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Father/Daughter Project - Part 6

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Father/Daughter Project - Part 6

Wednesday, December 4, 2019   |   Chris Blandford

Over the long holiday weekend, I had Mathilda down in the studio with me a bit. She’s a wiggly almost-four-year-old, so the patience for my show-and-tell is a little short. That said, she’s been enjoying seeing the progress made on her bike, and asks to see the little frame each time we head through my space down to the garage. She also really loves squishing stuff in my vise. I keep having to tell her that we will not be squishing her frame. She only seems mildly disappointed.


6. Miter Chainstays
7. Drill Vent Holes
8. Tin Seat Tube to Bottom Bracket
9. Final Fit & Prep for Brazing

I miter my chainstays in the same way as my main tubes. However, there’s one extra (semi-backwards step) that I take here. Back in BikeCAD, I now retroactively make the computer drawing match my chainstay hand drawing. All I care about here is the miter template. So, now that I have a chainstay miter angle (from my hand drawing), I can make the BikeCAD version match. It takes 30 seconds--and I don’t care about the overall shape of the chainstays in BikeCAD, just the diameter and angle. Once done, I print the templates, tape them onto the tubes, and set their distance from the rear axle (which is taken from my hand drawing). Trace, hacksaw to rough length, tin snip, and file.

These used to give me fits. I’ve found, however, that thinking of the chainstays as a single unit--instead of individual tubes--has helped me with their mitering quite a bit. Once I get the individual chainstays mitered, I place them onto their dummy axle--together--and lightly clamp them in my vise. Just a light swipe or two with the file--across both tube ends at once--saves a lot of tail-chasing in the jig.

Next I drill all of the vent holes. I first trace the joining tubes’ copes with a Sharpie, then drill the holes on my drill press. I start with a combination center drill/countersink bit, then move to a regular drill bit for each hole. Usually ½ inch for the main tube holes; smaller for the chainstay holes. I also drill vent holes at the dropout end of the chainstays, facing down (obviously) and about 10-20mm from the dropout tab. After drilling the holes, I clean everything up with 80-grit and a swipe of isopropyl.

Next I tin the seat tube to the bottom bracket. I load up the inside of the seat tube with blue flux, give the BB a light coat, and load the tubes into the jig. I center the seat tube on the BB by checking either side (between the seat tube and BB edge) with calipers. Here, I use the same Smith 203 tip and 1/16th bronze rod as was used on the dropout brazing. I start at the front of the frame, and braze from centerline to 90-degrees in both directions. Then I move around to the back of the frame, and braze from centerline to 90-degrees again, to meet. The goal is a small fillet with full internal penetration the entire circumference of the joint. I let the assembly cool in the jig, then it goes into the soak tank.

The little fillet now gets in the way of the downtube/BB miter. I take a couple swipes at the fillet, and round off the points of the downtube to return the fit to how it was prior to tinning. Again, this is another advantage to working in larger, more cohesive steps. The first couple frames I made, I mitered the downtube after tinning the seat tube to the BB. Getting good fit up was tricky, though, because I wasn’t sure if the miter was as it should be if the little fillet wasn’t there. Much easier now that I have a (slightly) better grasp of the overall process and can avoid this type of thing.

The frame bits are now ready for fillet brazing. Time to have some fun.