Carbide Drill Bits - High-speed Drill Bits (cont.)

Most common high-speed drill bits are high-temperature grade, solid tungsten carbide drill bits. Where chatter may be a problem, a good choice is a tool made from a finer grade of carbide. Cutting-tool manufacturers offer carbide drill bits with grain sizes of 0.0000197 in. (0.5 microns) or smaller, as compared to a more standard 0.0000984 in. (2.5-micron) grade. Carbide drill bits with smaller grains resist wear with less of a sacrifice in toughness.

Solid-carbide drill bits with two grades of cemented carbide sintered together also are available. These carbide drill bits have a core, which is a tough, high-cobalt, high-strength grade for low-speed center performance. The outer material is a harder, low-cobalt, wear-resistant grade for high-speeds. These special carbide drill bits allow shops to increase penetration rates substantially, especially for drilling stainless steel that normally requires low feedrates. High temperatures usually rule out using tools with steel bodies and brazed-on carbide drill bits because there is a danger of softening the braze that holds the tip to the tool body.

Although ceramic tools exhibit greater high-temperature hardness than carbide, they must be run on machine tools that can deliver sufficient speed and rigidity. Other tool materials exhibiting high wear resistance include polycrystalline diamond (PCD) and cubic boron nitride (CBN). However, these materials are expensive and, like ceramics, must run on high-performance machine tools.

Thin-film coatings also can contribute to significantly longer tool life, greater wear resistance, and faster operating speeds and feeds. Coatings increase surface hardness, reduce friction and heat buildup, and increase resistance to edge buildup, galling and fissure propagation. Common coatings for high-speed drill bits include titanium nitride (TiN), titanium carbonitride (TiCN), and titanium aluminum nitride (TiAlN).

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