The Hex-Socket Screw: A Head That Sinks Out of Harm's Way

Allen writes that it is "now generally recognized as very desirable, and numerous governments, State and national, make it obligatory" to guard or sink the heads of set screws on rotating parts. His object is screws "cheaper than the common headed set screws, which can be entirely sunk below the surface". Fig. 1: a blank 1 of round wire is cut, one end upset "in the ordinary way" to form an expanded head 2, and the blank is driven through a die 3 by a punch 4. The head is "drawn up to the original diameter of the wire, leaving a socket 5 in the end". Other figures start from a drilled or punched round socket and turn it hexagonal the same way, or make a headed machine screw (Fig. 4). The sockets are "shown as hexagonal" but "can be formed square, octagonal or any other angular or non-circular shape, depending on the shape of the punch". The claims are for the process: an enlarged end forced by an angular punch through a die smaller than that end, "compressing that end onto the punch".

Measure a socket head cap screw with the caliper: head diameter, the socket across the flats, and its depth. Find the hex key that fits and note the size. Slide a 16-tooth timing pulley onto a 6 mm steel shaft clamped in the bench vise. Tighten its set screw with the hex key — first lightly, then firmly. Each time, wind a string round the pulley's teeth, hook the spring scale to it, and pull until the pulley slips. Slip torque = scale reading × the pulley's pitch radius (about 5.1 mm for 16 teeth of 2 mm pitch). Compare with the notebook. File a small flat on the shaft under the set screw and repeat: the grip rises sharply.

**William G. Allen** of West Hartford, Connecticut, assignor of one-half to Ira Dimock, filed US 960,244 on 9 January 1909; it was granted on 7 June 1910. It claims a process of making socketed screws by forcing an enlarged end through a smaller die onto an angular punch. **Honest limits.** The head sizes, nut factor and friction in the notebook are examples; real set-screw holding depends on the point form, hardness and shaft finish.