Bicycle Framebuilding: Jig Design

Every year in the spring, I teach a one-credit co-curricular in which 6 or 7 engineering students learn to build bicycle frames. We keep it pretty simple – we only work with steel, and students must either fillet braze or capillary braze (using lugged construction). No exotic materials. No TIG welding. Just good old fashioned oxy-acetylene brazing.

In years past, I’ve had the good fortune of being able to use a jig built by a predecessor of mine, but this year I wanted to build my own jig to tune up my machining skills again and to make some improvements to the previous design. I opted for a sort of “S”-shaped design with it’s closest commercially available cousin being the Arctos jig (pictured below). It seemed like a well-proven design, and I really like the way the bottom bracket pivot floats thanks to a clever dual-curved-slot design that puts the center point of the slot arcs at the center of the bottom bracket.

The commercially-available Arctos bicycle frame building jig. Image credit: Oscar Camarena

Thankfully, I did not have to start entirely from scratch in copying the Arctos design. There is a wonderfully detailed Instructable created by Instructable user “Tanner W” that I also referred to for basic dimensions and ideas about how to fabricate different parts of the jig.

The aspect of this project that I am most excited about is that it gave me an excuse to go deep on my new favorite CAD software, Onshape. If you’ve never heard of Onshape, imagine a marriage of Solidworks, Google Drive, and Github that runs entirely in your browser (that’s right, I said IN YOUR BROWSER). (As a Mac user, can I just say what an absolute pleasure it is not to have to boot into Windows or run a VM to work in CAD?) Ok, permit me a momentary diversion to highlight just some of the myriad reasons Onshape is a delight to use.

  • Sharing – You can share your models with any other Onshape user. This has been sooooo helpful for checking my students’ bicycle frame models before they started notching tubes. We can collaboratively edit their model from any machine and it just updates and works. It’s the little things, people.
  • Mating – Onshape’s mating system for assemblies is dramatically more intuitive to use once you get the basic concept (attaching coordinate frames to parts and specifying relations between those frames). I suppose it helps that I teach dynamics, though, so coordinate frames are very familiar to me.
  • Multi-part design – In a single “Part Studio,” you can design multiple parts very easily. This makes the creation of assemblies very easy.
  • Updates – New features are released continuously. No downloading new releases. No hotfixes. No agonizingly slow local installs.
  • FeatureScript – This is THE game-changer for Onshape. Using a very JavaScript-like language, users can create new features that are native (NATIVE, PEOPLE!). They even get UI icons that show up in the toolbar. Because they are native, they automatically update when you change geometry and rebuild the model. It’s unbelievable. In the design of my frame jig, I used a couple FeatureScripts – one to create 8020 extrusions and another to make standard cap screws.

Okay, enough blathering on about Onshape. If you’re interested in using my jig design or just checking it out, it is publicly shared on Onshape. You can view it without logging in, and it is copyable and exportable for all Onshape users. The Onshape document includes models for all custom and COTS parts, an assembly, and engineering drawings with dimensions. Aspects of the assembly could probably be improved. Despite years of CAD experience, I’m still pretty new to Onshape’s system, so I suspect there are better ways of doing some things. Also, the drawings are still fairly rough (no tolerances, for example), but should give you a starting point for fabrication.

My Arctos-inspired bicycle frame building jig.

If you’re interested in building my design, drop me a note to let me know what doesn’t make sense or could be improved in some way.

Stay tuned for Part 2 of this post with more photos of the finished jig, its constituent parts, and short descriptions of how I manufactured each of the custom parts.

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