Multiple insert end mill
BACKGROUND OF THE INVENTION 
     Numerous companies have directed considerable effort toward designing small diameter long length indexable insert end mills. Small diameter end mills in particular have provided space limitations relative to the provision of flutes, gullets, insert pockets and provisions for insert retention which impose on the mass and strength of the cutter body proper. 
     In providing multiple teeth both circumferentially and axially it has been common practice, in order to derive three effective teeth circumferentially, for example in a long flute end mill, to provide six flutes in the body with adjacent inserts in each flute spaced apart axially at some distance less than the insert edge length; then, in alternate flutes, inserts are located to span the gap left between any two inserts in the adjacent flute. The result is that two flutes are required to achieve one effective flute. 
     It has also been common practice to mill helical or step flutes entirely along the axial length weakening the cross section torsional and bending strengths. In order to overcome the weakening effect of continuous helical flutes, prior art U.S. Pat. No. 4,093,392 provided individual isolated pockets for each of multiple round inserts arranged in a single circumferential helical path with staggered entry in each successive convolution involving four successive circumferentially spaced inserts thereby providing in effect four gradual helix interrupted axial flutes with intervening wall equal to the full diameter of the body extending between axially adjacent round cutter pockets. 
     U.S. Pat. No. 4,182,587 discloses a milling cutter employing multiple quadrilateral axially adjacent cutters extending along one edge of each of a plurality of continuous flutes forming a gradual helical angle. U.S. Pat. No. 4,265,574 discloses a combined boring and milling tool employing four turnable cutter blades each shaped like an equilateral triangle. Two of the blades with cutting edges extending parallel to the axis are circumferentially aligned and axially spaced, with a third blade spaced 180° circumferentially in an overlapping relation axially also having a cutting edge extending parallel of the axis, while the fourth blade has a cutting edge extending radially slightly beyond the end of the tool shank in the same plane as the third. 
     SUMMARY OF THE PRESENT INVENTION 
     The present invention employs a plurality of triangular inserts spaced axially in overlapping cutting relation in isolated pockets with chip clearing gullets which are in turn substantially isolated and arranged along a gradual helical angle extending axially of the end mill. Such pattern of axially spaced cutters is repeated at three 120° circumferentially spaced positions on the periphery of the body so that, in a typical case with the three helical rows of inserts each having a five inserts equally spaced axially in overlapping relation and circumferentially in 40° increments, three effective teeth are provided at each of the five axial positions. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a side elevation of an end mill constructed in accordance with the present invention; 
     FIG. 2 is an end view of the end mill shown in FIG. 1; 
     FIG. 3 is a fragmentary face view of a single triangular cutter insert located at the cutting end of the end mill shank. 
    
    
     DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT 
     With reference to FIG. 1 it will be understood that 15 cutter inserts are arranged in three rows with five inserts in each row having cutting edges overlapping in axial relationship and with three 120° circumferentially spaced inserts at each of the five axially spaced positions. End mill body 10 has cylindrical shank end 11 adapted for engagement in a tool holder with provision for axial retention by a screw engaging countersunk recess 12 and including drive flat 13. Cutting end 14 is provided with insert seats 15 for 15 inserts numbered 16a-16e, the progressive letters indicating progressive equal axial locations. Chip clearance gullets 17a-17e project from insert pocket shoulders 18a-18e. Each insert is retained in its pocket by a center screw typically shown at 19. 
     In a typical end mill having a 1.250&#34; cutting diameter, 15 pockets are provided for 0.250&#34; I.C. triangular inserts 0.094&#34; thick having 15° positive sides with 0.0312&#34; corner radii and &#34;iso&#34; screw holes. The pockets provide a 10° positive axial rake and 5° negative radial rake for each insert. 
     Correction angle of 0° 26 minutes, as shown in FIG. 3, together with cutting corner positioned 0.054&#34; ahead of centerline, as shown in FIG. 2, enable the cutter to cut a square shoulder within 0.001&#34;. 
     A second typical example which has been effectively tested with excellent results incorporates 0.375&#34; I.C. inserts with 0° radial rake and 20° axial rake in a 2&#34; diameter end mill. There are, of course, other variations possible within the scope of the invention. 
     From the foregoing description it will be seen that an end mill has been provided with three effective teeth for each axial location and a like number of rows of teeth, the adjacent inserts in each row having overlapping relation while insert pockets and gullets are substantially isolated to retain maximum torque and bending strength of the full diameter of cutter body.