Automatic Boring Tool. A convenient substitute for a brad-awl is represented by Fig. 122. The drill, or bit, A is held in a suitable chuck C, at the end of the bar D, which runs in B. The drill is brought into contact with the work, and pressure in the direction of the arrow, slides B down upon D, and this movement causes D with the drill to revolve. The full extent of the movement having been reached, a relaxing of pressure leaves D free to return to its first position, as shown, the rotary motion of A, meanwhile, being reversed. These impulses can be imparted to the drill with great rapidity, and the work is quickly done. The dots below the figure, 122, indicate the full diameter of the different drills which are furnished with the tool.

Fig. 122

1 Considered as a mechanical movement, this is known as Hooke's joint.

Miscellaneous Tools.

97. Winding-Sticks, or "parallel strips," are wooden strips of any convenient length, the edges of which are straight and parallel. When applied to a surface, they increase its breadth in effect, and by thus giving a better opportunity of comparison, show whether the surface is "in wind," or twisted. For an illustration of their use, see 75.

98. Hand Screw-Drivers are in form similar to that shown by Fig. 123. The part which is to engage the screw should have parallel sides, as shown by Fig. 124, and never be wedge shaped, Fig. 125. In the latter case, it will be seen that force applied in an attempt to turn a screw, will have a tendency toward lifting the screw-driver from its place.

Fig. 133

Fig. 134

A set of three or four screw-drivers, having blades varying in size to suit different-sized screws, so that a fairly good fit may always be made, are indispensable to good work where screws are much used.

99. Brace Screw-Drivers, instead of having wooden handles, are provided with shanks for use in a brace. A good form is shown by Fig. 126. The brace gives a continuous motion, and the screw may be set much more rapidly by its use than with the hand screw-driver. There are many cases, however, in which a brace is useless.