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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.

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.

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