I am back from my business trip in Canada and was able to get some shop time in this afternoon to get more work done on the S&S coupler retrofit. The last post left off with the coupler fluxed and ready for the brazing. Unfortunately I forgot to take pictures of some of the steps here, and I was unable to take a picture of myself actually brazing! The first piece I braze is the threaded side of the coupler to the down tube stub. You really have to be careful about the rotational orientation of the coupler on the stub. The non-threaded coupler is then brazed to the remaining portion of the down tube. The rotation orientation of this mating does not matter at all.
In the picture below you can see the inside of the coupler and the stainless steel disk. Note the ring of silver around the inside on the circumference of the disk. This ring of silver shows that you have complete silver penetration since you add from the other direction and ensures that the tube is completely sealed.
After a through hot water soak, drying and brief cleanup work we are finally ready to add the head tube miter to the down tube. For this next step the down with the coupler is treated as a standard down tube and mitered as one piece tube.
As you can see from the picture I mark a small dash of the top of the down tube to use when cutting the miter. It is a conservative eyeball measurement, so I always miter the tube long; knowing I can always recut if the tube is too long. The picture below is after the second cut and I was happy with it’s centrality, tube length and mating. After this picture was taken the miter was fully deburred and the entire tube was cleaned.
Since the stainless steel disc was brazed into the coupler if any water was to get into the tube it would have no place to exit the tube. Consequently, I use sealed water bottle bosses to keep the tube sealed. Adding sealed bottle bosses must be done before the tube is welded to the frame for two reasons: the air inside the tube will heat and expand to a point where it needs to escape and will blow a hole in the silver and you will likely have a rogue piece of flux rattling around inside the tube.
With the sealed bottle bosses brazed in and the tube completely cleaned the down tube is now ready to be welded to the frame. With the rest of the frame in the Anvil frame fixture I tack welded the down tube in place. The first tack weld is directly under the bottom bracket and this pulls the down tube down to that it firmly seats into the head tube. The next tack is on top of the down tube; mating it to the head tube. Additional tack welds are added in alternating patterns in multiple places at the bottom bracket and head tube. Since the tube is sealed you have to cognizant of the air inside heating and wanting to escape. It will not have the same easy of escaping as you would with silver, but if you get the air in the tube hot enough you will get an eruption in the molten TIG puddle.
I have become a very fast welder (you get faster with more experience), but I really have to take my time here with the down tube stub. If your miters are tight, as they should be, the air in the sealed tube heats up even faster. To get around all of this, I run quick beads about an inch long, do something else in the shop for a bit, then come back and run a few more beads.
I actually leave the last 1/4″ section of un-welded area overnight to allow the air inside the tube to settle back down to room temperature. I leave these welds in an area that is very easy to access and where the tubes meet at an acute angle so air has a more difficult time escaping. The next time I go back into the shop I am left with two easy 1/4″ welds to make. I do them fast and get the torch out of the area; leave it there too long and you get a little volcano.
Tomorrow when I get to the shop I will complete the afore mentioned two quick welds and then its cleanup work before I paint the frame. The couplers will get progressively more finely sanded; all the way down to 400 grit which will leave a beautiful brushed finish.