File Rectangular

Fig. 47.

all work where it is important that the screw heads should not be marred, as a result of the blade slipping and damaging the screw slot.

If good quality watchmaker's screwdrivers and those used by instrument makers are examined, they will be found to be hollow-ground, for in these crafts the heads of screws are usually highly finished, and any damage caused by an ill-fitting screwdriver becomes all too apparent.

Hollow-grinding is carried out by bringing the blade, when held at a suitable angle, into contact with the peri-

hollow ground

hollow ground

phery of a small grinding wheel, in a similar manner to that illustrated for grinding the end of a flat scraper. When grinding the blade, great care must be taken not to overheat the steel and draw its temper.

The screwdriver used in the gunmaking trade is of a special form adapted for turning the particular type of screws usually fitted. In best quality guns these screws are very accurately fitted and the heads are hand engraved. Moreover, for the sake of appearance, the slots are so narrow that a screwdriver with a parallel tip would be much too weak to turn them, in view of the fact that during assembly the screws are tightened until they " crack."

The gunsmith's screwdriver, therefore, has a tip of the

form shown in Fig. 48, with hollow-ground flanks meeting at a sharp edge. Needless to say, that if used in the ordinary way, this tool would have but little turning power and would readily rise out of the screw slot. The gunsmith therefore, operates the screwdriver in the manner illustrated in Fig. 49, which allows great downward pressure to be maintained while the full force of the forearm is used for the turning movement.

Centre Punches. When sharpening centre punches, it should be borne in mind that the point has to be ground to approximately the correct angle to suit the work, and, at the same time, it must be formed centrally. When used for marking-out, the punch should have a fine point of some 60 deg. included angle ; this may be followed, when locating drilling centres, by a punch, as illustrated in Fig, 50, with a 90 deg. point to afford an adequate bearing for the drill-point at starting.

Free-hand grinding of the point is the normal practice, and this will be greatly facilitated by using a support attached to the grinding rest, as depicted in Fig. 51. The V notch serves to locate the punch whilst it is rotated by the fingers against the surface of the grinding wheel.

When a general purpose grinding wheel is used in this way, the abrasion of the fine punch-point will be very rapid, and its surface will be left rather rough. To obviate

Fig. 50

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