As the coupling rods are going to be silver soldered together, and I haven't done any silver soldering for quite a while, I made up an assembly to experiment on using some offcuts. These are the same material as the actual coupling rods, but other than the joint itself the metal was just filed to a better (but still rough) finish, rather than machined accurately to size. I ended up pleased with the joint, but glad I had a test run. Here's the assembly about 5 minutes after removing the heat, still hot and flux everywhere: And then after a bit of cleaning up, this is pretty much how I was hoping the joint would turn out: I put a little too much silver solder into the joint as I wasn't sure there was enough to fully penetrate through to the other side of the rod. This is why I'm glad I did a test joint, as now I know how much solder to use, so I won't get this big lump on an actual rod. Still, it's nothing that couldn't be cleaned up with a dremel-like tool: The task basically required heating the steel to red hot, applying the solder, then adding a little more heat to the metal till the solder had flowed nicely into the joint. It took a lot of heat, I had to use my largest burner on the propane torch, and still took about 15 minutes to get up to heat.
Today I made the retaining collars for the rods, these were relatively simple turning jobs on the lathe. The front collars are completed, the rear/driving ones still need to be drilled and tapped for a grub screw to hold them in place.
Here's a really rough set of adjustable coupling rods, these were fitted on the loco, then adjusted till the wheels rotated freely, and will be used to jig drill the second crankpin hole in the coupling rods. I've just recently turned up the two pins at the front, the front one goes through the existing coupling rod hole, and one end of the jig, the second fits in the other hole of the jig and has a 5mm hole to spot drill the other end of the coupling rod.
Here's the first coupling rod partially assembled, at this point I had only machined and threaded one end of the rod, but both ends are now done. The pic' does give a good idea of how the finished rod will look though.
So I got some 12x7mm BMS offcuts from ebay, and am using these for the centre part of the coupling rods. A relatively simple matter of cutting to length, then turning the ends down to 6mm and threading them M6, so as to screw into the bosses. Only relatively simple as this is about as long as the little Taig II lathe can take between centres; the centre drilling being done by careful marking out, then drilling in the bench drill. Here's the rod being set up in the lathe, tool post still missing from the cross slide.