The boring of holes sometimes calls for a length and strength of tool that cannot be readily attained with the ordinary boring tool. A great deal of such boring is done with double-headed tools. These tools are held in bars, and cut at each end. An ordinary form of such tool is shown in Fig. 152. The tool A is turned and fitted so that when placed in the bar it is central with the centers of the latter. It is held in position by the key B. It cuts at each end. Such a tool may be made to do very rapid work. It is extensively used for boring in places where a piece of work must be duplicated a great number of times.
Tools of this kind are also used for finishing. After the cut has been started, the tool should not be stopped until the cut has been completed. If it is stopped, there will be a ledge in the bore at that point. The reason for this is found in the springing of the metal and the contraction due to cooling while at rest. The tools used for finishing usually have a broad surface. Those used for the roughing cut are narrower; they wear more rapidly than the finishing tools, and are usually adjustable. An excellent example of the use of boring bars is found in the boring of engine cylinders. Special machines are used for such work. The greater portion of the work is done with a boring bar such as that shown in Fig. 153. It consists of a heavy bar A, upon which there is a stiff traveling head B.
Fig. 152. Boring Bar.
The latter carries the tool C, which may or may not be capable of a transverse adjustment. The head moves longitudinally on the bar, and is held, adjusted, and fed by the screw D. At one end of the screw, there is a star wheel E, by which it is turned. As the bar revolves, one arm of the star strikes against a stop F at each revolution. This turns the screw by an amount proportional to the number of arms in the star. For example, if there are six arms in the star, the latter will be turned one-sixth of a revolution for each revolution of the boring bar. As the screw turns, it moves the head along the bar by an amount proportional to the pitch of its thread and the arms in the star. This forms the feed of the tool. For example, if a star has four arms, and is keyed to a screw of eight threads to the inch, then, for each revolution of the bar, the head will be advanced 1/32 of an inch. Another form of boring bar is shown in Fig. 154.
Fig. 153. Special Boring Bar for Boring an Engine Cylinder.
Boring bars with fixed tools are also used. In such cases the work is caused to travel beneath the bar as it is turned. A case of this kind occurs in the boring-out of brasses for railroad cars.
In general, it may be stated that all heavy work should' be machined in the position which it is eventually to occupy. This is to overcome its tendency to spring out of shape under the influence of its own weight. In small articles this tendency is inappreciable. For large pieces it is sometimes quite apparent.