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High-performance Carbide Drill Bits (cont.)
High-speed drills typically use a high-temperature grade, solid tungsten carbide drill bit. In operations in which chatter may become a problem, a tool made from a finer grade of carbide could be used. Cutting tool manufacturers offer carbide grades with grain sizes of 0.0000197 in. (0.5 µm) or smaller, as compared to a more standard 0.0000984 in. (2.5 µm) grade. Carbide drill bits with smaller grains resist wear with less of a sacrifice in toughness.
Shops can also obtain solid carbide drill bits with two grades of cemented carbide sintered together. The carbide drill bit's core is a tough, high-cobalt, high-strength grade for low-speed center performance, while the outside material is a harder, low-cobalt, wear-resistant grade for high-speeds. This carbide drill bit design lets shops increase penetration rates substantially, especially for drilling stainless steel materials that normally require low feedrates. High temperatures usually rule out the use of tools with steel bodies and any carbide drill bit tips that are brazed on because there of the danger of softening the braze that holds the tip to the tool body.
Although ceramic tools exhibit greater high-temperature hardness than carbide drill tools, they must be run on machine tools that can deliver sufficient speed and rigidity. Other tool materials that exhibit 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 significantly to 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 drilling include titanium nitride (TiN), titanium carbonitride (TiCN) and titanium aluminum nitride (TiAlN).
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