Abstract:
A method of bending a metal object, such as a tube, is provided that uses real time, closed-loop feedback of the actual springback of the object in order to modify the applied bending force or preprogrammed bending coordinates so that the final desired bend geometry is achieved. The variability of springback from object to object is thus accounted for and the number of objects that must be scrapped due to incorrect bends (over bend or under bend) is reduced. The method is carried out using an apparatus such as a rotary draw bender with a measuring device operable to measure actual bend coordinates of metal objects bent by the bender. A controller is operatively connected to the bender and the measuring device and is configured to control the bender to bend the metal objects at least partly based on measured bend coordinates provided by the measuring device.

Description:
TECHNICAL FIELD 
       [0001]    The invention relates to a method of bending a metal object that provides real time bend verification and correction, and a bending apparatus for same. 
       BACKGROUND OF THE INVENTION 
       [0002]    While bending metal objects, such as metal tubes, many variables are encountered that must be accounted for to ensure that the desired final geometry is achieved. One such variable is the natural variation of sheet metal from coil to coil and its associated springback changes. Other contributors to processing variations include ambient temperature, machine temperature, lubrication, wear and tear of the bend tooling, and tooling setup. Metal tubes are formed from sheet metal rolled into a tubular shape and welded along an axial seam. “Springback” is the tendency of sheet metal (or a metal tube formed from a sheet) to lose some of its shape when it is removed from a die. As the die is released, the work piece ends up with less bend than that on the die (i.e., an “under bend”). The amount of springback is dependent on the characteristics of the material, including thickness, grain and temper. Springback that is not properly predicted or corrected can lead to excessive scrap rates. 
       SUMMARY OF THE INVENTION 
       [0003]    A method of bending a metal object, such as a tube, is provided that uses real time, closed-loop feedback of the actual springback of the object in order to modify the applied bending force or preprogrammed bending coordinates so that the final desired bend geometry is achieved. The variability of springback from object to object is thus accounted for and the number of objects that must be scrapped due to incorrect bends (over bend or under bend) is reduced. The method is carried out using an apparatus that includes a stationary base and a measuring device that is secured to the base. A rotatable bend die, a clamp die secured to the bend die and a pressure die movable with respect to the rotatable base, such as may be present on a rotary draw bender, are configured to bend metal objects and are also included in the apparatus. The pressure die acts on a wiper die. Additionally, a particular bend may require a mandrel to be placed between the wiper die and the metal object. The measuring device is operable to measure actual bend coordinates of metal objects bent by the dies. A controller is operatively connected to the dies, the base, and the measuring device and is configured to control the dies to bend the metal objects at least partly based on measured bend coordinates (i.e., feedback of actual springback) provided by the measuring device. 
         [0004]    The method includes applying force to bend a first portion of a first metal object (such as a tube) a first time to a first predetermined bend coordinate. The first predetermined bend coordinate is based at least in part on expected springback (i.e., springback based on characteristics of the metal, but that has not been verified as actual springback of the particular metal tube). The force is then released, and the tube is allowed to springback. An actual bend coordinate is then measured after the springback. This measurement may be via a video camera. The controller then determines whether the tube is over bent, in which case it is scrapped, or under bent, in which case a first bend correction factor is calculated based on the first predetermined bend coordinate and the first actual (i.e., measured) bend coordinate. (If the tube is neither over nor under bent, then a predetermined bend coordinate, based on expected springback, is used for a subsequent bend without a bend correction factor being necessary.) If the tube was under bent, force is then reapplied via the dies to bend the first portion of the first metal object a second time (i.e., the first portion is rebent) based at least in part on the calculated first bend correction factor. When the force is released, the tube springback should result in the tube being at the desired bend coordinates and having the desired tube geometry. If subsequent bends in the same tube are desired, force may be applied to bend a second portion of the tube based on the calculated first bend correction factor (i.e., using the measured actual springback to obtain a more precise bend when the force is released). If a second metal object such as a second metal tube is to be bent to achieve the same desired bend coordinates as the first metal object, the controller “resets” in that it reverts to bending the second metal object to the predetermined bend coordinate based on expected springback. This allows the actual springback of the second metal object to be individually determined by measuring the actual bend coordinate of the second metal object after releasing the second metal object. A second bend correction factor is then calculated based on the predetermined coordinate and the second actual bend coordinate. Force is then reapplied to bend the first portion of the second object a second time (i.e., the second tube is rebent) to a second revised bend coordinate based at least in part on the second calculated bend correction factor. When the reapplied force is released, the second tube should springback to the desired coordinate. 
         [0005]    The above features and advantages and other features and advantages of the present invention are readily apparent from the following detailed description of the best modes for carrying out the invention when taken in connection with the accompanying drawings. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0006]      FIG. 1  is a schematic illustration in plan view of a rotary draw bender with a clamp die clamping an unbent metal tube; 
           [0007]      FIG. 2  is a schematic illustration in side view of the rotary draw bender of  FIG. 1 ; 
           [0008]      FIG. 3  is a schematic illustration in plan view of the rotary draw bender of  FIGS. 1 and 2  with the clamp die closed and a pressure die applied to bend a first portion of the metal tube to a predetermined bend coordinate; 
           [0009]      FIG. 4  is a schematic illustration in side view of the rotary draw bender and bent tube of  FIG. 3 ; 
           [0010]      FIG. 5  is a schematic illustration in plan view of the rotary draw bender and metal tube of  FIGS. 1-4  with the clamp die released and the metal tube having sprung back from the predetermined bend coordinate; 
           [0011]      FIG. 6  is a schematic illustration in plan view of the rotary draw bender and metal tube of  FIGS. 1-5  with the clamp die closed and the pressure die applied to bend the metal tube beyond the predetermined bend coordinate to correct an under bend; 
           [0012]      FIG. 7  is a schematic illustration in plan view of the rotary draw bender and metal tube of  FIGS. 1-6  with the clamp die released and the metal tube sprung back to a desired bend coordinate; 
           [0013]      FIG. 8  is a schematic illustration in plan view of the rotary draw bender of  FIGS. 1-7  with the metal tube repositioned and the clamp die clamping the metal tube; 
           [0014]      FIG. 9  is a schematic illustration in plan view of the rotary draw bender of  FIGS. 1-8  with the clamp die closed and the pressure die applied to bend a second portion of the metal tube to another predetermined bend coordinate; 
           [0015]      FIG. 10  is a schematic illustration in plan view of the rotary draw bender and metal tube of  FIGS. 1-9  with the clamp die released and the metal tube sprung back from the other predetermined bend coordinate to a desired bend coordinate; 
           [0016]      FIG. 11  is a schematic illustration in side view of the bent metal tube of  FIGS. 1-10  with the bends at the first and second portions; 
           [0017]      FIGS. 12A and 12B  are a flow diagram illustrating a method of bending metal tubes; and 
           [0018]      FIG. 13  is a flow diagram illustrating another method of bending metal tubes. 
       
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0019]    Referring to the drawings, wherein like reference numbers refer to like components,  FIG. 1  shows an apparatus  10  for bending objects that includes a rotary draw bender  11  shown with a bendable object in the form of a metal tube  12 . As can be seen in  FIG. 2 , the rotary draw bender  11  includes a stationary base  14  that supports a rotatable bend die  16 . Bending is accomplished by clamping the tube  12  with a clamp die  18  against the bend die  16  and the pressure die  20  against a wiper die  21 . The bend die  16  and the clamp die  18  are rotated as a unit starting plastic deformation of a first bend  30  in tube  12  (see  FIG. 3 ). The pressure die  20  is delayed to prevent it from colliding with the clamp die  18  and to allow for material elongation on the inner side (compression side) of the bend as it flows against the wiper die  21  to prevent wrinkling. The apparatus  10  also includes a measuring device, optionally in the form of a video camera  22 , positioned on a stationary support post  24  above the metal tube  12 . 
         [0020]    The apparatus  10  further includes a controller  26  that is operatively connected by electrical wires (not shown), radio frequency, wireless connections, or otherwise, to the clamp die  18 , pressure die  20  and bend die  16 , as well as to the video camera  22 . The video camera  22  records an image of the tube  12  and relays the position of the tube  12  derived from the image to the controller  26 . 
         [0021]    An algorithm is stored within the controller  26  that is configured to provide feedback on springback of the metal tube  12  to verify and correct bends applied by the bender  11  to ensure that the intended bend coordinates are achieved. The algorithm is described below with respect to  FIGS. 12A-12B  and  13  as a series of steps carried out by the apparatus  10  under the control of controller  26 . The algorithm may carry out a method of bending metal objects  100  illustrated in  FIGS. 12A and 12B  as a series of steps carried out by the apparatus  10  under the control of controller  26 . Similarly, the algorithm may carry out a method of manufacturing bent metal tubes  200  illustrated in  FIG. 13  as a series of steps carried out by the apparatus  10  under the control of the controller  26 . 
         [0022]    Referring to  FIGS. 12A and 12B , the method  100  will be described with respect to the apparatus  10  shown in  FIGS. 1-10  and the product of the apparatus, a bent tube  12  forming an automotive frame component such as a roll bar, shown in  FIG. 11 . The method  100  is illustrated in both  FIGS. 12A and 12B , with the flow diagram if  FIG. 12A  continuing in  FIG. 12B  at bullet F. The method  100  includes step  102 , applying force to bend a first portion of a first metal object a first time to a first predetermined bend coordinate; wherein the first predetermined bend coordinate is based at least in part on expected springback. Step  102  includes step  104 , clamping a first die (i.e., the clamp die  18 ). Steps  102  and  104  are illustrated in  FIGS. 3 and 4 . The clamp die  18  is closed and the pressure die  20  moves forward, applying force to the tube  12  as the bend die  16  rotates a predetermined amount to bend a first portion  30  of the tube  12 . The dies  16 ,  18 ,  20  are controlled such that the tube  12  is bent to a first predetermined coordinate stored in the controller  26 , which here is represented as a point A, centered under the video camera  22 , with the tube  12  bent until a centerline Cl of the tube  12  is aligned with the point A. Because it is understood that all ductile metals will possess some degree of springback, the first predetermined coordinate A is determined specifically taking into account the minimum springback for the given material being bent. As will be seen in the explanation below this will allow some tubes to flow through the bending apparatus  10  without the need for further corrections and reduce any impacts on cycle time. During the bending operation of step  102 , the camera  22  is active and records the position of the tube  12  at the end of the desired (first) bend. The data is sent to the controller  26  to determine the position of the tube  12  and the degree of bend. The recording of data is indicated in  FIG. 4  by view line  17  of the camera  22 . 
         [0023]    Referring again to  FIGS. 1 through 4 , following steps  102  and  104 , step  106  is carried out, releasing the force applied to the first metal object to allow actual springback. Step  106  includes step  108 , opening the first die (i.e., the clamp die  18 ). Thus, under step  106 , the clamp die  18  is opened, freeing the tube  12  to undergo an actual amount of springback, as illustrated in  FIG. 5  as the centerline of the tube  12  shifts slightly away from the predetermined point A to a position in which the centerline is referred to as C 2 . (The position of the centerline Cl prior to release of the dies is shown in phantom on  FIG. 5  to illustrate the amount of springback.) The method  100  includes step  110 , measuring a first actual bend coordinate on the first metal object resulting from the applied force and the actual springback of the first metal object. Step  110  may include step  112 , visually recording the first metal object, such as by using the camera  22  again to record the position of the tube  12  after the actual springback, and sending this data back to the controller  26 . The data on the position of the tube  12  recorded by the camera  22  after step  102  and again after step  106  may be an angle (e.g., the angle of the centerline C 2  relative to a predetermined line, such as the centerline when at the predetermined position C 1 , with the angle represented as θ), a distance (e.g., the distance B of the centerline C 2  from point A along a radius extending from point A), or any other data relating the relative positions. For purposes of this description, it will be assumed that the first actual bend coordinate measured by the camera  22  is the position of the centerline C 2 . Based on step  110 , the controller  26  can determine in step  114  whether the actual bend coordinate is indicative of an under bend or, in step  115 , an over bend by comparing the actual springback amount to the predetermined springback amount. In the case of an over bend (i.e., where the actual springback was less than that anticipated), the tube  2  is scrapped under step  116 . The occurrence of an over bend will alert the operator to an unexpected material condition that should warrant further investigation. Possible causes could include inadvertently using tubes of a different material, using tube material that is out of specification, or a need to revise the predetermined (minimum) springback setting. If neither an over bend nor under bend exists (i.e., the first actual bend coordinate is the same as the first predetermined bend coordinate), then the first bend is complete and the method  100  moves to step  117 , with force applied to bend a second portion of the first object to a second bend coordinate based at least in part on expected springback. The method then moves to step  126 , described below. 
         [0024]    In the case of an under bend determined under step  114 , then, under step  118 , the controller  26  calculates a first bend correction factor based on the difference between the actual springback and the expected springback. The actual springback is the difference between the first predetermined bend coordinate (e.g., A) and the first measured actual bend coordinate C 2 . In this embodiment, the actual springback is the distance between the position of centerline C 2  after actual springback and the predetermined coordinate A, e.g., the distance B along a radial line extending through the predetermined coordinate A. Because the expected amount of springback is already stored in the controller  26  and represents some percentage of distance B, the first bend correction factor is the portion of distance B that is unexpected (i.e., that represents excessive springback above and beyond that expected of the particular material). Based on the data measured in step  110 , if the actual springback of tube  12  is consistent with the expected springback, no corrections are needed, as the bend of the tube  12  at the first portion  30  is consistent with the desired parameters. However if the bent tube  12  is under bent (due to higher spring back) then the controller  26  corrects the stored bend data used to control movement of the dies  16 ,  18 ,  20  with a springback correction factor. The bend at the first portion  30  is corrected under step  120  in which force is reapplied via the dies  16 ,  18 ,  20  to bend the first portion  30  of the first tube  12  a second time to a revised bend coordinate based at least in part on the calculated first bend correction factor. That is, referring to  FIG. 6 , the clamp die  18  is closed and the pressure die  20  and bender die  16  are controlled to bend the tube  12  the incremental amount that the tube  12  is under bent plus a newly determined springback amount, as illustrated by moving the tube  12  until the centerline is in a position referred to as C 3 , past point A. Next, under step  122 , the reapplied force is released, and the tube  12  undergoes springback to the desired position, as illustrated in  FIG. 7  wherein the centerline is in the desired position and is referred to as C 4 . 
         [0025]    With the actual springback of the tube  12  now having been quantified, and the controller  26  having calculated the first bend correction factor to modify the preprogrammed bend coordinates that were based on the expected springback, all subsequent bends on tube  12  may now be bent more precisely as the controller  26  revises all of the predetermined bend coordinates for those subsequent bends using the actual measured springback. Thus, in order to bend a second portion of the tube  12 , the tube  12  is repositioned in the bender  11 , as illustrated in  FIG. 8 , and then, in step  124 , force is applied with the bend die  16 , the clamp die  18 , and the pressure die  20  to bend the second portion  20  to a second bend coordinate which here is represented as a point D, centered under the video camera  22 , with the tube  12  bent until a centerline C 5  of the tube  12  is aligned with the point D. Then, in step  126 , the applied force is released, and the tube  12  will springback to the desired bend location, shown in  FIG. 10  for purposes of illustration as being when a centerline of the tube  12  is in a position referred to as C 6  in which it intersects point E. No corrections (i.e., no “rebends”) will be required to the second portion  32 , as the bend of the second portion  32  was controlled based on the actual measured springback of the tube  12 . As shown in  FIG. 11 , as a result of the method  100 , the tube  12  now has proper bends at bend locations  30  and  32 , as desired. 
         [0026]    If additional tubes are to be produced to the bend specifications shown in  FIG. 11 , the actual springback of each tube is separately determined in order to account for any variations. For example, if a second tube is placed in the bender  11 , under step  128 , force is applied to bend a first portion of the second tube a first time to a first predetermined bend coordinate based in part on the same expected springback that was initially used in forming the first bend  30  of the first tube  12 . This will be well understood by those skilled in the art by viewing  FIG. 3  and assuming that the tube  12  is a second tube. Next, as in step  106  with the first tube, in step  130 , force is released to allow the second tube to springback, as represented with respect to the first tube in  FIG. 5 . The amount of springback occurring with the second tube may very well be different than the amount that occurred with the first tube  12 . A second actual bend coordinate of the second tube is measured in step  132 , and then a second bend correction factor is calculated in step  134  based on the actual measured springback of the second tube (i.e., the difference between the predetermined bend coordinate and the second actual bend coordinate). Force is then reapplied in step  136  to bend the first portion of the second tube a second time to a second revised bend coordinate that takes into account the second calculated bend correction factor. Finally, in step  138 , the force is released, and the second tube should springback an amount such that the first bend has the desired geometry. As the actual springback of the second tube is now quantified, any subsequent bends to the second tube may use the known actual springback and be based on revised bend coordinates. The method  100  should result in fewer scrapped metal tubes (e.g., scrapped due to over bends), as the assumed springback of each tube is separately verified, and corrected, if necessary, using a calculated springback correction factor. 
         [0027]    Referring now to  FIG. 13 , a method of manufacturing bent metal tubes  200  is described with respect to  FIGS. 1-12 . The method includes step  202 , placing a first metal tube  12  in a rotary draw bender  11 . Next, in step  204 , a first portion  30  of the first metal tube  12  is bent to a first predetermined bend coordinate (e.g., where centerline Cl of the tube  12  is aligned with the predetermined bend coordinate, point A, which is based at least in part on the expected springback of tube  12 ). Then, in step  206 , the force applied in step  204  is released (by releasing clamp die  18 ), allowing springback of metal tube  12  as in  FIG. 5 . After the springback, in step  208 , an actual bend coordinate of the first bent portion  30  of the metal tube  12  is measured. This may include visually recording the first metal tube  12  with the camera  22  and sending this data back to the controller  26 . The data recorded may be an angle (e.g., the angle of the centerline C 2  relative to a predetermined line, such as the centerline when at the predetermined position C 1 , with the angle represented as θ), a distance (e.g., the distance B of the centerline C 2  from point A along a radius extending from point A), or any other data relating the relative positions. For purposes of this description, it will be assumed that the first actual bend coordinate measured by the camera  22  is the position of the centerline C 2 . Under step  210 , the controller  26  may then calculate a first bend correction factor based on the actual springback (i.e., the difference between the measured bend coordinate and the predetermined bend coordinate) and its relation to the predetermined springback. Using the first bend correction factor, under step  212 , the first portion  30  of the first tube  12  is rebent with a second applied force (i.e., force applied by the dies  16 ,  18 ,  20 ,  21 ), as shown in  FIG. 6 , to a revised bend coordinate (represented by the location of the centerline C 3 ) that is based on the first bend correction factor. The force is then released in step  214 . In step  216 , the accuracy of the bend can now be verified by measuring a new actual bend coordinate, such as the position of the centerline C 4  shown in  FIG. 7 , after step  214 . With the accuracy verified, a second portion  32  of the metal tube  12  is then bent to another bend coordinate C 5  (as in  FIG. 9 ) that is based at least in part on the bend correction factor. When the tube is released, the second portion  32  should springback to a desired position in which the centerline is at the desired position without requiring a rebend, as the actual springback is now incorporated in the bend coordinates achieved via the dies  16 ,  18 ,  20 ,  21  under the control of the controller  26 . 
         [0028]    It should be noted that a minimal amount of cycle time may be added to the bending process under method  100  or  200 , but the overall uptime, elimination of scrap and quality improvement will more then offset this minimal cycle time increase. Therefore, this invention will reduce if not eliminate scrapped objects due to metal spring back issues in horizontal rotary draw benders and improve overall quality and bender uptime. 
         [0029]    While the best modes for carrying out the invention have been described in detail, those familiar with the art to which this invention relates will recognize various alternative designs and embodiments for practicing the invention within the scope of the appended claims.