Lead screws (worms) 1 and 2 rotate in opposite directions about fixed axes A and B, traversing carriage 3 with wire-feeding pulleys 4 and 5 along guides to the right or left, depending on which screw (1 or 2) half-nuts 6 engage. Link 7 turns about fixed axis C and is subject to the action of two spring-loaded plungers 8, which tend to hold it in the central (disengaged) position. The tip at the bottom of link 7 slides along bevelled bar 9 which, at the end of each stroke, swivels link 7 so that half-nuts 6 are taken out of engagement with one screw and into engagement with the other. This reverses carriage travel so that the wire is coiled in successive layers.
$2836$WG,FD$