The invention relates to a work-holding fixture for clamping a workpiece in position on a work-holding table, comprising a guide shoe, which can be fixed on the work-holding table and has recesses in symmetric arrangement, a clamping dog, which is designed as a rocking lever, has a clamping leg and a gripping leg and, with laterally projecting extensions arranged in the center region, engages into the recesses of the guide shoe and is supported there during clamping, and a clamping screw which passes through the clamping leg in the region of its free end and is supported on the guide shoe during clamping. Such work-holding fixtures are used in particular in the machining of workpieces by the workpiece being clamped in position on the work-holding table.
French Preliminary Published Specification 2,612,439 discloses a work-holding fixture of the type described at the beginning. In this work-holding fixture, a clamping dog is already allocated to a guide shoe. The guide shoe has two oblong-hole-like apertures and is clamped in position by means of two fixing screws in tapped holes which are provided in the work-holding table, the oblong-hole-like apertures enabling an infinitely variable displacement of the guide shoe in a single horizontal direction. The effective direction of the clamping dog in the horizontal direction is thus also established. The clamping dog is of plate-shaped design and, roughly in its center area, has two laterally projecting extensions, the plate-shaped clamping dog thus being designed as a rocking lever and having a clamping leg extending to the rear and a gripping leg extending forward from the extensions, with which gripping leg the workpiece is ultimately clamped in position. The laterally projecting extensions have a cylindrical cross-section and engage on their part into recesses of the guide shoe. These recesses, with the exception of an insertion shaft which can be closed by a headless setscrew or the like, are designed to be enclosed at the margins and enable the clamping dog to be displaced a certain limited distance in the single direction described above relative to the fixed guide shoe before the workpiece is clamped in position. The adjustability in this direction is only comparatively small. The recesses have an effective length which is arranged within the limits of the oblong-hole-like apertures and is smaller than the length of the oblong-hole-like apertures. A clamping screw passes through the free end of the gripping leg, which clamping screw is guided in a corresponding thread in the gripping leg and, when the workpiece is clamped in position, presses down on the rear end of the guide shoe. The clamping range of this known work-holding fixture is restricted vertically to a few bridgeable millimeters, since the clamping dog is of plate-shaped design and is pivotably mounted about the journal-like extensions essentially only to a very limited extent. Furthermore, it is disadvantageous that, due to the fixing of the guide shoe on the work-holding table, only a single direction in the horizontal plane is predetermined in which the work-holding fixture is infinitely adjustable. The displacement travel is here very small and it is necessary to provide a comparatively close grid of tapped holes in the work-holding table so that workpieces protruding to various widths can be clamped at the various locations in this single direction with one and the same work-holding fixture. In this arrangement, it is also disadvantageous that, as a result of the close grid, the guide shoe often has to be shifted on the work-holding table if different sizes of workpieces require this. Clamping in a horizontal direction other than that predetermined by the two fixing screws is not possible. The guide shoe cannot be rotated about a vertical axis. Furthermore, unfavorable force-transmission ratios also result if, for example, the fixing screws are inserted relatively far to the rear at the oblong-hole-like apertures and the extensions and thus the clamping dog come into use in a position displaced relatively far forward in the recesses. Furthermore, it is disadvantageous that the clamping screw, with the free end of its threaded shank, rests in a planar manner on the guide shoe only at a single, fixed workpiece height. At every deviating height of the workpiece, the clamping screw is supported only in a point-like manner on the guide shoe, in the course of which inadmissibly large surface pressures develop. With this known work-holding fixture, it is not possible to clamp workpieces of different height within a larger range, since the tilt angle of the plate-shaped clamping dog is limited to a few degrees.
U.S. Pat. No. 4,470,586 shows a work-holding fixture in which there is no guide shoe. Instead, the clamping dog, which here, too, has two laterally projecting extensions interacts directly with the slot of a slotted work-holding table. For this purpose, the clamping dog has a bent shape, the gripping leg and the clamping leg extending roughly in a straight line relative to one another and the extensions being arranged well below this straight line, since they have to engage into the slot of the work-holding table. A clamping screw is also provided here at the rear end of the clamping dog, that is, at the free end of the clamping leg, which clamping screw is supported on the root of the slot in the slotted table. The laterally projecting extensions provided on the clamping dog have a slightly oval shape, as a result of which the clamping height is limited. The clamping height ends here when the oval extensions abut against two opposite surfaces of the slot of the slotted table; higher workpieces can no longer be clamped with this work-holding fixture. It is thus possible to alternatively clamp workpieces in a height range which exceeds a few centimeters. In the horizontal plane, however, the work-holding fixture is tied to the direction which is predetermined by the direction of the slots in the slotted work-holding table. The work-holding fixture cannot be used in an angular position turned, for example, through 90.degree. thereto or also in any oblique angular position. The free end of the clamping screw here is certainly of spherical design; but here, too, the disadvantage of the point-like force transmission results. A precondition for the applicability of this work-holding fixture is a work-holding table provided with slots. In machining centers, the work-holding tables provided there are often only provided with tapped holes which are arranged in a certain grid. The known clamping dog cannot be used in combination therewith.