Per′sian drill.
A hand-drill (
a) which is sometimes known as the Persian drill has a shank with a
quick thread and a back-center in the head or handle.
The nut is moved back and forth on the screwshank, which is thus rotated, the action of the drill being rotary reciprocal.
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Persian drills. |
It is frequently used for fine work and in dentistry, the shank being made of
pinion wire twisted, and the spiral grooves inside the nut being made conformable to the screw-threads of the shank.
The
screw-stock drill (
b) is a modification of the preceding, in which the motion of the screw-stem is communicated by a pair of bevel-pinions of the drill, whose axis is at right angles to that of the drivingscrew.