Cutting tool with chisel edge

A twist drill has double margins and a chisel edge angle of between 80 degrees and 100 degrees to maximize hole straightness and to minimize “walking” of the drill upon a workpiece. The drill may be modular having a removable tip body or the drill may be of a solid configuration. A tip body is consumable and may be removed and replaced as required.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates to a cutting tool, such as a twist drill, having a uniquely oriented chisel edge.

2. Description of Related Art

It is important in the drilling of brittle materials such as grey cast iron to drill a circular hole at a desired location and in a straight direction with “hole straightness”. During the drilling process, not only is it possible for the drill to “walk” across the surface of a workpiece away from the target location but, furthermore, even when the drill is positioned at the proper target location, asymmetrical cutting forces upon the drill may cause the drill to wobble, thereby producing a non-circular hole, a hole that is not straight, or both. U.S. Pat. No. 3,977,807 issued Aug. 31, 1976, is directed to a double margin twist drill having a chisel edge that forms a chisel edge angle with the corner of the primary margin. WhileFIG. 2of this patent illustrates this feature, it is difficult to discern the chisel edge angle becauseFIG. 2is a perspective view. Nevertheless, the outer diameter of the primary margin reduces as the margin approaches the chisel edge and, as a result, some drill stability is sacrificed.

U.S. Pat. No. 7,267,514 issued Sep. 11, 2007, is directed to a self-centering bit drill with a pilot tip, wherein the drill has only a single margin. This patent discloses inFIG. 4, an arrangement, whereby the chisel edge is essentially parallel with the drill margin. While this reduces the “walking” of the drill, the same problem exists with respect to hole shape and hole straightness because the drill tends to wobble while drilling.

A drill design is needed that makes straighter and better located holes, and that stabilizes the drill within the hole during the hole making process.

SUMMARY OF THE INVENTION

The tip of a generally cylindrical cutting tool is used for making a hole within a workpiece, wherein the tip has a central axis and a cutting end. The tool is designed to rotate in a cutting direction. The tip comprises a body having two diametrically opposed flutes with lands therebetween, wherein the lands each have a pair of edges defined as a leading edge and a trailing edge defined by the rotation of the tool. A primary margin and a secondary margin are spaced apart from one another on each of the lands. The primary margin is adjacent to the leading edge and the margins have the margin diameter along the entire length of the body that is constant. A cutting lip extends inwardly from the leading edge of each land to an innermost end. A chisel edge is defined by a line connecting the innermost end of each cutting lip, wherein, when viewed along the central axis, the chisel edge forms a chisel edge angle with a line extending radially inward from the leading edge of the land to the midpoint of the chisel edge. The chisel edge angle is between 80 degrees to 100 degrees. The subject invention is also directed to a rotatable cutting tool having a tip as described above.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1illustrates a rotatable cutting tool10which, as an example, is a twist drill. The cutting tool10has a tip15at the cutting end20. The tip15has a central axis17extending therethrough. As illustrated inFIG. 2, the tip15has a tip body25with two diametrically opposed flutes30A,30B. In between each flute30A,30B about the periphery of the cutting tool10are lands35A,35B. Each land35A,35B is made up of a tip body land37A,37B and a prong land39A,39B.

The embodiment illustrated inFIGS. 1-4show a cutting tool10and a shank12, wherein the tip body25is removable from the shank12. As illustrated inFIG. 3, the tip body25is secured between two extending prongs14A,14B.

The cutting tool discussed herein is symmetric when viewed from the cutting end20and, as a result, only one-half of the cutting tool10will be discussed with the understanding that the symmetric side, which has so far been referenced using the “B” suffix, is symmetric. Therefore, the same discussion directed to the “A” side will apply to the “B” side.

The tip body land37A has a pair of edges defined as a leading edge40A and a trailing edge42A. The rotatable cutting tool10and the associated tip body25are designed to operate in a single rotational direction indicated by arrow “45”. As a result, the first portion of the tip body land37A to contact a workpiece is defined as the leading edge40A, while the remaining edge is the trailing edge42A. The tip body land37A is made up of a primary margin47A and a secondary margin49A spaced apart from one another on the tip body land37A. The primary margin47A is adjacent to the leading edge40A and each of the margins47A,49A form with the opposing primary margin47B and secondary margin49B a margin diameter MD which is identical to the outer diameter4. While the secondary margin49A, illustrated inFIG. 2, contacts the trailing edge42A, it is entirely possible and easily envisioned for the secondary margin49A to be spaced from the trailing edge42A in a direction closer to the primary margin47A.

The margin diameter50extends along the entire length of the tip body25and is substantially constant. In particular, it is standard in twist drill designs to provide the cutting end of the drill with an outer diameter slightly larger than the region behind the cutting end. As an example, a twist drill may have a cutting end outer diameter that is approximately 0.05 millimeters greater than the drill outer diameter away from the cutting end, thereby providing clearance behind the cutting end. It is in this respect that the margin diameter50extends along the entire length of the tip body25and is substantially constant.

It should be appreciated that, with respect toFIG. 2, the margin diameter MD is the largest diameter of not only the tip body25but also of the shank12and the remaining segments of the periphery of the tip body25and the prongs14have a diameter that is less than the margin diameter MD.

A cutting lip55A extends inwardly from the leading edge40A of the tip body land37A to an innermost end57A.

A chisel edge60is defined by a line63connecting the innermost ends57A,57B of each cutting lip55A,55B.

It should be appreciated that the chisel edge60is defined by both the cutting lip55A and the cutting lip55B and, for that reason, will not be identified as two parts associated with the symmetry of the tip body25.

When viewed along the central axis17, the chisel edge60forms a chisel edge angle A with a line65B extending radially inwardly from the leading edge40B of the land35B to the midpoint67of the chisel edge63. The chisel edge angle A is between 80 degrees to 100 degrees and preferably, is approximately 90 degrees.

As illustrated inFIG. 2, the chisel edge60has an “S” shape with a generally straight central portion69in curved ends70A,70B. For example, the straight central portion69of the chisel edge60has a length of 0.25 millimeters for a drill having an outer diameter of between 12 millimeter and 13 millimeters.

As illustrated inFIG. 2, the chisel edge60transitions into the cutting lips55A,55B with curved ends70A,70B. These curved ends70A,70B have a radius of 0.4 millimeters to 1.5 millimeters for the outer diameter of 12 millimeters to 32 millimeters.

A gash75A (FIG. 3) extends upwardly from the flute30A to the chisel edge60and, in one embodiment, defines a positive rake angle R at the chisel edge60of between 2 degrees and 10 degrees.

Between the primary margin47A and the secondary margin49A is a recess78A defined by a clearance diameter CD between opposing recesses78A,78B, which is less than the margin diameter MD.

The region between the land35A and the chisel edge60is generally referred to as the flank80A. Typically, the flank80A meets the land35A to form an edge. However, for improved strength, durability and to minimize blow out/breakout on exit for brittle materials, such as cast iron, a chamfer82A may be introduced between the land35A and the flank80A. The chamfer82A may form a chamfer angle CA (FIG. 4) of between70degrees and90degrees with the central axis17.

As illustrated inFIG. 1, the cutting tool is a modular twist drill having a shank12that accommodates the tip15. However, it is also possible and may be easily envisioned for the tip body25to be an integral part of the shank12, thereby providing a solid drill.

It should also be noted that so far discussed has been a twist drill having helical flutes. The subject invention may also be applied to a twist drill having straight flutes.

As illustrated inFIG. 3, the tip15may be secured within the prongs14A,14B of the shank12using different techniques. In the embodiment illustrated herein, the tip body opposite the cutting end20, as illustrated inFIG. 1A, is a threaded bore85along the central axis17for engagement with a mating bolt105having a head107which abuts against an internal shoulder109within the shank12. It should be appreciated that other arrangements are known by those skilled in the art of machine tools for securing the tip15to the shank12including, but not limited to, prongs14, which resiliently clamp the tip15to the shank12.

The configuration of the shank12within the region of the tip15is such that when the tip15is mounted within the shank12, the profile of the shank12conforms to the profile of the tip15, such that, as illustrated inFIG. 1, the combination appears to be a unitary piece having continuous surfaces.

It should be noted that the cutting tool10may have coolant passages110running the length of the shank12and supplied with coolant at the base13of the shank. Additionally, as shown, the shank12has a tang11secured to a spindle (not shown) used to impart rotation to the cutting tool10. The shank12may be secured to the spindle in any of a number of different ways known to those skilled in the art of drill manufacturing.

The tip body15may be made from a hard cemented carbide such as tungsten, titanium carbide or TiC—TiN. In general, the tip body15may be made from a hard wear-resistant material such as one of a number of refractory coated cemented carbide materials, which are well known in the art. Because of the expense associated with carbide tools, although it is possible, it is unlikely for both the tip body25and the shank12to be comprised of carbide materials and it is more likely that the tip body25is comprised of a carbide material, while the shank12is comprised of machine tool steel.

A cutting tool design in accordance with the subject invention provides superior results to other cutting tools. With respect to the hole straightness, tests were conducted using a modular twist drill having a diameter of 12.5 millimeters, a 140 degree point angle, and a 30 degree helix angle to drill blind holes having a depth of 125 millimeters. The workpiece was grey cast iron class40and the drill was advanced at a speed of 198 millimeters per minute and 0.35 millimeters per revolution. Additionally, coolant was introduced to the workpiece through internal passageways in the drill shank.

Under these circumstances, the following results were attained.

Case 1. A single margin drill having a chisel angle of 60 degrees achieved a hole straightness of 0.15 millimeters.

Case 2. A single margin drill having a chisel edge angle of 90 degrees achieved a hole straightness of 0.13 millimeters.

Case 3. A double margin drill having a chisel edge angle of 60 degrees achieved a hole straightness of 0.11 millimeters.

Case 4. A double margin drill having a chisel edge angle of 90 degrees, consistent with the subject invention, achieved a hole straightness of 0.04 millimeters. These unexpected results show the superiority of the design in accordance with the subject invention.

The chisel angle is also instrumental in minimizing “walking” of the drill along the workpiece. In particular, for a drill having a chisel angle of 60 degrees, the location error relative to the target location was 0.0042 millimeters while the location error utilizing a drill having a chisel edge angle of 90 degrees had a location error of 0.003 millimeters.

As can be seen, the drill in accordance with the subject invention provided not only unexpected but superior results to other designs.

As a result of the drill design disclosed herein, during the drilling process, the forces produced by the chisel edge encounter reaction forces on the margins from the hole wall along the same direction/line. This minimizes any imbalance in forces or moments on the drill, thereby reducing the tendency of the drill to walk and ensuring hole straightness and accurate hole positioning. This feature of the drill is very beneficial in a heterogeneous material such as grey cast iron, reputed for the difficulty in maintaining hole straightness. The chisel edge angle, as disclosed herein, provides a much superior hole straightness in conjunction with double margins, where secondary margins act to counterbalance any out-of-line forces. As illustrated in the cases described above, there is a 50% improvement in hole straightness utilizing a double-margin drill when the chisel angle is changed from 60 degrees to 90 degrees. Additionally, the hole location error is reduced by about 25% utilizing a drill having a chisel edge angle of 90 degrees as opposed to utilizing a drill having a chisel edge angle of 60 degrees.