Rock drill
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The invention relates to a rock drill having radial recessed grooves for locating cutting bodies therein which are to be brazed in, and in particular to a rock drill for break-throughs having a drill head body which is arranged at the end of a drill shank and has at least two radially projecting lobes provided with cutting bodies and also has a central extension which has cutting bodies and is axially arranged in the drilling direction in front of the lobes. 
     2. Background of the Art 
     In known rock drills, carbide cutting bodies are brazed by the brazing method into cutting body locating grooves of a steel drill head. At the same time, the depth of the cutting body locating groove is of such a size that the cutting body sits on the groove route during the brazing process in order to achieve a precisely defined position. With this method, it is accepted that, during the brazing process and as a result of the considerably different coefficients of expansion of carbide and steel (factor about 1:2), stresses will develop particularly in the lower area of the recessed groove which can lead to weakening of the connection during extreme loading. 
     This problem is equally known with normal rock drills as well as with rock drills for producing break-throughs, such as can be inferred, for example, from German Pat. No. 2,414,354. The center extension on such tools is principally constructed in the same way as normal carbide drills; that is, the center extension has an appropriate carbide cutting body. In addition, it is also necessary with the known rock drills for making break-throughs to incorporate grooves or holes in the lobes pointing radially outwards, which grooves or holes are used to locate the carbide cutting bodies in the lobes. These individually tip-locating grooves in the lobes must be made by means of end milling cutters or similar, which makes the manufacturing process more expensive. 
     SUMMARY OF THE INVENTION 
     The object of the invention is to remove the abovementioned disadvantages, that is, to create a seat, which is as stress-free as possible, for carbide cutting bodies in rock drills, and in this connection to simplify and thus arranged more cost-effectively the manufacturing process in particular of rock drills for producing break-throughs. 
     This object is achieved by providing a first embodiment of a rock drill having radial, recessed grooves for locating cutting bodies to be brazed in, wherein the depth of the recessed groove for the cutting body is made larger than the axial brazing-in depth of the cutting body, and, in particular, by the further provision of a second embodiment of a rock drill for making break-throughs having a drill head body which is arranged at the end of a drill shank and has at least two radially projecting lobes provided with cutting bodies and also has a center extension which has cutting bodies and is axially arranged in the drilling direction in front of the lobes, wherein the depth of the recessed groove extends through the axial center extension into the area of the radial lobes. 
     The installation according to the invention of a carbide cutting body without lower support has a favorable effect on the stress condition in the drill head. The reason for this can be seen as follows. 
     With the steel-carbide material pairing, the thermal expansion ratio is about 2:1. At room temperature and before the brazing process, the lengths of carbide and steel are initially all the same. During the heating up to brazing temperature, the steel then expands substantially more than the carbide. When the connection cools down to the solidification temperature of the brazing filler metal, the longitudinal expansion of the steel is always considerably greater than that of the carbide. Further cooling down to room temperature then causes the assembled connection to warp--in a similar way to a bi-metal. However, this bending cannot bake place with a drilling tool because in practise the carbide tip is enclosed on both sides by steel as a result of the slot brazing. Accordingly, tensile stresses must exist in the steel body, which stresses are greatest in the slot route. Tensile stresses also exist in the carbide tip in the transverse direction. 
     According to the invention, the carbide cutting body can now at least partially follow the shrinkages in the steel, so that the stresses are considerably reduced in both the steel and the carbide cutting body, and in particular do not exist directly in the slot route. This area is in any case greatly endanged as a fracture location as a result of stress concentration. 
     If a slot which penetrates deeper is made according to the invention for the above stated reasons, the previously discussed second inventive embodiment follows as a further development of this idea. 
     In contrast to known, one-piece rock drills for making break-throughs, the invention accordingly has the further advantage that, in the case of a rock drill with two lobes, all grooves for locating cutting bodies can be made in just one operation. For this purpose, the groove is axially made so deep, according to the invention, through the center extension by a side milling cutter that it engages at the same time into the lobes of the drill head body. A continuous, radial groove is accordingly developed which cuts through both the center extension along its full axial length and the lobes down to the specified depth for the cutting bodies. 
     The continuous groove according to the invention for forming the cutting tip seat in the lobes also facilitates, in an advantageous manner, optimum brazing of the cutting bodies into the lobes. This is achieved in that, as a result of the available space on both sides of the respective cutting element, it is possible to correctly measure out and feed the brazing filler metal. 
     The principle according to the invention, in the case of a one piece rock drill, can be applied both the two lobes and to lobes exceeding this number, provided the lobes are arranged diametrically to one another. According to the invention, the possibility of simplified manufacturing of one piece rock drills and thus the more economical manufacture of such break-through tools are decisive factors. 
     Advantageous further developments and improvements of the invention are possible by means of the measures stated in the further sub claims. In the case of a rock drill, an expendient length ratio for accomplishing the overall depth of the groove is preferably such that the difference between the slot depth (t) and the axial brazing-in depth of the cutting body is at least 0.5 times the slot width. 
     The previously discussed second embodiment of the basic idea according to the invention a rock drill in particular for producing break-throughs has production advantages because of a simple design. 
     Further, several cutting bodies can be arranged radially next to one another in one groove in order to increase consequently the cutting capacity if necessary. For this purpose, it is not necessary for new grooves, or slots, or holes, to be made in the lobes by expensive production processes. Thus, according to further variations of the second embodiment of the invention, the cutting body seat for the cutting bodies of the symmetrically arranged lobes and for the cutting body of the center extension may be formed by a continuous, radial recessed groove which can be made in one operation and/or each lobe has at least two cutting bodies arranged radially next to one another. 
     It is expedient in the case of the special rock drill for producing break-throughs to arrange the groove through the lobes in a displaced manner above a certain angle to the symmetrical plane. According to another variation of the second embodiment of the invention, the cutting body locating groove may be arranged in a displaced manner relative to the center longitudinal axis through the lobes about an angle α≈18°. In this way, for one rotary movement of the tool in the clockwise direction, early engagement of the cutting bodies in the material to be drilled and increased support of the cutting bodies by the drill head body are ensured. 
     Several radial grooves can be arranged relative to one another at a certain angle in one finger. Thus, the two oppositely located lobes and the intermediately located center extension may have several continuous recessed grooves displaced at an angle β. In this way, the cutting capacity can also be increased for special applications. 
     Four symmetrically arranged lobes may be provided which have recessed grooves running through across the center extension for locating cutting bodies. Thus, as is known per se, four symmetrically arranged lobes may be made, however, by means of the measures according to the invention. 
     The cutting body of the center extension may be designed as a cross bit. Thus, an advantageous embodiment of the invention also extends to cross bits. 
    
    
     BRIEF DESCRIPTION OF THE DRAWING 
     Illustrative embodiments are described in greater detail in the following description and shown in the drawing, wherein: 
     FIG. 1 shows an elevation side view of a first embodiment of a rock drill according to the invention with extended cutting body recessed groove, 
     FIG. 2 shows the representation according to FIG. 1 turned through 90°, 
     FIG. 3 shows an elevational side view of a second embodiment of a rock drill according to the invention for producing break-throughs, 
     FIG. 4 shows a top plan view of the rock drill according to FIG. 3, 
     FIG. 5 shows an elevational side view of a variation of a second embodiment of the rock drill having four radial sections which are symmetrically arranged lobes, 
     FIG. 6 shows a top plan view of the rock drill according to FIG. 5, 
     FIG. 7 shows an elevational side view of a variation of the rock drill wherein the cutting body of the center extension is a cross, bit and 
     FIG. 8 shows a top plan view of the rock drill according to FIG. 7. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The rock drill 10 shown in FIGS. 1 and 2 itself can be a standard twist drill, as well as being the center point or center extension 16 of a rock drill 10&#39; according to the representation in FIGS. 3 and 4. If significance is the largely stress-free seating of carbide cutting body-cutting element 23 in cutting body locating groove 17. According to the representation in FIGS. 1 and 2, it can be seen that depth t of the cutting body locating groove 17 or recessed groove 17, which has a pair of side walls 26a, 26b and is to be made by means of a side milling cutter, is greater than penetration depth t 2  of the carbide cutting body 23. Clearance step t 4  between the cutting body 23 and groove root, i.e., groove bottom surface 18 at least 0.5 times a slot width or cutting body width b. In this way, lower edge 27 of the cutting body 23 does not sit against the root 18 of the recessed groove 17. The width b of the slot or the groove 17 is constant. 
     The precondition for this arrangement is that the brazed surface, in conjunction with the shearing resistance of the brazing filler metal, can accommodate the loading on the cutting tip. With a drill having a nominal diameter of 25 millimeters, the following calculation can be made: brazed surface about 430 mm 2 , shearing resistance of brazing filler metal about 150 to 300 N/mm 2 . From this results the following loading capacity: 
     Minimum: 430×150=64,500N (≈6.45 tonnes) 
     Maximum: 430×300=129,000N (≈12.9 tonnes). 
     Depending on the hammer drill, the loads occurring in practise are in the range of about 2 to 4 tonnes. 
     From this it can be seen that the method according to the invention leads to a reduction in the stresses at an adequate loading capacity of the drill head. 
     The further embodiment shown in FIGS. 3 and 4 is a logical extension of the idea according to the invention to a drill for producing break-throughs and has the same advantages. The same parts are therefore stated with corresponding reference numbers. 
     The rock drill 10&#39;, shown in side view in FIG. 3, consists of a drill head body 11 which is integrally formed onto a cylindrical shank 12 of a break-through tool. 
     According to the representation in FIGS. 3 and 4, the drill head body 11 consists of two radial sections which are designated as lobes 13 and 14 and are made in a way known per se. The lobes 13 and 14 are made symmetrically with respect to axis plane 15. 
     A center extension 16, which is used for making a center hole, is located in the drilling direction in front of the lobes 13 and 14. 
     According to the invention, a continuous cutting body locating groove 17&#39; is produced, for example, by means of a side milling cutter, which locating groove 17&#39; extends in an aligned manner from the outermost radial point of the lobe 13 over the center extension 16 to the outermost radial point of the lobe 14. A lower edge or groove root 18&#39; of the locating groove 17&#39; which can be seen in top plan view in FIG. 4, is indicated as a dotted line in FIG. 3. The locating groove 17&#39; slits the center extension 16 along its entire length, so that the side milling cutter for making the locating groove 17&#39; must penetrate into the drill head body 11 to a depth t 1 . 
     Cutting bodies 19 and 20 are positioned in the lobe 13, and cutting bodies 21 and 22 are positioned in the lobe 14, and cutting body 23 of the center extension 16, which cutting body 23 is displaced in the axial direction, are brazed by the known brazing method into this continuous cutting body locating groove 17&#39; which is made in one operation. In this connection, it is important for manufacturing reasons that the cutting bodies 19 to 22 are easily accessible from the side so that the brazing filler metal can be optimally measured out and the brazing process optimally arranged. Also, according to the invention, the cutting body 23 of the center extension 16, because of the continuous groove 17&#39;, is not limited in the downward direction, so that less stress concentration occurs during brazing than during firm gripping. 
     According to the representation in FIG. 4, it is particularly advantageous that the cutting body locating groove 17&#39; is made in a displaced manner about an angle α≈180° relative to symmetrical plane 24 through the lobes 13 and 14. In this way, during one rotary movement of the tool in the clockwise direction (arrow 25), early engagement of the cutting bodies 19 to 22 into the material to nbe drilled and increased support of the cutting bodies by the drill head body 11 are ensured. 
     By means of this measure, it is also possible to provide a further cutting body locating groove 17&#34;, shown is phantom in FIG. 4, displaced at an angle, in the lobes 13 and 14 in order to achieve an increased cutting capacity with only two lobes. Of course, more than two lobes can also be used, that is, for example, an arrangement according to the literature mentioned at the beginning and as shown in FIGS. 5 and 6, for which arrangement, the cutting body 23 of the center extension 16 is a cross bit as shown in FIGS. 7 and 8. 
     The rock drill shown in the illustrative embodiment according to FIGS. 3 and 4 has, for example, an outside diameter of D=68 mm and a shank diameter of d=19 mm. The radius R shown in FIG. 2 is about 32 mm. The penetration depth t 2  in the lobes 13 and 14 is about 4.5 mm and the groove width b is also about 4.5 mm.