Abstract:
A spot drilling tool includes a drill holder and an insert removably mounted to the holder. The insert is a standardized part configured to support a drill point in a predetermined position forward of the drill holder and centered on the rotational axis of the drill holder. The drill point has two flutes and two substantially conical land surfaces extending between the flutes. The land surfaces have centers of curvature offset from the rotational axis of the insert to provide radial clearance for the trailing portion of the land. A chisel edge at the tip of the drill point is supported by a tapered web defined between the diagonally opposed and offset flutes. Linear cutting edges are formed at the junction of a planar flute surface and the conical land surface. The cutting edges define a narrow included point angle of between 50° and 80°.

Description:
FIELD OF THE INVENTION 
   The present invention generally relates to the field of cutting tools and more particularly to a spotting drill insert with improved point geometry. 
   DESCRIPTION OF THE RELATED ART 
   Hole making is the most common machining process, consuming 50 to 70 percent of all production time. Differences in material, hole diameter, depth, location and surface finish make each hole making operation unique. The selection of the proper machine tool, cutting tool, point geometry, feed rate, cutting speed and feed force have a significant impact on the efficiency of the hole making operation and the quality of the resulting holes. Drilling holes with a diameter of less than 0.010 inches, sometimes called “micro drilling”, is a particularly demanding type of hole making. The tiny drills used to make these holes require an accurately placed and configured spot or pilot hole to position the drill point on the workpiece so it won&#39;t walk. The pilot hole also acts as a guide for the drill during the initial stage of drilling. Setting the drill on a straight path is an important factor in the accuracy of the resulting hole. 
   Spot drilling is the process of forming the pilot hole or “spot” for a subsequent drilling operation. Spot drilling in preparation for micro drilling requires a spot with steeply angled side walls that will accurately center the drill and provide directional stability during the initial stage of drilling. The most common types of drills employed in spot drilling are twist drills and spade drills. Twist drills are rotary end-cutting tools having one or more cutting lips and one or more helical flutes for the passage of chips and cutting fluids. The included angle defined between the cutting lips of a twist drill is called the point angle. The standard point angle for a twisted drill is 118°, with smaller angles available for drilling softer materials such as magnesium, aluminum or plastic. As with any cutting tool, the cutting surfaces of a drill wear with use and must be renewed to ensure acceptable quality of the finished product. When worn, a twist drill must be removed from the drilling machine and sharpened or replaced. Removing the twist drill often requires a time consuming re-calibration of the machine tool. 
   Spade drills are alternative rotary end cutting tools in which a tool holder defines a socket for rigidly supporting a replaceable cutting insert. The cutting edges are ground on the insert. A major advantage of spade drills is that the cutting insert can be replaced quickly, minimizing machine downtime and reducing the cost of maintaining a sharp cutting edge. The tool holder, socket and mounting screws for the insert are configured to precisely and repeatably position the insert. Replacement of the insert can frequently be accomplished without re-calibrating the drilling machine. 
   As with all metal cutting edges, the cutting surfaces of a drill must be provided with a relief clearance. The purpose of relief clearance is to avoid interference and rubbing between the workpiece and trailing surfaces of the cutting tool. Relief clearance is typically provided by removing material behind the cutting edges of the drill point and is quantified in terms of a relief angle measured in degrees of clearance. In general, as the drill point angle decreases, the relief angle required for adequate clearance increases. A common procedure is to grind the tool surface behind the cutting edge into one or more facets to provide the necessary clearance. Twist drills may employ conical relief surfaces ground by rotating the drill about its axis relative to a grinding wheel. Spade drill inserts are frequently provided with angular relief surfaces. 
   As previously discussed, very small diameter drills function best when provided with a pilot that centers and guides the drill. Pilots formed by drills having point angles in excess of 100° may not have sufficient centering and guidance for very small diameter drills. As the point angle of the pilot forming drill is reduced to provide a more steeply tapered spot, the increased relief angle eventually weakens the cutting edges to an extent that the drill is no longer durable. There is a need in the art for a durable spot drilling tool having a point angle of less than 90° for the preparation of pilot holes for drills having a diameter of 0.006 inches or greater. 
   SUMMARY OF THE INVENTION 
   Briefly stated, the present invention comprises a new and improved point geometry for a spot drilling insert. An illustrated embodiment of the spot drilling insert has two cutting edges at a point angle of approximately 60° supported on a substantially conical drill point. The cutting edges are diametrically opposed and offset from each other. A chisel edge having a length of approximately 0.005 inches extends between the radially inward ends of the cutting edges. Angled linear flutes are defined by the intersection of substantially planar first and second flute surfaces. The intersection of the first and second flute surfaces is configured as a radius to promote chip curling and movement. The flutes converge toward the tip of the drill point. Conical land surfaces define the periphery of the drill point between the two flutes. 
   An aspect of the invention relates to conical land surfaces with an axis of curvature offset from the rotational axis of the drill point. The conical land surface intersects the first flute surface to define a substantially linear cutting edge. A trailing edge of the drill point is defined at the intersection of the conical land surface with the second flute surface of the second flute. Because of its offset axis of curvature, the conical land surface is closer to the rotational axis of the drill point at the trailing edge of the land. This reduced radial distance provides relief clearance for each cutting edge. The offset axis of curvature of the conical land surface is parallel to the rotational axis of the drill point so that the relief remains constant along the length of each cutting edge. 
   A conical relief surface leaves more tool material behind the cutting edge than is possible with angular relief. The conical land surface permits manufacture of a narrow point angle, small diameter spot drilling point with adequate clearance and a robust cutting edge not possible with prior art drill point geometries. 
   It is an object of the present invention to provide a new and improved spot drilling point geometry. 
   Another object of the present invention is to provide a new and improved spot drilling point geometry for pilot drilling in fine-gauge materials. 
   A further object of the present invention is to provide a new and improved method for manufacturing a drill point. 
   A still further object of the present invention is to provide a new and improved drill point geometry useful for drilling pilot holes for subsequent very small diameter drilling operations. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     These and other objects, features and advantages of the invention will become readily apparent to those skilled in the art upon reading the description of the preferred embodiments, in conjunction with the accompanying drawings in which: 
       FIG. 1  is a front view of a spot drilling insert according to aspects of the present invention in a tool holder; 
       FIG. 2  is a side view, partially in phantom, of the spot drilling insert and tool holder of  FIG. 1 ; 
       FIG. 3  is a right end view of the spot drilling insert and tool holder of  FIG. 1 ; 
       FIG. 4  is a partial enlarged view of the spot drilling insert and tool holder of  FIG. 3 ; 
       FIG. 5  is an enlarged isolated front view of a spot drilling insert according to aspects of the present invention; 
       FIG. 6  is a right end view of the spot drilling insert of  FIG. 5 ; 
       FIG. 7  is a greatly enlarged partial end view of a spot drilling insert according to aspects of the present invention; and 
       FIG. 8  is a schematic sectional representation through a drilling point having a geometry according to aspects of the present invention. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
   With reference to the drawings, wherein like numerals indicate like parts throughout the several figures, a spot drilling insert exemplary of aspects of the present invention is generally designated by the numeral  10 .  FIGS. 1-3  illustrate a drill holder  50  in conjunction with a spot drilling insert  10  according to aspects of the present invention. The drill holder  50  defines a socket that is configured to receive and secure the spot drilling insert. The insert  10  includes a tapered mounting hole that receives a mounting screw having a complementary taper. The tapered hole and screw ensure contact between mounting surfaces of the insert and the bottom and rear surfaces of the socket to consistently position the insert  10  relative to the holder  50 . The rear, side and bottom mounting faces of the insert  10  are ground to very close tolerances so that exchanging one spot drilling insert for another positions the drill point of the new insert substantially in the same location as the drilling point of the insert being replaced. This permits a fast and efficient renewal of the drill point cutting edges without adjustment or calibration of the machine tool carrying out the spot drilling operation. 
   An exemplary embodiment of the inventive spot drilling insert  10  is illustrated in  FIGS. 5 and 6 . The substantially conical drill point  12  supports two cutting edges  30  at an included angle of 60°. Two offset flutes  14  and two conical land surfaces  32  extending between the flutes define the drill point  12 . Each side of the drill point is a mirror image of the other, resulting in a rotationally symmetrical cutting tool. The flutes  14  are offset as best shown in  FIG. 6  and angled to converge toward the tip of the drill point  12 . Each flute  14  is substantially linear and includes first and second substantially planar flute surfaces F 1 , F 2  meeting at a radius R. Each cutting edge  30  is formed at the intersection of a first flute surface F 1  with the conical land surface  32 . The flute surface F 1  is substantially planar and the land surface  32  is conical, resulting in a linear cutting edge  30 . The first flute surfaces F 1  are parallel to each other but diametrically offset by a distance T to leave a web connecting the lands and supporting a chisel point  36 . A chisel point  36  extends between the radially inner ends of the cutting edges  30 . In the illustrated embodiment, the offset T is approximately 0.005 inches. The flutes diverge from each other at an angle of approximately 24° as they progress away from the chisel edge so that the web connecting the opposite sides of the drill point  12  is thickest at the intersection of the drill point  12  with the insert body  16  at the base of the drill point. A thin web at the tip of the drill point  12  supports a short chisel edge  36  with a small negative rake angle. The chisel edge  36  is oriented at an angle B of approximately 120° relative to a plane bisecting the drill tip  12  as shown in  FIGS. 6 and 7 . A chisel edge  36  of short length, angled orientation and relatively small negative rake angle combine to reduce the feed force necessary for the inventive drill point geometry to penetrate the material being drilled. 
   The first and second flute surfaces F 1 , F 2  define an obtuse included angle of approximately 140°. With reference to  FIG. 5 , the radius R at the junction of the two flute surfaces is oriented at an angle of approximately 20° with respect to the rotational axis A of the insert. The flutes  14  and their associated cutting and trailing edges  30 ,  34  cross (overlap angularly) beneath the chisel edge  36  as shown in  FIG. 7 . This results in a twisted drill point configuration that cuts in a clockwise direction of rotation also referred to as a right hand drill point. With reference to  FIG. 6 , the flutes are cut so that the first flute surfaces F 1  are oriented at an angle of approximately 7° relative to the front and rear faces of the insert body. This angular orientation ensures a small positive rake angle for the face of the cutting edge  30  (first flute surface F 1 ). During drilling, chips of the material being drilled form on the flute surface F 1  and are guided by the flute radius R and second flute surface F 2  away from the drill point  12 . The flutes  14  provide a path for chip flow away from the drill point  12  and cutting fluids toward the cutting edges  30  and chisel point  36 . 
   Manufacture of a spotting drill insert according to aspects of the present invention begins with a rectangular slab of micro-grain tungsten carbide. The slab is provided with a tapered mounting hole that will eventually receive the complementary mounting screw. The bottom, side, front and rear faces of the slab are precision ground to a particular rectangular configuration relative to the mounting hole. The calibrated carbide slab forms the rectangular insert body  16 . The flute surfaces F 1 , F 2  and the radius R at their junction are then ground, extending off one end of the insert body  12 . Each flute surface comprises two substantially planar surfaces F 1 , F 2  meeting at an angle of 140°. The radius R at the junction of the two flute surfaces is between five and ten thousandths of an inch (0.005-0.010). As best seen in  FIGS. 4 and 7 , flute surfaces F 1  are diametrically offset approximately five thousandths of an inch (0.005) at the tip of the drill point, e.g., each flute surface F 1  is offset from a center plane P of the insert body approximately 0.0025 inches beneath the chisel edge  36 . This leaves a web between the diametrically opposite portions of the drill point to support the chisel edge  36 . Flute surface F 1  is angled at 7° relative to a center plane P of the insert body. Flute surfaces F 1  and F 2  are ground so that the radius R at their junction is oriented at an angle of 20° relative to a longitudinal center line of the insert body as shown in  FIG. 5 . The angled and divergent configuration of the flutes  14  promotes chip movement away from the cutting edge  30  and the chisel edge  36 . 
   An aspect of the invention relates to providing the spotting drill insert with a hard coating of titanium nitride (TiN) or titanium aluminum nitride (TiAIN). Such coatings are known in the art to enhance the hardness and abrasion resistance of tool cutting surfaces. It has been observed that the thickness of the hard coating, though small, will define a radius covering a pre-ground cutting edge. This radius dulls the cutting edge, necessitating increased energy input for a given cutting operation which results in increased heat and reduced tool life. In the context of the present invention the hard coating is applied after grinding the flutes  14  and prior to a final grinding step which provides the cutting edge  30 . Grinding the cutting edge after hard coating leaves the hard coating on the working face (flute surface F 1 ) of the tool without covering the cutting edge  30 . This manufacturing sequence provides a clean, sharp cutting edge adjacent a hard, abrasion resistant chip forming face. Tool life is extended without impairing cutting performance. 
   After coating, the offset conical land surfaces  32  are ground to shape the drill point  12 . The coated insert with its pre-ground flutes  14  is installed in a swing jig at an angle calculated to produce center cutting edges  30  at the finished included angle of approximately 60°. The axis of rotation of the swing jig is then offset relative to the axis of rotation A of the spot drilling insert  10 . As best shown in  FIG. 8 , the offset is taken so that the cutting edges  30  are centered on the rotational axis A of the spotting drill insert but the conical land surface  32  is radially closer to the rotational axis A at its trailing edge  34 . In the illustrated embodiment, the offset is approximately five ten thousandths of an inch (0.0005). The resulting conical land surface  32  is five ten-thousandths of an inch closer to the rotational axis A of the spotting drill insert where it meets flute surface F 2  than it is at the cutting edge  30 . This small conical/radial relief is sufficient to prevent excessive rubbing contact between the land surface  32  of the drill point  12  and the material being cut. In a typical pilot drilling operation for a subsequent micro-drilling operation, only a very small axial portion of the 0.125 inch total axial length of the drill point  12  will be used. At the chisel edge  36 , the drill point has a diameter of approximately five thousandths of an inch (0.005). Thus, what appears to be a very small radial relief of five ten thousandths of an inch (0.0005) actually represents 10% of the drill point diameter at its tip. 
   The drill point geometry according to aspects of the present invention provides a small diameter, narrow point angle drill point  12  with cutting edges  30  strong enough for production runs. Experimentation has shown that the spot drilling insert according to the present invention operates best at rotational speed between 5,000 and 8,000 rpm. The feed rate for this style of insert is relatively low, between one-half and one inch per minute. However, since the spot drilling insert is used for shallow-depth spotting, this relatively slow feed rate does not significantly impede production rate. 
   While an exemplary embodiment of the present invention has been disclosed for purposes of description, it should not be deemed a limitation of the invention. Various modifications, adaptations and alternatives may occur to one skilled in the art without departing from the spirit and the scope of the present invention.