I usually make wishbone stays but I wanted to make this current cross frame more ‘classic’ looking and a little lighter, so I went with fastbacks. I’ve been filing my fastback seat stays as a way to “earn my chops.” (If you’re not a new framebuilder you don’t know this…but the old/er guys mostly recommend (want) you to learn the hard way, like they did. Kinda the “walked uphill to school in the rain – both ways” type of thing. I understand it, I respect it…and even get it. Even though I’m the first to admit I have acquired more tools and fixtures than some would have just starting out, I think it’s helped me progress faster than I would have otherwise with framebuilding and welding. Not in other ways of course (like making my own tools and understanding the intricacies of that craft) but I have been able to focus on building bikes which is what I want to do after all. Had I started by building my own tools, I’d likely have built only a few frames if that.)
The irony is now that I’m kinda know what I’m doing, I have a much better idea of what I actually need to build a better bike through a more efficient and accurate process, as well as what I need and want in my mitering fixtures. Had I started out from scratch, I would have had NO clue on what I would want in a fixture. I still have very little experience using machinery to actually MILL, but I at least understand more how it’s done and can do more basic processes. Yes, a community college course in machining would be a good idea, but really…who has the time? I live at least and hour or more from the nearest community college and have no one to ask that does this for a living nearby. So, I do what I’ve always done, trial and error.
I’ve seen lots of builders use 80/20 aluminum extrusion for anything from frame fixtures to chainstay fixtures. It looks pretty easy to work with since there are many parts and pieces available. I figured that’s what I’d try using for my first seatstay fixture. I got the materials from McMaster Carr, and their website is just awesome. It does take some getting used to, but the amount of info and product on there is endless. They even had the CAD drawings with specs on each item. The t-slotted framing and hardware is located together so you can figure out what goes with what and order it up together. For starters, I just got a 1.5″ solid extrusion that’s 2ft long and some fasteners. I had the 1/4″ piece of steel from another crappy “fixture” I used long ago to make unicrown forks but it needed some work. I cleaned up the middle slots with a 1/4″ endmill so I could slide the tube blocks side-to-side, and also get a 4mm allen key in there to tighten the blocks up from underneath. It’s not an extremely rigid fixture, but it actually works just fine (see pictures)!
The key with this fixture will be to create a top plate to hold down the tube blocks while mitering (like with the Sputnik stay fixtures) to improve rigidity. Keeping the shortest amount of the stay sticking out of the front of the tube blocks really helps with rigidity. I was getting some catching/snagging/popping of the hole saw on the tubes because of the long arbor flexing as well as the stays flexing.
I used just the single extrusion because of the tube blocks lying flat on the plate idea you see here. Most seem to build it on the wider double extrusion (i.e., Meade’s seatstay fixture) and make a better platform for the tube block. I may go there some day as well as make a better axle holder. My axle is totally clunky but is mostly just for the spacing since the tube blocks hold the spacing and tubes where they need to be. The axle is my Bringheli dummy axle that came with my alignment system, inside a 1 x 0.049″ split tube, inside the 1″ single clamping tube holder from McMaster. The total cost of new stuff was under $60. So, if you were to do this the right way and put more time into it (this took me a couple of hours) it’s possible to make a much stiffer, prettier, better fixture for around $100 total (and that’s without any fabrication of parts myself, which could lower costs even more). It’s pretty damned fun to piece this puzzle together and get a usable result.
Improvements: I want to use a Anvil indexed dummy axle so I will have to fabricate a different rear axle holder. They only sell the tube holders in 1″ diameter, so I’ll have to create a baseplate for the axle and use a toggle clamp for easy in/out. The modified (mutilated) Paragon tube blocks work but I’ll likely flip them and drill/tap holes in the bottom to attach to the steel plate to make the fixture quicker and easier to load up and get centered.