Drill for making flat bottom hole
The present invention is directed to a helical drill configuration that allows substantially flat bottomed holes to be machined. At the cutting end of the body member, there are at least two cutting edges that are symmetrically oriented on apposing sides of the rotational axis of the drill. In an embodiment of the invention, first and second cutting edge portions form a continuous surface which provides strength and tool stability. The height of the second portions of each of the cutting edges remains relatively consistent along the horizontal for formation of a generally flat bottom hole. In the embodiment, a center point is defined by two sloped peak surfaces. A central straight chisel edge is formed by the intersection of the two sloped peak surfaces. The first cutting edge section extends from the chisel edge to the second cutting edge section. The first cutting edge section for both cutting edges is formed by symmetrically thinning the two peak surface. In the invention, stress at the center portion of the helical cutting tool is limited by the chisel edge and first sections of the cutting edge near the center portion having a balanced geometry. The balanced geometry of the helical drill also prevents the drill from wobbling and creating deviations in the hole being formed.
1. A generally cylindrical cutting tool for making a flat bottom hole, said tool having a central axis, and a cutting end, said cutting end comprising:
two peak surface intersecting at and generally adjacent to said central axis and forming a chisel edge, a first curvilinear cutting edge portion formed by the intersection of a first thinning surface and at least one of said peak surfaces, said first curvilinear cutting edge portion extending radially from said chisel edge, a second straight cutting edge portion perpendicular to said central axis and extending radially from said first cutting edge portion, and a third cutting edge portion extending radially from said second straight cutting edge portion, wherein said third cutting edge portion is curvilinear or chamfered.
2. The cutting tool as in claim 1 wherein said first cutting edge portion has a constant radius of curvature.
3. The cutting tool as in claim 1 wherein said two peak surfaces are oriented at an angle gamma between approximately 10–25 degrees with respect to the second cutting edge portion.
4. The cutting tool as in claim 1 wherein said two peak surfaces are oriented at an angle gamma between approximately 25–45 degrees with respect to the horizontal second cutting edge portion.
5. The cutting tool as in claim 1 wherein said second cutting edge portion is formed by the intersection of a top flank surface and a flute.
6. The cutting tool as in claim 5 wherein said top flank surface includes top flank rearward sections and top flank forward sections,
the top flank forward sections are oriented at a first relief angle and the top flank rearward sections are oriented at a greater second relief angle.
7. The cutting tool as in claim 2 wherein said cutting tool is generally cylindrical and has an external diameter,
wherein said radius of curvature has a dimension that is between about 8 to 20 percent of said external diameter.
8. The cutting tool as in claim 1 wherein said chisel edge is substantially straight.
9. The cutting tool as in claim 1 wherein said first thinning surface is substantially parallel to said central axis.
10. The cutting tool as in claim 9 further comprising a second thinning surface, said second thinning surface is flat and planar.