Abstract:
The cutting rate of a spade bit is greatly improved by the provision of radial flutes on the lateral shoulders of the bit, extending transversely to the bit axis. Further improvements are obtained if the shoulder flutes are stopped by spurs positioned at the outer ends of the shoulders.

Description:
FIELD OF THE INVENTION 
     This invention relates to woodworking tools and more particularly to so-called &#34;spade bits,&#34; which are generally used with an electric drill for drilling large diameter holes in wood. 
     BACKGROUND 
     Spade bits (sometimes also known as &#34;fly cutters&#34;) are widely used for boring holes between approximately 1/4&#34; and 11/2&#34; diameter in wood and similar soft materials, because they are fast, true cutting, resharpenable, and relatively inexpensive. Their name derives from the shape of the spadelike blade or cutter, in contrast to the spiral shape of augers and twist drills. The blade, which is usually forged integrally from the shank, is relatively thin and flat and may have a width several times the diameter of the shank. 
     Robinson U.S. Pat. No. 2,782,824, issued Feb. 26, 1957, to the assignee of the present application, shows a bit of this general type, having a flat blade with straight outer sides extending parallel to the axis, shoulders which extend radially inwardly from the outer sides, and a sharp triangular central point which projects from the shoulders along the direction of the axis. The point centers and steadies the blade with respect to the workpiece and prevents it from wandering as it cuts; the shoulders cut the hole to the diameter of the outer sides; and the outer sides serve to stabilize or center the blade when it cuts through the workpiece. The outer sides, shoulders, and point are all slightly beveled along the edges to provide a &#34;positive rake&#34; (i.e., a slightly acute angle, typically about 86°, with the blade surface) for cutting. On each cutting or leading edge of the point (the two diagonally opposite edges in the direction of rotation) a groove or flute is formed which extends back to and somewhat beyond the shoulder, onto the flat face of the blade. These point flutes extend parallel to the cutting edges of the point; one flute is formed on each surface of the point. These flutes are nearly parallel to the axis and assist in the upward removal of chips cut by the point. This enables the point to penetrate and cut the wood with less force and thrust than would otherwise be required. 
     In use, spade bits of relatively small diameters, approximately 1/2&#34; or less, penetrate relatively quickly, for a given loading but for blade diameters above approximately 3/4&#34;, the rate of cutting is markedly slower. Moreover, as the blade emerges from the opposite (or back) side of the workpiece, having cut through it, the centering function of the point is progressively lost and the blade is then guided only by its outer sides. As the blade shoulders emerge on the opposite face of the workpiece, splitting and tearing tend to occur, with increased likelihood of jamming and damage to the tool and the workpiece. 
     It is known to form a forwardly projecting sharp cutting foot on auger bits, see for example, Kronwall U.S. Pat. No. 2,627,292. However, such forwardly projecting cutting edges require an increase in the thickness of the blade at the cutting edge, which in turn greatly increases cost of auger bits relative to that of a spade-type bit wherein the blade is forged to a uniform thickness. 
     BRIEF DESCRIPTION OF THE INVENTION 
     In attempts to improve cutting rate, experiments were made with sharper bevel angles on otherwise conventional spade bits. It was found that the provision of sharper (more acute) cutting edge on the shoulders would increase cutting rate, but also tended to cause the blade to pull itself rapidly into the wood and to cause jamming. Moreover, serious splitting occurred as the blade cut through the workpiece. 
     In an effort to overcome the problems resulting from use of greater shoulder rake, we have now discovered that the rate and smoothness of cutting are dramatically improved by providing essentially radial flutes on the spade portion of the bit, adjacent and parallel to the cutting edges of the shoulders. These flutes should run essentially perpendicularly to the axis of the bit, and thus perpendicularly (rather than parallel to) the direction of blade movement into the workpiece. These shoulder flutes should intersect the point flutes, and should extend toward but preferably not to the outer sides of the blade; that is, the shoulder flutes should terminate just inward of the outer corners of the shoulders. It is especially preferred that each shoulder flute be stopped or closed by a sharp cutting spur which extends from the outer corner of the shoulder, parallel to the blade axis. 
     Test have demonstrated that the provision of these shoulder flutes will in some cases more than double cutting rate, but without the splitting or tearing that accompanies use of a sharper shoulder bevel. The reason for their effectiveness is not fully understood, because they expand perpendicularly to the axis of the cutter, that is, transversely rather than parallel to the shank, whereas the movement of the chips is essentially axially or diagonally outward along the shank. In contrast to point flutes, which run parallel to the direction of chip movement and thereby facilitate the flow of movement of chips away from the cutting edge, the shoulder flutes run crosswise and thus might be thought not to assist, or to disrupt rather than facilitate, the flow of chips. 
     In tests we have found that such shoulder flutes, if extended to the outer sides of the blade, can cause tearing or ripping where the blade emerges from the workpiece; the cutting proceeds by a shearing action which propagates splits outwardly beyond the blade. However, we have found that if the flutes are &#34;stopped&#34;, that is, if they do not extend or &#34;open&#34; to the respective outer sides of the blade, and if cutting spurs are provided at the outer corners of the shoulders so that the spurs &#34;close&#34; the flutes, then this splitting is virtually eliminated and cutting proceeds more smoothly, with even less tearing where it cuts through the workpiece than with conventional spade bits of the type shown in the Robinson patent. 
     Thus, the provision of flutes on the lateral cutting edges of the shoulders does not by itself provide the effect that is obtained by the combined use of shoulder flutes and shoulder spurs. In other words, the combination of the shoulder flutes and the spurs together provides much better results than can be obtained by either alone. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a plan view of a bit having shoulder flutes and spurs in accordance with a preferred form of the invention; 
     FIG. 2 is an enlarged plan view of the cutting end of the bit; 
     FIG. 3 is an end view of the bit of FIG. 2; and 
     FIG. 4 is an enlarged longitudinal crosssection taken on line 4--4 of FIG. 2. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The drawings show a spade bit 10 in accordance with a preferred embodiment of the invention, having a generally cylindrical shank 12 with a hexagonal end 14 by which the bit can be gripped in an electric drill. A blade or spade 16 is formed at the opposite end of shank 12. Typically but not necessarily, blade 16 is forged integrally from the same piece as shank 12. Blade 16 is substantially flat with parallel opposite large faces 18 and 20, and is thinner and wider than the shank. The blade may be up to about four times wider than the diameter of the shank. 
     The blade may contain a hole 17 on the axis, by which it may be hung, as from a nail, for sale or storage. 
     When the shank is rotated at cutting speed, cutting occurs on cutting edges of the forwardly moving faces of the blade. As shown in FIG. 3, the blade is rotated counterclockwise in the direction of the arrows, so that cutting occurs on the left edges of the upper half of blade 16, and on the right edges of the lower half of the blade. The outer sides 22 of the blade are beveled, forming angles of about 86° with the respective blade surfaces 18 and 20, to provide sharp outer cutting edges 24. 
     The edges has radial shoulders 26, 26, which extend inwardly from outer sides 22, toward the axis 25. In practice these shoulders should be perpendicular, or nearly perpendicular, to the axis. The cutting edge of each shoulder is beveled with respect to the plane of the face of the blade, suitably about 86° (see FIG. 4). 
     A point 30, having converging side edges 32, 32, extends from between shoulders 26. The sides of point 30, meet at a tip 34. The faces of point 30 may be coplanar with the surfaces of the blade, that is, the point is no thicker or thinner than the blade and is not necessarily tapered. It has a sharp tip. 
     As shown in the Robinson patent previously referred to, point 30 may have a point flute or groove 36 adjacent each cutting edge thereof, extending parallel to the edge from tip 34 to and slightly beyond the intersection with shoulders 26, 26 (as shown in FIG. 2). These flutes are shown by dotted lines in FIG. 3 and may have a leading or outer surface portion 38 which forms an acute angle with respect to the beveled side 32 of the point. An inward (closer to the axis) point flute surface 40 defines the radially inward side of the flute. Each flute is generally parallel to side 32 of the point, and its outer surface 38 meets and forms a cutting edge with the point side 32. The cross-sectional width of the flute may increase in the direction toward the shoulder; or, as shown in FIG. 2, it may be of uniform width. 
     In accordance with this invention, a shoulder flute 44 is provided adjacent the cutting edge of each shoulder. This flute preferably has a cross-sectional configuration as shown in section in FIG. 4. This apparently acts to &#34;curl&#34; chips upwardly onto the blade surface. More specifically, the shoulder flute 44 may have an outer portion 46 and an inner surface portion 48. The outer portion joins the leading edge of the shoulder at an acute angle which is less than about 78°, and preferably is about 68° (see FIG. 4). The shoulder flutes join the respective point flutes 36. The leading edge of the shoulder need not project beyond or outwardly of (in the direction of rotation) the plane of blade surface. 
     It will be noted in FIG. 2 that the shoulder flutes need not, and preferably do not, extend all the way to the outer sides of the blade. The flutes are preferably stopped at ends 52, inwardly of the sides of the blade. We have found it very advantageous to provide a cutting spur 56 which projects parallel to the axis, at the corner or intersection of each shoulder and the respective outer side of the blade. This spur has an outer side which is essentially a continuation of the outer side of the blade. Its inside edge is beveled, as at 58, so that the spur presents a sharp or knife edge which scores and then cuts the workpiece at the periphery of the bore. This helps to hold the blade centered and greatly reduces chipping or tearing. 
     The flutes and flute ends (both the point flutes and the shoulder flutes) may be formed by milling or griding, but preferably are formed by forging. 
     Surprising results are obtained by providing shoulder flutes in accordance with the invention; and still greater benefit is obtained by providing both shoulder flutes and spurs in accordance with the preferred embodiment. This is shown by the following comparative tests. 
     I. Drilling Rate 
     Bits having both shoulder flutes and spurs in accordance with the preferred embodiment, of various sizes in the range of 9/16&#34; to 11/4&#34; diameter, were tested against bits of the type shown in Robinson U.S. Pat. No. 2,782,824, of the same sizes. The latter bits differed from those of the invention in that they had neither shoulder flutes nor spurs. The bits were compared, in standardized tests on a drill press, by drilling under a constant load into oak. 
     
         ______________________________________                             Rate                             Improvement                       Drill of Inven-Bit              Test       rate, tion OverDiameter   Bit      Duration   in/sec.                             Prior Art______________________________________9/16&#34;   Prior Art             5 sec.    0.17  41%   Invention             5 sec.    0.245/8&#34;    Prior Art             5 sec.    0.19  16%   Invention             5 sec.    0.227/8&#34;    Prior Art            15 sec.    0.16  31%   Invention            12 sec.    0.211&#34;      Prior Art            15 sec.    0.11  109%   Invention            12 sec.    0.2311/4&#34;   Prior Art            15 sec.    0.07  86%   Invention            15 sec.    0.13______________________________________ 
    
     As can be seen, in each size the new bits bored much faster; the improvement varied but in general increased dramatically with diameter. 
     II. Effect of Spurs 
     The combination of the shoulder spurs with shoulder flutes provides advantages over the use of shoulder flutes alone, without spurs. This is shown by the following drilling rate test: 
     
         ______________________________________                              Rate                        Drill Improvement              Test      Rate, OverSize  Bit          Duration  in/sec.                              Prior Art______________________________________9/16&#34; Point flutes only              10 sec.   .22 Shoulder flutes              10 sec.   .24    9% Shoulder flutes              10 sec.   .27   23% and spurs1&#34;    Point flutes only              10 sec.   .13 Shoulder grooves              10 sec.   .20   54% Shoulder grooves               9 sec.   .30   131% and spurs______________________________________ 
    
     As can be seen, the combination of the spurs with the shoulder flutes more than doubles the improvement over the prior art use of point flutes alone, and in the larger size the prior art rate is more than doubled. 
     III. Tear-out 
     In addition to improving drilling rate, two further advantages are obtained. Where the bit is used to bore all the way through a workpiece, the provision of the spurs produces a clean, crisp exit from the wood, whereas the prior art bit exits from the workpiece with severe splintering and tearing. 
     IV. Wear Rate 
     It is especially surprising that the spurs greatly reduce the wear rate of the new bits; the new bits perform substantially longer without &#34;burn out.&#34; This is difficult to quantify precisely, but one of the new bits continued to bore effectively after three prior art bits had burnt out and lost their cutting edges.