[Illustration: Fig. 3. Boring-cutters of Different Types]
Another method of holding a flat cutter is shown at _B_. The conical end of a screw bears against a conical seat in, the cutter, thus binding the latter in its slot. The conical seat also centers the cutter. A very simple and inexpensive form of cutter is shown at _C_. This is made from a piece of round steel, and it is held in the bar by a taper pin which bears against a circular recess in the side of the cutter. This form has the advantage of only requiring a hole through the boring-bar, whereas it is necessary to cut a rectangular slot for the flat cutter.
[Illustration: 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.