Fig. 1108.
Fig. 1109.
For making a recess round a hole, or, in other words, for cutting a flat-bottom countersink, a facing countersink, [Fig. 1108], may be used, its cutting edges being at a, b, c, &c. The clearance is given at the ends of the teeth only, being shown from b to d. The pin p steadies the tool, and is made a working fit to the hole in the work. Or if too small, a ferrule may be placed upon it, thus increasing the capacity of the tool. When a tool of this kind is to be used on iron, steel, or copper, and not upon brass, the front face of the teeth may be given rake by cutting the grooves at an angle, as in [Fig. 1109].
Boring Tools for Lathe Work.—The principal object in forming a boring tool to be held in a slide rest is to have the body of the tool as large as can be conveniently got into the size of the hole to be bored; hence the cutting edge should not stand above the level of the top of the steel. By this means the tool will be as stiff as possible, and less liable to spring away from its cut, as boring tools are apt to do, especially when the cut or hole is a long one.
It is so difficult a matter to bore a long hole parallel with a long boring tool that cutters of various forms are usually preferred, and these will be described [hereafter].
Fig. 1110.