3/2 (oops):

Or, an idea someone else just gave me: are there other people who would like to do hobbyist frame design but can't afford BikeCAD? Maybe we could start a co-op that does an old-school computer time share or something and we buy a copy of BikeCAD that we can all take turns using? Although I wonder if this is legal or if the BikeCAD guy wouldn't like it? I have no idea if it's sold "per-seat" or if that means "per-person" or "per-computer".

I wonder if people would purchase bike designs that don't include the actual frame?

Ie. in exchange for a donation you get a design consultation, fitment, schematics, and mitering templates. Even a printed blueprint if you want. If you want an actual frame you'd have everything you need to take to a machine shop (or I could pre-sell a huge discount on the frame if I ever raise the money for the facilities I'd need later, though that seems less likely).

2/2

And nows it's painted and is looking very bike-like
#framebuilding #biketooter

Glam video of a mostly finished test joint in actual bicycle tubing from a seventies SR Silstar frame.
The air filer removed enough steel you can see that the intersecting tube wall thickness is reduced, but once I started using the sanding bands by hand (and cut them in half lengthwise) it was way more controlled. I might build an adapter for a bowsaw to carry sandpaper bands.
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I'm still learning how to finish the brazed joints. I establish a line down the centerline of the intersection with a big round rough file, and keep using that for most of the removal, then finish up with a combo of an air filer and hand sanding with a long thin belt. This air filer is WAY too wide. The sanding belt needs to be max 5mm, better 3mm. And honestly, hand drawing the sanding belt was really nice, fast, accurate, and way more flexible than the air filer.
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But I have to be honest: some of my welded joints were so beautiful, so well filleted, that I'm not sure I need to braze them. If I could do every welded joint like this I would be thrilled. I added in a moderate amount of filler on a first pass, and then used a zigzag pass while adding a tiny bit more filler, and got this incredible fillet, with no post-weld finishing work. This is lighter than brazing. (It has a shorter moment arm so may be more susceptible to buckling.)
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And then it's on to BRAZING! and brazed joints are beautiful even when they're right off the torch. As noted elsewhere, I am using a TIG torch and silicon bronze filler. I was originally using AC with a 90% positive cycle for cleaning, but found that if I sand and scrub the joint well, DC does just as pretty a job and the material flows better. These need lots of finishing. But you can quickly add LOTS of brazing rod to get smooth fillets.
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Intent was to tack weld and then put in a brazed joint over the tacks, because welding is difficult and slow and puts way more heat into the tubing, and leaves stress concentrations along the weld edges, where you can braze/file to form a tangent curve to both intersecting tubes, which makes the joint much stronger even though the brazing material is a little weaker.
But I got a tiny tig tip and some 0.030" wire and started making tiny welds. I'm not proud of these but it's fun.
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The pseudo bottom bracket is just a steel plumbing nipple but it's close to the right diameter, so this is a case where I have to miter the tubes to meet a much larger diameter tube, and also the down and seat tubes intersect, so after mitering each I have to do a secondary miter on one to match the other. This was fussy. The picture shows a huge gap. I clamped this much tighter but there was still more gap than anywhere else on the frame. I need practice here.
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Real framebuilders have fancy alignment jigs. I may make one. Right now I'm using a piece of plywood to hold the front triangle flat (which only works because all the tubing is the same diameter.) I clamped the tubing to the plywood but it scooted around, so I used #freecad to design a little vblock and printed it. They sit on the plywood and let me securely clamp the tubing. In one picture here I'm trying to fit the downtube into three other already mitered tubes.
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What I have been doing is using a fancy jig I made for my lathe for line boring the intersection. This has the advantage of producing perfect joints: if you hold them together they're briefly air-tight. It's very slow and somewhat misuses the lathe since it's yanking the cross-slide into the work for part of the cut. I'm probably going to build a hole saw arbor for the lathe that'll reuse the tubing clamp system shown here, and then do a finish cut with the line boring tool.
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When I start a tube, I mark it with a line at the intersection angle with the tube it'll hit, and then subdivide that mark with two more, that are symmetric with respect to the intersection mark and at least 90 degrees to each other, wider if it's hitting a larger tube. Then I hacksaw all the way through the tube at those two lines and that'll remove 95% of the material needed for a decent miter.
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