Prior Art During drilling, chips are produced at the tip-forming end of the drill, the chips being formed and transported from the tip-forming end of the drill to the opposite end of the drill, said transportation taking place in the chip channels.

At drilling with a three-edged drill, the part of the drill that first reaches the workpiece forces the tip of the drill to rotate around an axis that does not coin- cide with the rotational axis of the drill shank. Thus, an entrance for the desired hole is obtained that develops outside the desired entrance. An example of said type of three-edged drill is disclosed in WO 01/07189. Furthermore, it is com- mon that the balancing of known drills is deteriorated when the drill is worn.

Objects of the invention An object of the present invention is to provide a three-edged drill, at which balancing of the drill is not effected as the drill is worn.

Another object of the present invention is to provide a three-edged drill having a good balance before the entire drill tip is in engagement.

An additional object of the present invention is to provide a three-edged drill of good strength.

These and other objects of the present invention-are realized by means of a three-edged drill of the above-mentioned type, having the features given in the appended claims.

Brief Description of the Figures Below, an embodiment example of the invention will be described, refer- ence being made to the appended drawings, wherein: Fig. 1A shows a three- edged drill according to the present invention in a perspective view. Fig. 1 B shows the axially front end in side view of the drill in Fig. 1A. Fig. 1C shows the axially front end in top view of the drill in Fig. 1. Fig. 2A shows a part of a first centre cutting edge. Fig. 2B shows a part of a second edge. Fig. 2C shows a

part of a third edge. Fig. 3 shows an alternative embodiment of a three-edged drill according to the present invention in a perspective view.

Detailed Description of Preferred Embodiments of the Invention The drill 10 according to Figs. 1A-1C comprises a substantially solid shank 11 having a longitudinal centre or tool axis CL. The elongate drill 10 has a first tip-forming end 12 and a second opposite end 13. The first tip-forming end 12 comprises three cutting edges 19,20 and 21 and three straight chip channels 22,23 and 24, i. e. one chip channel for each cutting edge. The cutting edges 19,20 and 21 are arranged having a substantially even partition, i. e. with 120° between each edge. The drill is made from solid high speed steel or preferably in solid cemented carbide. The expression"solid"includes drills that have shanks with holes for cutting or flushing medium in bodies being solid in other respects. The cemented carbide consists, e. g., of wolfram carbide, WC and a binder phase such as cobalt, Co and is ground, extruded or extrusion pressed.

The straight chip channels 22,23 and 24 of the drill are parallel to the centre axis CL and may extend along the entire drill or more often along a part thereof.

The shank 11 is to be fastened in a rotatable spindle, not shown. The drill has three upper flank surfaces 15,16 and 17. All external surfaces and appurtenant edges are preferably made of the same material. The core of the drill may be made of a more tenacious cemented carbide while more peripheral parts may be made in wear resistant cemented carbide.

Even if the described embodiment in the present patent application relates to a drill having a substantially solid shank, the present inventive idea also comprises drills having tube shaped shanks.

Thus, the drill 10 is provided with three cutting edges at the first tip-form- ing end 12. At the second end 13 of the drill 10, the shank 11 is shaped as a thicker portion, which is intended to be inserted and fastened in a holder, not shown. The direction of rotation of the drill is marked by the arrow R in Fig. 1 C.

The drill 10 has a substantially constant diameter D along the entire length thereof, the length of which is designated L1 in Fig. 1A. The chip channels 22, 23 and 24 have a substantially constant cross-section shape along the entire length L1 of the drill 10.

A flush duct 25, situated between the chip channels, runs centrally through the drill, said flush duct 25 extending from the second end 13 of the drill and through the drill 10 in the direction towards the tip-forming first end 12 to immediately inside the flank surfaces 15,16 and 17. There, three eccentric channels 26,27 and 28 connect for feeding flushing medium to the respective cutting edge. The channels 26,27 and 28 terminate in a preferably planar surface each, situated in the feeding direction F axially behind the associated flank surfaces 15-17. All three channels 26,27 and 28 terminate at the same distance from the centre axis CL. The flush duct 25 runs parallel to the centre axis of the shaft 10 for at least 80 % of the length L1 of the drill, i. e. the respective length of the eccentric channels is maximally 20 % of the length L1 of the drill.

With reference being made to foremost Figs. 2A-2C, enlarged views of the three cutting edges of the drill according to present invention are shown. The three cutting edges 19,20 and 21 are straight and have an extension from the periphery of the drill radially inwards towards the centre axis CL of the drill.

Each one of the cutting edges forms the same tip angle £ with the centre axis CL. The tip angle £ is 45° to 89°, preferably 70° to 85°. Each one of the straight cutting edges connects radially inwards at a breakpoint 30 forming an inner angle a with a rearwardly and inwardly directed straight edge 31,32 and 33.

The angle a is 90° to 179°, preferably 120° to 160°. The breakpoints 30 are located in a plane P, which is perpendicular to the centre axis CL and at the same distance L3 from the centre axis. By disposing the breakpoints at the same distance from the centre axis, the cutting forces are balanced around the rotational axis. Thus, the cutting edges 19,20, 21 comprise axially front parts 30 that are arranged in a common plane P. The distance L3 is less than a quarter and greater than a tenth of the diameter D of the drill. Each breakpoint 30 coincides with an imaginary circle C, which intersects three flush ducts 26, 27,28, each one terminating in an axially front end of a chip channel 22,23, 24, see Fig. 1 C. The closer to the rotational axis that the breakpoints are arranged, the less force is consumed in order to commence the formation of the hole.

However, if the breakpoints would be arranged on the rotational axis, the drilling force becomes very large by virtue of the low cutting speed in the centre at the same time as the metal in the work piece then sticks (built-up edge formation)

so that the quality of the hole is deteriorated. In order to decrease the cutting forces in the centre, the straight edges 32, 33 have been shortened radially inwards by means of a concave recess each, 34 and 35 respectively. The edges 32,33 are preferably equally long. Each cutting edge is followed by a flank surface in the direction of the rotation R. The drill has no power- consuming cross cutting edge but solely a first straight cutting edge 31 with an extension radially inwards to and somewhat past the centre axis CL.

Furthermore, the recesses 34,35 comprise no chip forming parts but may serve as clearance for a grinding wheel at the provision of the first straight centre cutting edge 31. The recess 35 is deeper in relation to the plane P than the recess 34 in order to substantially not come into contact with the workpiece.

The breakpoints 30 are the parts of the drill that first get into contact with the workpiece that is to be drilled. Said contact takes place simultaneously so that a circle initially is cut in the workpiece, which gives a good balance for the drill during the continued initial drilling thereof. If said circle is projected over to the drill, the circle C will intersect the flush ducts 26,27 and 28. Even if the breakpoints are somewhat worn down, said circle will be formed, even if the circle then becomes somewhat wider as a circle ring under comparable conditions in other respects so that balancing of the drill is not affected during wear of the drill. The arrangement of the breakpoints 30 also means that it becomes easier to manufacture the drill so that the outermost tips 30 land in the same plane P.

The alternative embodiment example of a drill 10'according to the invention, illustrated in Fig. 3, is a so-called twist drill having three helix chip channels 22', 23'and 24'. The tip-forming end 12 of the drill 10'is identical to the one that has been shown above. Therefore, the same reference numbers are used for the same features. The helix chip channels 22', 23'and 24'of the drill may extend along the entire drill or more often along a part thereof. In the illustrated embodiment, the three chip channels 22', 23'and 24'extend helically from the tip-forming end to the shank 11'. As is shown in Fig. 3, the chip channel 23'has a certain helix angle y, a preferred value of said angle y being in the interval 5° to 40°. The other chip channels 22'and 24'have an equally large helix angle y. In this connection, it should be pointed out that within the scope of the invention, it may be conceived that the helix angle of the chip

channels is negative for the same direction of rotation. A negative helix angle is advantageous for vibration damping of the drill. Thus, the present invention relates to a drill for chip forming metalworking, the drill having three cutting edges, the axially foremost tips of which have been arranged in a plane in order to jointly forming a circle during drilling, only one of the three cutting edges having an extension into the centre axis of the drill.