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6- Twist Drill - Specialty Drills
The desire to cut tooling and machining cost has become even more prevalent with numerical control machines. There are numerous, combination twist drills that have been modified so that more than one operation can be formed by one tool. The high speed steel, combination drill and tap (second from top) drills and taps in a single operation at a relatively high speeds

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High speed steel standard step drills (left) can be used for drilling holes of two diameters.

Self-centering combined tap drill and countersink tools (not shown) are designed for drilling mounting holes for flat-head cap screws. Dual cutting diameters permit both tapped drill hole and countersink to be machined in one operation

Coolant fed twist drills (bottom right) have two holes completely through the drill that exit at the lips. Coolant fed twist drills are available in straight and tapered shanks, mostly in fractional sizes, and only workable if there is a coolant or oil delivery system available on the machine.

Coolant fed twist drills combined with very high liquid pressure has allowed for extremely fast drilling of holes on CNC machines. R.S.
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