upper side of the lower part. It is important that the ribs are arranged crosswise irrespective of the particular mounting of the ribs.