Drill with improved cutting insert formation
A drill includes a tip-forming front end having at least two straight cutting edges and at least two chip flutes extending from the tip end towards a rear end. Each cutting edge includes an inner portion disposed near a center line of the drill, and an outer portion disposed farther from the center line. The peripheral end of each cutting edge is in the form of a smoothly curved nose portion protruding radially with respect to the center line. The nose portion is formed by several circular segments having mutually different radii (R 1 , R 2 , R 3 ). Adjacent ones of the radii are of different size.
1. A drill comprising a generally solid shaft which defines a center line and includes:
a rear mounting end portion;
a front end portion disposed opposite the mounting end portion and provided with at least two cutting edges, each cutting edge including a main edge extending from an inner end to a peripheral end wherein the inner end is located closer to the center line, each cutting edge being inclined wherein the inner end is located axially forward of the peripheral end; and
a plurality of flutes formed in an outer surface of the shaft, each flute extending from a respective cutting edge towards the rear mounting end portion for forming chips,
wherein the main edge at the peripheral end has a curved shape,
wherein said curved shape includes a plurality of circular segments having different radii, said plurality of segments including a front segment having a first radius, an intermediate segment having a second radius, and a rear segment having a third radius, said front segment having a radius which is smaller than the radius of the intermediate segment and greater than the radius of the rear segment,
wherein the intermediate segment is a secondary cutting edge of the cutting edge, and has a wiper effect and wherein the second radius is larger than either the first radius or the third radius and
wherein the intermediate segment is positioned between the front segment and the rear segment.
2. The drill according to claim 1 wherein the cutting edges are arranged symmetrically with respect to the center line and meet to form an obtuse angle.
3. The drill according to claim 1 , wherein the cutting edges are disposed on a body attached to the front end portion of the shaft.
4. The drill according to claim 3 , wherein the body is formed of cemented carbide and is soldered to the shaft.
5. The drill according to claim 3 , wherein the body is releasably attached to the shaft.
6. The drill according to claim 1 , wherein the shaft is in the form of a composite including a core portion formed of a tough first cemented carbide grade containing 5-10 weight % Co, and an outer tube-shaped portion which comprises the curved nose portion, the tube-shaped portion being formed of a second cemented carbide grade which is more wear resistant than the first grade and contains 8-20 weight % of at least one of carbides and carbonitrides of Ti, Ta or Nb.
7. The drill according to claim 6 , wherein the cutting edges are arranged symmetrically with respect to the center line and meet to form an obtuse angle.
8. The drill according to claim 6 , wherein the radius of the front segment of the curved secondary edge is in the range of 0.1-2.0 mm, the radius of the intermediate segment is in the range of 1.6-10.0 mm, and the radius of the rear segment is in the range of 0.1-1.0 mm, the front segment radius always being smaller than the intermediate segment radius, and greater than the rear segments radius irrespective of the absolute sizes thereof.
9. The drill according to claim 8 , wherein the axial length of the intermediate segment is in the range of 0.1 to 0.5 mm.
10. The drill according to claim 6 , wherein the cutting edges are disposed on a body attached to the front end portion of the shaft.
11. The drill according to claim 10 , wherein the body is releasably attached to the shaft.
12. The drill according to claim 10 , wherein the body is formed of cemented carbide and is soldered to the shaft.
13. The drill according to claim 1 , wherein the radius of the front segment of the curved secondary edge is in the range of 0.1-2.0 mm, the radius of the intermediate segment is in the range of 1.6-10.0 mm, and the radius of the rear segment is in the range of 0.1-1.0 mm, the front segment radius always being smaller than the intermediate segment radius, and greater than the rear segments radius irrespective of the absolute sizes thereof.
14. The drill according to claim 13 , wherein the axial length of the intermediate segment is in the range of 0.1 to 0.5 mm.
15. The drill according to claim 10 , wherein an inner end portion of the cutting edge is straight.