The present invention relates to a drilling tool for chip-breaking machining of metallic materials in accordance with claim 1.

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 it has not been possible to attain the optimal, desirous formation of 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, 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. This makes the brazing of the inserts more difficult, since the accessability to the inserts in axial direction is considerably reduced in the chip flute or canal. Moreover, it is difficult to access with the milling tool downwardly in the chip flute in the cases when the insert seats or pockets are to be milled out. A further drawback with this construction is that the axial extension of the inserts are bulky and that the chip flutes or canals become long and this increases the risk for chip jamming. Furthermore, the long chip flutes have the consequence that one has to mill relatively deeply, with large over-hangs as a result.

In EP-A-240 759 a drilling tool is disclosed with tangential positioning of the drilling inserts on the upper or end-side of the drill head. Admittedly, this has made possible shorter chip flutes and, consequently, a decreased risk for chip jamming. On the other hand, the drill inserts and/or the tangential insert seats is/are unnecessarily complicated. Thus, the inserts shown in figure 3 of this citation are apt to break,

on one hand because of the included 90°-angles which function a kerfs, and on the other hand because of the complicated geometry, which causes compacting problems when the inserts are pressed. Moreover, the insert seats necessitate a relatively complicated and very precision-demanding milling operation in order to obtain a satisfactory abutment against the insert, particularly since the latter is fastened by screwing and not b soldering.

Thus, a primary object of the present invention is to provide a drilling tool that is easy to manufacture.

A second object of the present invention is to as far as possible simplify the production of the insert pockets in a drill head, particularly in an ejector drill head, and also the production of the cutting inserts, particularly by avoiding all compacting problems when pressing them.

Another object of the present invention is to bring down the number of different cutting inserts to a minimum, in particular to only one design.

These and further objects have been attained in a surprising way by providing a drilling tool comprising 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 drilling 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.

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, life 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 10C 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 such a 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 cavity 25. On the opposed side of the central axis, the intermediate cutting insert 10B is in the insert pocket 9. Like the peripheral inser 10A, axially 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 cuttin inserts, in order to obtain a continuous cutting line from the central axis to the periphery.

Two chip canals, ducts or flutes end in the top side o the drill: one common, larger chip canal 11 for the peripheral and the central inserts, and one somewhat smaller chip canal 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 opening 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 a lighter construction. 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 canals, the milling tool has been angled relative to the central axis o 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 line 17 with said envelope surface. The drilling tool according to the invention is preferably produced of one single piece. The external, rotation- symmetrical surfaces are 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 pocket, 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 to 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 the rounded edge side of the cutting insert, it may be provided with 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 semi 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 of the insert is large in comparison to the insert width, a larger support area is obtained for taking up cutting forces. Moreover, the insert shape is very favourable being pressed 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, in the shape of a cylinder surface segment, in 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 certai dislocation laterally, which is a necessity for automated mounting.