The term boring as used in machine practice usually means methods of machining internal surfaces, other than those of common drilling and reaming. Also methods for holding the work other than those common to ordinary drilling and chucking operations are often used. When boring machine parts, use may be made of the common inside turning tools or of special appliances termed boring bars.

Fig. 128. General Form of Boring Tool.

When a hole is to be bored in lathe work, tools of a shape different from those used in turning should be used. The general form of the tool is shown in Fig. 128. The length of the shank depends on the depth of the hole to be bored, for it must be long enough to reach from the tool-post to the bottom of the hole. This overhang makes the tool more likely to spring, and necessitates a much lighter cut being taken than when removing the same amount of metal by outside turning tools. The result of this lighter cut is seen in the increase of time required to remove a given amount of stock. The shape of the cutting edge is practically the same as that of the tools for turning, except that the boring tool must have more clearance to avoid striking the work. Therefore, with the same solid angle, the tool will have less rake. The reason for this will be seen by comparing Figs. 114 and 129. In Fig. 114, it will be seen that the surface of the work is outside a tangent at the cutting point and can never interfere with the bottom of the tool. In Fig. 129, the surface of the work is inside the tangent, and, unless the tool has a large amount of clearance, it will cause trouble by striking the concave surface.

Fig. 129. Boring Tool Set for Clearance.

Fig. 130. Tool for Turning in Brass.

Tools for brass differ from those used on steel and iron in that they have no rake. A tool suited for working brass is shown in Fig. 130. Brass does not readily split, and the chips break off as soon as started from the main body. When turning wrought iron and steel, on the other hand, the metal does not break, but forms long spiral chips if the tool is in condition. If a tool with top rake is used in turning brass, the work will not only be rough in appearing; K-Boring; L-Inside Threading ance, but there is great danger of the tool gouging into the stock and spoiling the work or tool, possibly both. The finishing tools for brass may be square or round-nosed, without rake; in fact, a small amount of negative rake will produce a much better surface. When the brass contains a large percentage of copper, some rake to the tool may be required, owing to the ductility and toughness of the metal.

Fig. 131. Common Forms of Slide-Rest Tools. A-Left-Hand Side; B-Right-Hand Side; C-Right-Hand Bent; D-Right-Hand Diamond Point; E-Left-Hand Diamond Point; F-Round Nose; G-Cutting-Off; H-Roughing; I-Threading; J-Bent Thread-.

Fig. 131 shows common lathe tools for cast iron and steel.

The shape of the tool has a very important influence on the amount of work it can be made to do. As has already been explained, these shapes vary with the different metals that are being worked, and also with the class of work performed. It is highly important that the cutting angles be correctly formed. While hand-grinding on the emery wheel and grindstone is fairly satisfactory, the best results can be obtained only by the use of a regular tool-grinding machine, such as that shown in Fig. 132. In addition to the grinding, tools for fine finishing should be carefully whetted on a fine oil-stone.

Fig. 132. Tool-Grinding Machine.