Frame Fixture (2016-17)
I spent about nine months of nights and weekends in 2016-17 building this bicycle frame-welding fixture (a.k.a. frame jig). I put a lot of time into researching other designs that already existed, and then I mixed and matched ideas to get at my own thing. I wanted it to be rigid and easy to use, provide good welding access, and work on a wide variety of bicycle frames.
I’m not producing and selling this jig to the public. When I do end up selling a frame jig, it will look different from this. I was just building it for my own use, so I didn’t worry about if I was ripping off other people’s ideas. When it comes to selling products, I take intellectual property more seriously, and I wouldn’t copy someone’s ideas without at least advancing the tech or trying to improve it for the end user.
One of the big ideas for my design was to make the head tube always perpendicular to the main extrusion of the fixture as a way to remove one angular adjustment from the fixture. On my tool, the “seat tube angle” is actually the seat tube angle minus the head tube angle. You set up the fixture with a finalized bike frame design and five variables that BikeCAD spits out for you.
After I had the fixture together, I started to play with some convenience features like this. Here I have a plate that makes it a bit easier to hold the chainstays for welding, and the center boss represents the tire and helps you make sure the stays are symmetrical.
I painstakingly engraved the angular scale on the tool without a rotary table or CNC capability. I spent all day on this one detail. I would clock the plate half a degree, tram it in using trigonometry and dial indicators, and then clamp it down and machine another line. Here’s an old video I recorded of me working through the challenge of machining this. The reference indicator has a bit of adjustment side-to-side so I can calibrate the tool.
For the standoffs that hold the cones and pucks of the fixture, I wanted to go with really heavy duty bar stock for maximum rigidity. When you’re only building one of something, the material cost is not that big of a consideration. If it’s not required to be lightweight like a bike, why not overbuild it?
This axle and plate bolt to the back of the fixture and mate with the stand for rotation. The four big holes in the face of the plate were only for aesthetic, and they were a ton of work to machine manually, but the finished part was so pretty.
Similar to the angular scale, the reference pointer here is adjustable for calibration. The linear scale is one of four, as the whole fixture can be setup with an X,Y coordinate for the bottom center point of the head tube, and an X,Y coordinate for the center of the rear axle.
This is how the seat tube angle is locked into place. I wanted the clamps on the fixture to be as easy to operate with one hand as possible and to all have hand knobs so that you wouldn’t need wrenches to set it up and make adjustments.
A picture at the end of the long day when I manually machined all of the witness marks for the angular scale for the seat tube as described above. What a job!
The heavy duty standoffs for the head tube and the top of the seat tube. The parts that interface with the tubes have been plumbed for argon backpurge during welding.
Here I modified the seat tube standoff to allow a bent seat tube. This design is taken pretty blatantly from Sputnik Tool, and it was one of the first parts I machined with my new CNC mill when I got it up and running in the spring. To truly finish this part, I would need to engrave a linear scale on the part and a reference line on the stainless cone.