Thomas Industrial Library

Tool Materials

What materials are used for lathe tool bits and how are they designed?

• Individual Steel Tool Bits , Figure 7–25, are usually 2 to 3-inches long, square in cross section and made of HSS (high-speed steel) or a boronsteel alloy. They are available in a wide range of sizes, from 1/16 -inch to 1-inch square, both sharpened and unsharpened. The most common bit for medium-sized lathes is ⅜-inches square by 2½-inches long. These bits are supplied hardened and no further heat treatment is needed. The advantage of these over carbide bits is that they are readily sharpened on aluminum oxide grinding wheels, easily formed into custom shapes, and provide a better surface finish on many materials. They remain in use by tool and die makers and in prototype work, but not in production operations.

Smaller tool bits are held in tool bit holders that fit into the tool post, Figure 7–26. These now obsolete tool holders were introduced when alloy steel was expensive. They permitted the use of smaller tool bits. Today these tool bit materials are not expensive, and larger bits that fit directly into the tool post are most often used. However, because the cutters for these tool holders are smaller than cutters that fit directly into tool posts, they continue to be used because they save a lot of grinding time. This is especially true for small lathes where only a portion of the cutter is needed.

• Brazed Carbide Bits consist of a small piece of tungsten carbide brazed onto the end of a square piece of carbon steel as in Figure 7–27. Only the carbide tip performs cutting. In a comparable size, a brazed carbide bit costs about twice as much as a HSS bit, but cuts many times longer. They also fit directly into a tool post like those used for conventional tool steel bits and are available in similar shank dimensions as square tool steel bits. Since they cannot be ground and shaped to fit the particular turning or boring situation like steel bits, there are over 20 industry “standard styles” with different shaped carbide tips to cover most applications. Brazed carbide bits allow the use of carbide cutters without the expense of purchasing a matching tool holder which is needed for carbide inserts.

• Inserts are individual pieces of tungsten carbide (or other materials) held in a matching insert tool holder by a small Torx ® screw or clamp (or both), Figure 7–28. Like brazed carbide bits, carbide inserts cannot be reshaped and are used as supplied. Most inserts have multiple cutting edges accessed by rotating and/or flipping the insert in its holder. For example, a triangular insert usually has six cutting edges, three on each side. Such an insert with six cutting edges costs about the same as an HSS tool bit with a single cutting edge. Carbide insert cutting tools perform better than brazed carbide tools because the brazing process leaves residual stresses in the carbide chip causing them to crack sooner than inserts.

There are two reasons why the majority of production work is done with carbide insert tools:

– Inserts withstand much higher temperatures than HSS tool bits so work can be performed at cutting speeds two to three times faster than HSS.

– Because inserts are made to tight dimensional tolerances and are not resharpened, their cutting edges appear in the same position relative to the tool holder and workpiece. This means the tooling does not have to be reset whenever the insert is changed, a very important issue in DRO and CNC machines.

Taken together, these factors lead to much lower costs in a production setting than with HSS bits.

How are these three types of cutting bits sharpened?

See Table 7–1.

Since carbide inserts cannot be reshaped like HSS bits to fit the needs of a particular job, doesn’t choosing to use inserts restrict machining choices?

Because inserts are widely used in CNC lathes and machining centers, and many different tool holders and inserts are required to meet these needs, tool manufacturers offer dozens of tool holder designs and hundreds of insert sizes and shapes. Using inserts does not impose restrictions on their users. In an emergency, steel tool bits can be modified in the shop to get the job done.