Spade drill insert having curved cutting edges

#Drilling systems are frequently used to provide cylindrical holes in metallic workpieces. The cutting or boring action of the drill system may be carried out by an elongated, substantially cylindrical drilling tool, such as a combination of a tool holder and a spade drill insert, which is selectively attached thereto. Such an arrangement may then be used in an application wherein one end of the tool holder is securely mounted in a driving apparatus, which rotates the holder about its longitudinal axis. At the opposite end of the elongated tool holder, the cutting insert engages the material to be cut. Alternatively, the workpiece may be made to rotate relative to the holder and cutting insert, such as in positioning the holder in the tail stock of a lathe or the like. Further, the tool and workpiece may be made to rotate relative to one another. The use of spade drill cutting inserts allows for quick changing of the insert upon wear of the cutting surfaces instead of the entire tool, and allows for one tool to be used for a variety of different boring applications by simply changing the insert and not the entire drill assembly.

#Spade drill inserts are characterized in that they are generally flat having a pair of connection surfaces or faces on opposite sides of the insert that are parallel to each other and provide a register surface for the clamping arms of the holder. The leading end of the spade drill typically has a pair of straight, transverse cutting edges in the form of an obtuse V. The trailing end or base of the spade drill insert is typically planar and perpendicular to the pair of connection surfaces and engages the bottom of the holder slot. The sides across the width of the insert provide the margin and a clearance for the insert. The margin is cylindrically formed about the rotational axis of the spade drill and contacts the side of the hole during drilling operations.

#One problem with prior art spade drill inserts is that the straight cutting edges do not efficiently form and/or transport chips during drilling operations. With the prior art spade drills, the chips are formed radially outward toward the outside diameter of the insert due to their straight cutting edge. Another problem with prior art inserts utilizing a corner clip clearance, the furthermost outside cutting edge is closer to the centerline of the lip thus being closer to a neutral or less positive rake geometry, especially when an oversized corner clip is utilized. This results in less efficient cutting at the outside diameter of the spade drill insert. A further problem with the prior art inserts is that the straight cutting edge chip formation is not tight which requires that the operational ranges of the tool to be narrowly defined for proper tool operation. In addition, the geometry of the straight cutting edges of prior art spade drills are apt to be susceptible to chatter primarily due to the chip formation energy along rotating symmetrical planes at a constant speed developing harmonics at various operating speeds depending on the material being cut and the natural frequency of the tool and system. This may result in the operator having to change the operating conditions of the drilling procedure to less than optimal ranges.

#The invention and developments thereof are described in more detail in the following by way of embodiments with reference to the drawings, in which:

#FIG. 1 is an exploded assembly view of the drill tool assembly according to a preferred embodiment;

#FIG. 2 is a partial perspective view of the holder associated with the assembly;

#Turning now to a preferred embodiment of the invention, FIG. 1 illustrates a drill tool assembly 10 generally indicated. Drill tool assembly 10 comprises a holder 12, which has a body 14 and head portion 16 associated therewith. In the preferred embodiment, holder 12 has, in general, a cylindrical shape with a first end 20 and second end 22. As shown in FIG. 2, the first end 20 of holder 12 has a clamping or holder slot 30, which may extend across the entire diameter of the head portion 16 or, at least, over a center portion thereof at the general location of the rotational axis 18 of holder 12. The holder slot 30 has a bottom wall 32 positioned in substantially perpendicular orientation relative to the rotational axis 18 of the holder 12. In the preferred embodiment, the assembly 10 may further include a locating boss or dowel pin 24, which is positioned precisely with respect to the axis 18 and extends from the bottom wall 32 of the holder slot 30. The pin 24 may be positioned within a hole 26 extending downwardly from the bottom wall 32 of slot 30 along the axis 18 of the holder body in a press fit relationship to position pin 24. Alternatively, the locating boss, which, in the preferred embodiment, comprises pin 24, may be configured in another manner to achieve the corresponding functionality of pin 24, such as an integral member extending from bottom wall 32. Within the holder slot 30, a drill insert 35 is precisely positioned with respect to the holder 12 to perform the desired drilling function in conjunction therewith. As will be hereinafter described in more detail, the insert 35 has a point geometry comprising a plurality of cutting surfaces, which are precisely positioned with respect to the axis 18 of the holder 12 to minimize errors in a resulting drilling operation using assembly 10.

#More particularly, the preferred embodiment of holder 12 is shown in FIG. 2, and may be configured to include at its first end 20 a pair of clamping arms 34, which extend about holder slot 30. The clamping arms 34 preferably include apertures 36, which accommodate screws 38 (see FIG. 1) to secure the drill insert 35 in its position within the holder slot 30. In the preferred configuration, the holes 36 are threaded to engage screws 38, and mate with screw holes formed in the drill insert 35 in a predetermined manner to precisely locate the drill insert in a predetermined location within holder slot 30, as will be described in more detail. Each of the clamp arms 34 may also include a lubrication vent 28, which allows the application and flow of lubrication adjacent the cutting surfaces of the drill insert to facilitate the drilling operation. The clamp arms 34 may also include angled or curved surfaces, which facilitate chip removal via chip evacuating grooves 37 on each side of the holder 12. The seating surface 32 is also shown to be designed as a planar surface, which corresponds to the planar bottom portion of the preferred drill insert 35, although another configuration of bottom surface 32 may be employed and is contemplated herein. It is contemplated that the drill insert 35 is made of a sintered metallic hard material such as carbide, cermet, ceramic, monocrystalline and polycrystalline diamond, or boron nitride. However, the drill insert 35 may also be comprised of high speed steel.

#Turning to FIGS. 3A–3G, a first embodiment of the spade drill insert 35 is shown. The drill insert 35 comprises cutting edges 64 on its upper surface generally in the form of an obtuse V-shape, with a cutting edge 64 on each either side of the chisel 62 and radially outward from a web thin 63. Each cutting edge 64 is curved and formed in a cutting edge plane such that the planes formed through the cutting edges 64 are transverse to each other and form the included angle. The cutting edges 64 are curved in the direction of rotation.