The invention relates generally to drill bits and more particularly to an improved bit construction for a spade-type drill bit.
Rotary drill bits for boring holes of preselected diameters in a workpiece are known. Such bits come in a variety of styles such as twist bits, fluted bits, augers and the like. One type of bit is known as a spade bit or paddle bit. This type of bit includes a shank that has one end adapted to be releasably connected to a chuck of a rotary tool such as a drill or driver. The opposite end of the shank supports a cutting member where the cutting member has a substantially flat, relatively thin face formed as a paddle or spade. The width of the cutting member is selected such that the bit bores a hole of predeteremined diameter.
The cutting member is typically formed with a first cutting edge and a second cutting edge arranged substantially perpendicular to the direction of movement of the bit through the workpiece. The cutting member may also be formed with a pilot point along its longitudinal axis that has a sharpened edge for guiding the bit into and through the workpiece. The first and second cutting edges may be formed with spurs at the ends thereof to create a clean edge to the hole.
While such drill bits are known an improved spade bit that bores a cleaner, faster hole is desired.
The bit of the invention has a longitudinally extending shank having a quick release connection formed at one end thereof for attachment to a driving tool such as a drill. The opposite end of the shank is formed with a cutting member. The cutting member is formed as a twisted planar member having a generally helical shape. In one embodiment the twist angle is approximately 30 degrees. A screw tip is provided along the longitudinal axis of the bit for guiding the bit through the workpiece.
Referring to
Cutting member 6 is comprised of a member 8 having a first face 10 and a second face 12 arranged substantially parallel to the first face 10. A pair of side edges 14 and 16 are arranged generally parallel to one another and to the longitudinal axis A-A. In one embodiment a relief angle Δ may be provided where the side edges 14 and 16 angle inwardly toward the longitudinal axis A-A from the end of the bit toward shank 2 as best shown in
A twist angle α of up to 45 degrees provides performance benefits in the drill bit of the invention because the blade allows for the rapid and efficient ejection of chip swarf from the hole being drilled. The twist angle as used herein is the angle between one of the cutting edges (e.g. cutting edge 18a) and the opposite edge (e.g. edge 22) of the face creating that edge (e.g. face 10) about the longitudinal axis A. In one embodiment the cutting member 6 has a twist angle α of approximately 30 degrees, as best shown in
The cutting shoulders 18 and 20 are formed at the end of cutting member 6 and extend from side edges 14 and 16, respectively, toward the longitudinal axis A. The cutting edges are arranged at an oblique angle β of up to 20 degrees with a preferred angle of approximately 10 degrees relative to a line perpendicular to longitudinal axis A such that sharp scribing corners 26 and 28 are formed on cutting shoulders 18 and 20, respectively, that have an included angle of between approximately 80 to 90 degrees. Shoulders 18 and 20 are formed with beveled surfaces that define cutting edges 18a and 20a. Cutting edge 18a is formed as an acute rake angle λ between shoulder 18 and face 10 such that cutting edge 18a is formed as sharp edge that contacts the workpiece as the drill bit is turned in the direction of arrow D. Shoulder 20 and face 12 are arranged in the same manner to create a similarly sharp cutting edge 20a. In one embodiment rake angle λ is between 5 and 25 degrees and is preferably 18 degrees. The cutting edges 18a and 20a are offset (d) from the centerline CL of the cutting member 6 extending through longitudinal axis A-A as best shown in
The cutting shoulders 18 and 20 extend to the base 30b of pilot screw 30 where pilot screw 30 is arranged along the longitudinal axis A-A. Pilot screw 30 is generally conically shaped decreasing in diameter from shoulders 18 and 20 to point 30a. The pilot screw 30 includes tapered threads 32 that extend upwardly from base 30b that maintain equal thread pitch along the length of pilot screw 30. In a preferred embodiment, the point of the pilot screw 30 defines an included angle μ of 24 degrees although angles within a range of 15 to 35 degrees can also be used. An alternate embodiment of the tip is shown in
Specific embodiments of an invention are described herein. One of ordinary skill in the art will recognize that the invention has other applications in other environments. In fact, many embodiments and implementations are possible. The following claims are in no way intended to limit the scope of the invention to the specific embodiments described above.