I. Field of the Invention
The present invention relates to an automotive framing system, or any other geometry station, for accurately positioning vehicle body components relative to each other prior to securing the vehicle body components together. 
II.  Description of the Prior Art
In the manufacture of automotive vehicles, a conveyor system typically transports a body preassembly sequentially along a conveyor line.  Such body preassemblies supported by a vehicle carrier comprise various body components, such as an underbody, front structure, body sides, headers, etc.,  which are loosely attached to each other in their approximate final assembly position relative to each other. 
In order to fixedly secure the body components together, it is imperative that the body components be precisely positioned relative to each other to freeze their geometry by “tack welds” performed in this framing station, prior to a “respot” of the whole body in order to provide its final strength.  To accomplish such precision positioning of the body components, there have been previously known automotive framing systems. 
In one prior art automotive framing system, a gantry is positioned above the assembly station at a midpoint of the conveyor line.  The gantry includes swing arms which are movable between a raised and a lowered position.  In their raised position, the swing arms are positioned away from the body preassembly which enables the next preassembly to be moved by the conveyor system into the assembly station.  Conversely, in their engaged position, the arms swing downwardly approaching “damp units” supporting reference blocks and clamps to engage predetermined reference surfaces or location points of the various vehicle body components, and clamp the body components together at a predetermined position relative to each other.  With the body components clamped together, robotic welders or the like extend through openings in the reference frame and are used to fixedly secure the body components together by “tack welds”. 
In still a further type of previously known automotive framing system, a reference frame is positioned around the body preassembly when the preassembly is positioned at the assembly station.  In this type of previously known automotive framing system, pivoting or sliding units connected to the reference frame and supporting reference blocks and clamps extend into the interior of the automotive vehicle body components to engage the reference surfaces of the body components, and lock the body components together at a predetermined position relative to each other prior to welding. 
In still a further type of previously known automotive framing system, a side gate is positioned along each side of the assembly station.  These side gates are movable between a retracted position, in which the gates are positioned laterally outside the assembly station to permit the body preassembly to be moved into the assembly station, and an assembly position in which the gates are positioned along each outer side of the body preassembly.  Pivoting or siding units mounted onto the gates and supporting clamping assemblies then extend into the vehicle body components to secure the body components in the desired predetermined position relative to each other.  Thereafter, robotic welders extend through openings in the gate, into the vehicle and “tack weld” the vehicle body components together. 
All of these previously known automotive framing systems, however, suffer from a number of common disadvantages.  First, the wide area covered by the same tool structure, i.e. the gate or swing arm, does not enable a common approaching trajectory for the tool structure in which all the reference blocks and clamp units will remain stationary on the tool structure, and the clamps of simple design.  Further, to remain quasi-standard, the gates, frames, or swing arms supporting the pivoting or sliding units holding the reference blocks and clamping units will stay positioned remotely around the exterior of the body preassembly.  Since the clamping surfaces on the body components are frequently contained within the interior of the body preassembly, these previously known framing systems necessarily required complex, articulated clamping assemblies which must extend into the interior of the body preassembly in order to clamp the body components at their desired position relative to each other.  Such clamping assemblies are oftentimes necessarily articulated relative to their gate or reference frame.  As such, these clamping assemblies are both expensive to manufacture and subject to wear after prolonged use.  Such wear adversely affects the accuracy of the overall framing system. 
A still further disadvantage of these previously known framing systems is that, after the body preassembly has been moved into the assembly station and clamped at the desired position relative to each other, it is necessary for robotic welders to then extend through openings in either the gate or the reference frame in order to weld the body components together.  Due to interference between the robotic welders and either the gate or reference frame, the use of complex and time-consuming robot trajectories, and thus expensive robotic engineering study, has been required. 
A still further disadvantage of these previously known framing systems is that it is necessary to use a different reference frame or a different gate even for slightly different vehicle body styles.  Since multiple body styles are oftentimes assembled together at a single assembly station, it has been previously necessary to move either different reference frames or different gates to the assembly station in order to accommodate the different vehicle body styles.  Since these previously known reference frames and gates are massive in construction and require a long design and fabrication time, they are expensive and may delay the time to put a new vehicle on the market.  Furthermore, these systems require a large footprint on the shop floor to store the unused set of tools. 
Recently, a new generation of framing system has been developed to take advantage of the low cost, mass-produced robots.  All these framers try to reproduce the exact same tool change movement previously achieved with a dedicated piece of machinery, but by using a dedicated high load capacity robot.  The tooling used corresponds to the previous gates or frames, but is more simply built with lighter structure, material and components.  There is, of course, an initial saving achieved on the tool handing system, but because the tooling remains large and difficult to handle, the full agility of the robot cannot be exploited.  Furthermore, the tooling still requires a lot of pivoting or sliding units to bring some movable reference blocks into contact with their working surface, thus increasing the complexity of the tooling, its weight, compliance, cost, reliability and cycle time.