Making a watch case (part 2), soldering the ring joint

Making a watch case (part 2), soldering the ring joint

To solder the ring together I'll be using hard solder, hammered into a thin ribbon. This will be draped over the joint with plenty of borax flux. The flux can be mixed from this borax cone, shown above, using a porous dish. The cone is ground around the dish with some distilled water until a paste is formed.

Before I do any soldering, though, it's essential to clean and de-grease the joint. Any grease will prevent the flux, which is water based paste, from wetting the surface. The flux needs to cover the joint to prevent oxides from forming during the heating process. Here I'm leaving the ring to soak in a caustic bath for a good half-an-hour and agitating from time-to-time.

de-greasing the brass ring.

Next the ring is fluxed and the solder ribbons put in place and the ring is heated:

heating the ring for soldering.

There's a bit of technique required to get the solder to flow correctly. The whole ring needs to be gently heated, at first, so that all the flux can dry out slowly. If the flux dries out too quickly it bubbles and will quite likely throw off the solder ribbon, or chip itself off the brass. Once the flux is dry the whole ring can be heated a little more strongly until the flux melts. Then the heat can be concentrated at the joint until the solder melts and clearly flows down into the joint.

Solder has just flowed

Here, above, the solder has just flowed nicely so I can leave it to cool. When cooled down the joint can be inspected,

Joint after cooled,

this one is fine. You can see here the effect the flux has on the metal. The golden colour is where the flux was applied and here the metal has not oxidized, the blackened regions is where the oxides have formed. The flux, effectively, forms an acid, when molten, which dissolves any surface metal oxides that have formed, it also seals the surface and prevents oxidation this way too.

Now the ring is soldered, I put it back on the stake and gently hammer it to as near a circle as I can. I say 'gently' because too much force will thin out the metal and increase the diameter of the ring, which I don't want in this case.

The next stage is to turn the ring on a lathe.

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