The lathe is often used for boring, or internal turning and is as well adapted for that class of work as it is for the plainer and easier external work. The simpler operations of boring, such as holes that are too large to be drilled economically, can be performed with a simple design boring tool that can be attached directly to the tool post. A hole is first drilled in the piece of sufficient size to accommodate the boring tool, the work then going on just as in external turning as far as control of the tool is concerned. Special boring tools and supporting fixtures are necessary when the work demands and while the simple forged tool does good work in the smaller sizes, it is believed to be more economical to use tool holders with removable high speed steel tools on the heavier work.

The boring fixture shown at Fig. 330-A is an adjustable design for light work. The boring tool is supported in a swinging tool holder held at one end by a fulcrum screw, and the whole is supported in a frame adapted to be secured to the tail stock spindle. The amount of stock removed at a cut is regulated by the adjusting screws passing through the sides of the tool support frame, these permitting the boring tool to be brought in contact with the work with varying degrees of pressure. The boring tool is of round stock and is kept in place in the socket made to receive it in the tool holder by clamping screws.

Another design in which the cross slide of the lathe is used to support the boring bar is shown at Fig. 330-B. The tool support is of round stock, having a hole drilled through one end at an angle. The cutter is made of high speed steel, round section and is a good fit in the hole of the tool holder. A taper pin is provided to lock the cutter in the holder. The boring bar itself is held by a cast iron clamp member attached to the cross slide carriage instead of the usual tool post by bolts. The cutter or boring tool may be removed for regrinding by knocking out taper pin which releases the tool from its socket in the tool holder.

Fig. 330.   Boring Bar Construction and Use.

Fig. 330. - Boring Bar Construction and Use.

Two designs of boring bars for heavy work are shown at Fig. 331. That at A uses an adjustable boring tool which can be moved out by loosening the clamping screws and screwing in the adjusting screw. The tool support casting may be attached to the cross-slide of the lathe as indicated at Fig. 330-B. The boring bar at B, Fig. 331, is similar in principle to that shown at A as relates to manner of support. The cutter, however, is of rectangular section stock, has two cutting edges and is intended to make the desired hole with one cut. The cutter is held in place in cutter support by a locking wedge. This form of boring bar is more suitable for finishing and duplicate boring where it is desirable to have the bore come to the size determined by the cutter. Cutter supporting bars or boring bars of the form at A and B, Fig. 331, are used more in manufacturing than general work. They are also widely used in boring mills and turret lathes.

Fig. 331.   Boring Bars for Heavy Work.

Fig. 331. - Boring Bars for Heavy Work.