Abstract:
An apparatus and method to cradle and hem panels at an assembly-line station. A cradle assembly is configured to conform to the shape of a first sheet material and prevents deformation of the class-a surfaces. A first robotic arm operatively associated with said cradle assembly and a slide-assist assembly cooperate to stabilize the cradle and secure a cradle anvil against the first sheet material. A roller head for hemming said first sheet material supported on a second robotic arm. The second robotic arm moves the roller head around the periphery of the first sheet material and against the cradle anvil to form a hem.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims the benefit of U.S. Provisional Application No. 61/086,001, filed Aug. 4, 2008. The entire disclosure of the above application is incorporated herein by reference. 
     FIELD 
     The present disclosure relates to vehicle body assembly and, more particularly, to a system for in-situ locating, fixturing and hemming a body panel at an assembly-line station. 
     BACKGROUND 
     This section provides background information related to the present disclosure which is not necessarily prior art. 
     This specification describes a system that forms and joins sheet material and, in particular, a hemming device and method of use at an assembly-line station, which is considered on-line, where it is sometimes preferred versus off-line where an added station and event is expensed. The assembly-line station environment presents partially assembled panels oriented in vehicle position, which is reversed relative to off-line hemming. A hemming tool assembly having a roller hemming unit operates in this environment with unlimited hem flange orientation without physical attachment to a counter-pressure device, and with the flexibility to manage multiple vehicle body model shapes at a single assembly-line station. 
     One of the earliest operations required in the history of automobile assembly was on-line joining of panels to form a variety of body assemblies to build up a vehicle body. On-line welding pinches the panels together between two welding-tips via a common jaw, then a weld current is induced until securely joined. Similarly, on-line roller hemming pinches the panels together between two rollers via a common jaw, then multiple passes are made until securely joined. Welding and hemming are both widely used. Welding is the most popular method though it is not often used on class-a surfaces due to marking, while hemming is intended for class-a joining. An example of off-line hemming may be found in U.S. Pat. No. 5,454,261 issued on Oct. 3, 1995 to Campian for HEMMING MACHINE AND METHOD OF OPERATION. Examples of known on-line hemming are set forth in U.S. Pat. No. 7,017,268 issued on Mar. 16, 2006 to Lang for SEAL REFORMING METHOD AND APPARATUS, and U.S. Pat. No. 7,500,373 issued on Mar. 10, 2009 to Quell for FLANGING DEVICE AND FLANGING METHOD WITH COMPONENT PROTECTION. 
     While the above-referenced patents provided advancements in the state of the art of machines for joining two panels together, opportunity for design and feature improvement remained available. One of the difficulties of known on-line panel joining devices has to do with backing up the pressures induced to the hem flange to prevent panel deformation. As is known in the art, pinch rollers provide pressure to a pair of rollers, but are difficult to program and have angular manipulation restraints. The restraint on angular manipulation arises from the use of a common jaw supporting both rollers which results in an inability to make complex panel shapes or bow-tie-flanges—e.g., final flanges that vary the completeness of the folded flange from closed to partly open to fully open which are common to automotive assemblies. 
     Existing off-line hemming systems possess a stable and rigid lower anvil that is generally mounted horizontally with the floor. The incoming assembly is unattached to other assemblies allowing free and easy handling and orientation. The assembly drops onto the anvil from above, gravity assisted, with the class-a facing down on the anvil and the flanges to be hemmed facing up and out. The hemming head has open real estate to manipulate while pushing and driving the flange downward into the anvil. On the other hand, on-line hemming systems orient the part with the class-a facing outboard and the hemming work is done vertically to the floor which makes it impossible to use conventional off-line hemming heads and rollers due to the orientation of the part. There is also little or no room for the robot knuckle to manipulate from the inside of the vehicle to the outside, as the hemming forces dictate. Even if the robot knuckle has enough room to work from the inside out, the class-a metal would distort from the pressures required to hem. 
     To date there is no way to use a single conventional roller hemming tool in an on-line hemming station. On-line hemming has been performed using non-conventional pinch rollers that use a counter-pressure roller. It is a limited technique that does not yield high-quality flanges. The main issues are that the pressures applied are angular to the surface during the prehem passes, which have the greatest effect on the final hem quality. During prehem, the pinch rollers form a v-shape and the robotic paths attempt to compensate the angular skid of both paths simultaneously. Reviewing after a pinch rolling operation directly on class-a reveals scratches and skid marks evident on the class-a surface. 
     To protect the class-a surface from this damage, a known protective cover is used between the counter-pressure wheel and the class-a surface. It protects the class-a by having the counter-pressure wheel travel along a machined race track on the face of the cover, and at the same time limits the rotational freedom of the hemming tool as it is locked into following the track exactly. Reprogramming different angles within the path requires substantial re-tooling of the machined race track. To change angles if a path needs reprogramming requires the cover&#39;s surface to be welded and re-cut to the new angle. Abrupt changes to the attack angles are known to be done with pinch rollers by physically adding additional rollers to the head assembly. 
     Unfortunate features of pinch-rollers include: (a) the flange-side roller requires an attachment to a jaw with a direct relationship to a back-side counter-pressure roller also attached to same jaw for countering the applied force of the flange-side roller to prevent panel deformation; (b) the pinch rollers are locked in relationship to one another and have no opportunity to be quickly swapped out by rollers of different shapes to more robustly hem different panel shapes; (c) the pinch rollers possess an inherent inability to adjust the yaw, pitch or roll of the rollers during the hemming process to more robustly follow different panel shapes and associated attack angles; (d) the program path of both rollers are locked together via the jaw, and have extreme programming limitations of the counter-pressure roller with relationship to the panel surface and/or backer track; and (e) the hem attack angle is locked and non-adjustable, changing this requires introducing multiple rollers, each locked to different angles. 
     Another prior approach to address on-line hemming is the use of sliders, which offer low quality and little flexibility to manage multiple vehicle body model shapes on the same assembly-line. 
     Prior approaches to address on-line hemming have failed to overcome the aforementioned problems. Accordingly, an on-line hemming system that captures all the flexibility that off-line systems exhibit remains wanting. 
     SUMMARY 
     This section provides a general summary of the disclosure, and is not a comprehensive disclosure of its full scope or all of its features. 
     The specification describes an apparatus and method that overcomes the problems of known techniques for forming and joining standard hem flanges of a first sheet material to a second sheet material at an assembly-line station. 
     The apparatus provides tooling that uses a single roller which is disconnected from the counter-pressure device. One roller with a solid backing yields a more robust path trace and allows for variable tool angles during the course of the path, permitting any type of flange combination to be made, and reprogramming without machining. 
     In order to provide a solid backing, the first sheet material is cradled by a movable anvil when roller hemming to a second sheet material. Cradling prevents the class-a surface from being distorted by the hemming roller forces. Such distortion can be picked up either immediately or after the paint process that will reveal bolder and smoother light reflections than unfinished metal. Protection is achieved via the cradle anvil which is CNC manufactured from different materials dependant on the product quantities and duty cycle. The cradle anvil for high-quantity panel runs are preferably made from metals such as steel and iron, while low panel runs are preferably made from kirksite or even plastics such as urethane. 
     The roller hem tooling described herein is flexible enough to accommodate panels of various sizes, shapes, contours and accessibility at the same station. To this end, the roller hem tooling may be used in conjunction with a robotic arm in operation with a variety of stations. A variety of panel orientations, flange lengths, flange shapes, part thickness, and material type are all variables. As a result, the roller hem tooling has unlimited attack angle capability to produce completed seams with variable shapes as desired, and with easy program changes to chase part changes as they may be received. 
     Because the panel flanges in an on-line system are generally hidden behind the class-a surface, the system preferably utilizes a positional pressure variance unit (PPVU) similar to that disclosed in U.S. Pat. No. 7,254,973 operatively associated with a programmable positioning apparatus in the form of a robotic arm. A biasing element in the form of a compression spring is operably disposed within the cylinder and atop the piston. As further described below, the capture of the biasing element in the present invention is rearranged from the PPVU disclosed in U.S. Pat. No. 7,254,973 to urge the piston to a retracted position rather than an extended position. 
     The described system also includes at least one cradle assembly unit operatively associated with a programmable positioning apparatus in the form of a robotic arm. Multiple cradle assemblies may be fitted on one robot arm for fast model change and easy storage. Disconnected and independent of the PPVU hemming tool, the cradle assembly is fit tight and rigid against the body panel to be hemmed, but not so tight as to damage or mark class-a surface. To date robots have not been configured to achieve a level of rigidity required to keep adequate positional resistance on the cradle assembly to counteract the roller hemming tool. Gear backlash, bearing clearances, encoder accuracy, motor drive control and arm-cantilever-flex add to make a typical commodity robot incapable of sustaining the required positional resistance to support the present cradle stability requirement. 
     The described system employs slide-assists that are releasably coupled to the cradle assembly to enhance a secure match of the cradle to the part surface. Slide-assists are urged against cradle frame corner pins via a linear cylinder. The force applied through the pins to the frame in turn urges the anvil and part surface to conform to one another but not so much as to deform the class-a surface. 
     After secure matching between the anvil and the first part surface is achieved, a brake is applied to the cylinders of the slide-assists. The cylinder will now remain stationary holding the slide-assist and anvil with sustained resistance. A rigid cradle anvil is achieved with fixed points from the slide-assists at the bottom of the cradle frame and fixed points at the top of the cradle frame from the robot arm. 
     The described system may employ a docking fixture at the assembly-line station to temporarily dock the body-in-white assembly during the hemming operation. The accuracy of the body dimensions are held close, though the welding inaccuracies and the docking inaccuracies between the frame and the trolley, and the trolley and the track, result in significant dimensional variance that must be accommodated in order to provide a quality hem. 
     The docking fixture may include a commercially available positional recognition system to assist the robots to best align the cradle and PPVU hemming programs to play out. The positional recognition system software and hardware can pass offset values to the robots simultaneously to keep them in sync with each other and the panel. 
     Further areas of applicability will become apparent from the description provided herein. The description and specific examples in this summary are intended for purposes of illustration only and are not intended to limit the scope of the present disclosure. 
    
    
     
       DRAWINGS 
       The drawings described herein are for illustrative purposes only of selected embodiments and not all possible implementations, and are not intended to limit the scope of the present disclosure. 
         FIG. 1  is a perspective view of the cell at an assembly-line station; 
         FIG. 2  is an isometric view of a single cradle; 
         FIG. 3  is a perspective view of the cradle assembly, which is reversed from  FIG. 1 ; 
         FIG. 4  is a sectional view IV-IV from  FIG. 1  showing the final hem forming process; 
         FIG. 5  is a sectional view similar to that shown in  FIG. 4  showing the pre-hem forming process; 
         FIG. 6  is a section of a pull-type PPVU in a relaxed or retracted position; 
         FIG. 7  is a section of the PPVU shown in  FIG. 6  in a pulled or hemming position; and 
         FIG. 8  is a Sequence Flow-Chart of the hemming operation. 
     
    
    
     Corresponding reference numerals indicate corresponding parts throughout the several views of the drawings. 
     DETAILED DESCRIPTION 
     Example embodiments will now be described more fully with reference to the accompanying drawings. 
     With reference first to  FIG. 1 , an embodiment of an on-line hemming station  10  is illustrated in a perspective view. The assembly-line hemming station  10  includes a body-in-white  20  docked at a hemming station  10 . The body  20  is received from the assembly-line track  80  with a partially flanged first sheet material  30  requiring a hemming operation that is located primarily about the rear wheel opening. 
     A roller head  400  for hemming the first sheet material  30  is outlined in the sectional views of  FIGS. 4 ,  6  and  7 . The roller head  400  is of a pull-type, different than those used off-line in that it has no compliance when pushed. A shaft extends from one end of the cylinder and supports a roller that creates biasing compliance when pulled on. By pulling instead of the traditional pushing of the biasing element, the present invention can achieve the feature of unlimited hemming attack angles that an off-line system enjoys; a feature that pinch rollers can not achieve. 
     When the wheel  410  is not in contact with the first sheet material  30 , the spring  420  expands between a spring-plate  430  and a t-head bolt  440 , urging the cylinder  450  and roller  410  toward the robotic arm  50 . When the wheel  410  is hooked in contact with the first sheet material  30  and the robot  50  pulls on it, the spring  420  compresses between a spring-plate  430  and a t-head bolt  440 , thus producing the positional pressure of a selected amount to produce a quality hem. 
     A robotic arm  50  operatively associated with the roller head  400  for hemming a first sheet material  30  is bolted to the floor as exhibited in  FIG. 1 . A positional recognition camera  300  mounted to the center hub  150  reviews the first sheet material  30  from its pounce position. The camera  300  is configured to send offset data to both robots  50  and  140  to manipulate the robots reference frame to align the hemming station  10  with the first sheet material  30 . A presently preferred positional recognition camera  300  is the single camera, 3-D recognition system providing robotic guidance without the use of calibration targets or structured lighting, available from Comau, Inc. of Southfield, Mich. as the RecogniSense system. 
     The roller head  400  must wait until the cradle  100  is engaged prior to any flanging operation at the on-line assembly. As best viewed in  FIG. 2 , the cradle assembly  100  comprises a frame  110  onto which mounts pins  120  for pressure application across the lower portion of the frame  100 , and a cradle anvil  130  made from either plastics or metal, dependent on the life expectancy of the tooling. The cradle anvil  130  conforms rigidly to the shape of a first sheet material  30  to prevent deformation of the class-a surfaces during roller hemming. The cradle assembly  100  is affixed to the robotic arm  140  via a center hub  150 , as best viewed in  FIG. 3 . The center hub  150  may be configured to accept up to four cradle assemblies  100 . Changing panel models only requires robot  140  to rotate about its wrist axis in 90° increments. 
     Slide-assist assemblies  200  are mounted below first sheet metal  30  (see  FIGS. 1 and 4 ) and operatively associate with cradle frame pins  120  that slip loosely into guide pockets  220  to form a structure to assist stabilizing the cradle assembly against the roller hemming pressures. While  FIGS. 2-4  illustrate the use of two slide-assists, one skilled in the art will recognize that the number of slide-assists may vary as required by the size and configuration of the sheet material being formed. The slide-assist assembly includes (see  FIG. 4 ) a slide rail  210  and a pocket-guide  220  mounted to the slide rail  210  that accepts the frame pin  120 . Once the pin  120  is introduced via robot  140 , the slide rail  210  is dragged via the pin  120  from its retracted position until the cradle assembly  100  encroaches the first sheet material  30 . The cylinder  230  operatively associated with the robot program and mounted to the slide rail  210  is then charged to drive the pocket-guide  220  and push the pin  120  that in turn moves the entire cradle assembly  100 , including the anvil  130 , against the first sheet material  30 . The pressure in the slide cylinder  230  must be regulated so as not to disturb the class-a surface. After the cradle anvil  130  and the first sheet material  30  are matched snugly, and operatively associated with the robot program, a cylinder brake  240  mounted to the cylinder  230  is applied to lock the cradle frame  110  rigid during the hemming operation. 
     The prehem and final hem passes are then executed by initially looping the roller  410  around the material  30  with a fish-hook movement. The hemming process may be completed with a single pass or multiple passes, depending on the configuration and materials to be joined. The contact relationship is viewed in  FIGS. 5 and 4 , respectfully, while the robotic arm  140  and slide-assists  200  are kept immobile. Once the hemming is complete, robot  50  moves the PPVU  400  to a pounce position. The brakes  240  then release and the cylinders  230  release to atmosphere as the pins  120  pull the slide-assists  200  away from the first sheet metal  30 . The robot  140  swings the cradle assembly  100  out of the guide pockets  200  and back to a pounce position. At this point, the body-in-white is moved away from the assembly-line hemming station and a new body-in-white is moved into the assembly-line hemming station. The process then recycles. 
     While the present description will focus and describe forming a pre-hem Hp and a final hem Hf of a rear wheel arch  22  of a vehicle body  20 , it is not limited to simple flange formations such as a wheel arch  22 . Therefore, one skilled in the art will recognize that the system so described has application to a variety of body panel configurations. Furthermore, the system may have application to the assembly of non-vehicle-related products; e.g., appliances or metal cabinets. 
     The foregoing description of the embodiments has been provided for purposes of illustration and description. It is not intended to be exhaustive or to limit the invention. Individual elements or features of a particular embodiment are generally not limited to that particular embodiment, but, where applicable, are interchangeable and can be used in a selected embodiment, even if not specifically shown or described. The same may also be varied in many ways. Such variations are not to be regarded as a departure from the invention, and all such modifications are intended to be included within the scope of the invention. 
     Example embodiments are provided so that this disclosure will be thorough, and will fully convey the scope to those who are skilled in the art. Numerous specific details are set forth such as examples of specific components, devices, and methods, to provide a thorough understanding of embodiments of the present disclosure. The terminology used herein is for the purpose of describing particular example embodiments only and is not intended to be limiting. As used herein, the singular forms “a”, “an” and “the” may be intended to include the plural forms as well, unless the context clearly indicates otherwise. It will be apparent to those skilled in the art that specific details need not be employed, that example embodiments may be embodied in many different forms and that neither should be construed to limit the scope of the disclosure. In some example embodiments, well-known processes, well-known device structures, and well-known technologies are not described in detail. 
     When an element or layer is referred to as being “on”, “engaged to”, “connected to” or “coupled to” or “operatively associated with” another element or layer, it may be directly on, engaged, connected or coupled to the other element or layer, or intervening elements or layers may be present. Other words used to describe the relationship between elements should be interpreted in a like fashion (e.g., “between” versus “directly between,” “adjacent” versus “directly adjacent,” etc.). As used herein, the term “and/or” includes any and all combinations of one or more of the associated listed items. 
     Although the terms first, second, third, etc. may be used herein to describe various materials, elements, components, regions, layers and/or sections, these elements, components, regions, layers and/or sections should not be limited by these terms. These terms may be only used to distinguish one material, element, component, region, layer or section from another region, layer or section. Terms such as “first,” “second,” and other numerical terms when used herein do not imply a sequence or order unless clearly indicated by the context. Thus, a first element, component, region, layer or section discussed below could be termed a second element, component, region, layer or section without departing from the teachings of the example embodiments. 
     Spatially relative terms, such as “inner,” “outer,” “beneath”, “below”, “lower”, “above”, “upper” and the like, may be used herein for ease of description to describe one element or feature&#39;s relationship to another element(s) or feature(s) as illustrated in the figures. Spatially relative terms may be intended to encompass different orientations of the device in use or operation in addition to the orientation depicted in the figures. For example, if the device in the figures is turned over, elements described as “below” or “beneath” other elements or features would then be oriented “above” the other elements or features. Thus, the example term “below” can encompass both an orientation of above and below. The device may be otherwise oriented (rotated 90° or at other orientations) and the spatially relative descriptors used herein interpreted accordingly.