1. Field of the Invention
This invention relates to a twist drill bit having a pair of cutting edges at a forward end thereof.
2. Prior Art
One conventional twist drill bit 10 shown in FIG. 1 has a cylindrical body 12, and a pair of spiral grooves or flutes 14 formed in the outer peripheral surface of the body 12, the grooves 14 serving to discharge cuttings or chips therethrough during a drilling operation. Each of the spiral grooves 14 has a forward end portion which is defined by a substantially straight rake surface 16 extending radially inwardly from the outer periphery of the bit body 12, and a concavely-curved surface 18 having a relatively large radius of curvature and extending from the inner end of the rake surface 16 to the outer periphery of the bit body 12, as seen in FIG. 1. Each rake surface 16 terminates at its forward end in a cutting edge the outer end of which is designated at 20.
FIG. 2 shows another conventional twist drill bit 10a in which a forward end portion of each of spiral groove 14a has a rake surface 16a similar to the rake surface 16 of the drill bit 10 of FIG. 1, and a concavely-curved surface 18a having a relatively small radius of curvature and extending from the inner end of the rake surface 16a toward the outer periphery of the bit body 12a.
FIG. 3 shows a further conventional twist drill bit 10b in which a forward end portion of each of spiral grooves 14b is defined by a rake surface 16b extending radially inwardly from the outer periphery of the bit body 12b and curving concavely gently and a concavely-curved surface 18b having a relatively small radius of curvature and extending from the inner end of the rake surface 16b to the outer periphery of the bit body 12b.
FIG. 4 shows a still further conventional twist drill bit 10c in which a forward end portion of each of spiral grooves 14c has a convex rake surface 16c extending from the outer periphery of the bit body 12c and curving convexly gently and a concavely-curved surface 18c having a relatively large radius of curvature extending to the outer periphery of the bit body 12c.
FIG. 5 shows a further conventional twist drill bit 10d in which a forward end portion of each of spiral grooves 14d is defined by a convex rake surface 16d and a concavely-curved surface 18d extending from the inner end of the rake surface 16d to the outer periphery of the bit body 12d, the convex rake surface 16d being defined by a first straight outer surface 16e extending from the outer periphery of the bit body and a generally straight inner surface 16f extending between the outer surface 16e and the curved surface 18d.
Although not shown in the drawings, in the conventional drill bit 10 of FIG. 1, a projection is formed on each of the rake surface 16 so as to serve as a chip breaker for breaking a chip or cutting, cut by the cutting edges from a workpiece, into pieces to facilitate the discharge of the chips through the spiral grooves 14.
On the other hand, in the conventional drill bit 10a of FIG. 2, a chip or cutting is extended along the rake surface 16a and is abruptly curved or bent by the concavely-curved surface 18a, so that the chip is divided into pieces, thereby facilitating the discharge of the chip. Similarly, in the conventional drill bit 10b of FIG. 3, a chip or cutting is extended along the rake surface 16b and is abruptly curved or bent by the concavely-curved surface 18b, so that the chip is divided into pieces.
In the conventional drill bit 10c of FIG. 4, although not shown in the drawings, each rake surface 16c is ground off at its forward end portion, so that each rake surface 16c is stepped or offset inwardly to provide a flat portion disposed along the axis of the bit body 12c and extending to the cutting edge, so that the stepped portion serves as a chip breaker. The conventional drill bit 10d of FIG. 5 has a chip breaker similar to that of the drill bit 10c of FIG. 4.
Recently, it has been desired to operate drills at high speeds to enhance drilling efficiency. To meet such requirements, solid-type drills made of cemented carbide have been extensively used. However, such solid-type drills of cemented carbide have the disadvantage that they have a lower resistance to breakage. Therefore, if the conventional drill 10 of FIG. 1 is designed as a solid-type and is made of cemented carbide, there is a possibility that the drill will be broken during a high-speed drilling operation in which an increased cutting resistance is encountered, since such a drill has a relatively small web thickness and hence a lowered rigidity. If the web thickness is increased to prevent such a drill breakage, then the spiral grooves 14 becomes so small in size that the chip breaker can not be provided on the rake surface 16.
As regards the conventional drill bits 10a and 10b of FIGS. 2 and 3, it is rather difficult that these drill bits are made of cemented carbide due to their configuration. In addition, the rake surfaces 16a and 16b are generally flat, so that the web thickness is rather small. Therefore, even if these drill bits are made of cemented carbide, the rigidity of the drill bits are reduced as described above for the drill bit 10 of FIG. 1.
The conventional drill bits 10c and 10d of FIGS. 4 and 5 can have a greater web thickness as compared with the conventional drill bits 10, 10a and 10b of FIGS. 1, 2 and 3. However, the cutting edges have a negative rake angle, and therefore an increased cutting resistance is encountered during the drilling operation. In addition, if these drills 10c and 10d are made of cemented carbide which is hard, much time and labor are required to grind off the the forward end portion of the rake surface 16c, 16d to provide the stepped portion serving as the chip breaker.