Effect of point geometry and dimensional symmetry on drill performance
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Because the chisel edge (the edge across the web of the drill connecting the inner ends of the main cutting lips) is virtually a straight line perpendicular to the drill axis, the conventional chisel-point drill (Fig. 1) has no true point. Consequently the drill has no selfcentering action and has a tendency to walk to one side or the other. This causes the hole to start at other than its desired location, to have a bellmouth, to be out of round, to be crooked, to have poor finish, and to be more oversize. It is common practice then, to use a center-punched or center-drilled hole to provide a means for starting the drill on its desired location and to use guide bushings for holding the drill in line. Also, because of the chisel edge, the cutting action is extremely poor near the center of a conventionally ground drill, due to the very large negative rake in this region. An illustration of this is shown in Fig. 2 which is a photomicrograph of a section taken perpendicular to the chisel edge at a radius of 0.030 in. from the center of a 1⁄2 in, chisel point drill, while in the act of forming a chip. As shown the rake angle at this point is about minus 56 degrees. Because of the low cutting speed near the drill center the high negative rake angle is particularly bad since it has been previously shown [1] that high cutting speed is necessary for good chip formation when a cutting tool has negative rake. Furthermore, as the machined metal is virtually trapped between the tool face and the bottom of the hole, its escape sideways into the flute is very difficult. As a result, the action in this area is more an extruding action than a free cutting action. The thrust force in penetrating the work is therefore very high and a great deal of heat is generated. This also may result in excessive workpiece distortion. In view of these limitations of the conventional drill, research studies were undertaken, aimed at the elimination of the chisel edge. As a result of this research, a new drill point geometry and a practical method for applying the geometry to the common twist drill have been developed. Figure 3 shows the drill point configuration which has been developed as a result of this investigation. This point shape is produced, in a single grinding operation, by a mechanism which provides a controlled relative movement of the grinding wheel and drill, axially, radially, and circumferentially, all in properly timed relationship to the flutes of the drill. As indicated by the name "spiral point", the entire flank surface extended rearwardly
[1] S. A. Tobias. Machine tool vibration research , 1961 .