Abstract:
A workpiece holder comprises a stationary workpiece holding fixture and a workpiece rest which is releasably attachable thereon and on which can be mounted a workpiece. In all the three coordinate directions, the workpiece rest bears by counterpart surfaces on bearing surfaces formed on the workpiece holding fixture and is aligned and fixed by an oblique chucking appliance in two coordinate directions and by at least one holding-down appliance in the third coordinate direction.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The invention relates to a workpiece holder. 
     2. Background Art 
     A workpiece that is to be machined in several steps of working on various machine tools is fixed in the machine tools by means of chucking appliances for being machined on a workpiece holder. For an ensuing step of working on the following machine tool to be performed, it is detached from the workpiece holder and transported to the following machine tool where it is again fixed on a workpiece holder. 
     U.S. Pat. No. 3,606,300 teaches a workpiece rest with a workpiece aligned and fixed on it. To this end, the workpiece rest has quite a number of holes. The workpieces and the workpiece rest that carries them are transported together from one machine tool to the next, nothing being said on how fixing takes place on the individual machine tools. 
     It is known from U.S. Pat. No. 5,558,784 to fix a work that is to be machined on a workpiece rest. The workpiece rest possesses chucking elements in the form of twistlocks which simultaneously constitute centering elements for at least roughly centering and distinctly aligning the workpiece rest on the processing machine tool, it being absolutely necessary to fit the workpiece rest accurately into the machine tool. In the machine tool, the coordinate values i.e., the position of the workpiece rests in the three-dimensional system of coordinates, are measured and offset against the data stored in the machine tool in order for the precise position of the workpiece to be obtained in relation to the coordinate systems of the machine tool. 
     This familiar method exhibits several drawbacks in installations with various machine tools, for instance transfer machines, or short-cycle flexible manufacturing systems, because the periods needed for accurately fitting in the workpiece rests add to the cycle times of the entire installation. 
     SUMMARY OF THE INVENTION 
     It is an object of the invention to embody a workpiece holder that does not require any accurate fitting of the workpiece during or after the chucking process. 
     According to the invention, this object is attained in a workpiece holder, comprising a workpiece holding fixture; and a workpiece rest; wherein the workpiece rest is provided with arrangements for fixing a workpiece in a given position relative to the workpiece rest; wherein the workpiece holding fixture is provided with bearing surfaces and the workpiece rest with counterpart surfaces that rest on the bearing surfaces for the workpiece rest to have an accurately given position in a first, second and third coordinate direction; and wherein chucking appliances are provided, chucking the workpiece rest relative to the workpiece holding fixture in the first, second and third coordinate direction. The workpiece takes an accurately defined position on the workpiece rest and keeps this position throughout the steps of machining. When transported from one machine tool to the other, the workpiece rest is chucked accurately in all the three coordinate directions, there being no need for renewed fitting of the workpiece into a machine tool. 
     Further features, advantages and details of the invention will become apparent from the ensuing description of an exemplary embodiment, taken in conjunction with the drawing. 
    
    
     BRIEF DESCRIPTION OF THE DRAWING 
     FIG. 1 is a plan view of a workpiece holder with a workpiece rest mounted on a workpiece holding fixture; 
     FIG. 2 is a vertical cross-sectional view through the workpiece rest on the line II—II of FIG. 1 with a workpiece mounted on the workpiece rest; 
     FIG. 3 is a perspective diagonal view of the workpiece holder; 
     FIG. 4 is another perspective diagonal view of the workpiece holder; 
     FIG. 5 is a perspective diagonal view of the workpiece holding fixture in accordance with the representation of FIG. 4; 
     FIG. 6 is a perspective diagonal view of the workpiece rest; 
     FIG. 7 is a cross-sectional view through the workpiece holder on the line VII—VII of FIG. 1; 
     FIG. 8 is a cross-sectional view through the workpiece holder on the line VIII—VIII of FIG. 1; and 
     FIG. 9 is a partial view of a holding element on an enlarged scale. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     The workpiece holder seen in the drawing is fundamentally comprised of two parts. It consists a box-type workpiece holding fixture  1  and a workpiece rest  2 . The workpiece holding fixture  1  has the shape of an angular column with a foot  3  and a rear supporting wall  4 . The workpiece holding fixture  1  is arranged in a given position on the machining side of a machine tool, for instance a universal machine center. 
     The workpiece rest  2  has a basic shape of a flat cuboid and may be comprised of several pieces for manufacturing reasons. It has a workpiece  6  supporting surface  5  which faces away from, and is approximately parallel to, the supporting wall  4 , lying on a x-y plane. It is provided with two index pins  7  which stand out from the supporting surface  5  in the z direction, engaging with index holes  8  of identical diameter in the reference surface  9  of the workpiece  6  that is turned towards the supporting surface  5 , thereby defining the position of the workpiece  6  relative to the workpiece rest  2  in the plane given by the supporting surface  5 . The workpiece  6  is clamped against the supporting surface  5  of the workpiece rest  2  by means of fasteners  11 . The fasteners  11  reach through drilled holes  12  in the workpiece rest  2  into the workpiece  6 . In the way described, the workpiece  6  takes an accurately defined position relative to the workpiece rest  2 . 
     Means are provided on the workpiece rest  2  and the fixture  1 , enabling the fixture  1  and the rest  2  to be connected rapidly and positionally accurately so that the workpiece  6 , when transported together with the workpiece rest  2  from a machine tool to another machine tool linked thereto and from there to yet another machine tool linked thereto, has an accurately defined position relative to each machine tool once the workpiece rest  2  has been fixed to an identical fixture  1  allocated to each machine tool. 
     Two lateral holding elements  13  of identical design are arranged one above the other on the supporting wall  4 , each possessing a lower bearing surface  14 , these surfaces  14  being arranged in parallel to each other and approximately parallel to the supporting surface  5  i.e., in an x-y plane. The two bearing surfaces  14 ,  15  of each holding element  13  open in the form of a wedge toward the workpiece rest  2 . The upper bearing surface  15  inclines from the workpiece rest  2  downwardly toward the lower bearing surface  14 . 
     Mounted on the work-rest- 2  side wall  16  that is turned toward the lateral holding elements  13  are two counterpart holding elements  17  which have counterpart surfaces  18 ,  19  arranged in the shape of a wedge one relative to the other; upon correct insertion of the counterpart holding elements  17  into the lateral holding elements  13 , these counterpart surfaces  18 ,  19  rest planely and tightly on the bearing surfaces  14  and  15 , respectively. 
     Mounted on the supporting wall  4  of the fixture  1  is a lower support  20  which has a lower bearing surface  21 . Allotted to this support  20  is a counterpart support  22  which is formed on the lower side wall  23  of the workpiece rest  2  and has a counterpart surface  24 , by means of which the workpiece rest  2  takes an accurate position on the lower support  20 . 
     On the side of the workpiece rest  2  that faces away from the lateral holding elements  13 , two bearing elements  25  are provided on the supporting wall  4  of the fixture  1 , lying opposite the holding elements  13 . They each have a bearing surface  26  extending parallel to the lower bearing surfaces  14  i.e., in the x-y plane. On the associated side wall  27  of the workpiece rest  2 , counterpart bearing elements  28  are formed, having a counterpart surface  29  that rests planely on the respective bearing surface  26 . When the workpiece rest  2  rests by its counterpart surfaces  18 ,  19 ,  24 ,  29  on the bearing surfaces  14 ,  15 ,  21 ,  26  of the fixture  1 , then it has a precisely defined position—any time reproducible—toward each identical workpiece holding fixture  1  in all the three coordinate directions, namely the x direction, y direction and z direction. 
     The job of detachably fixing the workpiece rest  2  on the fixture  1  in the precisely defined position mentioned takes place as follows: 
     An oblique chucking appliance  30  is provided on the supporting wall  4  of the fixture  1 , working on the x-y plane diagonally of the x direction and diagonally of the y direction. It comprises a pressure-fluid actuated piston-cylinder drive  31 , the cylinder  32  of which is mounted on the supporting wall  4  and on the piston rod  33  of which is mounted a chucking hook  34  that bears against an abutment  35  in the vicinity of the side wall  27  of the workpiece rest  2 . When the drive  31  is pressure-fluid actuated, then the workpiece rest  2  is forced by this single oblique chucking appliance  30  into the lateral holding elements  13  on the one hand and against the lower support  20  on the other i.e., a single chucking appliance  30  is sufficient for aligning and fixing the workpiece rest  2  relative to the fixture  1  in the x direction as well as in the y direction. In this case, the workpiece rest  2  is aligned and fixed in the lateral holding elements  13  in the z direction. 
     Provision is made for two holding-down appliances  36  which are allocated to the bearing elements  25  and the counterpart bearing elements  28  and by means of which the workpiece rest  2  is held down on the fixture  1 . They comprise pressure-fluid actuated pivot drives  37  which are arranged in the supporting wall  4  and the pivoted shaft  38  of which stands out from the supporting wall  4  in the z direction and is equipped with a chucking lever  39 . The chucking lever  39  has a chucking surface  40  which inclines in the z direction and cooperates with a counterpart chucking surface  41  on the respective counterpart bearing element  28 . The chucking surface  40  and the counterpart chucking surface  41  slope from the workpiece rest  2  away toward the supporting wall  4  so that, when the workpiece rest  2  is chucked, pressure is exercised by the counterpart surfaces  29  on the bearing surfaces  26  in the direction toward the lateral holding elements  13  for tightly bearing fit to take place there. The pressure-fluid actuated drives  31 ,  37 , which are preferably hydraulically actuated, can be operated very rapidly so that the production of a locking connection between the workpiece rest  2  and the fixture  1  as well as a detachment thereof may take place within fractions of seconds. 
     Furthermore, arrangements are provided, ensuring that the workpiece rest  2  is being accurately aligned and fixed on the fixture  1  in the way described. Machining a workpiece  6  causes chips or other dirt particles to crop up which might place themselves between a bearing surface and a counterpart surface, causing the position of the workpiece rest  2  relative to the fixture  1  not to correspond precisely to the nominal position. For this to be avoided, pressure-fluid actuated bearing-contact testing ducts  42  are provided in the bearing surfaces  14 ,  15 ,  21 ,  26 , of which the drawing only illustrates those in the lower bearing surfaces  14  and  15  of a lateral holding element  13 . Once the workpiece rest  2  has been fixed on the fixture  1  by the oblique chucking appliance  30  and the holding-down appliances  36 , compressed air is supplied from a compressed-air piping  43  via this bearing-contact testing duct  42 . Any pressure drop occurrence means that the respective counterpart surface  18 ,  19 ,  24 ,  29  does not rest planely and tightly on the bearing surface  14 ,  15 ,  21 ,  26 . This is detected by a manometric switch that is connected to the piping  43 . 
     For possible cleaning of the bearing surfaces and the counterpart surfaces, rinsing ducts  45  are provided in the bearing surfaces  14 ,  15 ,  21 ,  26 ; rinsing liquid is supplied via a rinsing pipe  46 , cleaning the bearing surfaces and the counterpart surfaces. Control takes place via a solenoid valve  47  in the pipe  46 . Suitably, such a cleaning job is performed prior to the chucking operation by means of the oblique chucking appliance  30  and the holding-down appliances  36 . In order to prevent a film of rinsing liquid on the bearing surfaces  14 ,  15  and the counterpart surfaces from hampering any tight contact between these surfaces, a groove-type drain  48  is provided, leading from the respective rinsing duct  45  outwards through the bearing surfaces  14  and  15 .