Secondary point angles reduce the acute angles to reduce corner break down. Consequently, drill points break down most frequently at these corners. The weakest areas of the point are the outside-cutting lip corners, which are also exposed to the greatest amount of stress and travel. Figure 4 shows the fifth and sixth facets forming secondary drill point angles. The six-faceted drill with secondary point angles (SPA) is the most durable of points, but least understood and utilized. Primary facet angles are determined by the nature of the drilled material while the secondary facet angles are at 20 degrees. It is especially applicable to NC machines and can be ground on the largest drills. This point will not walk and is the most accurate of all the points. A self-centering point is produced, eliminating center punching and pilot holes. Extending the secondary facets to the midway point of the chisel produces the apex (point) at the center of the chisel's long axis, Figure 3. As a result, the chisel has outstanding extruding properties permitting less thrust, less heat, and greater productivity than the conventional drill.įour-faceted drill points consist of separate cutting lip (primary) and secondary heel clearance (relief) facets.
Furthermore, the flat ground facets produce a pronounced sharp edge on the chisel.
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When drilling a hole, the chisel's full length contacts metal, and the corners simultaneously augur into the metal immediately producing chips. Sharp, acute angled corners are formed where the chisel ends join with the web surface. In Figure 2, the chisel's profile is flat (linear), running straight across the drill.
With larger drills of the same diameter, the spread is increased accordingly. Specifically, the lip length of a l-1/2 inch-diameter drill with 140-degree included angles will measure 0.865-inch per lip and the same drill with 118-degree included angle will measure 0.956-inch length per lip, or the equivalent of drilling with a 0.182-inch larger diameter drill. This means that Figure B would drill the same size hole, but with longer lip lengths and a wider ribbon, thus creating more torque for the same size drill. Therefore, the more blunt the point, the sooner the lips are involved in full cutting action.Īlthough both A and B have the same width denoting the same drill diameter, the more blunt point of Figure A has shorter over-all cutting lip length than the one in Figure B. Maximum rate of penetration into metal occurs only when the full length of the cutting lips are involved in the material. In Figure 1-A, the full lengths of the cutting lips are involved in cutting action while, in Figure 1-B, only 64 percent of the lips lengths are involved in the same depth of parent metal. Lip cutting edge clearance angles must be adjusted to the metal's degree of hardness and machinability (generally, the harder the metal, the less required cutting edge clearance.) A compromise between high- and low-lip clearance angles, not the non-cutting point angles, is the determining factor for a general purpose cutting tool. Varying the point angles has nothing to do with cutting action. The 118-degree point, when the full length of the cutting for example, is erroneously assumed to be a good compromise or general-purpose point for drilling a variety of different metals. Drill point standards are accepted based on inflexible precedent rather than logical deduction and experimentation. The twist drill is one of the important, least understood, and most neglected of all cutting tools.
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