Machinery's Handbook, 31st Edition
1064 TOOL WEAR AND SHARPENING The relief angle is usually measured at the periphery of the twist drill and is also specified by that value. It is not a constant but should increase toward the center of the drill. The relief grinding of the flank surfaces will generate the chisel angle on the web of the twist drill. The value of that angle, typically 55 ° , which can be measured, for example, with the protractor of an optical projector, is indicative of the correctness of the relief grinding.
Margin
Chisel Edge Angle
Point Angle
Lip Relief Angle Land
Web Thickness
Helix Angle
Standard Point
Fig. 1. The principal elements of tool geometry on twist drills.
T
C 55° Fig. 3. The chisel edge C after thinning the web by grinding off area T .
f
/2
A
Fig. 4. Split-point or “crankshaft” type web thinning.
Fig. 2. In grinding the face of the twist drill, the tool is swung around the axis A of an imaginary cone, while resting in a support tilted by half of the point angle b with respect to the face of the grinding wheel. Feed f for stock removal is in the direction of the drill axis.
Drill Point Thinning.— The chisel edge is the least efficient operating surface element of the twist drill because it does not cut, but actually squeezes or extrudes, the work material. To improve the inefficient cutting conditions caused by the chisel edge, the point width is often reduced in a drill-point thinning operation, resulting in a condition such as that shown in Fig. 3. Point thinning is particularly desirable on larger size drills and also on those which become shorter in usage, because the thickness of the web increases toward the shaft of the twist drill, thereby adding to the length of the chisel edge. The extent of point thinning is limited by the minimum strength of the web needed to avoid splitting of the drill point under the influence of cutting forces.
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