Ceramic bit for high speed drilling

1. A ceramic drill bit comprising:

a cylindrical shank; and

a shaped portion extending axially in line with the shank and having a free end forming the tip of the bit, the shaped portion being in the form of a truncated cone whose larger base is situated beside the tip of the bit,

the tip comprising two main cutting edges and a central edge between the two main cutting edges, the shaped portion comprising two lips and two flutes extending in alternation around the axis of rotation of the bit, the lips and the flutes extending from the tip towards the shank, each lip including a land and each flute including a main cutting face adjacent to a land and to a main cutting edge, at least the shaped portion of the bit being made of ceramic material, the drill bit being wherein:

the cutting angle of the bit is positive;

a relief face extends from each main cutting edge at a relief angle lying in the range 4° to 10° approximately relative to a plane perpendicular to the axis of rotation of the bit; and

two notches forming two secondary cutting faces extend from the central edge with positive cutting angles lying in the range 1° to 7° approximately relative to the axis of rotation of the bit; and

wherein the taper angle of the shaped portion lies in the range 1° to 5° approximately.

2. A ceramic bit according to claim 1 , wherein the lips and the flutes wind helically about the axis of rotation of the bit with a helix angle lying in the range 20° to 30° approximately relative to the axis of the bit.

3. A ceramic bit according to claim 2 , wherein the helix angle lies in the range 20° to 25° approximately.

4. A ceramic bit according to claim 1 , wherein the taper angle of the shaped portion lies in the range 2° to 4° approximately.

5. A ceramic bit according to claim 1 , wherein the width of each land lies in the range 0.2 mm to 0.8 mm.

6. A ceramic bit according to claim 5 , wherein the width of each land lies in the range 0.4 mm to 0.8 mm.

7. A ceramic bit according to claim 1 , wherein each land is of width less than or equal to about one-twentieth of the outside diameter of the bit.

8. A ceramic bit according to claim 1 , wherein the relief angle lies in the range 6° to 8° relative to a plane perpendicular to the axis of rotation of the bit.

9. A ceramic bit according to claim 1 , wherein each relief face is extended by an undercut face.

10. A ceramic bit according to claim 1 , wherein each main cutting edge of the bit and the leading edge of each land forming an intersection with each land and a cutting face is rounded with a radius lying in the range 2 μm to 40 μm approximately.

11. A ceramic tip according to claim 1 , wherein the angles of the bit include respective chamfers of about 0.5 mm at about 20° relative to the axis of the bit.

12. A ceramic bit according to claim 1 , wherein the apex angle of the bit corresponding to the angle formed by the two main cutting edges lies in the range 140° to 155° approximately.

13. A ceramic bit according to claim 1 , wherein the shank and the shaped portion of the bit are made of ceramic.

14. A ceramic tip according to claim 1 , wherein the ceramic material is based on alumina, on zirconia, on silicon nitride, or on a mixture of ceramics.

15. A ceramic bit according to claim 1 , wherein the ceramic material is reinforced by silicon carbide fibers.

16. A method of drilling a material, said method comprising a step of drilling with a ceramic bit described in claim 1 , wherein said step of drilling is performed with a peripheral cutting speed of the ceramic bit which lies in a range of 400 m/min to 1000 m/min approximately, and wherein said step of drilling is performed with a speed of advance of the ceramic bit which lies in a range 0.04 to 0.1 millimeters per revolution.

17. A drilling method according to claim 16 , wherein the peripheral cutting speed of the bit lies in the range 400 m/min to 600 m/min approximately.

18. A drilling method according to claim 16 , wherein drilling is performed dry.

19. A drilling method according to claim 16 , wherein drilling is a roughing operation that does not require prior centering.

20. A drilling method according to claim 16 , wherein a single drilling operation is needed to make the final hole.

21. A drilling method according to claim 16 , wherein the drilling depth is greater than the diameter of the shaped portion of the bit.

22. A drilling method according to claim 16 , wherein said step of drilling is performed on a refractory material.