Fig. 1787.
[Figs. 1786] and [1787] represent the brace arranged to have a gear-wheel connected or disconnected at will, the object of this addition being to enable a quick speed to the chuck when the same is advantageous.
Fig. 1788.
For drilling small holes in metal, the breast drill shown in [Fig. 1788] is employed. It consists of a spindle having journal bearing in a breast-plate at the head, and in a frame carrying a bevel gear-wheel engaging with two gear-pinions that are fast upon the spindle, this frame and the bevel gear-wheel being steadied by the handle shown on the right. At the lower end of the spindle is a chuck for holding and driving the drill, which is obviously operated by revolving the handled crank which is fast upon the large bevel gear. The feed is put on by pressing the body against the breast-plate.
It is obvious that but one bevel pinion would serve, but it is found that if one only is used the spindle is apt to wear so as to run out of true, and the bore of the gear-wheel rapidly enlarges from the strain falling on one side only. To avoid this the spindle is driven by two pinions, one on each side of the driving gear as in figure.
Breast-drills do not possess enough driving power to capacitate them for drills of above about quarter inch in diameter, for which various forms of drill cranks are employed.
