I. Field of the Invention
The present invention relates generally to hemming tools for performing a hem between two metal panels.
II. Description of Related Art
In the automotive industry, hemming machines are conventionally used to attach two metal panels together. These metal panels include, for example, the metal panels to form the automotive hood, door panels, quarter panels and the like.
In one type of previously known hemming machine, the outer panel is mounted on a nest and includes an almost perpendicularly upwardly extending flange formed about its outer periphery. The inner panel is then positioned on top of the outer panel and within the flange.
In order to hem the two metal panels together, typically the entire hemming operation is performed in two steps, namely a prehemming step and a final hemming step. These two operations can be either performed in two totally separate sequences by a specific hemmer, or on the fly during the single stroke of an actuator. The first method allows a perfect control of both prehemming speed and stroke orientation, but is by nature more time consuming. The second method, generally implemented on a classical vertical press, uses a cam-activated linkage system to move in and out on the fly the prehemmed steel just before the impact of the final hem. This cycle time efficient method, however, imposes a compromise in the way the prehemming operation is achieved, especially the orientation of the prehemming stroke. A single linkage mechanism is generally used to generate a curvilinear motion of the prehem steel, impacting on the side the flange to be prehemmed. This side impact causes in turn an excessive creep of the panel during prehemming.
Such excessive creep results in an unacceptable panel construction in situations, such as the automotive industry, which demand high quality standards of its formed body panels.
There are other types of hemming machines that are used in the automotive industry. These other types of hemming machines include both a tabletop hemmer as well as a roller hemmer. However, both the tabletop and roller hemmer disadvantageously require long cycle time thus resulting in low throughput for the hemming machine. Such long cycle times are sometimes unacceptable in automotive industry large volume production.