1. Field of the Invention
The invention is directed to a device for producing compacted or pressed articles with a main cylindrically-shaped element and a secondary helically-shaped element from powdered material such as powdered metal, and in particular for producing helical gearwheels in which the helical gearing or toothing is the secondary element.
2. Description of the Related Art
A conventional device as described in European Patent Publication 0 528 761 A1 produces pressed articles, e.g., helical-toothed gearwheels, from metal powder. This known metal powder press has a linearly movable upper ram in which is supported a top punch which is rotatable about the longitudinal axis in the pressing direction and a bottom ram which is also moveable linearly against a bottom stop and in which a bottom punch is rotatably supported. A die plate forms a mold cavity and is movable linearly in the pressing cycle. The rotatable bottom punch and the rotatable top punch each have a toothing profile corresponding to the toothing profile or helical toothing of the mold shell or casing, i.e., the mold cavity. The bottom punch which is supported so as to be freely rotatable is constantly engaged with the profile of the mold cavity and therefore rotates compulsorily in a corresponding manner when linear relative movements occur between the bottom punch and die plate during the press cycle. In contrast, a rotational movement corresponding to the helical toothing is externally forced upon the top punch during the press cycle corresponding to its penetration depth in order to reduce the friction between the outer surfaces of the top punch and mold cavity of the female mold. The toothed-wheel mechanism provided for this purpose is driven via a mechanical linkage control corresponding to the desired helical toothing of the pressed article. The linkage control contains linkage cores, rigidly connected to a guide plate and guided in a positive engagement and in a sliding manner in the coaxially arranged driving wheels of the toothed-wheel mechanism. During the press cycle, the guide plate is temporarily rigidly coupled with the die plate and moves jointly therewith. A withdrawal process is used to remove the produced pressed articles from the mold.
This known metal powder press gives rise to considerable costs with respect to mechanical construction and also retooling since for every different pressed article a set of linkage cores corresponding to this pressed article must be prepared and exchanged, aside from the special tool set including the female mold, top punch and bottom punch. Added to this is the cost of the guide plate and the mechanically operated locking device for rigidly coupling the guide plate to the die plate. There also remains the problem of friction between the bottom punch with respect to its rotational movement and the female mold, the bottom punch not being positively driven externally. This not only results in increased tool wear in this region, but also leads to an uneven density distribution in the pressed article.
A press with electronically controlled movements which is used for the rotary press process is described in the publication entitled "Quality control through process monitoring of rotary forming press", Metal Powder industries Federation, Volume 6, May 6-11, 1994, 125-137. A press of this type is used for subsequent treatment of already sintered molded articles produced by powder metallurgy in order to give them a density in the range of 95% to 98% of the theoretically possible density of the material in question. The special construction of these presses makes it possible to generate extremely high local pressing pressures in the pressing tool with a comparatively low overall pressing force of the press. The special construction for this purpose includes an upper punch die that moves in a gyrating and rotating manner and applies locally defined extremely high pressing forces on the workpiece in order to compact the latter in directed manner. This reference contains no suggestion that the top punch and bottom punch, which participate directly in the shaping of the helically shaped secondary element of the article to be pressed, may be controlled with respect to their movement in the mold cavity of the female mold by electronic means for the purpose of producing compact with main cylindrically shaped elements and secondary helically shaped elements from powdered material.