The Square-Socket Screw: A Driver That Holds the Screw for You

**Cummings, US 161,390**, dated 30 March 1875. The object is a screw that can be driven home "without any danger of injuring the slot or head of the screw by the slipping of the screw-driver." He forms in the centre of the head "a cavity of suitable shape and depth", usually tapering from top to bottom, or triangular (Fig. 5), either with the usual slot or without it — the slotless form being "particularly adapted to the capped screws" whose thin plated caps a slipping driver would ruin. He notes the cavity "may be formed in the screw machine at the time the screw is made, by any of the well-known mechanical processes, this being no part of my invention". **Robertson, US 1,003,657.** He explains why earlier attempts failed: punching a closed slot or a cross left the metal ahead of the punch "crystallized thus fatally weakening the screw at its neck", and the recess too shallow or uneven to hold the driver. His punch e is reversible, with a male die f at each end, "of square cross-section and all the sides whereof are of equal area" converging at a very slight angle, "while the end converges at an angle of forty five (45°) degrees and terminates in a point preferably of pyramidal form." A bumper block of softer metal behind the punch cushions it. The wire gets two blows: "the first (Fig. 2) upsetting the protruding end and the second (Fig. 3) punching the socket and thereby forming and facing the head". The blank is then threaded "in the usual way".

Drill pilot holes in a pine board in three rows. Drive square-drive screws with a square-drive screwdriver, cross-head wood screws with a Phillips driver, and slotted screws with a flat screwdriver. 1. **Carry test.** Press a screw onto each driver, then hold the driver horizontal. Record which screws stay on the tip. 2. **One-hand start.** Start each screw using only the driver hand, without holding the screw. 3. **Cam-out.** Drive each screw until the head seats, then keep turning with light pressure. Note which driver rides out of its recess first, and inspect the heads with a lens. Measure the square socket's taper with the caliper: across the flats at the mouth, and the depth, against the tip of the driver.

**Allan Cummings**'s US 161,390 (dated 30 March 1875, filed 15 February 1875) claimed a screw whose head has an angular cavity at its centre together with the usual slot. **Peter Lymburner Robertson** patented the tooling to punch a square, slightly tapered socket cold, with a first application filed on 8 November 1907 and US 1,003,657 granted on 19 September 1911. **Honest limits.** The friction coefficient in the notebook is an assumed value; real grip depends on finish, plating and dirt. The patent gives no figure for the taper, only "a very slight angle".