Abstract:
For retrofitting conventional machining apparatuses, in particular cross-cut machines, with a workpiece holder, the invention proposes a workpiece holder with at least one continuous clamping chain that is provided on one side with a plurality of clamping jaws.

Description:
BACKGROUND OF THE INVENTION 
     The invention relates to a workpiece holder for a machining apparatus, wherein a succession of workpieces is supplied to a least one machining unit and wherein during the machining operation each workpiece is held by the workpiece holder, as well as a machining apparatus having such a workpiece holder. 
     Different forms, sizes and designs of workpiece holders of the aforedescribed type are known in the art. In particular, workpiece holders are known which have active gripping arms and receive workpieces from a feed unit, hold the workpieces during the machining operation, and subsequently transfer the workpieces to a transport device. Although these workpiece holders have proven successful in practice, they are expensive difficult to maintain due to their moveable active gripping arms. In addition, the conventional workpiece holders come to a stop at the machining unit to allow machining of the workpiece. This limits the throughput of machining apparatuses equipped with corresponding workpiece holders. 
     In certain applications, the form and structure of the workpiece is not machined, so that the workpiece does not have to be held in a precisely defined position relative to the machining unit. This may be the case where, for example, the workpiece is only cleaned or filled. These applications can be found, for example in the beverage industry, wherein workpieces, such as bottles, are continuously moved past a processing unit, for example a filling machine or a cleaning machine. Here conventional transport units can be used, where in general the position of the workpiece relative to the processing unit does not have to be accurately established. However, if it becomes necessary to fix the position, then the processing unit itself can be provided with grippers which grip the workpiece and hold the workpiece in place for the duration of the operation. Such devices allow a very high throughput, but tend to have rather complex mechanisms. Moreover, such devices cannot be easily employed in or added to conventional machining apparatuses, in particular milling machines, saws or chamfering machines. 
     It is therefore an object of the invention to provide a workpiece holder that can also be easily employed with and added to simple conventional machining apparatuses, in particular cross-cut machines that include milling and sawing tools, and that provides a high throughput. 
     SUMMARY OF THE INVENTION 
     The object is solved by a workpiece holder for a machining apparatus of the aforedescribed type, in which at least one continuous clamping chain with a plurality of clamping jaws is provided. 
     By using a workpiece holder designed in this way, workpieces from a corresponding feed unit can be received continuously using the clamping jaws and guided past a machining unit, such as a saw or milling machine, without a need to stop the machining operation. It should be noted, that a continuous belt, similar to a conveyor belt, can also be used instead of the continuous chain. In the simplest embodiment, it may be sufficient to employ only a single clamping jaw. 
     The clamping jaw can be an active clamping jaw with moveable clamping elements. However, a passive clamping jaw is preferable due to its simpler design, ease of maintenance and inexpensive manufacture. In this case, the clamping action is effected solely by, for example, guiding of the clamping chain along a slide rail disposed opposite the open side of the clamping jaws. 
     The clamping chain can be guided and driven in various ways. Preferably, the workpiece holder includes drive and guide means in the form of two cylinders that drive and guide the clamping chain along a predefined pathway. If a slide rail which acts as a counter pressure element is provided along the region of the pathway, where the workpiece to be machined has to be securely clamped, then the workpiece holder needs only have a single clamping chain. 
     In a preferred embodiment, the workpiece holder includes two clamping chains with complementary clamping jaws. Under normal operating conditions of the workpiece holder, the two clamping chains move in opposite directions and parallel to one another along a section of the respective path, thereby providing a secure support of the workpiece to be machined along this section. A workpiece holder implemented in this way not only provides an especially high throughput, but—in cooperation with suitably formed feed means—also grips and guides the workpieces so as to prevent the workpieces from being damaged. The clamping chains can revolve continuously, so that the workpiece holder does not have to be constantly braked and then again accelerated, which improves its durability. 
     According to another preferred embodiment of the invention, pressing means can be provided that press the clamping jaws towards a workpiece along a machining path. The pressing means can have any form that is optimized for the respective application. In particular with cross-cut machines, it has proven advantageous to implement the pressing means in form a pressing rail. The pressing rail can be formed so that the pressure exerted on the clamping jaws is variable. This arrangement allows, for example, workpieces with different diameters and/or different friction coefficients to be securely held by the clamping jaws without damaging the workpiece. 
     If the pressure exerted by the pressing rail on the clamping jaws is to be variable, then the pressing rail is preferably designed so as to be formed of a plurality of pressing elements, wherein each pressing element can be pre-biased and/or moved towards the clamping jaws using its own mechanical, hydraulic or pneumatic pressing means, in particular springs. The pressing elements can be arranged in an overlapping fashion, so that the resulting pressure that is applied via partitioned rams using the individual pressing means is uniform. The pressing elements can be used to apply a preferably variable pressure to each revolving clamping jaw, thereby optionally forming a feed and discharge region with a low pressing force and a holding region with a high pressing force. The pressing force can also be varied from location to location during the machining operation depending on the specific machining task. 
     In another preferred embodiment, each clamping jaw has at least two clamping pockets, preferably with different profiles. These pockets can be formed, for example, as recesses with different apex angles, e.g., 90° and 120°, so that such clamping jaw can be used to grip and clamp round profiles as well as hexagonal and rectangular profiles. 
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     Additional details and advantages of the invention are described in the exemplary embodiment of the invention and in the accompanying drawing. It is shown in: 
     FIG. 1 a schematic diagram of a machining apparatus with a workpiece holder according to the invention, 
     FIG. 2 an enlarged detail of FIG. 1, 
     FIG. 3 a schematic top view of a clamping chain according to the invention that is guided past two machining units, and 
     FIG. 4 a schematic top view of a clamping chain according to the invention that is guided past three machining units. 
    
    
     FIGS. 1 and 2 depict a machining apparatus wherein a feed screw  10  that feeds the workpieces, is employed to feed the workpieces to be processed via a slide  12  to a workpiece holder  5  having two revolving clamping chains  14  and  16  which are only partially shown. The workpiece holder  5  includes drive and guide means in the form of cylinders  23 ,  25  that drive and guide the clamping chains  14 ,  16  along a predefined pathway. Each clamping chain  14  and  16  includes a number of clamping jaws  18 . The reference numerals of most clamping jaws have been omitted for sake of clarity. In this embodiment, the two clamping chains  14  and  16  have an identical length, and each clamping jaw of the upper clamping chain  16  is associated with a respective clamping jaw of the lower chain  14 . It should be noted that this feature is not necessarily required, since if a secure clamping action is required only during a relatively short processing path, then one of the two clamping chains can be formed to be active only along the short machining path, whereas the other of the two clamping chains can be longer and can operate following the processing path as a transport belt on which the processed workpieces are loosely supported. 
     In this embodiment, each clamping jaw  18  has clamping pockets  20  and  22  which—when viewed from the side—have a triangular profile with different apex angles. 
     To effect a particularly secure clamping action in the machining region, a pressing rail  24  is provided in this embodiment which is made of individual pressing elements that are biased by springs  26 . 
     The machining unit, past which the workpieces are continuously guided by the workpiece holder, can be, for example, a milling machine  28  disposed on a support plate  30 . 
     As indicated in FIGS. 3 and 4, workpieces  40  that are supplied to the workpiece holder, for example, via the slide shown in FIG. 2, be guided past different processing units in a feed direction  42  using a single workpiece holder. Advantageously, a centering aid  44  can be provided in the region where the workpieces  40  transition from the slide to the workpiece holder. The workpieces are then gripped by the clamping jaws of the workpiece holder and guided past a first processing unit, for example a milling machine  46 , whereafter the workpieces move to a positioning device  48  and subsequently past a second milling machine  50 . In this way, both ends of the workpieces  40  which in the illustrated example are elongated tubular or rod-shaped, can be machined. 
     In FIG. 4, the workpieces are guided past three processing units, namely the aforedescribed milling machines  46  and  50  and a saw  52 . 
     While the invention has been disclosed in connection with the preferred embodiments shown and described in detail, various modifications and improvements of the clamping jaws will become readily apparent to those skilled in the art. In particular, instead of the illustrated clamping jaws with two clamping pockets, clamping jaws with three or more clamping pockets can be employed.