My wife has been wanting a casual bike for riding around the village or going to the market. She has a restored 61 Rudge 3 spd that she loves but she wants more gear but not the fuss of 21-30 gears. I knew that Sheldon Brown had been fooling around with a Nexus 8 spd internal hub setup so I called him up and ordered one. I first built up the Nexus hub on a 26" narrow MTB rim and then fished out an old Tange MTB frame that was my first attempt at tig welding. The welds looked like chicken crap but that frame has stayed together over 14 years. It ended up in the scrap pile after I smashed it into the garage door header .The frame was too big for my wife so after I cut the badly dented top tube out, I then shortened the head and seat tube. I fashioned a seat binder /reinforcement from an old stamped steel seat lug.
I fillet brazed in a new top tube that was made from a 531 down tube on a wrecked Trek . I cut down the seatstay wishbone. I replaced the MTB dropouts with a pair of horizontals salvaged from a Puch. I borrowed the chain guard from a 72 Raleigh and brazed on mounts and tabs for the chain guard. All brazing was done with a Benzomatic mapp gas torch
Paint is PPG impact red metallic base coat, clear coat is a cycloaliphatic ( epoxy with hardner)epoxy. The epoxy gives a really tough finish but needs wet sanding and a lot of buffing to get a good gloss.
My fillet brazed joints look better than my welds, You don't get to see my welds
Here is a quick and dirty description of how I fillet braze with mapp gas.
This is the setup: a section from an old top tube. The mitered end is .9mm wall the butt is probaly .6mm wall. The torch is a Benzomatic ts4000 , the blue rod is nickle bronze flux coated it is liquid at about 1750-1800 degrees It doesn't flow well so I use it for tacking.
As I mentioned in other posts , If you redirect some of the wasted heat back to the work it will get plenty hot enough, The iron table saw top under the work is doing the reflecting here. It only took about 10 seconds to get the tubes too hot.
here is the first layer using a brass rod now. Problem with most bottle torches is the tips is usually 9mm and the flame isn't well directed so the heat goes all over the place. in turn the filler tends to go all over the place. Afterwards you spend a week trying to file it all off. A traditional A/O setup with the correct tips would make this a much neater Job . For my purposes I figured out if I dipped the rod in flux as I go I could control the flow better. in the picture below the top of the joint got lots of flux and has brass spread out well beyond the joint. The bottom of the joint I dipped the rod in flux and used just enough to tie the joint together you can see the defined ridges in the brass.
Here I am ready for the next layer of brass, what I will do is dip my rod in flux as needed and use the ridge on the bottom as a dam. I now let gravity help me out. I melt the brass into the previous layer and allow it to flow down against the ridge, as it builds up I keep moving the torch and adding filler to the molten pool till I get to the opposite side . If this were a real frame I would rotate the frame and continue all the way around the joint
All filled in, I brushed it a bit with a wire brush so you could see the work, All the pits and voids are there because I didn't clean the tubing at all before I brazed it,nasty looking!
Done right the joint would look smoother and have no pits. proper preparation is key to a strong and easy to finish joint regardless of the heat source.
Flux soaked off and the joint wire wheeled. Unfortunately the glare from the flash killed the detail but you can see there is a nice fat joint of brass there just waiting for a rat tail file!
One comment though, my understanding is that you should slather the joint in paste flux before heating to protect the metal from oxidation. From the look of the photos you skiped this step, no?
Yes that is correct normally you would have flux paste 2" out from the joint, but as you see in the third picture the brass follows the gas. The benzomatic torch heats a large area, therefore I only put flux on the rod to lay a bead and then repeat to lay another one. that is why thorough cleaning of the joint prior to brazing is a must IMO. Downside is the impurities from oxidation will end up in a big hard crust of flux to clean up. I usually soak a joint about 45 mins in hot water and the flux is gone
For lugs the Benzomatic torch works just fine. though small detail areas like the little points on Newvex Lugs such as I used on the Green bike can be a challenge to keep the silver from running all over the place. You just have to practice on keeping the heat inside the lug .
Finally there are better quality mapp gas torches that use the 1lb bottles. Turbo torch is one brand, it has a several diffrent tips available and has a better defined flame. You can find them online for about $80 and up. Turbotorch also makes a traditional hose, handle and tank set, It starts around $150, it is pretty sweet to use.