Abstract:
A computer-based apparatus for performing a multiaxial forging method is disclosed. The apparatus comprises a process control computer and a pair of forging dies connected thereto, wherein each of the dies comprises a piston ( 1, 2 ), said pistons being moveable along a common axis (z), and wherein a shaping anvil ( 3 ) is mounted on each of the pistons ( 1, 2 ), and the pistons ( 1, 2 ) are equipped with position measuring sensors. The apparatus further comprises a measuring instrument ( 20 ) attached to the pistons ( 1, 2 ) of the dies for directly measuring the distance between the two pistons ( 1, 2 ) at a location adjacent to the workpiece.

Description:
FIELD OF THE INVENTION 
       [0001]    The present invention relates to a method of computer-controlled multiaxial forging, an apparatus for carrying out the method, and a non-transitory computer program product storing a computer program for carrying out the method. 
       BACKGROUND ART 
       [0002]    In the computer-controlled multiaxial forging, a workpiece to be forged is shaped along its one or more axis by means of forging dies moveable along a tool axis. The forging dies are moved by hydraulic pistons in a manner that the extent of displacement of each die (anvil) is determined according to the dimensions of the surfaces of the workpiece to be forged. When a workpiece is subject to a multiaxial shaping, the gripped workpiece is to be rotated around an axis perpendicular to the axis of the tool motion during the forging process. The angles of rotation depend on the shape of the surface to be produced. In order to avoid causing damage in the dies by the workpiece when the workpiece is being rotated between the successive forging steps, the dies must be refracted to a substantial extent. The greatest extent of retraction is necessary when the workpiece is rotated by 90 degrees, since the largest extension, i.e. generally, the diagonal extension, of the cross-section of the workpiece necessarily occurs within the volume between the dies. 
         [0003]    In  FIG. 1 , a perspective view of a conventional forging tool is schematically illustrated. The tool comprises two pistons  1 ,  2 , which may be displaced along a common axis z. Each of the pistons  1 ,  2  is provided with a respective anvil  3 , wherein said anvils are used to forge two opposite surfaces of a workpiece  4 . The gripped workpiece  4  may be rotated along an axis y perpendicular to the axis z. The displacement of the pistons  1 ,  2  is measured by position sensors (not shown in the drawings) mounted on a remote rear ends of the pistons  1  and  2 , respectively, which position sensors, in turn, forward the measured displacement positions of the pistons to a computer (not shown in the drawings) that functions to control the overall forging process. The pressing force exerted to the workpiece  4  is measured by a load cell  10  that forwards the measured force values also to the process control computer. 
         [0004]    Before starting the forging process the two pistons  1 ,  2  are caused to approach the workpiece  4  in z-direction step by step, advancing by predetermined distances, for example by 0.001 or 0.002 mm, under the control of the value provided by their associated position sensors, and then after bringing them into contact with the workpiece  4 , they are caused to proceed until the compressive force measured by the load cell  10  of the tool achieves a predetermined value, a so called contact force. The programmable values of the positions of the two pistons  1 ,  2  are set to zero at the end of this initial moving cycle, i.e. at reaching the contact force. (The programmable position values of the position sensors of the two pistons is not identical to those absolute position values of the position sensors that belong to a particular location thereof.) 
         [0005]    In the following part of the shaping process, in each forging step, the position of the pistons may be specified as a function of time relatively to these zero positions (serving as initial positions), and thus the end position of the dies and the advancing speed of the dies may be determined. For example, if for a workpiece having an initial width of 10 mm, the forging end position is programmed to −1.31 mm for both of the dies, the workpiece will be pushed by the two dies to a nominal width of (10−1.31−1.31)=7.38 mm. 
         [0006]    The return position of the pistons after forging (i.e. their retracted position) may be specified relatively to the aforementioned initial positions, which is a necessary condition for that the workpiece can be freely rotated between the dies. 
         [0007]    In the subsequent forging steps, the end positions of the two dies may be specified in every case relatively to the initial positions determined by the contact forces measured on the workpiece right before starting the respective forging steps. 
         [0008]    A drawback of the above mentioned conventional control of advancing is that the position of the surfaces of the workpiece in the direction of tool motion, i.e. in z-direction, is known only in the first forging step, meaning that the extent of the z-directional deformation associated with the shaping to be carried out can be exactly specified only in this case. As shown in  FIG. 2.A , before the first forging step the workpiece has a width h0 in the z-direction and a height s0 in the x-direction. After the first forging step, as a result of a forging action, the workpiece will have a width h1 in the z-direction and a height s1 in the x-direction. Before the next forging step, the dies are retracted and the workpiece is rotated, for example, by 90 degrees, which means that the z-directional width of the workpiece will change to s1, whereas its x-directional height will change to h1. 
         [0009]    In the next forging step, the pistons  1 ,  2  start from their rear (retracted) position, while the z-directional dimension of the workpiece, i.e. the dimension s1 produced in the first step, is not known because of the rotation of the workpiece  4 , therefore the forging end positions of the dies can be specified only relatively to the initial width h0 of the workpiece  4 . As a result, in the second and every subsequent forging steps, neither the extent nor the speed of the actual z-directional deformation can be known in advance, because the pistons  1 ,  2  carry out the forging operation specified for a given step (i.e. they move to their specified end positions) independently of the actual z-directional width of the workpiece  4 . This raises a problem because neither the effect nor the extent of the shaping to the microstructure of the material of the workpiece  4  and to the mechanical properties of the workpiece  4  can be precisely determined. 
         [0010]    It is an object of the present invention to provide a control mechanism that allows an exact determination of the desired extent and speed of deformation of the workpiece in every forging step as a function of the actual z-directional width of the workpiece. 
       SUMMARY OF THE INVENTION 
       [0011]    In a first aspect of the present invention, the above objects of the invention are achieved by providing a method of computer-controlled multiaxial forging of a workpiece, wherein a workpiece having a z-directional initial width is subject to multiple-step forging by means of a pair of forging dies moving along a first axis, and wherein after each forging step, the opposite dies are moved away from the workpiece and the workpiece is rotated around a second axis, wherein before starting each forging step:
       finding the surfaces of the workpiece to be shaped by moving the dies by predetermined intervals to the opposite surfaces of the workpiece to be shaped until a predetermined contact force thereof is achieved,   upon reaching the contact forces, storing an actual starting position for each of the working surfaces of the forging dies,   based on said actual starting positions and the initial z-directional width of the workpiece, calculating and storing the z-directional width of the workpiece along the first axis is before the actual shaping step,   calculating a z-directional cyclical advancement dz(k) of the dies belonging to the actual forging step according to the equation:       
 
         [0000]        dz ( k )=0.5* δt* *{dot over (φ)}( i )* e   −{dot over (φ)}(i)*(δt*k)  
 
         [0000]    where δt is the time period of one advancement cycle [s], {dot over (φ)}(i) is the deformation rate [1/s] specified for the i-th forging step, w(i) is the z-directional width [mm] of the workpiece before carrying out the shaping in the i-th step, i is the serial number of the forging step, and k is the cycle number of advancement which is incremented after each cycle, and
       moving both dies from their actual initial positions by cyclical advancements dz(k) until the deformation specified for the actual i-th forging step is achieved.       
 
         [0017]    Preferably, the forging steps are repeated until a predetermined z-directional deformation of the workpiece is achieved. 
         [0018]    In another preferred embodiment of the method, in each shaping step, the current z-directional width of the workpiece is determined by directly measuring the distance between the two pistons at a location adjacent to the workpiece. 
         [0019]    In a second aspect of the present invention, it is provided a computer-based apparatus for performing the above mentioned multiaxial forging method, wherein the apparatus comprises a process control computer and a pair of forging dies connected thereto, wherein each of the dies comprises a piston, said pistons being moveable along a common axis, and wherein a shaping anvil is mounted on each of the pistons, and the pistons are equipped with position measuring sensors, and wherein the apparatus further comprises a measuring instrument attached to the pistons of the dies for directly measuring the distance between the two pistons at a location adjacent to the workpiece. 
         [0020]    A preferred embodiment of the apparatus further comprises a camera connected to the process control computer for taking photos of the workpiece in predetermined states thereof. 
         [0021]    Finally, in third aspect of the present invention, it is provided a non-transitory computer program product for computer-controlled multiaxial forging, the computer program product comprising a computer-readable medium having a plurality of computer program instructions stored therein, which are operable to cause a computer to perform the steps of the method according to the invention. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0022]    The present invention will now be described in detail through its preferred embodiments with reference to the accompanying drawings, in which: 
           [0023]      FIG. 1  schematically illustrates the structural elements of a conventional pair of forging dies in a perspective view, 
           [0024]      FIG. 2A to 2C  illustrate deformation of a workpiece in course of a uniaxial forging process schematically, partly in a perspective view and partly in a cross-sectional view, accompanied by photos taken in front view, in accordance with the method of the invention, before the first forging step ( FIG. 2A ), at the end of the first forging step ( FIG. 2B ) and before starting the second forging step ( FIG. 2C ), 
           [0025]      FIG. 3  illustrates a flow diagram including the main steps of the method according to the invention, 
           [0026]      FIGS. 4A to 4F  are photos showing various phases of a forging step in a preferred embodiment of the method according to the invention, and 
           [0027]      FIG. 5  schematically illustrates the structural elements of a pair of forging dies used in the apparatus according to the invention, in a perspective view. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0028]    The main steps of the method according to the invention are shown in the flow diagram of  FIG. 3 . A key feature of the method is that the actual z-directional width of the workpiece is determined not only once, at the very starting of the forging process, but before each forging step. 
         [0029]    In a first step S 100  of the method, the surfaces of the workpiece to be shaped are found by using the working surfaces of the dies in so manner that the dies are advanced by predetermined intervals f to the respective surfaces of the workpiece to be shaped until the specific predetermined contact forces F R , and F L  associated with the right-sided and left-sided pistons, respectively, are reached. The length of the advancing intervals f may, for example, be the above mentioned 0.001 or 0.002 mm in laboratory conditions, but in an industrial environment it may have a value of several orders higher. The displacement positions of the opposite dies, which positions are provided by the position sensors mounted on the rear end of the pistons, are stored in variables P R  and P L . 
         [0030]    Upon reaching the contact forces F R  and F L , the actual starting position P R (i) and P L (i), respectively, of the working surfaces of the forging dies belonging to the i-th forging step are stored in step S 110 . By using these actual starting positions P R (i) and P L (i), and the initial z-directional width h0 of the workpiece, the z-directional width w(i) of the workpiece before the actual forging step is calculated and stored in step S 120  according to the following equation: 
         [0000]        w ( i )= h   O   +P   R ( i )+ P   L ( i )  (1)
 
         [0031]    In the next step S 130 , a z-directional cyclical advancement dz(k) of the dies belonging to the actual i-th forging step is calculated according to the following equation: 
         [0000]        dz ( k )=0.5* δt *{dot over (φ)}( i )* w ( i )* e   −{dot over (φ)}(i)*(δt*k)   (2)
 
         [0000]    where δt is the duration of one advancement cycle [s], {dot over (φ)}(i) is the deformation rate [1/s] specified for the i-th forging step, w(i) is the z-directional width [mm] of the workpiece before carrying out the shaping operation in the i-th step, i is the number of the forging step, and k is the cycle number of advancement which is incremented after each cycle. According to the above Equation (2), the distance dz(k), i.e. the advancement of the dies specified for a particular forging step, is gradually decreased in every cycle so that the equivalent deformation rate within the shaped volume is nearly a predetermined value. Although the above Equation (2) provides a nearly constant equivalent deformation rate of the workpiece for a uniaxial state of stress, it also provides a good estimation for the multiaxial forging with a multiaxial state of stress. 
         [0032]    In the next step S 140 , the shaping is accomplished, wherein both of the pistons are simultaneously moved ahead by a specific advancement dz(k) determined using the motion function according to Equation (2) in every time interval (cycle) δt until the z-directional deformation specified for a given step is achieved. During this shaping step both dies are moved from their actual initial positions P R (i), P L (i) by cyclical advancements dz(k) to their end positions belonging to the actual i-th forging step, thereby the specified forging step is completed. 
         [0033]    In each forging step, during the shaping process the already produced instantaneous z-directional deformation is computed in every cycle, i.e. at time intervals δt, and it is checked whether the specified deformation is achieved, by comparing the calculated values with the deformation values specified for the given step. In the shaping step S 140  of the method according to the invention, instead of position data provided by the position sensors mounted on the remote rear end of the pistons, a distance between the pistons (or more precisely, between the shaping anvils) is directly measured by means of a supplementary distance-measuring instrument to determine the z-directional deformation of the workpiece at a higher accuracy, said supplementary instrument being mounted onto the tool adjacent to the ends of the pistons proximate to the workpiece. This supplementary distance-measuring instrument is capable of measuring the relative distance P LVDT  between the working surfaces of the dies at a much higher accuracy relatively to the afore-mentioned position sensors since it eliminates the displacement errors resulted from the elastic deformation of the long pistons and the plastic deformation of the workpiece during applying the compressive force. 
         [0034]    The deformation is calculated from the z-directional width w(i) measured before starting the actual forging step and the instantaneous width w(t) of the workpiece measured in every advancement cycle, i.e. at intervals δt, according to the following equation: 
         [0000]    
       
         
           
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         [0000]    where w(i) is the z-directional width of the workpiece before shaping in the i-th step, w(t) is the instantaneous width of the workpiece during shaping, P LVDT (0) is the distance between the two pistons before carrying out the i-th forging step, and P LVDT (t) is the instantaneous distance between the two pistons in the given k-th advancement cycle. 
         [0035]    To determine the z-directional width w(t) of the workpiece in each advancement cycle, the distance between the opposite forging surfaces of the tool is preferably measured by a supplementary distance-measuring instrument fixed to the pistons adjacent to the shaping anvils, for example a uniquely designed LVDT-type distance-measuring instrument (see  FIG. 5 ). The direct measurement of the distance between the two pistons in the proximity of the workpiece is necessary because the conventional position sensors measuring the positions of the pistons at their remote rear end output an aggregate displacement resulted from the elastic deformation of the pistons and the plastic deformation of the workpiece, therefore the deformation of the workpiece cannot be calculated at the required accuracy from those measurements. As it can be seen in  FIGS. 2A to 2C  relating to the first forging step, in the forging method according to the invention, the shaping anvils of the dies are pushed to the faces of the workpiece to be shaped before starting a shaping step, until reaching the specified contact force ( FIG. 2.A ). Then the workpiece is subject to shaping wherein in the actual forging step, each of the anvils are cyclically moved towards the workpiece by the advancement displacements dz(k) according to the advancement function defined by Equation (2) and calculated relatively to the starting position associated with the contact force, wherein the workpiece is being moved until reaching the z-directional deformation specified for the actual forging step ( FIG. 2B ). After finishing the actual forging step, the pistons are retracted and the workpiece is rotated by, for example, 90 degrees, and before the next forging step, the anvils are pushed again to the workpiece&#39;s faces to be shaped until reaching a specific right-sided and left-sided contact force ( FIG. 2C ) so that the unknown z-directional width of the workpiece can be measured. Based on the actually measured z-directional width of the workpiece, the necessary extent of the advancement of the dies for the next step can be readily determined. 
         [0036]    In a particularly preferred embodiment of the present invention, a camera is settled adjacent to the dies so that it can observe the workpiece located between the dies at an appropriate angle. The forging steps are now divided into two sub-steps, namely a pre-shaping step and a main shaping step. In the pre-shaping step the lateral surfaces of the workpiece which have become arcuate in the previous step and which are the surfaces to be shaped in the current step, are formed to be planar surfaces with providing a slight deformation thereof (e.g. φ=0.1). In  FIG. 4A , the moment of reaching the contact force at the beginning of the pre-shaping is shown, and  FIG. 4B  shows the situation in which the tool is in its end position of the pre-shaping step. As show in  FIG. 4C , after the pre-shaping step the workpiece is rotated by 90 degrees and a photo of one of the two faces which have been formed to be planar in the pre-shaping step is taken by the camera. 
         [0037]    Next, the workpiece is rotated back by 90 degrees and the main shaping step is started.  FIG. 4D  shows the situation in which the anvils of the dies reach the contact force while being pressed to the already planar lateral surfaces of the workpiece, and  FIG. 4E  shows the situation in which the dies are in their end positions associated with the main shaping step. After completing the main shaping step, the workpiece is rotated again by 90 degrees in order to carry out the next forging step as shown in  FIG. 4F . At this time, another photo of the shaped lateral surfaces is taken by the camera, wherein said shaped lateral surfaces have undergone a substantial deformation, and as a result of the high-power impact, the free lateral surfaces of the workpiece have become again outwardly arcuate (barrel-like surfaces). 
         [0038]    The above mentioned step of pre-shaping is necessary in order to start shaping in the main shaping step with forming a planar surface. Since the area of the surface to be shaped can be calculated, for example, by means of scales placed on the dies, any change in the surface compression stress during the main shaping step can be determined if a photo is taken of the forged surface at the end of both of the pre-shaping step and the main shaping step, and those photos are then analyzed in a similar way. 
         [0039]      FIG. 5  schematically illustrates, in a perspective view, the structural elements of a pair of dies comprised in the forging apparatus according to the invention. The dies according to the invention differ from the conventional pair of dies shown in  FIG. 1  in that a distance-measuring instrument  20  is mounted on the pistons  1 ,  2  adjacent to their ends proximate to the workpiece for cyclically and precisely measuring the z-directional width of the workpiece  4  in each forging step. This instrument comprises a measuring solenoid  7  fixed to a support assembly  6  and a measuring bar  8  fixed to another support assembly  9 . Although for the sake of simplicity, in  FIG. 5  the computer adapted for receiving data from the distance-measuring and position measuring elements of the tool and for controlling motion of the dies based on the received data is not shown, the way of connection of the computer with the dies and other details of how the dies are controlled by the computer are all obvious for a person skilled in the art. 
         [0040]    Due to a novel way of controlling the advancement of the dies, the method and apparatus according to the invention allow the execution of the sequence of forging steps in a more precisely controlled manner as compared to the prior art, while also allowing the specification of the extent of deformation and the speed of deformation for each forging step in advance. 
         [0041]    Finally, the invention relates to a non-transitory computer program product for computer controlled forging, the computer program product comprising computer-readable medium having a plurality of computer program instructions stored therein, which are operable to cause a computer to perform the steps of the method according to the invention.