Abstract:
The cutting insert disclosed is a block of hard metal configured on at least one of its major faces to provide peripheral cutting edges successively index able about a central fastener location to position each edge serial for cutting. The major face slopes downwardly and inwardly from the cutting edges to provide rake surfaces of positive rake behind the cutting edges, and then rises as a sloping ledge to a central boss. A series of depressions disposed along the rake surface produce a ribbed chip which breaks more readily, and into smaller chips, easier to flush or otherwise remove from the cutting site.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application is a continuation of prior application Ser. No. 09/256,514, filed Feb. 23, 1999, now abandoned, which is hereby incorporated herein by reference in its entirety. 
    
    
     FIELD OF THE INVENTION 
     The invention disclosed and claimed relates generally to index able hard metal cutting inserts for ductile metals. In particular, it concerns the configuration of the rake face of the insert to cut a chip from a workpiece of ductile material in a manner such as to cause the curling chip to break off in a short curl over a practical working range of feed rates, i.e., chips both thin and thick, to facilitate their ready removal from the cutting site. 
     BACKGROUND OF THE INVENTION 
     In general, the interruption of the rake face of cutting inserts with laterally spaced depressions in the chip path from the cutting edge has been appreciated for its ability to render chips removed from ductile work material more readily breakable. The effect of properly placed depressions is to produce longitudinal ridges in the chip while it remains in a somewhat plastic state due to the high temperature produced by the velocity of the forceable shearing of the chip from the workpiece. The ridges, in turn, stiffen the chip against bending and hasten the attainment of a sufficiently high level of bending stress or strain at a point in the chip remote from the cutting site to cause it to break off. 
     The effect varies with the flow properties of the material, and with the cutting speed and feed rate, which influence the strain rate and heat generated in the chip as well as determining its thickness. 
     The object of this invention is the provision of an insert, suitable for use in drills and other applications, that will break chips of ductile materials into short lengths over a practical working range of feed rates and chip thicknesses. 
     SUMMARY OF THE INVENTION 
     The cutting insert of the invention provides a rake surface of positive inclination when mounted in a drill body. The rake surface recedes from the cutting edge to a sloped ledge of significant height, while the rake surface proper between the cutting edge and the ledge is interrupted by a series of spaced depressions along and adjacent to the cutting edge in the path of chip flow. The depressions serve to corrugate and curl a chip thin enough to follow their contour as it emerges from the cutting edge, while the ledge behind them serves to curl thicker chips which are stiff enough to override the depressions. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention is explained in the following description of a preferred embodiment shown in the following drawings, in which: 
     FIG. 1 is an elevational view of a drill employing the cutting insert of the invention; 
     FIG. 2 is a bottom end view thereof; 
     FIG. 3 is an enlarged isometric view of the insert; 
     FIG. 4 is a plan view of the same; 
     FIGS. 5 and 6 are fragmentary sectional views of the insert taken on the lines  5 — 5  and  6 — 6  of FIG. 4, respectively; and 
     FIG. 7 is an enlarged fragmentary elevation of the drill of FIG. 1, in the process of generating the corrugated form of chip produced by the insert of the invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring initially to FIGS. 3 to  6  of the drawings, the cutting insert  10  of the invention in preferred form is a modified parallelepiped, namely a square block having two major faces, an upper or front face  12  configured to provide the peripheral cutting edges  14  and the associated rake surfaces  16 , and a parallel plane seating face  18 , both bounded by identical sloping, minor side surfaces  20 . The insert is pierced by a central hole  22  on an axis perpendicular to the major faces  12  and  18 . 
     The upper or front face  12  of the insert recedes from peripheral cutting edges  14  as the downwardly sloping rake surfaces  16 , and then rises as a sloping surface at approximately forty-five degrees (45°) to the plane of an elevated central boss  24  from which the central hole  22  is countersunk to seat the customary screw  21  to secure the insert in a suitable pocket in a fluted drill body  23 . The downward and inward slope of the rake surface  16  (FIGS. 5 and 6) provides positive rake with respect to the planar boss  24  and the parallel seating surface  18 . 
     The identical minor side surfaces  20  of the block taper inwardly from the periphery of the upper major face of the insert to the lower seating face  18 . This taper proceeds in two steps, a narrow upper band  26 , at a smaller angle to a perpendicular to the lower face, and a broader band  28  at a somewhat greater angle to that perpendicular. The intersections of the side and bottom surfaces of the insert are relieved by a slight chamfer. The narrow band  26  and the broader band  28  serve respectively as primary and secondary clearance surfaces for the cutting edges  14  of the insert on the periphery of its upper major face  12 . 
     In the preferred form illustrated, each cutting edge  14 , formed at the intersection of the narrow band  26  of the side surface and the receding upper rake surfaces  16  of the insert, is essentially linear, and may be slightly blunted by a cutting edge land  30 . 
     Behind each cutting edge  14  is a series of depressions, or “dimples”,  32  in the rake surface  16 , five behind each cutting edge in the illustrated case. These depressions in preferred form are spherical, their spacing being derived as though a line of imaginary tangent contacting spheres of equal size had been impressed into the rake surface  16  to a depth of forty percent of the spherical radius, sufficient to crater the rake surface from the cutting edge  14  to the base of the upwardly sloping peripheral ledge surfaces  34  of the elevated central boss  24  on the upper major surface  12  of the insert In terms of relative dimensions, the depressions  32  occupy approximately forty percent of the area of each rake surface  16 . The height of said ledge surfaces relative to the cutting edge is at least as great as the depth of the depressions relative to the cutting edges. 
     The spaced spherical depressions  32  of the receding peripheral rake surface  16  of the upper major face  12 , transform that peripheral surface into a composite rake surface of varying rake angle along the cutting edge  14 , with a greater positive rake angle in the depressions  32  (FIG. 5) than in the residual rake surfaces  16  between them (FIG.  6 ). The result is the formation of a chip  40  (FIG. 7) which is corrugated or ribbed, with the ribs  42  between adjacent flutes  44  formed by the residual rake surfaces between the depressions. 
     At the four corners of the insert  10 , the junctures of the several peripheral cutting edges  14  are rounded off, and the immediately adjacent corner rake surface  46 , distinguished from the positive rake surfaces  16  by its relatively neutral rake angle, strengthens the cutting edge at the corner. That feature is conventional practice rather than a novel aspect of the invention. When used for drilling, it benefits the radially innermost insert particularly, upon which the relatively lower cutting speeds result in greater cutting-force reaction. 
     Typically, two such inserts are mounted in the drill body  23  (FIG. 1) so as to sweep overlapping cutting paths as the drill is fed into a workpiece,  48  (FIG.  7 ). FIG. 7 shows a chip  40  being formed by the insert nearer the center of rotation, where the cutting speed is zero at the center of rotation and increases linearly along the cutting edge, resulting in a distinctly conical chip  40 . 
     When the feed of the drill  23  into the workpiece  48  is light, say of the order of a cutting depth of six thousandths inches, the chip tends to follow the curvature of the depressions  32  in the rake surface  16 , and to curl readily as it passes through those depressions. 
     Under conditions of higher feed rates, say at ten thousandths inches or greater, the thicker chip tends to pass over the depressions  32  of the rake face with little conforming thereto. Such thicker chips strike the sloping ledge  34  of the central boss  24  standing as a continuous barrier to the progress of the chip and deflecting it away from the rake surface. The resulting bending effect upon the chip, in its hot and relative plastic state upon being progressively sheared from the workpiece, causes the thicker chip to curl as well, although typically not as tightly as the thinner chip of lighter feed. 
     The ribbed effect of the composite rake surface upon the chip is produced in chips both thin and thick, and promotes the breakage of the chip sooner than would be the case with a continuous uninterrupted rake surface from end to end of the cutting edge. The time required for a ribbed chip to reach ultimate material strain is less than for an unbarbed chip, resulting in smaller chip curls, i.e., fewer turns, than with an unbarbed, non fluted chip. 
     For example, at a feed of 0.006″ per revolution of the drill, the ribbed chip produced by the composite or “dimpled” rake surface broke off after one-and-one-half convolutions, typically, of its conical helix configuration, whereas those produced by an otherwise identical insert of the same size operated at the same speed and feed, but having no dimples, typically produced a chip of more than three turns of the conical helix before breaking off. 
     The consistently smaller chips produced by the insert of the invention are more easily flushed from the hole by the cutting liquid issuing from the tip of the drill, greatly reducing the incidence of the jamming of the drill from failure to break off the chip for easy removal. 
     The preferred embodiment, illustrated as an essentially square block, is preferred for its ruggedness in the drilling context, as well as for the multiple cutting edges it provides, as the insert is successively indexed about the axis of its central hole  22  to present each of the cutting edges in the active position as its predecessor, wears to an unacceptable degree, reflected usually by excess power consumption. However, it is also feasible to press the insert in double-sided configuration providing eight cutting edges in a square-block insert, and wherein the central boss  24 , duplicated on the reverse side of the insert, becomes the seating surface of the insert in the alternate position which presents the identical cutting edges of the opposite face for cutting service. Heeling clearance behind the cutting edges of both faces, in such cases, requires a more forward leaning attitude of the active face of the insert, which may to some extent be compensated by a larger positive rake angle of the peripheral rake surfaces  16  of both faces of the insert. 
     Those skilled in this art will recognize that the principles illustrated and explained are adaptable as well to other polygonal shapes, and to other cutting applications. 
     The features of the invention believed new and patent able are set forth in the following claims.