Abstract:
A cutting insert for a during includes a broken main cutting edge, the ends of which join respective additional edges of the insert by curved nose portions. Each curved nose portion forms a curved cutting edge defined by at least three sequentially arranged curved cutting edge segments defined by respective radii of different sizes. A first of the segments joins one end of the main cutting edge and has a first radius. The next segment has a radius of different length than the first radius and projects farther laterally than the first segment to perform a wiping function.

Description:
[0001]    This application is a continuation-in-part of International Application No. PCT/SE99/01776 filed on Oct. 5, 1999, which International Application designates the United States and was published by the International Bureau in English Apr. 20, 2000. 
     
    
     
       BACKGROUND OF THE INVENTION  
         [0002]    The present invention relates to a cutting insert for metal drilling. The insert is intended to be secured into a drill body that can be equipped with several identically shaped inserts. The drill body is normally provided with chip flutes, an inner insert site disposed close to the drill center and an outer insert site disposed close to the periphery. This type of drill has cylindrical basic shape defined by a diameter and is rotatable around a central axis.  
           [0003]    A drill of this type is disclosed in EP-A-1 81 844 (corresponding to Karlsson et al. U.S. Pat. No. 4,648,760). The inserts therein are basically of rectangular shape where the short end surfaces are confined by two short cutting edges which intersect one another at an obtuse angle, wherein the radially outer insert is located such that it defines the diameter of the drilled hole. The longitudinal axis of the radially inner insert extends parallel with the center axis of the drill whilst overlapping the same.  
           [0004]    When providing drills of the aforementioned type it is often difficult to achieve desirable small diameters and at the same time achieve desirable surface finish when drilling in metallic workpieces. Thus, a primary object of the present invention is to provide a new type of cutting insert for drills that enables an improved surface finish to be achieved when drilling in metal. Another object of the present invention is to provide a new type of insert for drills with such optimized geometry that improved “feel” is possible during drilling without simultaneously impaired surface finish of the hole wall in the hole to be drilled.  
         SUMMARY OF THE INVENTION  
         [0005]    These and other objects are achieved by the insert of the present invention, which comprises a body having a polygonal shape with substantially flat upper and lower surfaces, and edge surfaces adjoining the upper and lower surfaces. The lower surface has a smaller area than the upper surface to provide the insert with a positive cutting geometry. A first of the edge surfaces intersects the upper surface to form a main cutting edge. The main cutting edge comprises short cutting edge portions converging from respective ends of the cutting edge and forming an obtuse angle with one another. An end of the first edge surface joins a second one of the edge surfaces by a curved nose portion. The curved nose portion intersects the upper surface to form a curved cutting edge defined by a plurality of sequentially arranged curved cutting edge segments having respective radii. The radii have different respective lengths. 
       
    
    
     BRIEF DESCRIPTION OF DRAWINGS  
       [0006]    The invention will be described in further detail in connection with the following drawings showing preferred embodiments of the invention:  
         [0007]    [0007]FIG. 1 shows a side view of the cutting portion of a drill shank intended to be equipped with an insert of the invention.  
         [0008]    [0008]FIG. 2 shows a perspective view of the front portion of a drill equipped with inserts of the invention.  
         [0009]    [0009]FIG. 3 shows a side view of a drill with inserts as in FIG. 2.  
         [0010]    [0010]FIG. 4 shows a sectional view along the line IV-IV in FIG. 3.  
         [0011]    [0011]FIG. 5 shows an enlarged view of a nose portion of a drill insert of the invention.  
         [0012]    [0012]FIG. 5A is an enlarged view of FIG. 5.  
         [0013]    [0013]FIG. 6 shows an enlarged side view of a nose portion of a drill insert according to an alternative embodiment of the invention.  
         [0014]    [0014]FIG. 6A is an enlarged fragmentary view of FIG. 6.  
         [0015]    [0015]FIG. 7 shows a drill shank with a drill insert according to an alternative embodiment of the invention, and  
         [0016]    [0016]FIG. 8 shows an enlarged plan view of a drill insert according to FIG. 5. 
     
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS  
       [0017]    FIGS.  1 - 3  show a drill comprising a cylindrical drill shaft  10 , the front end of which is provided with two radially spaced insert sites  11  and  12  located on respective sides of the center line CL of the drill.  
         [0018]    The drill shank  10  has straight or spirally shaped passages  13 ,  14  for the flow of chips next to respective ones of the insert sites  11 ,  12 . The drill shank  10  also has channels  15  for transporting cooling medium to the front cutting end of the drill.  
         [0019]    The insert sites  11 ,  12  are provided to receive two inserts  16 ,  17  which are centrally apertured and intended to be secured by a central screw  18  that is threadably engaged in the drill shank  10 . One insert  17  is a central (inner) insert and the other is a peripherical (outer) insert  16  obliquely arranged in relation to the center line CL such that it defines the size of the drilled hole. There remains a waist portion M of the shank situated between said outer and inner inserts and having such width that sufficient strength is achieved. The drilling diameter of the drill is defined by diameter D. Each insert site comprises a tangential support surface  19 , an axial support surface  20  and a radial support surface  21 .  
         [0020]    Each insert has polygonal basic shape, in this case a rhomboidal basic shape, with two opposed planar parallel flat lower and upper surfaces  22  and  23 , which are joined by four edge surfaces  1 ,  2 ,  3 ,  4  corresponding to short and elongated sides of the rectangle. The lower surface  22  of the insert is smaller than the upper surface  23  such that the insert has a positive clearance angle. The intersection lines between the upper flat surface and the two opposing ones of the end surfaces  1 ,  4  represent main cutting edges  24 ,  25 , wherein each main cutting edge is angularly broken at an obtuse angle at the middle thereof, said obtuse angle designated δ. Adjacent pairs of the edge surfaces  1 - 4  are joined to one another by respective curved nose portions which intersect the upper surface  23  to form curved corner cutting edges  26 - 29 . Alternatively, instead of having a four-sided shape, the insert could have so called trigonal shape, i.e. a basically triangular insert with angularly broken edge surfaces. The largest width W 1  of the insert is measured across either of the main cutting edges from the tip of one curved cutting edge  26  (or  28 ) to the other  27  (or  29 ). A smallest width W 2  of the insert is defined by the shortest spacing between the edge surfaces  2  and  4 .  
         [0021]    As appears from FIG. 3 both the outer insert  16  and the inner insert  17 , due to their inclined rhomboidal shape, are inclined in relation to the central axis. The central insert extends axially ahead of the outer insert so as to improve the control of the radial forces such that the intended diameter can be kept. The outer insert  16  is inclined at an angle α and the inner insert at an angle β such that α&lt;β. The insert could, alternatively, have straight rectangular basic shape in accordance with the embodiment described in Swedish Patent 455 676 (corresponding to Karlsson et al. U.S. Pat. No. 4,648,760), which means that the insert could be considered as consisting of two regular triangles having broken sides whereby two of its corners and one side of said triangle coincide with the cutting edge of the insert and the third corner of each triangle overlap each other with a distance that corresponds with the straight portion of the longitudinal sides of the insert, said distance being 0.4-0.8 mm times the total length L of said insert. In such case the insert obtains an elongated polygonal basic shape such that the upper and lower flat surfaces are joined by two short and two elongated edge surfaces where cutting edges are provided along portions of the intersection lines between the short edge surfaces and the upper flat surface. In that case the short edge surfaces are also broken at the middle of the cutting edges so that each cutting edge appears at an inner obtuse angle α at said broken point.  
         [0022]    In accordance with the invention each corner cutting edge  26 ,  27  is defined by a plurality of circular segments of radii R 1 , R 2  and R 3  as shown in enlarged views in FIGS. 5 and 5A. Adjacent ones of the radii are of different length. In particular, the radius R 2  shall always be larger than R 1 , which contributes to a surface finish improving “wiper” effect along the secondary edge  30 . With reference to FIG. 5A, the center C 2  of radius R 2  lies on a line D which passes through: (i) a terminal end T of the segment defined by the radius R 1 , and (ii) the center C 1  of radius R 1 . The cutting edge segment defined by the radius R 2  extends farther laterally outwardly (i.e., farther to the left in FIG. 5A) than the cutting edge segment defined by radius R 1 . The insert is positioned in the drill bit to maintain that relationship, i.e., such that the cutting edge segment defined by radius R 2  extends farther than the segment defined by the radius R 1  in a direction perpendicular to the axis of rotation of the bit. Thus, the cutting edge segment defined by R 2  will perform a wiper effect. This applies both during machining by axial drilling forwards into the workpiece as well as during axial displacement of the drill in the opposite (reverse) direction when the drill is being withdrawn from the bore. The radius R 3  is somewhat smaller than each of R 1  and R 2  and its center C 3  lies on a line E extending through the center C 1  and the terminus of the segment defined by radius R 2 . When using the idea of the invention on an inclined rhomboidal insert as shown in FIG. 3, the size of radiused edges R 1 , R 2 , R 3  of diagonally opposed nose portions  26  and  29  that define an outer diameter of the bore when in an active cutting position should be somewhat larger than corresponding radiused edges provided at the other pair of diagonally opposed nose portions  27 ,  28 .  
         [0023]    In FIG. 7- 8  are shown an insert and a drill tool equipped with an insert according to an alternative embodiment of the invention. As appears therefrom, the insert has in this case a regular square shape where each curved corner cutting edge transforms into a straight edge surface  31  that is inclined inwards towards the insert which at an obtuse angle γ meets at the middle of the insert with an edge surface  32  that extends straightly from the opposite corner such that a distinguished waist portion is provided on the insert along each of the four edge surfaces. The appearance of the nose radius surface at each corner portion is, in all other aspects, identical with the appearance shown in FIG. 5 comprising a plurality, in this case three circular segments of different radii R 1 , R 2 , R 3 .  
         [0024]    In FIG. 6 there is depicted an enlarged portion of an alternative embodiment of the invention where the nose  26 ′ defines a corner cutting edge comprising four different radii circular segments of radii R 1 , R 2 , R 3  and R 4 .  
         [0025]    The radius R 1  of the segment located closest to the main cutting edge  24  should be shorter than the radius R 2  of secondary edge  30 . Further, there should be provided one radiused edge with a radius R 4  shorter than the radius R 1  and provided between the radii R 1  and R 2 . With reference to FIG. 6A, the center of radius R 4  lies on a line F which passes through: (i) a terminal end T′ of the segment defined by the radius R 1 , and (ii) the center of radius R 1 . Likewise, the center of R 2  lies on a line G which passes through; (i) a terminal end T″ of the segment defined by the radius R 4 , and (ii) the center of radius R 4 . The center of radius R 3  lies on a line H which passes through: (i) a terminal end of the segment defined by the radius R 2  and (ii) the center of radius R 2 .  
         [0026]    In order to achieve optimized chip control, the radii of the various radiused edges ought to lie in the following ranges:  
         [0027]    R 1 : 0.2-1.6 mm  
         [0028]    R 2 : 1.6-10.0 mm  
         [0029]    R 3 : 0.1-1.0 mm  
         [0030]    R 4 : 0-1.0 mm  
         [0031]    As regards radii R 1  and R 2  the choice of dimensions depends on which drill diameter shall be used. For drills in the diameter range 13-18 mm the radius R 1  should amount to 0.2-0.4 mm and R 2  should amount to 1.6-4.0 mm. For drills in diameter range 18-58 mm the radius R 1  should be 0.4-1.6 mm and radius R 2  should then be 2.5-10 mm.  
         [0032]    The length of secondary edge segment  30  should have a value in the range 0.1-0.5 mm.  
         [0033]    Although the present invention has been described in connection with preferred embodiments thereof, it will be appreciated by those skilled in the art that additions, modifications, substitutions and deletions not specifically described may be made without departing from the spirit and scope of the invention as defined in the appended claims.