Fig. 4. Boring with a Flat Double-ended Cutter
[Fig. 4] shows how a hole is bored by cutters of the type referred to. The bar rotates as indicated by the arrow a and at the same time feeds longitudinally as shown by arrow b. The speed of rotation depends upon the diameter of the hole and the kind of material being bored, and the feed per revolution must also be varied to suit conditions. No definite rule can be given for speed or feed. On some classes of work a long boring-bar is used, which passes through the hole to be bored and is steadied at its outer end by the back-rest B, [Figs. 1] and [2]. On other work, a short bar is inserted in the spindle having a cutter at the outer end. An inexpensive method of holding a cutter at the end of a bar is shown at D, [Fig. 3]. The cutter passes through a slot and is clamped by a bolt as shown. When it is necessary to bore holes that are “blind” or closed at the bottom, a long boring-bar which passes through the work cannot, of course, be used.
Sometimes it is necessary to have a cutter mounted at the extreme end of a bar in order to bore close to a shoulder or the bottom of a hole. One method of holding a cutter so that it projects beyond the end of a bar is indicated at E. A screw similar to the one shown at B is used, and the conical end bears in a conical hole in the cutter. This hole should be slightly offset so that the cutter will be forced back against its seat. The tool shown at F has adjustable cutters. The inner end of each cutter is tapering and bears against a conical-headed screw b which gives the required outward adjustment. The cutters are held against the central bolt by fillister-head screws f and they are clamped by the screws c. Boring tools are made in many different designs and the number and form of the cutters is varied somewhat for different kinds of work.
Fig. 5. Cutter-heads for Boring Large Holes
Cutter-heads for Boring Large Holes.—When large holes are to be bored, the cutters are usually held in a cast-iron head which is mounted on the boring-bar. One type of cutter-head is shown in [Fig. 5]. This particular head is double-ended and carries two cutters c. The cutter-head is bored to fit the bar closely and it is prevented from turning by a key against which a set-screw is tightened. By referring to the end view, it will be seen that each cutter is offset with relation to the center of the bar, in order to locate the front of the tool on a radial line. The number of cutters used in a cutter-head varies. By having several cutters, the work of removing a given amount of metal in boring is distributed, and holes can be bored more quickly with a multiple cutter-head, although more power is required to drive the boring-bar. The boring-bar is also steadied by a multiple cutter-head, because the tendency of any one cutter to deflect the bar is counteracted by the cutters on the opposite side.
