Abstract:
The present invention relates to a forming press of crankshafts which is safe, has a slide of large strength and high forming precision, and is capable of high-speed cyclic operation. A forming press of crankshafts includes a forging unit for twisting and a forging unit for reshaping, wherein (i) a twisting oil-hydraulic cylinder is mounted on the crown, (ii) twisting push rods are disposed along outer surfaces of the slide, and (iii) the bottom ends of the twisting push rods are connected by twisting-down beams and untwisting beams. The slide is of inverted T-form in section, and the pressing block is of H-form as seen from above and of inverted U-form as seen from one side. Therefore, they do not interfere with each other and a large stroke can be secured. The twisting angle is set using an upper stopper provided in the pressing block and an automatic telescopic lower stopper provided in the slide and freely movable toward and away from the upper stopper. An upper guide and a lower guide are provided between the pressing block and the slide. Each guide directs four portions, around the lower stopper, of the pressing block against its forward, backward, rightward, and leftward deviation.

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates to a forming press of crankshafts. 
     A one-piece crankshaft is forged in stages of closed die forging, twisting, and reshaping. Although the stage of closed die forging consists of the substages of preforming, rough closed die forging, finishing closed die forging, and trimming, all the four substages are being performed with a single forging press since it is required in all the four substages to apply pressure to the workpiece in only the downward direction. On the other hand, because the twisting stage and the reshaping stage require different pressing modes, the two stages were being performed with two different forging presses. However, because using two different forging presses for the two stages is disadvantageous in terms of equipment cost and man-hours, forming presses have recently been developed which, in a single unit, are capable of performing both the twisting and reshaping stages. The present invention relates to such a forming press. 
     A forming press of crankshafts is disclosed in the Japanese Examined Patent Publication No. 4640/H7 (1995), which is shown in FIG.  25 . 
     In the above forming press, a slide  106  is secured to the lower ends of the piston rods of piston cylinders  108 , which are mounted on the crown. An upper die holder  151  is secured to the bottom of the slide  106 . The upper section of a twisting die  101  and the upper section of a reshaping die  102  are fixed to the bottom of the upper die holder  151 . On the other hand, the lower sections of the twisting die  101  and the reshaping die  102  are mounted on a lower die holder  152 , which is mounted on the base of the forming press. The twisting die  101  and the reshaping die  102  are built in so that the upper and lower sections of the twisting die  101  and those of the reshaping die  102  join at one and the same height when the slide  106  comes down. When the slide  106  descends, a workpiece is caught between the upper and lower sections of the twisting die  101  and, at the same time, another workpiece in the reshaping die is pressed and reshaped. While the slide  106  is in its bottom position, the twisting oil-hydraulic cylinder  109  provided in the slide  106  contracts to lower push rods  171  penetrating the slide  106  and twist the workpiece in the twisting die  101 . 
     In this way, a workpiece can be reshaped in the reshaping die  102  and, at the same time, another workpiece can be caught between the upper and lower sections of the twisting die  101 . Then, the workpiece in the twisting die  101  can be twisted by contracting the twisting oil-hydraulic cylinder  109 , pushing down the push rods  171 , and thereby turning parts of the twisting die  101  anticlockwise and the other parts of the twisting die  101  clockwise. 
     In case of the above forming press, because the twisting oil-hydraulic cylinder  109  is disposed in the slide  106 , it is necessary for feeding hydraulic oil to the twisting oil -hydraulic cylinder  109  to use long flexible hoses  100  which cover the stroke of the slide  106 . Such configuration is not desirable for a high-speed production machine which requires a large flow of hydraulic oil, posing a large danger of hydraulic oil leaking from the long flexible hoses  100 , igniting from the heat of the hot forging, and causing a fire. 
     Besides, to bring the lower ends of the push rods  171  to the push-down arms  104  of movable retainers holding the twisting die  101 , the slide  106  has to allow the push rods  171  to penetrate itself. In this configuration, the following shortcomings occur. 
     (1) The adjustment and maintenance of the guiding parts of the push rods are difficult. 
     (2) Because the slide  106  is divided into three by the push rods  171 , its strength is reduced. To solve this problem, the height of the slide  106  has to be increased to recover its strength. 
     (3) Accordingly, the slide system (moving section) becomes heavy. Its large force of inertia impedes the control of high-speed cyclic operation. 
     One of the main objects of developing a forming press which performs both the twisting and reshaping stages is to achieve a high-speed operation. In case of the above forming press of the prior art, the object is difficult to achieve due to the reasons described in the above paragraphs (2) and (3). If the forming press achieves the object, it would present problems in forming precision and maintenance. 
     On the other hand, the twisting angle of such a forming press has to be changed in accordance with the types of crankshafts. The twisting angle varies depending on the numbers of cylinders, etc. of engines. 
     In conventional means for setting the twisting angle, stoppers are set, below the push rods, on the top of the bed to regulate the twisting angle. To shift the production from one type of crankshaft to another, stoppers have to be changed and adjusted to attain the twisting angle required by the latter. 
     Such stoppers have to be changed and adjusted by workers in accordance with twisting angles required by various types of crankshafts. Besides, because the space in the forming press is limited, the upper and lower die holders are usually taken out of the press in order to make fine adjustment of the stoppers. Accordingly, working efficiency is low. 
     Under the circumstances, another forming press is disclosed [Japanese Unexamined Utility Model Publication 104215/H4 (1992)]. The forming press has, under its bed, a driving mechanism which moves threaded shafts up and down to adjust the height of the stoppers. 
     However, because the driving mechanism is disposed under the bed, die lubricant and scale stick to the driving mechanism and hence it is liable to develop trouble. Besides, because the forming press has four stoppers, they require fine height adjustment among them. Therefore, the driving mechanism for adjusting the height of stoppers is very complex and expensive. 
     In accordance with the above, an object of the present invention is to provide a forming press of crankshafts of which the safety is high, of which the slide has large strength ensuring high precision, and which is capable of high-speed operation and inexpensive. 
     Another object of the present invention is to provide a forming press of crankshafts of which the twisting angle can be set freely and precisely, of which the construction is simple, and which is inexpensive to make and is capable of high-precision forming. 
     SUMMARY OF THE INVENTION 
     According to the first aspect of the present invention, there is provided a forming press of the crankshafts having a crown, a slide, a forging unit for twisting with a twisting die, and a forging unit for reshaping, wherein (i) a pressing oil-hydraulic cylinder and a lift-up oil-hydraulic cylinder for lowering and raising the slide and a twisting oil-hydraulic cylinder for twisting a workpiece in the forging unit for twisting are mounted on the crown, (ii) the piston rod of the twisting oil-hydraulic cylinder is connected to a pressing block, (iii) a twisting push rod is secured to each of the four corners of the pressing block, two of the four twisting push rods being disposed in front and back of the slide on the right side of the twisting die, the other two being disposed in front and back of the slide on the left side of the twisting die, and (iv) the lower ends of the two twisting push rods on each side of the twisting die are connected to each other by a twisting-down beam and an untwisting beam below the slide. 
     According to the second aspect of the present invention, there is provided the forming press of the first aspect, wherein the slide is of inverted T-form in section, the pressing block is of inverted U-form as seen from one side, and the slide and the pressing block are so disposed to each other that the former can enter the latter. 
     According to the third aspect of the present invention, there is provided the forming press of the second aspect, wherein (i) the pressing block comprises a guide shoe and a pair of block members secured to the front and the back of the guide shoe, (ii) a pair of guide members for guiding the guide shoe upward and downward is mounted on the slide, and (iii) the slide has four guide pipes for guiding the four twisting push rods. 
     According to the fourth aspect of the present invention, there is provided the forming press of the third aspect, wherein the slide and a member provided as a unit with the slide are mounted with a lower stopper to define the descending limit of the pressing block relative to the slide and an upper stopper to define the ascending limit of the pressing block relative to the slide. 
     According to the fifth aspect of the present invention, there is provided the forming press of the fourth aspect, wherein (i) a directional control valve is put in a hydraulic-oil feeding and discharging circuit which connects the head-side and rod-side oil chambers of the twisting oil-hydraulic cylinder to a pressurized-oil source, (ii) an opening and closing valve is put in a hydraulic-oil line which connects the head-side oil chamber of the twisting oil-hydraulic cylinder to an oil tank, and (iii) a relief valve and an opening and closing valve are put in series in a hydraulic-oil line which connects the rod-side oil chamber of the twisting oil-hydraulic cylinder to the oil tank. 
     According to the sixth aspect of the present invention, there is provided the forming press of the first aspect, wherein a means for setting the twisting angle is provided between the pressing block and the slide, the means comprising an upper stopper provided in the pressing block and an automatic telescopic lower stopper provided in the slide and freely movable toward and away from the upper stopper. 
     According to the seventh aspect of the present invention, there is provided the forming press of the sixth aspect, wherein the automatic telescopic lower stopper of the means for setting the twisting angle comprises (i) an internally threaded cylinder freely rotatable about its vertical axis in a casing which is provided in the slide, (ii) an externally threaded member which engages with the internal-thread portion of the internally threaded cylinder, (iii) a stopper block fixed on the top of the externally threaded member, and (iv) a driving mechanism for rotating the internally threaded cylinder. 
     According to the eighth aspect of the present invention, there is provided the forming press of the seventh aspect, wherein the driving mechanism of the automatic telescopic lower stopper comprises (i) a worm wheel formed on the outer surface of the internally threaded cylinder, (ii) a worm which engages with the worm wheel, and (iii) a geared motor for rotating the worm. 
     According to the ninth aspect of the present invention, there is provided the forming press of the sixth aspect, wherein a means for guiding the ascent and descent of the pressing block is provided between the pressing block and the slide, the means comprising (i) an upper guiding means which includes four guide sliders attached to the four corners of the upper portion of the pressing block, a guide structure mounted fixedly on the top of the slide, and four guide rails attached to the guide structure for guiding the guide sliders and (ii) a lower guiding means which includes four guide sliders attached to the four corners of the lower portion of the pressing block and four guide rails attached to the slide for guiding the four guide sliders. 
     According to the tenth aspect of the present invention, there is provided the forming press of the ninth aspect, wherein the guide slider and the guide rail, at each corner of the pressing block, of each of the upper guiding means and the lower guiding means are so configured that the guide slider consists of two parts and the guide rail consists of two parts, and one set of a guide-slider part and a guide-rail part guides the pressing block against its deviation to the front or the rear, as the case may be, of the forming press and the other set of a guide-slider part and a guide-rail part guides the pressing block against its deviation to the right or the left, as the case may be, of the forming press. 
     The advantages offered by the first aspect of the invention are mainly as follows. Because the twisting oil-hydraulic cylinder is fixed to the press frame by mounting the cylinder on the press crown, it is not necessary to use flexible hoses; fixed piping can be made. Therefore, the forming press is safe and easy in maintenance and poses no danger of fire from the heat of hot forging. Besides, hydraulic oil can be fed in a large flow rate to achieve high-speed operation of the forming press. Moreover, because the slide is not divided by the twisting push rods, the slide can be designed compact yet strong. By making the slide system light in this way, the controllable operating speed of the forming press can be increased. Furthermore, its high rigidity ensures a high precision. In addition, by securing the upper ends of the twisting push rods to the pressing block and connecting the lower ends of the twisting push rods with the twisting-down beams and the untwisting beams, high rigidity can be given to the twisting push rods, which contributes to weight reduction and high-speed operation. 
     The advantages offered by the second aspect of the invention are mainly as follows. Because the slide and the pressing block are so configured and disposed to each other that the former can enter the latter, the up and down strokes of the slide and the pressing block can easily be secured and the forming press can be made compact yet strong. Besides, guides and stoppers for the pressing block can easily be arranged. 
     The advantage offered by the third aspect of the invention is as follows. Because the pressing block is guided by the pair of guide members and the twisting push rods are guided by the guide pipes, their up and down movement is regulated orthogonally to the slide. Thus, the forming precision increases. 
     The advantages offered by the fourth aspect of the invention are as follows. Because the upper and lower limits of the stroke of the pressing block are mechanically defined, the safety operation is ensured. In addition, because the relative position of the twisting push rods to the slide can be determined exactly at their top and bottom positions, the forming precision increases. 
     The advantage offered by the fifth aspect of the invention is mainly as follows. The directional control valve controls the extension and contraction of the twisting oil-hydraulic cylinder. In addition, the opening and closing valve opened allows the hydraulic oil in the twisting oil-hydraulic cylinder to return to the oil tank and, on the other hand, the relief valve generates back pressure. All these actions cause the twisting mechanism to follow the slide. Thus, the relative position of the twisting mechanism to the slide is kept constant, and thereby the forming precision is increased. 
     The advantages offered by the sixth aspect of the invention are as follows. With only one means for setting the twisting angle provided between the pressing block and the slide, the stroke of the pressing block can be defined. Such a mechanism for fine height adjustment among multiple lower stoppers as conventional forging presses require is unnecessary. Therefore, the forming press can be constructed simply and inexpensively. Besides, by extending and contracting the lower stopper provided in the slide, the spacing between the upper stopper of the pressing block and the lower stopper can be changed. Thus, the descending stroke of the twisting push rods fixed to the pressing block and hence the twisting angle can be adjusted as desired. Moreover, because the lower stopper extends and contracts automatically, the twisting angle is adjusted automatically, which raises the press-work productivity. 
     The advantages offered by the seventh aspect of the invention are as follows. When the driving mechanism rotates the internally threaded cylinder about its vertical axis, the externally threaded member engaging with the internal-thread portion of the internally threaded cylinder moves up and down together with the stopper block on it. In this way, just by rotating an internally threaded cylinder, the spacing between the upper stopper and the stopper block and hence the twisting angle can be adjusted. Such a mechanism for fine height adjustment among multiple lower stoppers as conventional presses require is unnecessary. Therefore, the forming press can be constructed simply and inexpensively. Besides, the threaded fitting of the externally threaded member into the internally threaded cylinder can be configured so as to prohibit them from turning in the reverse direction when the upper stopper is pressed on the stopper block. Therefore, the stopper block remains in its set position, and hence high forming precision can be achieved. 
     The advantages offered by the eighth aspect of the invention are as follows. Because a worm and a worm wheel are used, a minute feed of the lower stopper is possible, enabling high-precision setting of the twisting angle. Besides, because the worm and the worm wheel do not turn in the reverse direction under the downward force exerted by the upper stopper, the stopper block remains in its set position. Therefore, high forming precision is achieved. 
     The advantage offered by the ninth aspect of the invention is mainly as follows. Because the upper guiding means guides the four corners of the upper portion of the pressing block relatively to the slide and the lower guiding means guides the four corners of the lower portion of the pressing block relatively to the slide, the pressing block does not slant and hence the twisting push rods are pressed down exactly vertically. Accordingly, with only one telescopic lower stopper as the means for setting the twisting angle, the four twisting push rods can be pressed down to one and the same level. Therefore, an exact twisting angle can be given to workpieces. 
     The advantage offered by the tenth aspect of the invention is mainly as follows. Each of the upper guiding means and the lower guiding means guides the pressing block at every corner with a set of a guide-slider part and a guide-rail part against forward and backward deviation and another set of a guide-slider part and a guide-rail part against rightward and leftward deviation. Therefore, the pressing block slants neither forward or backward nor rightward or leftward. Accordingly, the twisting push rods do not slant and, therefore, press work of a precise twisting angle can be accomplished. 
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The features and advantages of the present invention will become more clearly appreciated from the following description in conjunction with the accompanying drawings, in which: 
     FIG. 1 is a front view of an embodiment of forming press of crankshafts of the present invention; 
     FIG. 2 is a front view of the main part of the forming press of FIG. 1, its twisting oil-hydraulic cylinder extended; 
     FIG.  3 (A) is a view taken along the arrowed line A of FIG. 2, and FIG.  3 (B) is a view taken along the arrowed line B of FIG. 2; 
     FIG. 4 is a perspective view of the forging unit for twisting of the forming press of FIG. 1; 
     FIG. 5 is an oil-hydraulic circuit diagram of the twisting oil-hydraulic cylinder of the forming press of FIG. 1; 
     FIG. 6 is an explanatory drawing of the reshaping stage I of the forming press of FIG. 1; 
     FIG. 7 is an explanatory drawing of the twisting stage II of the forming press of FIG. 1; 
     FIG. 8 is an explanatory drawing of the die-opening stage III of the forming press of FIG. 1; 
     FIG. 9 is an explanatory drawing of the upper die section-untwisting stage IV of the forming press of FIG. 1; 
     FIG. 10 shows the above four stages I-IV in sequence; 
     FIG. 11 is a front view of another embodiment of forming press of crankshafts of the present invention; 
     FIG. 12 is a front view of the main part of the forming press of FIG. 11, its twisting oil-hydraulic cylinder fully extended; 
     FIG. 13 is a view taken along the arrowed line III of FIG. 12; 
     FIG. 14 is a side view of the means for setting the twisting angle of the forming press of FIG. 11 
     FIG.  15 (A) is a sectional view of the twisting angle-setting means of FIG. 14, and FIG. 15 (B) is a plan view of the stopper block  68  of FIG.  15 (A); 
     FIG. 16 is a plan view of the twisting angle-setting means of FIG. 14; 
     FIG. 17 is a front view of the means for guiding the pressing block of the forming press of FIG. 11, the view taken along the arrowed line VIII—VIII of FIG. 20; 
     FIG. 18 is a side view of the guiding means of FIG. 17; 
     FIG. 19 is a front view of the upper guiding means of the guiding means FIG. 17; 
     FIG. 20 is a plan view of the guiding means of FIG. 17; 
     FIG. 21 is an explanatory drawing of the reshaping stage I of the forming press of FIG. 11; 
     FIG. 22 is an explanatory drawing of the twisting stage II of the forming press of FIG. 11; 
     FIG. 23 is an explanatory drawing of the die-opening stage III of the forming press of FIG. 11; 
     FIG. 24 is an explanatory drawing of the upper die section-untwisting stage IV of the forming press of FIG. 11; and 
     FIG. 25 is a sectional front view of the main part of the forming press of crankshafts of a prior art. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     With reference to the drawings, preferred embodiments of the present invention will now be described. 
     (First Embodiment of the Invention) 
     Referring to FIG. 1, the basic structure of an embodiment of forming press of crankshafts of the invention will first be described. 
     The numeral  1  represents a bed hard plate;  2 , a crown;  3 , columns connecting the bed hard plate  1  and the crown  2 . The numeral  4  indicates a slide. The signs  5 A and  5 B represent a pressing oil-hydraulic cylinder and lift-up oil-hydraulic cylinders, respectively, both mounted on the crown  2 . When the rod-side chambers of the lift-up cylinders  5 B are opened, the slide  4  descends, hydraulic oil being sucked from a tank into the pressing cylinder  5 A. The slide  4  is of inverted T-form in section. 
     An upper die holder  6  is secured to the bottom of the slide  4 . A lower die holder  7  is mounted on the top of the bed hard plate  1 . An upper rotation guide  8  and the upper section  18  of a reshaping die  17  are secured to the bottom of the upper die holder  6 . A lower rotation guide  9  and the lower section  19  of the reshaping die  17  are mounted on the top of the lower die holder  7 . 
     The twisting die  10  consists of an upper section  11  and a lower section  12 . The upper section  11  is fitted in an upper retainer  13 , which is journaled in the upper rotation guide  8 . The lower section  12  is fitted in a lower retainer  14 , which is journaled in the lower rotation guide  9 . 
     With reference to FIG. 4, the details of the forging unit for twisting will now be described. The upper rotation guide  8  consists of three guide elements  8   a ,  8   b , and  8   c . The upper retainer  13  also consists of three retainer elements  13   a ,  13   b , and  13   c . Each retainer element has a push-down arm  15 . The push-down arms  15  project in upper right and upper left directions alternately. A pair of rollers  15   a  is journaled on the upper end of each push-down arm  15 . Disposed between the retainer elements  13   a ,  13   b , and  13   c  are upper-fixed-die-section holders  16   a  and  16   b , which are fixed to the upper die holder  6  (see FIG.  1 ). 
     The lower rotation guide  9  also consists of three guide elements  9   a ,  9   b , and  9   c . The lower retainer  14  also consists of three retainer elements  14   a ,  14   b , and  14   c . Disposed between the retainer elements  14   a ,  14   b , and  14   c  are lower-fixed-die-section holders  16   c  and  16   d , which are fixed to the lower die holder  7  (see FIG.  1 ). 
     The upper retainer  13  and the lower retainer  14  are disposed opposite to each other, the former accommodating the upper die section  11 , the latter accommodating the lower die section  12 . A workpiece is put between the upper and lower die sections  11  and  12 , and pressure is applied to the upper die section  11  to fix the workpiece with the upper- and lower-fixed-die-section holders  16   a ,  16   b ,  16   c , and  16   d . Then, the push-down arms  15  are pushed down to turn the upper and lower retainer  13  and  14  and hence the twisting die  10  between the upper and lower rotation guides  8  and  9 . Thus, the workpiece caught between the upper and lower die sections  11  and  12  is twisted. 
     Referring to FIG. 1, the forging unit for reshaping will now be described. A workpiece twisted in the forging unit for twisting is put between the upper and lower sections  18  and  19  of the reshaping die  17 , and pressure is applied to the upper die section  18  to reshape the workpiece. 
     The characteristic portion of the forming press will now be described. 
     In FIG. 1, the numeral  20  represents a twisting oil-hydraulic cylinder, which is mounted on the crown  2 . The cylinder may be mounted on the crown either fixedly or by means of trunnions. 
     In case that the twisting oil-hydraulic cylinder  20  is fixed to the crown  2 , the oil pipes to the twisting oil-hydraulic cylinder  20  can be laid fixedly, which eliminates almost completely the danger of fire due to the heat of hot forging. In case that the twisting oil-hydraulic cylinder  20  is mounted on the crown  2  by means of trunnions, flexible hoses are required. However, because the purpose of using the flexible hoses is only to absorb vibration, shorter flexible hoses suffice for the purpose compared with the flexible hoses used for conventional forming presses. 
     The lower end of the piston rod  21  of the twisting oil-hydraulic cylinder  20  is connected to a pressing block  23 . A twisting push rod (hereinafter referred to as “push rod”)  31  is secured to each of the four corners of the pressing block  23 . These components constitute a twisting mechanism. 
     Referring to FIGS. 2 and 3, the details of the twisting mechanism will be described. 
     The pressing block  23  comprises a guide shoe  23   c  and block members  23   a  and  23   b  secured to the front and the back of the guide shoe  23   c , taking an H-like shape as seen from above and an inverted U-like shape as seen from one side. The pressing block  23  is disposed so as to bestride the slide  4 . Thus, the twisting mechanism and the slide  4  do not interfere with each other, and a large up-and-down stroke can be secured. Besides, two column-like guide members  25  are erected on the top of the slide  4 , and the guide shoe  23   c  moves up and down, sliding in a guide gap  26  formed between the guide members  25  facing to each other. Thus, the pressing block  23  is guided properly in moving up and down relatively to the slide  4 . Moreover, guide pipes  41  are provided at right, left, front, and rear parts of the base plate of the slide  4 , the base plate being the bottom portion of the slide  4  and formed wide. The four push rods  31  move up and down, guided by the guide pipes  41 . Thus, the push rods  31  move up and down, keeping a certain distance from the twisting die  10  (refer to FIG.  1 ). 
     An upper stopper  27  is mounted on the guide members  25 , and a lower stopper  28  is disposed, on the bottom portion of the slide  4 , below each of the block members  23   a  and  23   b . The up and down strokes of the pressing block  23 , or the twisting mechanism, are restricted by these upper and lower stoppers  27  and  28 . 
     As mentioned earlier, four push rods  31  are secured into bosses at the four corners of the pressing block  23 . Two push rods  31  are disposed in front and back of the slide  4  (see FIG. 3) on the right side of the twisting die  10  (see FIG.  1 ); the other two, in front and back of the slide  4  on the left side of the twisting die  10 . The bottom portions of the front and rear push rods  31  and  31  on the right side of the twisting die  10  are connected to each other by a twisting-down beam  32  and an untwisting beam  33 . In the same way, the bottom portions of the front and rear push rods  31  and  31  on the left side of the twisting die  10  are connected to each other by a twisting-down beam  32  and an untwisting beam  33 . 
     Thus, because the upper ends of the push rods  31  are secured to the pressing block  23  and their lower ends are connected to each other by the twisting-down beams  32  and the untwisting beams  33 , the twisting mechanism can be constructed with higher rigidity and less weight. 
     The rollers  15   a  of the push-down arms  15  of the upper retainer  13  are put between the twisting-down beam  32  and the untwisting beam  33  on each side of the twisting die  10 . 
     Thus, because two or more push-down arms  15  are arranged side by side along the twisting-down beam  32  and the untwisting beam  33 , both having a suitable length, on each side of the twisting die  10 , it is unnecessary to change the push rods  31 , the twisting-down beams  32 , and the untwisting beams  33  regardless of different sizes of crankshafts, and hence different sizes of the upper and lower retainers  13  and  14 , and hence different positions of the push-down arms  15 . 
     Referring to FIG. 5, the oil-hydraulic circuit of the twisting oil-hydraulic cylinder  20  will be described. 
     A four-port, three-position directional control valve SV 1  is put in oil lines  52  and  53  between the twisting oil-hydraulic cylinder  20  and an oil-hydraulic pump  51  so that hydraulic oil can be fed to the head-side chamber  20   a  and the rod-side chamber  20   b  of the twisting oil-hydraulic cylinder  20 . The head-side chamber  20   a  is connected to an oil tank  54  through an opening and closing valve SV 2 . The rod-side chamber  20   b  is connected to the oil tank  54  through an oil line with an opening and closing valve SV 3  and another oil line with an opening and closing valve SV 5 . A relief valve  56  is put between the rod-side chamber  20   b  and the opening and closing valve SV 3 . An opening and closing valve SV 4  is put in a bypass between the oil lines  52  and  53 . The opening and closing valve Sv 4  constitutes a differential circuit. 
     The directional control valve SV 1  causes the twisting oil-hydraulic cylinder  20  to extend for twisting movement and contract for untwisting movement. When the directional control valve SV 1  shifts to its {circle around (1+L )} position, hydraulic oil is fed to the rod-side chamber  20   b  to contract the twisting oil-hydraulic cylinder  20 . When the directional control valve SV 1  shifts to its {circle around (2+L )} position, hydraulic oil is fed to the head-side chamber  20   a  to extend the twisting oil-hydraulic cylinder  20 , and thus a workpiece in the twisting die  10  is given a twist. In addition, when the twisting oil-hydraulic cylinder  20  contracts, the opening and closing valve SV 2  opens to return hydraulic oil from the cylinder  20  to the oil tank  54 . When the twisting oil-hydraulic cylinder  20  extends, the opening and closing valve SV 5  opens to return hydraulic oil from the cylinder  20  to the oil tank  54 . Thus, hydraulic oil in the twisting oil-hydraulic cylinder  20  is returned to the oil tank  54  through the valves SV 2  and SV 5  in addition to the valve SV 1 . 
     During a free downward stroke, or high-speed descent, of the slide  4 , it is necessary to keep constant the relative position of the four push rods  31  to the slide  4 . To accomplish this, the directional control valve SV 1  is set neutral, and the opening and closing valves SV 2 , SV 3 , and SV 4  are opened. The upper stopper  27  rigidly connected to the slide  4  pushes down the guide shoe  23   c , which pulls down, or extends, the twisting oil-hydraulic cylinder  20 . The hydraulic oil in the rod-side chamber  20   b  opens the relief valve  56  to return to the oil tank  54  through the opening and closing valve SV 3  and also flows toward the head-side chamber  20   a  through the opening and closing valve SV 4 . However, as the opening and closing valve SV 4  constitutes a differential circuit, back pressure develops between the rod-side chamber  20   b  and the relief valve  56 . Therefore, the push rods  31  do not descend more quickly than the slide  4  but descend keeping constant their relative position to the slide  4 . 
     To raise the slide  4 , the directional control valve SV 1  is set neutral, the opening and closing valves SV 2  and SV 4  are opened, and the opening and closing valve SV 3  is closed. The lower stoppers  28  of the slide  4  being raised by the lift-up oil-hydraulic cylinders  5 B push up the pressing block  23 , contracting the twisting oil-hydraulic cylinder  20 . The hydraulic oil in the head-side chamber  20   a  returns to the oil tank  54  through the opening and closing valve SV 2 . 
     On the other hand, hydraulic oil is sucked into the rod-side chamber  20   b  by opening the opening and closing valve SV 5 . Because the lower stoppers  28  are in contact with the pressing block  23 , the relative position of the push rods  31  to the slide  4  is kept constant. 
     The following table shows the relation between the opening and closing of the valves SV 1  to SV 4  and the stages in the forming process. 
     
       
         
               
               
               
               
               
               
             
           
               
                   
               
               
                 Stages in forming process 
                 SV1 {circle around (1)}   
                 SV1 {circle around (2)}   
                 SV2 
                 SV3 
                 SV4 
               
               
                   
               
             
             
               
                 Twisting 
                 X 
                 ◯ 
                 X 
                 ◯ 
                 X 
               
               
                 Untwisting 
                 ◯ 
                 X 
                 ◯ 
                 X 
                 X 
               
               
                 Pressing of Slide 4 
                 X 
                 X 
                 ◯ 
                 ◯ 
                 X 
               
               
                 Slide 4&#39;s idie descent 
                 X 
                 X 
                 ◯ 
                 ◯ 
                 ◯ 
               
               
                 Slide 4&#39;s ascent 
                 X 
                 X 
                 ◯ 
                 X 
                 ◯ 
               
               
                   
               
               
                 ◯:open  
               
               
                 X: close  
               
             
          
         
       
     
     Referring to FIGS. 6 to  10 , the crankshaft-forming process of the forming press will be described. 
     In the reshaping stage I shown in FIG. 6, the pressing oil-hydraulic cylinder  5 A extends to push down the slide  4 , a workpiece not yet twisted is caught between the upper and lower sections  11  and  12  of the twisting die  10 , and another workpiece already twisted is pressed and reshaped in the reshaping die  17 . 
     In the twisting stage II shown in FIG. 7, the twisting oil-hydraulic cylinder  20  extends to push down the pressing block  23 , the push rods  31 , and the twisting-down beams  32 , which push down the push-down arms  15  of the upper retainer  13  to give a twist to the workpiece in the twisting die  10 . 
     In the die-opening stage III shown in FIG. 8, the lift-up oil-hydraulic cylinders  5 B raise the slide  4 , which raises the pressing block  23 , the push rods  31 , and the untwisting beams  33 . Accordingly, the upper retainer  13  is raised from the lower retainer  14 , and hence the upper and lower sections  11  and  12  of the twisting die  10  are separated from each other. 
     In the upper die section-untwisting stage IV shown in FIG. 9, the twisting oil-hydraulic cylinder  2  is contracted to raise the push rods  31  and the untwisting beams  33 , which turn upward the push-down arms  15  of the upper retainer  13 . 
     In this way, workpieces are twisted and formed into crankshafts. FIG. 10 shows all the four stages in sequence. 
     (Second Embodiment of the Invention) 
     Now, the basic structure of another preferred embodiment of forming press of crankshafts of the present invention will be described. 
     In FIG. 11, the numeral  1  represents a bed hard plate;  2 , a crown;  3 , columns connecting the bed hard plate  1  and the crown  2 . The numeral  4  indicates a slide. The signs  5 A and  5 B represent a pressing oil-hydraulic cylinder and lift-up oil-hydraulic cylinders, respectively, both mounted on the crown  2 . When the rod-side chambers of the lift-up cylinders  5 B are opened and, at the same time, the pressing cylinder  5 A is allowed to suck in hydraulic oil from a tank, the slide  4  descends. The slide  4  has, in cross section, a tall rectangular body and a wide base plate, the two portions generally constituting an inverted T-form. 
     An upper die holder  6  is secured to the bottom of the slide  4 . A lower die holder  7  is mounted on the top of the bed hard plate  1 . An upper rotation guide  8  and the upper section  18  of a reshaping die  17  are secured to the bottom of the upper die holder  6 . A lower rotation guide  9  and the lower section  19  of the reshaping die  17  are mounted on the top of the lower die holder  7 . 
     The construction of the forging unit for twisting is as follow. The twisting die  10  consists of an upper section  11  and a lower section  12 . The upper section  11  is fitted in an upper retainer  13 , which is journaled in the upper rotation guide  8 . The lower section  12  is fitted in a lower retainer  14 , which is journaled in the lower rotation guide  9 . The detailed construction of the forging unit for twisting is the same as the detailed construction shown in FIG.  4 . 
     In FIGS. 11 to  13 , the numeral  20  represents a twisting oil-hydraulic cylinder, which is mounted on the crown  2 . The oil-hydraulic circuit for operating the twisting cylinder  20  is the same as the circuit shown in FIG.  5 . 
     The lower end of the piston rod  21  of the twisting oil-hydraulic cylinder  20  is connected to a pressing block  23 . Secured to each of the four corners of the pressing block  23  is the upper end of a twisting push rod (hereinafter referred to as “push rod”)  31 . 
     Two of the four push rods  31  are disposed in front and back of the body of the slide  4  (see FIG. 13) on the right side of the twisting die  10  (see FIG.  11 ); the other two, in front and back of the body of the slide  4  on the left side of the twisting die  10 . The bottom portions of the front and rear push rods  31  and  31  on the right side of the twisting die  10  are connected to each other by a twisting-down beam  32  and an untwisting beam  33 . In the same way, the bottom portions of the front and rear push rods  31  and  31  on the left side of the twisting die  10  are connected to each other by a twisting-down beam  32  and an untwisting beam  33 . 
     Thus, because the upper ends of the push rods  31  are secured to the pressing block  23  and their lower ends are connected to each other by the twisting-down beams  32  and the untwisting beams  33 , the twisting mechanism can be constructed with higher rigidity and less weight. 
     The rollers  15   a  of the push-down arms  15  of the upper retainer  13  are put between the twisting-down beam  32  and the untwisting beam  33  on each side of the twisting die  10 . 
     Thus, because two or more push-down arms  15  are arranged side by side along the twisting-down beam  32  and the untwisting beam  33 , both having a suitable length, on each side of the twisting die  10 , it is unnecessary to change the push rods  31 , the twisting-down beams  32 , and the untwisting beams  33  regardless of different sizes of crankshafts, and hence different sizes of the upper and lower retainers  13  and  14 , and hence different positions of the push-down arms  15 . 
     Now the means for setting the twisting angle of the forming press will be described. 
     As shown in FIG. 14, the means for setting the twisting angle comprises an upper stopper  23   d  on the pressing block  23 &#39;s side and an automatic telescopic lower stopper  60  on the slide  4 &#39;s side. 
     The construction of the pressing block  23  with the upper stopper  23   d  will be described first. The lower end of the piston rod  21  of the twisting oil-hydraulic cylinder  20  is connected to a bracket  23   e  which is mounted on the upper stopper  23   d . Block members  23   a  and  23   b  are secured to the front and the back, respectively, of the upper stopper  23   d . These three members are arranged in an H-like shape as seen from above them and in an inverted U-like shape as seen from one side. The pressing block  23  is disposed astride of the body  4   a  of the slide  4 . A push rod  31  is secured at each of the right and left end portions of each of the block members  23   a  and  23   b  as mentioned earlier (see FIGS.  12  and  13 ). The upper stopper  23   d  plays also the roll of connecting member between the block members  23   a  and  23   b.    
     The body  4   a  of the slide  4  is provided with the automatic telescopic lower stopper  60 , which takes a position below the center of the upper stopper  23   d  and of which the center is aligned with the axis of the twisting oil-hydraulic cylinder  20 . 
     As shown in FIGS. 15 and 16, a cylindrical casing  61  is secured to the body  4   a  of the slide  4 . An internally threaded cylinder  62  is inserted in the casing  61  so as to be freely rotatable about its vertical axis. Internal threads  63  are formed on the inner surface of the cylinder  62 . Besides, a worm wheel  64  is formed on the outer surface of the internally threaded cylinder  62 . Engaged with the worm wheel  64  is a worm  65 , which can be rotated in either direction by an electric motor  66 . On the other hand, an externally threaded member  67  is engaged with the internal threads  63  of the internally threaded cylinder  62 . A stopper block  68  is fixed on the top of the externally threaded member  67 . Four sliding surfaces  68   a  are formed in the vertical direction on the periphery of the stopper block  68 . Provided on the upper end of the casing  61  is a guide member  69 , which has four shoes  69   a  slidingly in contact with the sliding surfaces  68   a  to prohibit the stopper block  68  from turning and guide the stopper block  68  up and down. 
     With the above configuration, when the electric motor  66  rotates in one or the other direction, the internally threaded cylinder  62  rotates to raise or lower the externally threaded member  67  and hence the stopper block  68 . Thus, as shown in FIG. 14, the spacing “d” between the upper stopper  23   d  and the stopper block  68 , namely, the descending stroke of the pressing block  23 , or the push rods  31 , relative to the slide  4  can be adjusted. 
     Although the worm  65  can turn the worm wheel  64 , the worm wheel  64  cannot rotate the worm  65 . Therefore, the externally threaded member  67  and the internally threaded cylinder  62  do not turn in the reverse direction under the downward force acting on the stopper block  68 , and hence the set twisting angle is not disturbed during the operation of the forming press. Besides, by setting the lead angle of the threads of the externally threaded member  67  and the internally threaded cylinder  62  small, the stopper block  68  can be prevented from turning under the downward force working on it. Thus, its stability can be further increased. 
     Referring to FIGS. 17 to  20 , the means for guiding the pressing block  23  will now be described. In FIG. 17, the numeral  25  represents a guide structure, which is mounted fixedly on the top of the slide  4 . As shown in FIGS. 18 to  20 , the guide structure  25  comprises two columns  25   a  and  25   a  and a member  25   b  connecting the upper ends of the two columns  25   a  and  25   a . An opening  25   c  is formed in the center of the connecting member  25   b  to let through the piston rod  21  of the twisting oil-hydraulic cylinder  20 . The upper stopper  23   d  of the pressing block  23  is so disposed below the connecting member  25   b  of the guide structure  25  that the connecting member  25   b  and the upper stopper  23   d  cross each other at a right angle as seen from above them. The block members  23   a  and  23   b  are disposed in front and back of, and in parallel with, the connecting member  25   b  (see FIG.  20 ). 
     With the pressing block  23  and the guide structure  25  so constructed and disposed, an upper guiding means  70  and a lower guiding means  75 , both constituting the above-mentioned means for guiding the pressing block  23 , are constructed as follows. 
     As shown in FIGS. 17 to  20 , four guide sliders  71  are attached to such four spots of the upper portion of the pressing block  23  (that is, of the upper stopper  23   d  and the upper portions of the block members  23   a  and  23   b ) as are diagonally symmetrical about the connection between the lower end of the piston rod  21  of the twisting oil-hydraulic cylinder  20  and the upper stopper  23   d . Each guide slider  71  consists of a part facing inward of the forming press, that is, to the front or the rear of the forming press, as the case may be, and a part facing outward of the forming press, that is, to the right or the left of the forming press, as the case may be. 
     On the other hand, four guide rails  72  are so attached to the right and left columns  25   a  and  25   a  of the guide structure  25  that their positions are diagonally symmetrical about the center of the guide structure  25  as seen from above them. The length of the guide rails  72  corresponds to the stroke of the pressing block  23 . Each guide rail  72  consists of a art facing outward of the forming press, that is, to the rear or the front of the forming press, as the case may be, and a part facing inward of the forming press, that is, to the left or the right of the forming press, as the case may be. The four guide sliders  71  slide on the four guide rails  72 , and thereby the pressing block  23  is guided and prevented from slanting back and forth or right and left while it is moving up and down. 
     The construction of the lower guiding means  75  is as follows. As shown in FIGS. 17,  18 , and  20 , four guide sliders  76  are attached to both ends of the front block member  23   a  and both ends of the rear block member  23   b , namely, the four corners of the pressing block  23 , each corner holding a push rod  31 . Each guide slider  76  consists of a part facing inward of the forming press, that is, to the front or the rear of the forming press, as the case may be, and a part facing outward of the forming press, that is, to the right or the left of the forming press, as the case may be. 
     On the other hand, four guide rails  77  are so attached to the body  4   a  of the slide  4  that their positions correspond to those of the four guide sliders  76 . The length of the guide rails  77  also corresponds to the stroke of the pressing block  23 . Each guide rail  77  consists of a part facing outward of the forming press, that is, to the rear or the front of the forming press, as the case may be, and a part facing inward of the forming press, that is, to the left or the right of the forming press, as the case may be. 
     The four guide sliders  76  slide on the four guide rails  77 , and thereby the pressing block  23  is guided and prevented from slanting back and forth or right and left while it is moving up and down. 
     As described above, because the upper and lower guiding means guide the upper and lower portions of the pressing block  23 , the pressing block  23  does not slant and the lower ends of the four push rods  31  descend to one and the same level. Besides, although only one set of upper and lower stoppers  23   d  and  60  is provided directly below the axis of the twisting oil-hydraulic cylinder  20 , the pressing block  23  presents no slant and the lower ends of the four push rods  31  are at one and the same level when the pressing block  23  has descended to the bottom of its stroke and been checked by the lower stopper  60  because each of the upper and lower guiding means guides the pressing block  23  at four spots around the lower stopper  60 . Thus, high forming precision can be achieved. 
     Referring to FIGS. 21 to  24 , the crankshaft-forming process of the forming press of the present embodiment will be described. 
     In the reshaping stage I shown in FIG. 21, the pressing oil-hydraulic cylinder  5 A extends to push down the slide  4 , a workpiece not yet twisted is caught between the upper and lower sections  11  and  12  of the twisting die  10 , and another workpiece already twisted is pressed and reshaped in the reshaping die  17 . 
     In the twisting stage II shown in FIG. 22, the twisting oil-hydraulic cylinder  20  extends to push down the pressing block  23 , the push rods  31 , and the twisting-down beams  32 , which push down the push-down arms  15  of the upper retainer  13  to give a twist to the workpiece in the twisting die  10 . 
     In the die-opening stage III shown in FIG. 23, the lift-up oil-hydraulic cylinders  5 B raise the slide  4 , which raises the pressing block  23 , the push rods  31 , and the untwisting beams  33 . Accordingly, the upper retainer  13  is raised from the lower retainer  14 , and hence the upper and lower sections  11  and  12  of the twisting die  10  are separated from each other. 
     In the upper die section-untwisting stage IV shown in FIG. 24, the twisting oil-hydraulic cylinder  20  is contracted to raise the push rods  31  and the untwisting beams  33 , which turn upward the push-down arms  15  of the upper retainer  13 . 
     In this way, workpieces are twisted and formed into crankshafts. 
     The invention may be embodied in other specific forms without departing from the spirit or essential characteristics thereof. The above 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.