Different types for different diameters
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There are three types of drills: High-speed drills, Solid carbide drills and indexable type drills.
Most of the drills that are marketed today are PVD (physical vapour deposition) coated. PVD coating prevents damage caused by welding between the tool and the workpiece material and improves the overall wear resistance properties.
High-speed steel drills
Depending on the workpiece material and cutting conditions then the type of drill to be selected will vary. A commonly used material is JIS SKH51. Recently, cobalt-based dissolution type, high vanadium or high cobalt-based powder-metal is used for longer tool life, higher machining efficiency and longer drilling length. Vanadium (V) is effective in increasing wear resistance and cobalt (Co) is effective in increasing hot hardness.
In general, increasing quenching hardness improves wear resistance and tool life. However increased quenching hardness can result in problems such as breakage and chipping, depending on how a drill is used. Drill material and quenching hardness are important factors in determining drill performance.
Indexable type drills
Indexable type drills are drills that use inserts as the cutting edge. As the inserts of indexable type drills are changeable, it is possible to select a tool grade and a breaker that are suitable for a particular workpiece material. The cutting conditions can also be selected from a wide range. In addition, the need for a regrinding process and the regrinding facilities becomes irrelevant.
Due to some technical difficulties the indexable drills only came into practical use 10 to 15 years after turning and milling tools were introduced. In the early 1980s indexable drills started to become popular.
Solid carbide drills
Solid carbide drills, if used correctly, can play a decisive role in increasing machining efficiency, tool life and hole accuracy. One downfall with solid carbide drills is that they have lower toughness when compared to high-speed steel drills.
With recent improvements and developments of the machines that use of solid carbide drills however the use of solid carbide drills is becoming more commonplace.
Drills for steel machining are specially designed. They have a higher-rigidity flute-cross-section than drills designed for machining cast iron or light alloy. They also have a special chisel edge geometry to improve the initial machining performance. The cutting edge and flute shape are designed to improve chip control. They are also designed with coolant holes to improve chip disposal and cooling of the cutting edge.