Cutting tools

Cutting tools

Metal-cutting tools are classified as single point or multiple point. A single-point cutting tool can be used for increasing the size of holes, or boring. Turning and boring are performed on lathes and boring mills. Multiple-point cutting tools have two or more cutting edges and include milling cutters, drills, and broaches.

There are two types of operation; either the tool is moving on a straight path against the stationary workpiece, as on a shaper, or the workpiece is moving against the stationary tool, as on a planer. Relief or clearance angles must be provided to prevent the tool surface below the cutting edge from rubbing against the workpiece. Rake angles are often provided on cutting tools to cause a wedging action in the formation of chips and to reduce friction and heat.

Tool materials

In order to remove chips from a workpiece, a cutting tool must be harder than the workpiece and must maintain a cutting edge at the temperature produced by the friction of the cutting action.

Carbon steel

Steel with a carbon content ranging from 1 to 1.2 percent was the earliest material used in machine tools. Tools made of this carbon steel are comparatively inexpensive but tend to lose cutting ability at temperatures at about 400° F (205° C).

High-speed steel

In 1900 the introduction of high-speed steel permitted the operation of tools at twice or three times the speeds allowable with carbon steel, thus doubling or trebling the capacities of the world’s machine shops. One of the most common types of high-speed steel contains 18 percent tungsten, 4 percent chromium, 1 percent vanadium, and only 0.5 to 0.8 percent carbon.

Cast alloys

A number of cast-alloy cutting-tool materials have been developed; these nonferrous alloys contain cobalt, chromium, and tungsten and are particularly effective in penetrating the hard skin on cast iron and retaining their cutting ability even when red hot.

Cemented tungsten carbide

This material was first used for metal cutting in Germany in 1926. Its principal ingredient is finely divided tungsten carbide held in a binder of cobalt; its hardness approaches that of a diamond. Tungsten carbide tools can be operated at cutting speeds many times higher than those used with high-speed steel.

Oxides

Ceramic, or oxide, tool tips are one of the newest developments in cutting-tool materials. They consist primarily of fine aluminum oxide grains, which are bonded together.

Diamonds

Diamonds have been used for many years for truing grinding wheels, in wire-drawing dies, and as cutting tools. For cutting applications they are used largely for taking light finishing cuts at high speed on hard or abrasive materials and for finish-boring bronze and babbitt-metal bearings.