Twist Drill - 4: Materials
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WARNING: Machine tools present a safety hazard. Improper operation can result in severe injury. These topics are for non-laboratory study only and are not to be used in conjunction with the operation of any tool or machine described herein. Never use a machine tool without the supervision of a qualified instructor.
Before starting a drill operation the machinist must determine what material has been specified for the work piece and what material the drill bit is made of. Work piece material that is harder than the drill the drill will cause the drill to overheat. It can literally become molten and fuse to the work piece
Carbon Steel Twist Drills (above) are inexpensive. They are relatively soft and are limited as to what type of materials they can be used on. The RPM must be kept at a lower speed for these drills to prevent overheating and annealing The life of these drills can be extended by coating the drill with wear-resistant material such as Titanium carbide, aluminum oxide or titanium nitrite. Like all cutting operations drilling needs steady, uninterrupted feed and an appropriate coolant to keep down the heat. It is appropriate however to withdraw the drill from the whole to clear the chips and to relieve some of the heat being built up. The basic rule also applies that the heat should be transferred to the chip not the part to.

High Speed Steel Twist Drills are the most commonly used drills, since they can be operated at twice the RPM of carbon steel and can withstand temperatures up to a thousand degrees Fahrenheit. Cutting edges can be extended by coating the drill with wear-resistant material and extend the cutting speeds from 25 to 90 percent. High speed steel twist drills are often coated with titanium nitrite which gives it a gold colored look (left).

Drills are often marked with the size and what type of material they are made from. However if the markings are missing you can identify the material by touching the top of the drill to grinding wheel and observe the sparks. Carbon steel will produce white sparks. High-speed steel dull red sparks.

.Cobalt High-Speed Steel/Cobalt Twist Drills (above left) are a combination of cobalt and high-speed steel and produce heat resistance properties that are necessary for general purposes drilling of ferrous metal*s, tough casting-sand forgings, armored plate, working-hardened stainless steel, other hard steels and tough high tensile strength nonferrous metal*s like titanium. This drill works well with CNC work for it can be run at higher cutting speeds and feeds at a steady uninterrupted feed at high temperatures without breaking down.

Tungsten Carbide Twist Drills ( right) are available as solid tungsten carbide drill or with inserts made of this material. Inserts are positioned and cemented at the tip of the carbide or high-speed drill and resharpen. Insert drills are less expensive than the solid drills but able to perform equally on ferrous metal*s and nonferrous metal*s like glass bonded plastics; brass, aluminum, bronze, and other nonferrous metal*s; and hard scaled castings. Due to the hardness of the tungsten carbide drill, grinding wheels used to sharpen them are diamond-impregnated or made of silicon.

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label:Drill, Twist drill, drill geometery, shanks, drill materials, specialty