Figs. 163-167.
Fig. 168.
Fig. 169.
The tool is shown in Figs. [163] to [169], and is composed of a wrought-iron body D, connected by a screwed end E to the boring rod, and carrying the chisels F F, fixed in separate sockets and secured by nuts above; two or four chisels are used, or sometimes even a greater number, according to the size of the hole to be bored. This construction allows of any broken chisel being easily replaced; and also, by changing the breadth of the two outer chisels, the diameter of the hole bored can be regulated exactly as may be desired. When four chisels are used, the two centre ones are made a little longer than the others, as shown in [Fig. 167], to form a leading hole as a guide to the boring rod. A cross-bar G, of the same width as the tool, guides it in the hole in the direction at right-angles to the tool; and in the case of the larger and longer tools a second cross-bar higher up, at right-angles to the first and parallel to the striking edge of the tool, is also added.