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What is a Tool Bit?

By Paul Scott
Updated May 17, 2024
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The term tool bit generally applies to non-rotary type cutting bits for power tools such as milling machines and lathes. Tool bits are specially shaped to execute a cut of specific profile, depth, and width such as screw threads, grooves, and flat cuts. They are usually made of hardened high carbon tool steel and can be resharpened when blunt or reprofiled for different cuts. Although bit profiles differ considerably between applications, there are several standard profile geometry standards which are applied to most bits. These ensure operator safety and efficient chip removal while cutting.

A tool bit is the cutting face of most power tools and machines. The non-rotary tool bit is typically held securely in a clamping device known as a tool box. The tool bit may execute a static cut with the workpiece being rotated against it as is the case with a lathe. Alternatively, in applications such as milling machines, the tool bit may reciprocate or move back and forwards across a static workpiece.

Tool bits are made from high carbon tool steel stock and are hardened and tempered to hold a keen cutting edge and withstand the stress of cutting. The shape of the bit is determined by the type of cut it has to make and the material it is used to cut. Although there are many different tool bit designs, most feature several common geometry characteristics. These include a rounded nose radius on the cutting edge which ensures a smooth cut and strengthens the bit.

Other important standards are the back and side rake angles. The back rack angle of a tool bit controls the direction in which the swarf or cuttings are cleared away from the cutting face. It also effectively pulls the tool bit into the workpiece, thereby relieving operator pressure on the bit. The side rake angle reduces resistance of the workpiece to the movement of the tool bit. These angles are particularly important considerations when shaping a bit for a specific material. For example, a bit intended for use on brass would have no side or back rake while one used to machine aluminum would feature a side rake angle of 15 degrees and a back rake of 35 degrees.

Not only do these design considerations ensure efficiency but also prevent bit failures which could cause operator injury. Tool bits may be reground to different profiles if and when the need arises. They can also be resharpened when they become dull. Some tool bits even feature several cutting edges and only require turning around in the tool clamp when they become blunt. There are also several types of specialist bits which use ceramic or diamond cutting inserts mounted on a steel bit shank.

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