It's starting to look like a bicycle frame…
I brazed the front triangle of my first frame this weekend.
Things went pretty well. I did have one mistake which has made a fairly minor change to the geometry. My brazing order around the downtube, seattube, bottom bracket area should have brazed the front of the downtube and the back of the seat tube first before any of the crotch in between them. The fillet in the crotch pulled them them together, which made the seat tube angle a bit tighter. I think that this is all okay, I’ll just build the bike with a 72.5 HTA and a 73 STA instead of the opposite as I had planned. The seat tube is 56cm C-T and the top tube is 56cm C-C (giving me a virtual of around 57cm because it has a sloping top tube). The top tube slopes at roughly 4 degrees. This is a learning frame, so I’m okay with a few little mistakes.
The seat tube to top tube junction was one that I spent a lot of time thinking about. I wanted to put a sleeve here to keep heat distortion down and I don’t like brass brazing long sleeves. Mark Bulgier had the best suggestion for handling this area, but I didn’t have the right materials on hand. My solution (a pretty common one) was to sleeve out of 1 1/4″ x 0.058″ (which sleeves perfectly over a 1 1/8″ seat tube). The top tube was brazed to the sleeve first with brass, then the sleeve was brazed to the seat tube using silver. The silver inside the joint will melt a bit when I braze on the seat stays, but the sleeve is so long that the top and bottom will stay solid and the silver won’t be able to go anywhere. That is the theory anyway, we’ll see if it is true in a few weeks. My biggest concern is that I probably don’t have good silver penetration between the sleeve and seat tube behind the top tube fillet. I’m not worried about the lack of strength there, but I hope that it doesn’t create a good area for rust.
I used a pin through the sleeve to align the vent hole in the top tube with the one in the sleeve.
Everything came out fairly straight when I checked it on my alignment setup. The head tube has a very slight twist when compared to the seat tube. I might try to cold set it out of there, but I’m not too worried about it.
I mitered everything on my milling machine. My setup for this was really simple,but seemed pretty effective. I used two of Alex Meade’s clamping blocks and clamped the tube into the milling machine. I set the angle using the machine’s head (I don’t really have a good angle table to adjust the angle of the tube itself). To keep the miters in phase I always kept one block locked onto the tube at a time when I moved the blocks from one end of the tube to the other. The final miters came out nicely. I only touched up one miter witha file,and that was the top tube to seat tube miter because the top tube length shortened a bit when my ST/DT angle tightened up.
If you aren’t bored with this project by now you can see tons of other photos in my smugmug gallery.
Nice work Alex! Your miters(copes?) and fillets look great.
Tip: If you flux 3-4″ down the tubes you don”t get the heat discoloration, making clean-up a little easier.
Keep it up, you”ll have a rideable frame before you know it!
Jerry
Jerry — More flux is a message that I got on the framebuilders list too. We”ll see how I do in that department on the rear triangle.
Alex, I am completely not bored by this project. It’’s looking fantastic, thanks for the step-by step updates. What’’s the blob of silly-putty looking stuff on the mitering setup?
Also -what kind of distortion does the sleeve help avoid? Alignment issues, or seat post fit or ?
wouldnt it look better with a 6 degree sloping top tube?
Jimmy — the blob of silly putty is silly putty (actually it is plumbing putty). The Paterak framebuilding book recommended this as a way to keep chatter down. It doesn”t hurt, not sure if it helps.
The sleeve reinforces the thin tubing (.5mm at that portion) where the seatpost slot is and keeps it from distorting too much during brazing. There are a minimum of three brazing operations there (top tube, seat stays, and binder). Early TIG welded bicycles also used a sleeve, newer ones usually use externally butted tubing that is larger diameter there than in the thin section. Lugged bicycles have a lug which does the same job.
Hello. I think the article is really interesting. I am even interested in reading more. How soon will you update your blog?
I”ve been enjoying the outside too much, so I haven”t done much on the frame recently. I”ll have some updates when I”ve finished up the chainstays.