Sometimes the geometry just works, other times, the parts need to be adjusted, tweeked, changed and refined so that the moving parts fit together properly.
This model has been one of the awkward ones. I reckon, though, that I now have a nice mechanism which is easy to assemble and works with a reasonable tolerance.
I also have a grave yard of non-working, slightly working and broken prototypes ready for the bin.
Colour next, then instructions, then back to the Dodo!