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Father/Daughter Project - Part 4

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Father/Daughter Project - Part 4

Monday, November 25, 2019   |   Chris Blandford

Ok, enough of the planning and ordering. Off we go. I’m going to start with the Daughter bike, and--as such--have set aside the Father bike’s materials for now.

To this point, I’ve always started with my bicycles’ frames. So, same here.

Steps for today:

1. Clean and Inspect Main Tubes
2. Miter Front Triangle
...

First, I clean the head, seat, top (though not in this case), and down tubes, inside and out. For this, I use 80 grit emery cloth (shoe shine style) and 91% isopropyl alcohol.

Next, I mark the tubes for orientation. I have one of the small Bringheli surface plates. I roll each tube across this plate--sighting between the tube and the surface--to find its bow. I mark the “top” of the bow. I’ll orient the bows in the vertical plane (? sorry, I should’ve paid more attention in Geometry) of the bike/jig.

Continuing on, I Sharpie and scribe a line along the “top” of the tubes. I do this by holding the tube against a piece of U-channel aluminum. I like the Sharpie for visibility, but the scribe carries through the build process better. So I do both.

At the same time, I cut the head tube to length (based on the full-scale drawing) and square off its ends on the disk-side of my belt sander. I also mark the center of the bottom bracket, using my calipers as a scribe. If any of the tubes I’m using are butted, this is also where I’ll mark the butts. I do this by referencing the spec sheets (found on the supplier’s website). I also confirm the specs by holding the tube up to my skylight and sighting the internal butts. The main tubes of this bike, however, are straight gauge, so I skipped that step here.

Next, I move on to mitering.

I hand-miter everything. First, I print the little paper templates from BikeCAD, cut them out, and tape them to the tubes. The templates have indicator lines that I line up with my scribed “top” line. I get the distance between miters from my full-scale drawing. Then I trace the templates with a sharpie and slide the templates off of the tubes.

The miters are first cut out (within a few mm’s) with hardware store-bought tin snips. Then they’re shaped with half-round files (the size of which matches the mating tube’s diameter). Finally, I hone them in with the mating tube wrapped in a piece of 80-grit emery cloth. I load the tubes into the jig as I go (which is now set, I should have mentioned, based on my drawing/BikeCAD).

If there’s one tip I could give someone new to hand mitering, it would be this: act like a machine! I treat my file strokes like a machinist trams the head of mill (I think). I always try to file horizontal to the ground, and either parallel or 90-degrees to the edge of my workbench. If a tube requires an angled cope, I adjust the angle of the tube in the vise, not the angle of my file stroke. If a tube requires an actually-angled miter (like a unicrown fork blade with rake built-in), I adjust the angle of the vise in relation to my bench; I don’t change where I stand or the direction of my filing. In addition, I affix two Paragon tubing blocks to each tube (using the surface table to keep them in phase), and I keep these blocks on the tubes until all the mitering is done, just like a machinist would. I do this so that as I go back and forth between vise and jig, I’m not having to re-orient the tube in a block, potentially messing with the phase. Finally, I use a digital angle finder and my eyes (sighting along the vise or bench top) to check the miters.

All of that, above, was an epiphany I had a couple frames ago. Act like a machine. (I realize that these types of things are probably very obvious to someone with fabrication experience.) As a result, my mitering has gotten much more accurate as of late. This time around, I only had to check the downtube in the jig a couple times before it fit nicely. On my first couple frames I chased my tail for hours. Maybe days. It’s really satisfying to improve at something like that, even if just a bit. I wish I’d had this epiphany earlier… My files would be much sharper than they are now!

The front triangle is now mitered up. Chainstays are next.