Abstract:
A forming press for external high-pressure forming is proposed, in which the fluid box ( 10 ) is arranged stationarily. Working cylinders ( 6 ) are integrated in the ram ( 2 ) so as to achieve a compact design, and the sheetholding and closing force is applied by short-stroke cylinders ( 7 ).

Description:
FIELD OF THE INVENTION 
     The invention relates to a press, in particular a forming press which works by the method of external high-pressure forming, carries out the forming process with a small number of operating steps by virtue of the proposed arrangement of the driving and forming subassemblies and is distinguished by a compact rigid design. A sheet holder system having high flexibility, along with a low overall height, is also proposed. 
     DESCRIPTION OF THE RELATED ART 
     External high-pressure forming (EHF), also designated as a hydromechanical drawing method or hydrostatic cold-forming method, has been known for many years. The method is already described in detail in DE 12 40 801 of 1961 and reference is made to this description. The increased use of external high-pressure forming as an inexpensive alternative in small-lot production leads to a growing demand for appropriately equipped forming presses. Developments in hydraulic and electronic control engineering also have a beneficial effect on improvements in method. A multi-purpose press working by external high-pressure forming is disclosed in DE 198 19 950. This machine, designed as a hydraulic press, is also designed additionally for forming by internal high pressure and as a deep-drawing press. For the external high-pressure forming application, following method steps then take place: 
     1 Insert sheet billet 
     2 Move the holding-down device downward 
     3 Move the fluid box upward by means of the working cylinders 
     4 Apply the holding-down force 
     5 Move the ram downward 
     6 Move the working cylinders upward shortly before the ram reaches bottom dead center (BDC) 
     7 Regulation of the fluid pressure in the fluid box 
     8 The ram moves to BDC, and engage interlocks 
     9 The working cylinders move further upward and the holding-down cylinders are displaced 
     10 By the fluid box being moved further up, the clamped sheet billet is drawn via the die located on the fixed ram 
     11 The holding-down device is pressed to blocking with the spacers and the top spar 
     12 The working cylinders increase the closing pressure 
     13 A fluidic high pressure is built up in the fluid box, and the workpiece contour is produced by means of a platicizing operation 
     14 Relief of pressure 
     15 The working cylinders lower the fluid box 
     16 The interlock is disengaged 
     17 The ram together with the holding-down device is moved up 
     18 The workpiece is extracted 
     Quite apart from the multiplicity of process steps, the press is, of course, not designed optimally for the respective intended use, on the basis of the object of operating it as a universal or multi-purpose press. 
     SUMMARY OF THE INVENTION 
     The object on which the invention is based is to propose a press of high rigidity for external high-pressure forming, which makes it possible to reduce the process steps and in which the fluid box can be arranged stationarily and which has available a freely selectable and demand-related arrangement of the sheetholder cylinders. 
     The invention is based on the idea of selecting the arrangement of the fluid cylinders in such a way that a rigid and compact design is ensured as a result of a direct position in the force flux. Since the ram and sheetholder are moved jointly, only one movement cylinder is necessary and the sheetholder cylinders may be designed as straightforward short-stroke cylinders. These short-stroke cylinders are not arranged in a rigid system, but, instead, the number and arrangement of these can be selected optimally for the respective workpieces according to the requirements of the forming process. Corresponding coupleable connections to the power line or lines are provided. 
     The working cylinders carrying the die are integrated directly in the ram. The counterpressure is regulated in a stationary fluid box, thus making it possible to have fixed pipework, this being a further advantage during high-pressure operation. 
     The process sequence is as follows: 
     1 Insert a sheet billet 
     2 Joint downward movement of the ram and sheetholder 
     3 Interlocking of the ram with sheetholder by means of engaging spacer pieces 
     4 Downward movement of the die into the fluid box by means of working cylinders, at the same time 
     4.1 Pressure regulation in the fluid box 
     4.2 Regulation of the sheet holding force via short-stroke cylinders 
     5 Interlocking of the die at bottom dead center 
     6 Build-up of the maximum sheet holding force via short-stroke cylinders 
     7 High-pressure build-up in the fluid box and operation of plasticizing the workpiece (calibration) 
     8 Relief of pressure 
     9 The interlock is disengaged 
     10 The ram together with the holding-down device is moved up 
     11 The workpiece is extracted 
     The entire process sequence consequently consists of altogether 11 operating steps which also even partially overlap. This also ensures, as a further advantage, a high cycle rate and an economical use of the press. 
     A favorable energy balance due to small oil quantities being required is achieved as a result of the common movement cylinder for the ram and sheetholder and the use of the short-stroke cylinders for regulating the sheet holding force and for building up the maximum locking force. Since the short-stroke cylinders do not themselves carry out the forming travel, but execute merely a stroke of a few millimeters, an inexpensive design with a low overall height is obtained. This is also advantageous when multi-point support is necessary on account of the workpiece geometry. 
     It is to be understood that both the foregoing general description and the following detailed description are examples and explanatory and are intended to provide further explanation of the invention as claimed. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The accompanying drawings, which are included provide a further understanding of the invention and are incorporated in and constitute a part of this specification, illustrate embodiments of the invention and together with the description above to explain the principles of the invention. 
     FIG. 1 shows a front view of the forming press in sectional illustration along the sectional line A—A in FIG. 3; 
     FIG. 2 shows a side view of the forming press in a sectional illustration B—B, but with the die at bottom dead center; and 
     FIG. 3 shows a top view of the forming press along sectional line C—C. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIG. 1 shows the basic design of the forming press, consisting of the press column  1 , ram  2 , sheetholder  3 , platen  4 , working cylinder  5 , working cylinders  6 , short-stroke cylinders  7 , interlock  8 , die  9 , fluid box  10  and sheet billet  11 . 
     The start of the following operation can be seen in the illustration. The movement cylinder  5  has lowered the ram  2  together with the sheetholder  3  until the sheetholder  3  sits on the sheet billet  11 . A regulated pressure build-up for the sheetholding force is carried out, depending on the forming operation, via short-stoke cylinders  7 . The number and arrangement of the short-stroke cylinders  7  are selected in dependence on the die and on the sheet billet. Determining parameters for the die are, for example, the die size and the die contour, and, as regards the sheet billet, inter alia, mechanical material properties, forming speeds and tribological actions are important. In order to achieve the desired forming results, each short-stroke cylinder  7  may be regulated individually or be closed together in groups. 
     In the further process sequence, the working cylinders  6 , integrated in the ram  2 , move, together with the die  9 , toward the fluid cushion located in the fluid box  10 . A pressure regulating device, not illustrated in any more detail, sets the necessary pressure of the fluid cushion. 
     The die is moved into its lowest position  13  which is limited, for example, by stops, and is interlocked by means of engageable die spacers  12  illustrated in FIG.  3 . This interlock is necessary in order to support reliably the high pressure which is built up in the fluid box  10  for the conclusion of the forming operation. By means of this pressure, the workpiece is pressed into the die contour in a kind of calibration and is shaped. For this purpose, short-stroke cylinders  7  generate a maximum sheetholding force which ensures that the edge of the sheet billet or of the workpiece is clamped reliably. The expansions or compressions of the press components which occur due to these forces are advantageously compensated by means of the arrangement and the stroke of the short-stroke cylinders  7 . Fluid couplings  16  serve for connecting the short-stroke cylinders necessary in each case to the power supply. A holding or die top plate  17  is connected to the ram  2  and makes it possible to mount and demount the working cylinders  6  in a simple way. 
     FIG. 2 shows the situation at bottom dead center. The pressure medium  14  is subjected in the fluid box  10  to a maximum pressure at which the ultimate component geommetry is shaped in a plasticizing forming operation. An after flow of the clamped edge of the sheet billet is undesirable in this calibrating operation and is reliably prevented by the maximum locking force generated by the short-stroke cylinders  7 . The high forces occurring in this phase are absorbed in a kind of closed system as a result of interaction with the engaged ram interlocks  8  and die spacers  12 . Another advantage of the selected arrangement becomes clear here, since, in this concept, the compressibility of the fluid medium is essentially ruled out. 
     FIG. 3 is a sectional illustration which shows an arrangement of the short-stroke cylinder  7 . As already mentioned, both the arrangement and the number of short-stroke cylinders  7  are freely selectable according to the component. The moveable die spacers are designated by  12  and the associated displacement cylinders by  15 . In the position shown, the die spacers  12  are disengaged, that is to say the die  9  is no longer interlocked. 
     The invention is not restricted to the exemplary embodiment described and illustrated. It also embraces, within the scope of the current claim  1 , all refinements which are available to a person skilled in the art.