Abstract:
A grinding tool has radially projecting grinding elements separated by spacers. The grinding elements are provided with radially extending slots. Some of the slots are open-ended. Axially spaced slots alternate in different radial positions.

Description:
BACKGROUND OF THE INVENTION 
     This invention relates to a grinding tool with a drive shaft and radial grinding elements. 
     There exist numerous grinding tools for machine-operated abrasive surface finishing. Examples of such traditional machines include rotary grinders employing a grinding medium in the form of a grinding sheet with an abrasive surface on a fabric, cellulose-based or plastic backing, radially extending from a motor-driven cylindrical base unit. The rotating tool is passed across the surface to be finished as the free ends of the grinding medium successively strike the surface and are moved across it. The tool may be part of an automatic system or it may be manually operated. 
     This conventional design is particularly useful when configured with a small tool diameter for finishing hard workpieces, for instance metal objects. When grinding workpieces of a soft material such as wood, there is always the danger of producing an uneven surface since the individual grinding elements attack the material at different force levels. This problem takes on increasingly significant proportions after the break-in period of the tool when due to usage the grinding elements have differently shaped ends. For finishing larger workpieces it becomes necessary to apply the grinder in machine-controlled fashion or to manually go over the entire surface repeatedly to cover the whole area, a process which is time-consuming and once again tends to pose a problem in achieving a homogeneous finish. 
     For larger workpieces the dimensions of the tool should be larger as well, especially with respect to its operating width. For conventional grinding tools, however, that would create major problems in terms of grinding performance since they would necessarily be quite stiff and inflexible over their width, making the grinding of non-planar, curved surfaces of a soft material virtually impossible. 
     The U.S. Pat. No. 3,869,833 describes a grinding tool with a drive shaft from which grinding elements, mounted on a carrier band, project radially outward. Each grinding element is attached to a retaining pin that is positioned at a distance from and parallel to the drive shaft. The retaining pins are spaced from one another and attached, along a circular path around the drive shaft, to a base unit. 
     BRIEF SUMMARY OF THE INVENTION 
     It is the objective of this invention to introduce a grinding tool which is capable of also finishing workpieces of a soft material such as wood and having a curved surface. 
     According to the invention, this capability is provided by a grinding tool with the features per claim  1 . 
     Other preferred design versions are characterized by the features described in the subordinated claims  2  to  11 . 
     By virtue of the design of the grinding tool according to this invention, a tool is provided which, even when it is large or has a large working width, is capable of following curved contours as a well and indeed lends itself particularly well to the surfacing of workpieces of a soft material such as wood. It is an advantage of this design that it permits the tool to be relatively wide, i.e. with a large effective operating width, thus permitting even large, curved workpieces to be surface-finished in just a few steps. It is especially the surfaces of three-dimensionally cambered objects that can be finished quickly and in optimal fashion even when the workpiece consists of a soft material such as wood. 
     As an added benefit, the tool according to this invention is highly durable and is relatively easy to maintain. 
     While the tool according to this invention is particularly suitable for the finishing of soft workpieces, it is also well suited to the grinding of workpieces with a hard and/or straight surface. 
    
    
     BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING 
     The following describes a design example of this invention in more detail with the aid of the attached drawings in which 
     FIG. 1 is a schematic view of the grinding tool per this invention; 
     FIG. 2 is a partial, detailed illustration of the grinding elements of a tool per FIG. 1; 
     FIG. 3 is a partial view of the basic, retaining-pin-equipped base unit of the grinding tool per FIG. 1; 
     FIG. 4 is a longitudinal section through a design variant of the grinding tool in the area of the grinding element; 
     FIG. 5 shows a design variation of the grinding element; and 
     FIG. 6 is a section view of a grinding element with retaining pin and spacers. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     FIG. 1 is a schematic illustration of a grinding tool according to this invention. The grinding tool incorporates a cylindrical base unit with drive shaft  2  attached to which are blade-shaped grinding elements  3  radially protruding at an angle. Located between the grinding elements  3  are spacers  4  which serve to keep the grinding elements  3  apart and in position. 
     The rotary grinding tool is set in motion by a motor via the drive shaft  2  and then tangentially brought up to the workpiece to be processed. Alternatively, the grinding tool may be set up in a stationary position and the workpiece to be processed is tangentially brought up to the rotating grinding tool for instance by a pick-up arm which may be numerically controlled. 
     FIG. 2 is a partial view of one of the blade-shaped grinding elements  3 . In this case, the grinding element  3  is essentially reactangular and is provided with slots  5 ,  6  which extend in an essentially radial direction relative to the drive shaft  2  of the grinding tool. These slots  5 ,  6  may be for instance rectangular. The slots  5  extending toward the free end of the grinding element  3  preferably reach all the way to the open, distal edge of the grinding element  3 , i.e. the slots are open-ended. The slots  6  which are located further inward in the direction of the drive shaft  2  are arranged in a way that their upper edge  6 ′ is farther away from the drive shaft  2  than the lower edges  5 ′ of the slots  5 , meaning that the neighboring slots  5 ,  6  overlap in a mutually offset fashion. 
     This configuration of the grinding elements  3  makes up a continuous array of contiguous grinding elements  3  whose slots  5  and  6  virtually separate their free ends into individually and independently deflectable vanes, thus forming a flexible grinding edge which can follow even strongly curved three-dimensional contours. This provides in advantageous fashion for a good, homogeneous grinding effect without applying the excessive pressure that would be necessary in the case of a one-piece grinding element. It follows that even workpieces of a relatively soft material such as wood can be processed without an uneven or excessive removal of material by the grinding elements  3 . 
     During the grinding process, the grinding elements  3  striking the workpiece to be processed are deflected, i.e. pushed away, against the direction of rotation of the drive unit. In practice it has been found that the positioning of the slots  5 ,  6  per this invention causes the individual edge sections of all grinding elements  3  to butt against one another, creating in desirable fashion a virtually uninterrupted grinding surface. For a homogeneous finish on strongly contoured workpieces it is desirable to mount the grinding elements  3  on the base unit  1  in staggered fashion so that for juxtapositioned grinding elements  3  their slots  5  and/or  6  are not directly aligned one behind the other but are laterally offset by at least a slot width or more. 
     The grinding elements are composed, in conventional fashion, of a flat carrier or substrate material supporting on one side or both sides an abrasive grinding layer consisting for instance of emery or corundum granules. The carrier substrate or backing may consist of a cellulose or synthetic material. 
     According to the invention, the grinding elements  3  are attached to the grinding tool, i.e. the base unit  1  of the grinding tool, by means of retaining pins  7 . FIG. 3 is a partial view of a section of the base unit  1 , showing the mounting disk  8  for the retaining pins  7 . The preferably cylindrical retaining pins  7 , individually rotatable around their longitudinal axes, are seated in holes drilled into the mounting disk  8 . For simplicity&#39;s sake, FIG. 3 shows only one mounting disk  8  with a few retaining pins  7  while the opposite mounting disk  8 ′ is indicated by a dotted line only. 
     FIG. 4 shows in a more detailed longitudinal section view the outer region of the base unit  1  with both mounting disks  8  and  8 ′ and a mounted retaining pin  7 . The retaining pins  7  are secured against axial displacement by means of rings  9  provided on the outside of the mounting disks  8  and  8 ′. The rings  9  are clamped together by way of clamping provisions  10  for instance in the form of through-bolts which thus also connect the mounting disks  8  and  8 ′. 
     Since for weight and stability reasons the retaining pins  7  must be kept small, it may in certain cases be desirable to interposition between the two mounting disks  8  and  8 ′ additional support disks  11 . Instead of drilled round holes as in the case of the mounting disks  8  and  8 ′, these support disks  11  are provided with semicircular or U-shaped slots opening toward the outside, through which the retaining pins  7  can be radially inserted from the outside. As a preferred additional provision, a retaining wire  12  is attached around the perimeter of the support disk  11 . Its purpose, especially in high-speed operation, is to prevent the retaining pins  7  from bending outward under the load to which they are exposed or, in extreme situations, from being pulled out of the corresponding holes in the mounting disks  8  or  8 ′. The retaining wire thus serves primarily as a safety device which can be secured against the drive shaft  2  of the base unit  1  of the grinding tool by means of suitable fasteners. 
     The corners of the grinding elements  3  may be radiused as in the design variant illustrated in FIG.  5 . Similarly, the slots  5  may have rounded corners, they may be lens-shaped or follow a slightly curved or undulating longitudinal line. 
     FIG. 6 is a section view of the grinding element  3  mounted on a retaining pin  7 . As an example, the lower section of the grinding element  3  may be provided with an eye  3 ′ which can be slipped over the retaining pin  7 . Of course, the grinding elements  3  may be attached to the retaining pins  7  in other ways as well, for instance by insertion in mounting slots in the pins  7 . 
     Two spacers  4  are mounted for instance on both sides of the grinding element as shown in FIG.  4 . The spacers  4  are preferably of the same width as the grinding elements  3  but shorter. The spacers  4  may be directly connected to the adjoining sides of the grinding elements  3 , for instance by gluing, or they may be mutually interconnected in which case the connector, for instance a clamp or a rivet, extends through a corresponding perforation in the grinding element  3 . Preferably, the spacers  4  are connected to the grinding element  3  or interconnected with one another only at the bottom of the grinding element  3  and rest freely, in a radial arrangement, against the outside surface of the grinding element  3 . 
     The grinding tool per this invention is particularly suitable for the one-step processing of large, three-dimensionally shaped workpieces. The design according to this invention offers the benefit of allowing for large dimensions of the grinding tool, with a diameter preferably greater than 500 mm (19.7″), which makes the processing of such workpieces in one single operation possible. In spite of their large dimensions, the grinding elements  3 , provided according to this invention with slots  5 ,  6 , constitute a flexible effective grinding surface over the entire width of the tool, thus also permitting the processing of soft materials, especially wood.