Cutting head having tip portion with radially extending front cutting edges provided with both negative and positive rake angles, and rotary cutting tool
A cutting head rotatable about a first axis, comprising an intermediate portion and a tip portion. The intermediate portion has a plurality of leading edges defining a cutting diameter, and the tip portion has an axially forwardmost tip point and a plurality of front surfaces with outer and inner cutting edges. An outer rake surface adjacent to each outer cutting edge has a positive outer rake angle, and an inner rake surface adjacent to each inner cutting edge has a negative inner rake angle. Each outer rake surface is disposed on a head flute intersecting one of the leading edges, and each inner rake surface is disposed on a gash intersecting one of the head flutes. Each gash extends to a gash path end point located a first distance axially rearward of the tip point, and the first distance is greater than thirty percent of the cutting diameter.
1. A cutting head ( 20 ) rotatable about a first axis (A 1 ) in a direction of rotation (DR), the cutting head having three-fold rotational symmetry about the first axis (A 1 ) and comprising:
an intermediate portion ( 22 ) having an integer number N=3 circumferentially spaced apart peripheral surfaces ( 26 ), each peripheral surface ( 26 ) having a leading edge ( 28 ), and the plurality of leading edges ( 28 ) defining a cutting diameter (DC); and
a tip portion ( 24 ) having an axially forwardmost tip point (NT) contained in the first axis (A 1 ) and N front surfaces ( 30 ), each front surface ( 30 ) having a radially extending front cutting edge ( 31 ) which comprises an outer cutting edge ( 32 ) extending radially inwardly from one of the leading edges ( 28 ) and an inner cutting edge ( 34 ) extending radially inwardly from said outer cutting edge ( 32 ), each front surface ( 30 ) including a clearance surface ( 36 ) adjacent its associated outer and inner cutting edges ( 32 , 34 ), each inner cutting edge ( 34 ) adjoining its associated outer cutting edge ( 32 ) at a cutting edge transition point (NR),
wherein:
in a first cross-section taken parallel to the first axis (A 1 ) and intersecting any one of the outer cutting edges ( 32 ), an outer rake surface ( 40 ) adjacent to said outer cutting edge ( 32 ) is inclined at a positive outer rake angle (α 1 ); and
in any second cross-section taken parallel to the first axis (A 1 ) and intersecting any one of the inner cutting edges ( 34 ), an inner rake surface ( 42 ) adjacent to said inner cutting edge ( 34 ) is inclined at a negative inner rake angle (α 2 ),
wherein:
each outer rake surface ( 40 ) is disposed on a head flute ( 44 ) extending axially rearwardly from the tip portion ( 24 ) and intersecting one of the leading edges ( 28 );
the cutting head ( 20 ) has an axially rearward facing bottom surface ( 56 ), and each head flute ( 44 ) intersects the bottom surface ( 56 );
each inner rake surface ( 42 ) is disposed on a gash ( 46 ) extending axially rearwardly from the tip portion ( 24 ) and intersecting one of the head flutes ( 44 ),
each gash ( 46 ) has a gash path (GP) defined by a plurality of gash midpoints from a series of cross-sections taken in planes perpendicular to the first axis (A 1 ) and intersecting the gash ( 46 ) along its axial extent, each gash midpoint being located at a midpoint of a segment of the associated cross-section having a minimum radius;
each gash path (GP) extends to a gash path end point (NP) located a first distance (d 1 ) axially rearward of the forwardmost tip point (NT);
the first distance (d 1 ) is greater than thirty percent of the cutting diameter (DC); and
each gash path end point (NP) is located radially further from the first axis (A 1 ) than any one of the cutting edge transition points (NR);
wherein:
in a cross-section taken in a first horizontal plane (PH 1 ) perpendicular to the first axis (A 1 ) and intersecting the plurality of inner cutting edges ( 34 ), each gash ( 46 ) has a concave shaped first profile (P 1 ) which is continuously curved along the entire length of said profile (P 1 );
the first profile (P 1 ) has a minimum first radius (R 1 ) measured along a first segment (S 1 ) thereof, the first segment (S 1 ) containing a first gash midpoint (NA 1 ); and
the minimum first radius (R 1 ) is greater than six percent of the cutting diameter (DC), and
wherein:
the bottom surface ( 56 ) is perpendicular to the first axis (A 1 ) and located a second distance (d 2 ) axially rearward of the tip point (NT),
the first distance (d 1 ) is greater than seventy percent of the second distance (d 2 ), and
the first distance (d 1 ) is less than ninety percent of the second distance (d 2 ).
2. The cutting head ( 20 ) according to claim 1 , wherein:
each first profile (P 1 ) has a radially innermost first point (NI 1 ) contained in its first segment (S 1 ).
3. The cutting head ( 20 ) according to claim 1 , wherein:
in a cross-section taken in a second horizontal plane (PH 2 ) perpendicular to the first axis (A 1 ) and intersecting the plurality of leading edges ( 28 ), each gash ( 46 ) has a concave shaped second profile (P 2 ),
the second profile (P 2 ) has a minimum second radius (R 2 ) measured along a second segment (S 2 ) thereof, the second segment (S 2 ) containing a second gash midpoint (NA 2 ); and
the minimum second radius (R 2 ) is greater than six percent of the cutting diameter (DC).
4. The cutting head ( 20 ) according to claim 3 , wherein:
each second profile (P 2 ) has a radially innermost second point (NI 2 ) contained in its second segment (S 2 ).
5. The cutting head ( 20 ) according to claim 3 , wherein:
each second profile (P 2 ) is continuously curved.
6. The cutting head ( 20 ) according to claim 1 , wherein:
in any cross-section taken in a vertical plane parallel to the first axis (A 1 ) and intersecting any one of the outer cutting edges ( 32 ), the outer rake surface ( 40 ) adjacent to said outer cutting edge ( 32 ) is inclined at a positive outer rake angle (α 1 ).
7. The cutting head ( 20 ) according to claim 1 , wherein:
in said any second cross-section, the negative inner rake angle (α 2 ) has a magnitude of greater than 5 degrees.
8. The cutting head ( 20 ) according to claim 1 , wherein:
each gash path (GP) extends in a direction opposite the direction of rotation (DR), as it extends axially rearwardly from the tip portion ( 24 ).
9. The cutting head ( 20 ) according to claim 1 , wherein:
in a third cross-section containing the first axis (A 1 ) and intersecting the clearance surface ( 36 ), the clearance surface ( 36 ) has a concave shaped clearance profile (PC).
10. The cutting head ( 20 ) according to claim 9 , wherein:
each concave shaped clearance profile (PC) is continuously curved and extends step-free to the first axis (A 1 ).
11. The cutting head ( 20 ) according to claim 1 , wherein:
the tip portion ( 24 ) includes N chisel edges ( 38 ), and
each chisel edge ( 38 ) is formed by two adjacent ones of said N clearance surfaces ( 36 ) and extends radially away from the tip point (NT) to one of the inner cutting edges ( 34 ).
12. A rotary cutting tool ( 48 ) comprising:
the cutting head ( 20 ) according to claim 1 ; and
a shank ( 50 ) having a longitudinal axis (L), and N shank flutes ( 52 ) circumferentially alternating with N lands ( 54 ).
13. The rotary cutting tool ( 48 ) according to claim 12 , wherein:
the shank ( 50 ) has a support surface ( 58 ) transverse to the longitudinal axis (L), and
the cutting head ( 20 ) is removably mounted to the shank ( 50 ) with the bottom surface ( 56 ) in contact with the support surface ( 58 ).
14. The cutting head ( 20 ) according to claim 1 , wherein:
in the first cross-section, the clearance surface ( 36 ) is inclined at a positive outer clearance angle (β 1 ),
in the second cross-section, the clearance surface ( 36 ) is inclined at a positive inner clearance angle (β 2 ), and the inner clearance angle (β 2 ) is greater than the outer clearance angle (β 1 ).
15. The cutting head ( 20 ) according to claim 14 , wherein:
the inner clearance angle (β 2 ) continuously increases when measured at a series of parallel cross-sections taken in parallel vertical planes located progressively closer to the first axis (A 1 ).
16. The cutting head ( 20 ) according to claim 1 , wherein:
the cutting edge transition point (NR) is located at the juncture of the clearance surface ( 36 ), the head flute ( 44 ) and the gash ( 46 ); and
the inner cutting edge ( 34 ) extends for the full length of a boundary between the clearance surface ( 36 ) and the gash ( 46 ).
17. The cutting head ( 20 ) according to claim 1 , wherein:
the plurality of cutting edge transition points (NR) define an imaginary first circle (C 1 ) having a first diameter (D 1 ), and
the first diameter (D 1 ) is greater than thirty percent of the cutting diameter (DC).
18. The cutting head ( 20 ) according to claim 1 , wherein:
the cutting head ( 20 ) includes a central mounting protuberance ( 60 ) intersected by the first axis (A 1 ) and extending axially rearwardly from the bottom surface ( 56 ).
19. The cutting head ( 20 ) according to claim 1 , wherein:
from an associated leading edge ( 28 ), and toward the tip point NT, points along each main cutting edge ( 31 ), increase in distance in both an axially forward direction and a radially inward direction.
20. The cutting head ( 20 ) according to claim 1 , wherein:
the intermediate portion ( 22 ) of the cutting head ( 20 ) includes a plurality of N torque transmission surfaces ( 62 ) facing opposite the direction of rotation (DR).
21. The cutting head ( 20 ) according to claim 20 , wherein: each gash ( 46 ) intersects one of the torque transmission surfaces ( 62 ) at a radially outermost gash point (NO).
22. The cutting head ( 20 ) according to claim 21 , wherein:
the three radially outermost gash points (NO) define an imaginary second circle (C 2 ) having a second diameter (D 2 ) greater than seventy percent of the cutting diameter (DC).