I'm going to curve the back section to get rid of the right angled edge. I've written this g-code routine already for the top watch section so I'll use it again. Strictly speaking the back section is not supposed to be an ellipse shape, but if I tweak the parameters then it becomes a very shallow ellipse and will be fine for this initial design.
I'll be using a flat watch glass for the back section which is quite a bit smaller, in diameter, than the front watch glass. First, though, I cut the back ring diameter to match the middle watch section.
Now the ring is made it needs to be fit into the middle section.
In this case I'm cutting the inside of the ring first, to get this fully concentric, and clamping the outside. There is no particular preference or benefit of doing the inside or outside first since both should be near circular from the hammering.
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.
The watch is going to need some sort of protection against water and dust. I'm not planning on making this a diving watch but some sort of gasket mechanism needs to be put into place.
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.
Let's get the top of the watch finished, it's getting to look more and more like a watch so I'm getting quite excited!
I made some simple plans, above, with the key dimensions. I know it's a bit rough and ready but I didn't see much point in carefully drawing out plans, at this stage, when the real planning is going on as I make the watch itself. The next iteration may be a bit neater!
The top ring section is made in pretty much the same way as the centre section. I get a bar of brass and bend it into a ring and solder the joints. This technique is worth developing here because it can be extended to precious metals even though, with brass, the relatively inexpensive cost of the metal means the ring section could be machined from a solid piece of metal.