The present invention relates to a drilling tool for chip-breaking machining of metallic materials according to claim 1, which tool primarily is intended for so called ejector drilling. However, it may also advantageously be used for so called BTA-drilling.

It is known to use cutting inserts of cemented carbide for drills, which inserts are fastened by mechanical fastening arrangements, said inserts being provided with one or more recesses in the chip surface for chip-breaking purposes. Such drills are for instance known from US-A-4 215 157. However, it has turned out that one has not been able to attain the optimal, desirous forming of the chips. Thus, it has turned out to be impossible to attain the desired short comma-formed chips, at the same time as it has been impossible to reduce the effect consumption when running the drill in the desired way. Further, sometimes the chip canals have turned out to be too narrow for the created chips, which has resulted in chip stoppage and jamming.

Further, in EP-A-491 670 a drilling tool is described comprising a drill body on which two or more cutting inserts are mounted. The inserts are substantially formed as parallel- trapezoids and are axially mounted, i.e., the abutment surfaces of the cutting inserts extend axially, the inserts suitably- being fixed by brazing. However, after, a certain time of wear, also this drill body has sometimes caused chip jamming in the area where the two chip canals and the central hole meet. Moreover, the drill body consists of two parts which are joined by welding, namely the drill head as such or the drill crown, and the cylindrical, partly threaded part. This weld joint in combination with the fact that the drill crown has been cast, has with a certain frequency resulted in an imperfect roundness of the final product. This has in turn implied that some customers have required a finishing grinding in order to attain a perfect roundness and rotation symmetry around the central axis, which unnecessarily increases the production cost of the drill. A further drawback of this weld joint has turned out to

be that chips occasionally get stuck in the weld joint, since i practice a weld joint is never fully through but leaves a certain gap on the interior side. It is often enough that one single chip gets wedged to make the following chips pile up and cause chip jamming and in worst case a tool breakdown.

Thus, a primary object of the present invention is to provide a drill body, particularly a drill body for ejector drilling, that practically eliminates any risk for chip jamming

A further object of the present invention is to eliminate any unevenness on the inside, in which a chip could get jammed.

Another object of the present invention is to provide drill body with practically perfect roundness.

It has been succeeded to attain these and further objects by forming the drill body with the features as defined in the characterizing clause of claim 1.

For illustrative but non-limiting purposes, a preferre embodiment of the invention will now be further described with reference to the appended drawings. These are herewith presented:

Figure 1 shows a mounted drilling tool according to th invention in a perspective view obliquely from above.

Figure 2 shows a drill cutting insert according to the invention in a perspective view obliquely from above. Figure 3 shows the same drilling tool as in figure 1 i a side view, however unmounted.

Figure 4 shows the drilling tool straight from above.

Figure 5 shows the same view as figure 4, but with the different views and sections in the figures 5 to 8 defined. Figure 6 shows the cross-section VI-VI according to figure 5, of the upper part of the tool.

Figure 7 shows the view VII according to figure 5 of the upper part of the tool.

Figure 8 shows the view VIII according to figure 5 of the upper part of the tool.

Figure 9 shows the cross-section IX-IX according to figure 5 of the upper part of the tool.

In figure 1 a drilling tool of ejector type is

generally designated by reference numeral 1. Advantageously, it may also be generally used for so called BTA-drilling. The tool comprises a drilling crown or head 2, an intermediate part 3 and a shaft 4. The shaft 4 is provided with an outer thread 5, which is intended to, in a way known per se, be fixed by threading it upon a fastening outer tube (not shown) . An inner tube (not shown) that is concentrical with said outer tube is inserted in a way known per se into the inner, substantially cylindrical cavity of the drill, past the cooling medium holes 6, whereby formed chips follow the cutting medium through said inner tube.

According to previously known technique (for instance EP-A-491 670) , the drill head 2 per se is cast while the shaft 4 is turned, whereafter these two parts are joined by welding. A welding always causes deformations due to heat expansion and an uneven retraction during the following cooling. These inconveniences are further accentuated for thin parts of a workpiece. Further the head may become somewhat unround, in spite of the precision casting. These inconveniences are fully overcome by the present invention by producing the whole drill body in one single piece by turning, thereby avoiding all welding, which in turn brings the advantage that any risk for remaining weld joint gaps on the inside are avoided.

As may be seen in figures 3 and 4, the top side of the drill head is provided with three cutting insert seats or pockets 7, 8 and 9 each intended to accomodate a drill cutting insert 10. Advantageously, the three cutting inserts are equal, the only difference being that the central cutting insert is reversed in comparison to the peripheral and intermediate cutting inserts. The number of cutting inserts in an ejector drill may be chosen between one and five. However, the disadvantage with one single cutting insert is that the cutting forces that the support pads have to endure become large since the drill becomes unbalanced. It has been found that the number of three is a good compromise between complicity, li-fe and out- balancing. The ejector drill is usually produced as a one-way drill and the cemented carbide inserts according to figure 2 are therefore soldered or brazed in the cutting pockets. Since it is of one-way type, the drill should be worn as long as possible

without the product quality and the cassation risk becoming disturbing. The periphery insert 10A determines the diameter of the drilled hole, which is usually between 20 and 65 mm. Radially inwardly the cutting edge of this insert is inclined upwardly. The adjacent central cutting insert IOC in the cuttin pocket 8 overlaps the center axis of the drill, since no remaining core is desired. Contrary to the peripheral insert, axially downwardly its cutting edge is inclined radially inwardly, since otherwise the trailing cutting insert would be submitted to so large stress that it would very soon break. In agreement with the inclination of the central cutting edge, the head tip is provided with a conical recess 25. On the opposed side of the central axis, the intermediate cutting insert 10B i in the insert pocket 9. Like the peri-pheral insert 10A, axiall upwardly its cutting edge is inclined in direction radially inwardly. At rotation, the revolution path of the cutting edge of the intermediate insert overlaps somewhat with both the cutting edges of the peripheral and the central cutting inserts, in order to obtain a continuous cutting line from the central axis to the periphery. According to the present invention, the inserts may be either tangentially positioned, as illustrated i the appended figures, or axially positioned, as for instance disclosed in EP-A-491 670. However, they are preferably arrange in accordance with the appended figures. Two chip canals, ducts or flutes end in the top side o the drill: one common, larger chip canals 11 for the peripheral and the central inserts, and one somewhat smaller chip flute 12 for the intermediate cutting insert. In conformity with a preferred embodiment of the present invention, the opposed, lower ends of these chip canals end in a turned-out inner chip space 13, which has a frustoconical shape, with the bottom surface turned upwards in direction towards the top side of the drill. By this chip space 13, the central and intermediate cutting inserts will be on a bridge-shaped device 14, that extends transversely over the space 13 and connects to two, substantially diametrically opposed parts of the drill's top side. Since the whole drill 1 is preferably made in one single piece, this space 13 is turned by a turning tool being

introduced through the opening or substantially cylindrical cavity 15 in the rear end side of the drill. This space 13 results in several advantages, of which may be mentioned increased chip space with minimized risk for chip jamming, and lighter construction. It would be impossible to cast. However, thanks to the fact that the whole drill body is cast in one single turned piece, cavities may be formed which increase inwardly. The chip canals 11 and 12 have been milled out from above, from the drill's top side. In order to optimize the available chip space in the chip flutes, the milling tool has been angled relative to the central axis of the drill, adjacent to the periphery of the drill, so that outwardly angled, bevelled surfaces have been obtained, which adjoin either to the immediate proximity of the outer envelope surface of the drill via a small land portion 16, or which directly form a break lin 17 with said envelope surface.

In view of the above description, the combination effect should be evident of an integral drilling tool together with the turned-out chip space 13, namely that both cooperate to achieve maximal and fully unhindered chip flow. If for instance the chip space 13 were formed in a welded drill, the weld joint would be located on the conical envelope surface of said space, where the weld joint gap earlier or later would cause a wedging of a chip. On the other hand, if the boring 15 would continue uniformly without any chip space 13, the available chip flow space would diminish and thereby the risk for chip jamming woul increase.

The rotation-symmetrical outer surface of the drill is suitably made by turning while the other external surface portions are formed by milling. As best seen in figures 3 and 4, the insert pockets or seats 7, 8 and 9 are made in the simplest possible way, namely by one single short, straight end milling operation per insert seat, with one and the same end mill. Thereby, the rear abutment surface of the insert pocket of course gets a rounded, semi circle-formed shape corresponding t the cutting diameter of the end mill. The inner cavity 15 is bored, whereafter as mentioned above, the chip space 13 is turned out. It may be pointed out that also the part that is

occupied by the chip space 13 before then consists of a continuous portion of the boring 15.

As mentioned, figure 2 reproduces a cutting insert 10 according to the present invention. Inter alia, it comprises a relief surface 18 and a rounded edge side 19. The chip surface comprises an extended chip breaker 20 and below that a substantially plane chip surface portion 21. At the rear, on th rounded edge side of the cutting insert, it may be provided wit a distance knob 22 which sets aside any interferences when positioning the insert in the insert pocket due to unevennesses that may arise when the inserts are pressed. Moreover, the distance knob 22 minimizes the risk for any positioning discrepancies caused by the varying thickness of the solder layer, by the fact that the contact between the two opposed sem circle-shaped surfaces becomes minimal.

The rounded back of the cutting insert gives a considerably reduced risk for the formation of cracks, since it permits a favourable stress picture without any sharp corners which involve stess concentrations. Further, since the length o the insert is large in comparison to the insert width, a larger support is obtained for taking up cutting forces. Moreover, the insert is very favourable at the pressing per se and does not cause any compacting problems whatsoever.

In order to absorb radial cutting forces, the drill according to the invention is equipped with support pads 23 which are soldered or brazed in the support pad pockets 24. Als these support pad pockets are suitably milled out by one sole straight milling operation with an end mill, in the same way as the insert pockets 7, 8 and 9. The support pad may suitably hav a matching shape, i.e., an elongated body with a rounded end.

Furthermore, the outer side of the support pad is suitably give a rounded form, with the shape of a cylinder surface segment, i order to substantially conform with the substantially cylindrical envelope surface of the drill. Both at the mounting of the inserts and of the support pads, the rounded rear abutment surface functions for guidance in the initial stage of the mounting, i.e., it permits a certain dislocation laterally, which is a necessity for automated mounting.