High-performance Drill Bits (cont.)

TiN is a cost-effective, universally applicable coating that increases tool hardness to over 80 Rc. TiCN is a multilayer structure that inhibits surface fractures from propagating to the tool or the wear part substrate. It works best for machining hard materials and in high-shock applications such as interrupted drilling cuts. TiAlN provides extra hardness and heat resistance for machining such abrasive materials as cast iron and high-silicon-content aluminum alloys. This coating performs well in high thermal-stress conditions, such as in dry and near-dry machining along with deep and small-hole drilling where cutting fluids have difficulty penetrating.

In addition, proprietary coatings are available that manufacturers claim provide all the advantages of these three standard coatings. For example, combination coatings with a hard and soft layer can enhance machining capabilities. The soft layer, a lubricity coating, optimizes chip evacuation along the flutes and from the hole by eliminating edge build-up.

Near-dry/dry high-speed drilling

High-speed drilling and dry drilling have a lot in common. For both processes, the key is protecting the tool from heat. For machining with exposed edges (turning and milling), dry cutting is more easily accomplished than in the enclosed confines of a drilled hole. When cutting edges are exposed, chips leave the cutting zone quickly and have little contact with the workpiece or the tool. Both remain relatively cool. But in drilling, cutting edges are subjected to high temperatures that arise from the cutting process and hot chips. The effectiveness of dry drilling varies considerably from metal to metal. Cast iron is the material most often dry machined. Carbide is the tool material typically used for cutting dry. Ceramic tooling also is appropriate because it retains its hardness at high temperature and runs without coolant or lubricant. However, it can be used only with materials that form small, easily managed chips. Ceramic tools are brittle and require close control of runout.

When eliminating a high-pressure coolant stream, some provision must be made for lubrication and chip removal. A soft lubricant coating, applied over the tool's hard coating, keeps hot chips from adhering to the tool and facilitates chip evacuation.

An alternative to machining completely dry is suspending an extremely small amount of coolant in a pressurized, air-coolant mist and directing it at the tool's cutting edge either as an external spray or through the tool. Flows of less than 1.7 oz/hr are typical, as compared to a flood-coolant flow of 1.6 gallon/min. Other, more expensive techniques for meeting dry-drilling requirements include chip suction systems, more efficient through-the-spindle minimum lubrication systems and drilling upward so gravity removes chips.

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