Fig. 387.
A machine screw is a small screw, such as in [Fig. 387], the diameter of the body being made to the Birmingham wire gauge, the heads being formed by upsetting the wire of which they are made. They have saw slots s for a screw driver, the threads having special pitches, which are given hereafter. The forms of the heads are as in [Fig. 387], a being termed a Fillister, b a countersink, and c a round head. The difference between a Fillister head of a machine screw and the same form of head in a cap screw is that the former is upset cold, and the latter is either forged or cut out of the solid metal.
Fig. 388. Fig. 389.
When the end of a screw abuts against the work to secure it, it is termed a set screw. The ordinary form of set screw is shown in [Fig. 389], the head being square and either black or polished as may be required. The ends of the set screws of commerce, that is to say, that are kept on sale, are usually either pointed as at a, [Fig. 388], slightly bevelled as at b, or cupped as at d. If left flat or only slightly bevelled as at b, they are liable, if of steel and not hardened, or if of iron and case-hardened only, to bulge out as at c. This prevents them from slacking back easily or prevents removal if necessary, and even though of hardened steel they do not grip very firmly. On this account their points are sometimes made conical, as at a. This form, however, possesses a disadvantage when applied to a piece of work that requires accurate adjustment for position, inasmuch as it makes a conical indentation in the work, and unless the point be moved sufficiently to clear this indentation the point will fall back into it; hence the conical point is not desirable when the piece may require temporary fixture to find the adjustment before being finally screwed home. For these reasons the best form of set screw end is shown at d, the outside of the end being chamfered off and the inside being cupped, as denoted by the dotted lines. This form cuts a ring in the work, but will hold sufficiently for purposes of adjustment without being screwed home firmly.
