New anti-backlash and coupling

As "Titch" is out of action, I've put together a new stage driver with anti-backlash on it. It checks out to 10μm as near as I can tell by averaging and eyeballing the 0.2mm layers of 3D print on the stage as it moves. Better accuracy test with calibration slides coming when I make a Metriccano slide holder mount.

The lugs for the bands have been moved above the nut, which drags the nut rather than pushing it and removes the wobble. The bands are "Loom Bands" which stretch quite a lot and are stable over time (unlike rubber bands). They are quite weak, however, and this allows me to add several to alternate sides without stressing any flexures.

The crossbar conceals an M3 nut drilled out to 3mm. As a stop on the thread I'm using an M3 Nylok because dual lock nuts in confined spaces are just too damn hard. The upward flex in the coupling ensures the bottom of the screw does not pop out of the coupling.

The coupling does need to be manually centred by pressing hard on the high side until the unconstrained screw rotates centrally. Seems to work though.

There are a couple of other things going on, but I'll cover those later when I'm a bit more sure of not making an arse of myself. More than usual, anyway.