Abstract:
A percussion or hammer drill is provided including a drill plate arranged in the head end of a drill shaft, extending completely across the diameter of the drill shaft and comprising an open front face. Wedge-shaped cutting edges and flanks form two main cutters on the front face, whereby a plane running through the drill axis forms a mid-plane-of both main cutters. The main cutters form a tip angle in the range of approximately 140° to 180° and are separated by a central point.

Description:
BACKGROUND OF THE INVENTION 
   This invention concerns a percussion drill or hammer drill. 
   During drilling, a percussion or hammer drill, also known as a masonry, concrete or stone drill, performs a percussive movement along the drilling axis and a rotary movement about the drilling axis. Both components of its movement contribute to the removal of material in the drill hole. The axial movement shatters the material in the drill hole. The rotary movement, by abrasion, causes a reduction of the material into drilling dust and carries the drilling dust away out of the drill hole. 
   Known percussion or hammer drills consist of a drill shaft with a hardened metal plate, the cutting plate, set in it. Spiral grooves run along the shaft to evacuate the drilling dust from the drill hole. The cutting plate extends across the diameter of the drill shaft. At its exposed front surface, chip surfaces and free surfaces arranged in a wedge shape form cutting edges. These consist of two linear main edges offset parallel to a plane in which the drilling axis lies, and a transverse edge linking the two main edges through the drilling axis. In order to achieve satisfactory centering when starting to drill, there is an angle not exceeding 130° between the two main cutting edges. 
   BRIEF SUMMARY OF THE INVENTION 
   A task of this invention was to optimize removal of material in the drill hole by suitable design of the cutting plate. This task is fulfilled by a percussion or hammer drill in accordance with claim  1  or  15 . 
   Such a percussion or hammer drill, also designated by the generic term “masonry drill”, consists of the standard layout of a drill shaft with a cutting plate set in its head end. This cutting plate extends right across the diameter of the drill shaft and displays an exposed front surface. In this front surface—in a first embodiment—chip surfaces and free surfaces arranged in a wedge shape form linear main cutting edges diametrically opposite to each other. In this arrangement, a plane running through the drilling axis forms a central plane of both main cutting edges. With this arrangement of the main cutting edges, the percussive energy is transmitted into the material more advantageously than with two main cutting edges offset parallel to a plane in which the drilling axis lies. Thus the percussive energy is applied more effectively for demolishing material in the drill hole. 
   Further optimization of material removal in the drill hole is achieved by the fact that the two main cutting edges meet at an apex angle that is greater than 130° and lies preferably in the region of 150° to 170°, for example 155° to 165°. The increase in apex angle similarly yields more effective application of the percussive energy for removal of material in the drill hole. 
   In order to achieve satisfactory centering when starting to drill with a wide apex angle, it is advantageous if a centering point is provided between the two diametrically opposite main cutting edges. The apex angle of this central point is then smaller than the apex angle between the two main cutting edges. It may lie, for example, in the range of 80° to 130°. 
   Between the centering point and the chip surfaces or the free surfaces, as the case may be, it is advantageous to provide rounded transition zones, in order to prevent stress concentrations between the centering point and the main cutting edges. 
   The centering point may be formed in plane symmetry with two planes perpendicular to each other, both of which pass through the drilling axis and one of which also constitutes the central plane of the two main cutting edges. Such plane symmetry makes it possible to design a centering point that contributes to high material removal performance, high stability and outstanding resistance to wear of the cutting plate. As an alternative, the centering point can be designed with rotational symmetry. 
   According to another embodiment of the invention, removal of material from the drill hole is optimized through corresponding shaping of the cutting plate. If the apex angle between the main cutting edges increase radially from the inside towards the outside, the main cutting edges are better matched to the prevailing loads and removal of material is optimized. The apex angle is then increased steadily in an outward direction along a main cutting edge by 20° to 40°. In this arrangement, the smallest apex angle then advantageously lies in the range between 70° and 90°, and the greatest apex angle lies advantageously in the range between 90° and 130°. 
   In this connection it should be mentioned that in this embodiment, the two main cutting edges may, but need not, lie diametrically opposite to each other. 
   Furthermore, the wear resistance of the cutting plate can be improved if the angle between the angle bisector of the cutting wedge and the mid-plane of the two main cutting edges increases in an outward direction along the main cutting edges. In this respect, it has been shown to be advantageous if this angle is increased from about 5° to 25°. 
   The stability of the cutting plate is further enhanced by a cutting wedge that is rounded off at the tip, the radius of this rounding being greater in the outer zone than in the inner zone. 
   In addition, it has proved advantageous, with the tip angles of 150° to 170° used here, to increase the stability of the cutting edge by a protective chamfer of the outer edge. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     In what follows, an example of an embodiment of the invention will be described with reference to the attached figures. These show the following: 
       FIG. 1  A section of a percussion drill according to the invention in a drill hole; 
       FIG. 2  An elevation of a cutting plate; 
       FIG. 3  A top view of the front surface of the cutting plate shown in  FIG. 2 ; 
       FIG. 4  A side view of the cutting plate shown in  FIG. 2 ; 
       FIG. 5  A perspective view of the cutting plate shown in  FIG. 2 ; 
       FIG. 6  An enlarged section of a perspective view of the cutting plate shown in  FIG. 2 ; 
       FIG. 7  An elevation of a structural variant of the cutting plate shown in  FIG. 1 ; 
       FIG. 8  A top view of the front surface of the cutting plate shown in  FIG. 7 ; 
       FIG. 9  A side view of the front surface of the cutting plate shown in  FIG. 7 ; 
       FIG. 10A  perspective view of the cutting plate shown in  FIG. 7 . 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   The percussion or hammer drill  12  shown in  FIG. 1  in a drill hole  10  consists of a drill shaft  14  in the head end  16  of which is set a hard metal plate  18 , the drill plate or cutting plate as it is also known. This extends right across the diameter of the head end  16 . Spiral grooves  20 ,  20 ′ run along the drill shaft  14  to carry away the drilling dust out of the drill hole  10 . The drilling axis is designated by the reference number  21 . The percussion drill  12  performs a percussive movement along the direction of the drilling axis  21  and a rotary movement (see arrow  22 ) about the drilling axis  21 . Both of these components of its motion contribute to the destruction of material in the drill hole  10 . The axial movement shatters the material in the drill hole  10 . The rotary movement causes reduction of the material into drilling dust by abrasion and carries away the drilling dust out of the drill hole  10 . 
   A first embodiment of a cutting plate  18  for a percussion or hammer drill  12  according to the invention will be described with reference to  FIGS. 2 to 6 . Such a cutting plate  18  comprises a fixing shaft  24 , to all intents and purposes prismatic in shape, which is welded into a corresponding slit in the head end  16  of the drill shaft  14  (see also  FIG. 3 , in which the outlines of the head end  16  are indicated by a broken line). The fixing shaft  24  is provided with narrow sides  26  and  26 ′, each formed of a cylindrical face  28 ,  28 ′ and a flat face  30 ,  30 ′. The cylindrical face  28 ,  28 ′ in each case precedes the flat face  30 ,  30 ′ in its direction of rotation and ensures that the drill  12  follows the drill hole  10 . The flat face  30 ,  30 ′ is set slightly back with respect to the diameter of the drill hole  10 , thus reducing the friction of the narrow sides  26  and  26 ′ in the drill hole. 
   The end of the cutting plate  18 , which protrudes axially from the head end  16  of the drill shaft  14 , displays a profiled front face  32 , the profile of which is described in greater detail below. In this exposed front face  32 , the chip surfaces  34 ,  34 ′ in combination with the free surfaces  36 ,  36 ′ respectively are arranged together in a wedge shape so as to form the main cutting edges  38 ,  38 ′. As can be seen most clearly in  FIG. 3 , the cutting plate  18  is provided with two linear main cutting edges  38 ,  38 ′, arranged diametrically opposite to each other so that the plane  40  through the drilling axis  21  constitutes a central plane of the two main cutting edges  38 ,  38 ′. In the cutting plate  18  shown, this central plane  40  forms an angle  44  of approximately 8° with the central plane  42  of the fixing shaft  24 . In this arrangement, the central plane  42  intersects each of the two narrow sides  26 ,  26 ′ just behind the transition point between the flat faces  30 ,  30 ′ and the cylindrical faces  28 ,  28 ′. 
     FIG. 2  shows how the two main cutting edges  38 ,  38 ′ slope down from the inside towards the outside. In the central plane  40 , they form what may be termed an apex angle  46 , of 162°, for example, in the cutting plate  18  shown (in previous hammer drills this apex angle was no greater than 130°). The effect of the very blunt apex angle  46 , together with the common central plane  40  of the two main cutting edges  38 ,  38 ′, is that the percussive energy during drilling is strongly concentrated in the material being drilled, while friction is low. These two features thus contribute to a significant optimization of shattering of the material in the drill hole  10 . 
   As can best be seen in  FIGS. 2 and 5 , the two main cutting edges  38 ,  38 ′ are separated by a centering point  48  which is centered on the drilling axis  21 . This centering point  48  is in plane symmetry with respect to the two planes  40 ,  70 , which are perpendicular to each other. The first plane  40  is the central plane described in greater detail above. The second plane  70  also includes the drilling axis  21  and is perpendicular to the central plane  40 . As can be seen in  FIG. 3 , the centering point  48  is oval in cross-section, the longer axis of the oval lying in the central plane  40  and the shorter axis in the plane  70 . As can be seen in  FIG. 5 , the centering point  48  displays more or less the shape of a cutter such as is commonly used in mining. However, it is significantly more blunt in shape in the direction of the central plane  40 . It should also be noted that the centering point  48  contributes to higher boring performance, greater stability and outstanding resistance to wear of the cutting plate  18 . The rounded transition surfaces  52 ,  52 ′ are designed to prevent phenomena of stress concentration between the centering point  48  and the main cutting edges  38 ,  38 ′, which could generate stress peaks leading to fracture during drilling. Another point to note is that the transition surfaces  52 ,  52 ′ in the transition zones between the centering point and the chip surfaces can have a different radius of curvature from those between the centering point and the free surfaces. 
   The cutting wedge constituted by the chip surface  34  and the free surface  36  will now be described in greater detail with reference to  FIG. 6 . This cutting wedge will be defined, for each point on a main cutting edge  38 ,  38 ′, by a tangent  54  to the free surface  36  and a tangent  56  to the free surface  36 , in a sectional plane which is perpendicular to the central plane  40  and parallel to the axis of rotation  21 . As the projections of the chip surfaces  34 ,  34 ′ and the free surfaces  36 ,  36 ′ in the cutting plate  18  of  FIG. 6  in the sectional plane are largely flat, the tangents  54 ,  56  effectively constitute the lines of intersection between the chip surface  34 ,  34 ′, resp. free surface  36 ,  36 ′ and the sectional plane. 
   It should be noted that the point of the cutting wedge is rounded, or to put it another way, each of the main cutting edges  38 ,  38 ′ is rounded off. A large edge radius here favors the stability of the cutting plate. A smaller edge radius, on the other hand, favors drilling performance. In the cutting plate  18 , the edge radius of the main cutting edges  38 ,  38 ′, as can best be seen in  FIG. 3 , is fairly constant in the inner zone, but becomes significantly greater in proximity to the narrow sides  26 ,  26 ′. By this means, the main cutting edges  38 ,  38 ′ are strengthened in a particularly critical outer zone, but in the inner zone display a relatively small edge radius, which favors drilling performance. 
   Returning to  FIG. 6 , it may also be noted that the angle of the cutting wedge (referred to from now on as the wedge angle  57 ) along the main cutting edges  38 ,  38 ′ is not constant, but increases from the inside to the outside. In the cutting plate  18  in  FIG. 6 , for example, the wedge angle  57  shows a linear increase with the radius, from about 80° at the two transition surfaces  52 ,  52 ′ to about 110° at the two narrow sides  26 ,  26 ′. It can also be observed that the orientation of the cutting wedge along the main cutting edges  38 ,  38 ′ is not constant either. This orientation is measured as the angle  58  between the angle bisector  60  of the cutting wedge and the central plane  40 . In the cutting plate  18  in  FIG. 6 , this angle  58  increases with the radius, from about 5° at the two transition surfaces  52 ,  52 ′ to about 25° at the two narrow sides  26 ,  26 ′. Both the radially varying orientation of the cutting wedge and the radially varying wedge angle  57  of the cutting wedge give improved stability to the cutting plate  18 . This therefore becomes significantly stronger in its radially outer zone, that is to say where its tangential velocity is highest, and nevertheless displays outstanding drilling performance. It should also be noted that a larger wedge angle  57  in the outer zone results in a larger amount of material, reducing wear on the corners of the cutting plate, something that has to be kept in view during percussion or hammer drilling. Such wear on the corners leads, among other effects, to a reduction in the diameter of the drill hole  10 , so that slowing down wear on the corners extends the life of the drill  12 . 
   In addition, with the very blunt apex angle  46  used here, it has proved advantageous to provide the outer edge of cutting edge, chip face and free face with a protective chamfer  54 ,  54 ′ as a means of further increasing the stability of the cutting plate. The shape illustrated for the protective chamfer  54 ,  54 ′ is only one of a variety of possible embodiments. 
   The cutting plate  18  in  FIGS. 7 to 10  differs from the cutting plate  18  shown in  FIGS. 2 to 6  primarily in the design of the centering point  48 . This no longer displays plane symmetry with two planes perpendicular to each other, but displays rotational symmetry instead. In this arrangement, the centering point  48  is given an apex angle  50  which is significantly smaller than the apex angle  46  between the two main cutting edges  38 ,  38 ′, in order to enable satisfactory centering of the drill when starting a hole. In the cutting plate  18  illustrated, the apex angle  50  shown as an example for the centering point  48  is 90°, that is to say 72° less than the apex angle  46  of the two main cutting edges  38 ,  38 ′. The centering point  48  of the cutting plate  18  is made to all intents and purposes rotationally symmetrical, with transition surfaces  52 ,  52 ′ enabling a rounded transition towards the chip surfaces  34 ,  34 ′ and the free surfaces  36 ,  36 ′. 
   
     
       
             
           
             
             
             
           
         
             
                 
             
             
               Key to references 
             
             
                 
             
           
           
             
                 
             
           
        
         
             
                 
               10 
               Drill hole 
             
             
                 
               12 
               Percussion or hammer drill 
             
             
                 
               14 
               Drill shaft 
             
             
                 
               16 
               Head end 
             
             
                 
               18 
               Hard metal plate 
             
             
                 
               20, 20′ 
               Spiral grooves 
             
             
                 
               21 
               Drilling axis 
             
             
                 
               22 
               Direction of rotation 
             
             
                 
               24 
               Fixing shaft 
             
             
                 
               26, 26′ 
               Two narrow sides 
             
             
                 
               28, 28′ 
               Cylindrical face 
             
             
                 
               30, 30′ 
               Flat face 
             
             
                 
               32 
               Front face 
             
             
                 
               34, 34′ 
               Chip surface 
             
             
                 
               36, 36′ 
               Free surface 
             
             
                 
               38, 38′ 
               Main cutting edges 
             
             
                 
               40 
               Central plane 
             
             
                 
               42 
               Central plane 
             
             
                 
               44 
               Angle 
             
             
                 
               46 
               Apex angle 
             
             
                 
               48 
               Centring point 
             
             
                 
               50 
               Apex angle 
             
             
                 
               52, 52′ 
               Transition surfaces 
             
             
                 
               54, 54′ 
               Protective chamfer 
             
             
                 
               57 
               Wedge angle 
             
             
                 
               58 
               Angle 
             
             
                 
               60 
               Angle bisector