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.

This is a better shot as the lugs are not too blurred so you can tell it's the middle watch section

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After the cut I have a nice recess ready to receive the back watch section

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But I need to make it first!

Okay, I've done this before now quite a few times. In fact quite a few times more than I should due to improper measurements! I cut out a thin bar of brass,

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a bit wider than before to make a thicker ring. This is because the back watch section will have a smaller watch-glass diameter, hence the rest of the space has to be made up with metal.

I anneal the brass then hammer it into a rough ring shape:

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Then I bend the ring like this:

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to get access to the end edges so that I may file them to a close fit.

I'm getting good at this, here's the top view

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and the side view

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The ring has a ribbon of solder draped over the join and fluxed everywhere that needs it

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Now the heat is applied, in the way described in an earlier post (soldering the ring joint),

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Once it's soldered I'll just check that the ring is wider enough

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I'm comparing it with my test ring and, as you can see, the newly made ring is wider with quite a bit more material on the inside. This is just what I need, no mistakes this time!