RELATED APPLICATIONS

[0001] This application claims the benefit of United States Provisional Patent Application Serial No. 62/160,898 filed May 13, 2015, the entire content of which is hereby incorporated by reference herein.

BACKGROUND

[0002] The present invention relates to drill bits.

[0003] Drill bits are widely used in the manufacturing industry. Cutting performance is largely driven by the drill tip geometry. Many different tip geometries have been developed to meet material and process requests for metal cutting production. Fig. 1 illustrates a portion of a conventional drill bit tip. Its radial rake angle 1, which is defined between the drill cutting edge face/flute face 2 and a line 4 running from the lead or front corner of the helical outer face of the drill bit and passing through the drill bit's central axis 3, is always negative (trailing in relation to the drill's rotation direction and as is understood by those skilled in the drill art), which results in scratching on the side wall of the hole. This geometry has little or no material shearing effect and sometimes makes the drilled hole surface roughness high enough to be out of specified tolerances. This often happens in the case of small hole drilling since the smaller the drill bit diameter, the more negative the radial rake angle 1. A post reaming process is typically needed if the hole's roughness is not within specification. This not only increases process tooling cost, but also increases cycle time.

[0004] Some drill and reamer combination tools have been proposed and developed. Those tools combined a drill portion and reamer portion into one tool in axial progression to perform drilling and reaming functions in series in one pass. However, such tools only work when forming some through holes. For blind holes or situations with limited space at the hole exit, these combination tools do not work. Furthermore, these combined drill and reamer tools are expensive to make and buy. SUMMARY

[0005] In this disclosure, a novel drill bit concept is disclosed. The new drill bit design has a special flute section in which the radial rake angle in the outside diameter edge portion is always positive (in relation to the drill rotation direction). This provides a shearing effect for material removal on the side wall of the drilled hole, which improves surface roughness significantly. With the improved surface roughness, hole size can be controlled more accurately. This inventive concept applies not only to helical flute drill bits, but is also suitable in straight flute drill bits.

[0006] In one embodiment, the invention provides a drill bit having a flute face defining a leading surface of the drill bit in the direction of rotation, and a radial rake face offset from the flute face in a direction opposite the direction of rotation of the drill bit and located radially outwardly from the flute face. A radial rake angle is defined between the radial rake face and a line running from a lead comer of the radial rake face and passing through a central axis of the drill bit. The radial rake angle is positive.

[0007] Other aspects of the invention will become apparent by consideration of the detailed description and accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

[0008] Fig. 1 is a partial end view of a prior art drill bit.

[0009] Fig. 2 is a partial perspective view of a drill bit embodying the present invention. [0010] Fig. 3 is an end view of the drill bit of Fig. 2.

[0011] Fig. 4 is a partial perspective view of the drill bit of Fig. 2 showing a paralleled helical surface for reference.

[0012] Fig. 5 is a cross-sectional view of the drill bit of Fig. 2.

[0013] Fig. 6 is an enlarged partial cross-sectional view of Fig. 5.

[0014] Fig. 7 illustrates chips cut using a prior art drill bit and the drill bit of Figs. 2-6. DETAILED DESCRIPTION

[0015] Before any embodiments of the invention are explained in detail, it is to be understood that the invention is not limited in its application to the details of construction and the arrangement of components set forth in the following description or illustrated in the following drawings. The invention is capable of other embodiments and of being practiced or of being carried out in various ways.

[0016] Figures 2 and 3 show a drill bit 10 according to the invention in perspective and end views, respectively. While the illustrated drill bit 10 is a helical flute drill bit, the invention is also applicable for straight flute drill bits. In the outside diameter of the drill bit 10, there are two helical steps defining lands 14 radially outward of the respective flute outside or leading faces 18. In the illustrated embodiment, the two lands 14 and the two respective flute leading faces 18 are spaced one-hundred eighty degrees apart about the circumference of the drill bit 10, however in other embodiments, more than two lands and flute leading faces may be present. The lands 14 each have a width W (see Fig. 6) of about 10-30% of the radius of the drill bit 10, and in some embodiments, each have a width W of about 20-30% of the radius of the drill bit 10. When the drill bit 10 is cutting, the lead or front corner 22 nearest the tip in each land 14 is working as a diameter control cutting edge, which mainly controls the drilled hole's diameter and side wall surface roughness Ra. The radially outside portion of the lands 14 each form or define a helical side cutting edge 26 (see Fig. 2) extending from the lead comer 22. The surface of the lands 14 can also be referred to as a radial rake face 30. The radial rake face 30 is parallel with the flute leading face 18, but is offset lower (i.e., using drill rotation direction as a reference vector) than the paralleled helical surface 32 containing from the flute leading face 18 (see Fig. 4 and 5) and passing through the central axis 34 of the drill bit 10. As best shown in Fig. 6, by providing the step and the associated land 14, the radial rake angle 38, which is defined between the radial rake face 30 and a line 42 running from the lead or front comer 22 of the land 14 and passing through the central axis 34, is always positive (leading in relation to the drill rotation direction and as is understood by those skilled in the drill art). The positive radial rake angle 38 can be one to five degrees, as best shown in Fig. 6. The desired positive radial rake angle 38 drives the amount (i.e., height) of offset between the flute leading face 18 and the rake face 30, depending on the size of the drill bit 10. This positive radial rake angle 38 means that the drill bit 10 can cut the side wall of the hole with a shearing effect and with less scratching of the side wall than prior art drills having a negative radial rake angle.

[0017] Another aspect of this design is that the main lead cutting edge has been broken into two separate edge segments, a first edge 46 of the flute leading face 18 and a second, lower offset edge 48 of the radial rake face 30, as shown in Figs. 2 and 3. The edge 48 of the radial rake face 30 is offset in a direction opposite the direction of rotation from the edge 46 of the flute leading face 18, and is radially outward from the edge 46 of the flute leading face 18. This feature of the two edges 46, 48 permits the drill bit 10 to break drilled chips into two smaller and narrower pieces along the main cutting edge width. Smaller and narrower chips are easier to drive out of the cutting zone. This new drill design has been tested and Fig. 7 illustrates the narrower chips 50 from the inventive drill bit as compared to the wider chips 54 from a prior art drill.

[0018] The inventive drill bit 10 has been found to reduce the surface roughness of hole side walls as compared to prior art drill bits like that shown in Fig. 1. In a steel sample, testing has shown the reduction in the surface roughness Ra of the hole's side wall from 115.2 micro-inches for the prior art drill bit, down to 65.2 micro-inches for the inventive drill bit 10. In an aluminum sample, the reduction in the surface roughness Ra of the hole's side wall went from 25.8 micro-inches for the prior art drill bit, down to 6.2 micro-inches for the inventive drill bit 10.

[0019] Various features and advantages of the invention are set forth in the following claims.