Interior cable routing is quite a bit of work, but very rewarding when it works well. I use a two-step process, where I first solder the tunnel to the top tube, and then solder a reinforcement piece over the top. It makes for a lot of work, but I think the peace of mind knowing that the join is nice and solid inside is worth it.
It's important to add all the bits and pieces to the frame before doing the main lugs. When you attach something to the side of a tube, the heat tends to make the tube bend a little around the thing you've added. So if you put front derailleur tabs etc on after brazing the frame up, you can stuff the alignment.
I start by slotting the top tube - I just use a drill and file for this - I aim to get a really neat fit to the angled cable tunnel end. Then I assemble the cable tunnel by cutting the middle piece to length, and soldering the larger end pieces on with 56 % silver.
Next the cable tunnel is inserted into the tube. This can be tricky. Ben's frame has the entry point horizontal on the right side, to make for neat cable routing around the head tube, and then horizontal again on the left at the back. I'm leery of having the cable exit at the top, as that's asking for water to work it's way in.
Apply plenty of flux (I'm using the System 48 flux here, with 56 % silver filler).
And solder the tunnel to the tube, aiming for a small fillet all around.
After cleanup, it looks nice and clean, with a good fillet.
Next, I put the reinforcing diamond on top, ensuring it fits neatly. A little bit of blacksmithing was required to form the diamond so that it neatly fits the 31.75mm XL tube, as they're designed to go on 28.6mm tubes. I forgot to take a photo before fluxing, so the fluxed one will have to do.
And solder it in, again using 56% silver filler and System 48 flux. It's hard to do these neatly, as you have to ensure there's enough filler to fillet the tunnel end, and it tends to go everywhere. Also they're tiny pieces, and easy to overheat. I use a much smaller tip (no. 8) than my usual no. 15 for this work.
Finally, wash the flux off, cut the tunnel down neatly and use emery and needle files to remove the excess filler.
