Fig. 1121.
In place of a steel tool, a tool holder turned cylindrically true and parallel may be used to carry a short boring tool, as shown in [Fig. 1121], in which a is the tool secured by the set-screw b into the holder c. The latter may be provided with a line running true longitudinally, and may have a fine groove similar to a thread, and having a pitch measuring some part of an inch, as 1⁄8, 1⁄4, 1⁄2 inch, &c., so that the distance the tool projects from the holder may be known without measuring the same. But when a tool and holder of this description are used, the tool cannot be employed unless the hole passes entirely through the work, which occurs because of the presence of the set-screw b.
It is obvious that for a tool-holding bar such as this, a clamping device such as shown in [Fig. 1120] is requisite, and that the position of the clamping device may be adjusted to suit the work by setting it more or less through the tool post.
Fig. 1122.
The manner in which the deflection of a boring tool will affect the bore of the work depends upon the height of the boring tool in the work. If the tool is above the horizontal centre of the work, as in [Fig. 1122], the spring vertically will cause it to leave the cut, and bore the hole to a corresponding amount smaller; and since the tool gets duller as the wear proceeds, it will spring more at the latter end of each tool traverse, leaving the end of the hole last cut of smallest diameter.
Fig. 1123.