I make a recess in the middle watch section to take the back piece. I'm cutting down 2.0mm into the back of the middle section to leave a 1.7mm thick shelf inside the middle section. This gives strength and acts as a border between the top and bottom watch sections but I think it could be slimmer in future.
The middle section wall thickness is 0.5mm, so it's important to make sure the middle watch section is properly centered on the lathe before making this cut.
To cut the ellipsoid shape into the top ring section I've programmed the ellipse in g-code so that the lathe table will follow this shape. The cut gets incrementally closer to the final cut. In the diagram, above, the solid white lines are the tool paths yet to be followed and the red ones are the paths already done.
On the final cut I increase the lathe spindle speed to give the finest finish.
Going back over the photo's I remembered that I'd forgotten to include this bit, apologies about the 'part 10.5' but it fits between the part 10 and 11.
I'd like the edge of the watch-glass shelf to have some curvature so that it doesn't just end abruptly with a square edge. I'm going to use the lathe and some cnc to cut the curve.
The ring, now with lugs attached, goes back on the lathe. Since I'll be cutting the outside of the watch I put the clamps on the inside of the ring.
The ring is placed into a three jaw self-centering chuck. The three jaws automatically move together towards the centre of the chuck so the piece is automatically centred. If I've hammered the ring to a good circle then the ring should be pretty near centre too.
This is an experimental project to see if adding a border ring to a domed silver disc improves the overall result of the finished enamel. When I created the set of enamel doorknobs I didn't add a border ring, so the cloisonné wires had to be ground back to be quite thin near the perimeter of the piece where the enamel became thinner, or in some cases, left with a slightly non-smooth surface. This doorknob (above) shows the problem, albeit minor, but to the perfectionist something to improve!