Having a single spring and a fixed upper plate works much better than the first version:
The (lubricated!) nyloc nuts under the plate provide a little friction and stabilize the whole affair.
The solid model has the same stylin’ tapered snout as the LM12UU drag knife mount:
The spring seats in the plate recess, with the 3 mm shank passing through the hole as the tool holder presses the tip against the workpiece.
I diamond-filed a broken carbide end mill to make a slotting tool:
Lacking any better method (“a tiny clip spreader tool”), I rammed the Jesus clip the length of the shank with a (loose-fitting) chuck in the tailstock:
Even without nyloc nuts, the first test worked fine:
The 53 g/mm spring rate may be too low for serious engraving, but it suffices for subtle Guilloché patterns on scrap platters.
The OpenSCAD source code as a GitHub Gist: