This is where the rubber meets the road. Your tubes are filed, fitted, and ready to weld. You have you favorite welding or brazing setup. How to turn this pile of metal pieces into a component that you can bolt parts to?
There are fancy commercial fixtures out there. Anvil, and Sputnik are the two big names that I’m aware of, though I’m sure there are more. These are sweet. They’re made for pro framebuilders who need serious efficiency. I’ve never used one to build a bike, but I have met many builders who swear by them.
Mid range Commercial Fixtures:
Joe Bringheli sells some reasonably priced fixtures. I have met many hobby builders who have used his fixtures, and they seem great. It’s unclear to me how easy they are to set or how repeatable they are.
I love solving meta problems. I’ll happily make tools to make tools to make tools. This is sometimes referred to as yak shaving. With that in mind, I’ve made a number of iterations of DIY fixtures. My first one was a straight ahead Andrew Hague copy ripped straight from an illustration in the Paterek Manual.
This worked well, but was limited to fixed tube diameters. Some might poo poo the square tube used, but they show up very straight from your local steel distributor and are very inexpensive. If you’ve done any other welding fabrication work outside of bikes, you know how nice square tube like this can be to use.
A couple iterations on this landed the design at the fixture in this rambling video, where I set it up.
Around 2014 I was trying to sell tools like this in an ill advised business. While the business never really took off, it made me refine these tooling designs into a coherent system using 80/20 extrusions. 80/20 extrusions are nice because they require much less machining and they’re nice and straight and flat. They’re priced for business to business sales, but they also have an ebay store where they sell off surplus at very reasonable prices.
(I still need to gather up these designs and post links to the assemblies.)
A flat surface.
The basic building block of all these fixtures is a flat surface with standoffs of known length. If you have both of those things and the ability to make precise measurements over the distance of your frame, you might not need a fixture. It’s entirely possible to make a nice straight frame using some blocks, a flat surface, and lots of care. A full scale drawing helps with this too.
In the machining industry, flat surfaces like this are ubiquitous for measurement. They are referred to as “Surface Plates.” Vendors like McMaster, MSC, Shars, and Grizzly sell them in varying sizes and flatness ratings.
In the welding industry, great big flat tables are really useful for exactly the same task as welding bike frames. Lots of stuff needs to be aligned to a plane and then welded. Acorn welding platens are the canonical table for this task. Sadly, they are no longer in business. Weldsale sells a similar table. I have used them and they’re really nice. There are a couple of options geared toward casual users. Stronghand tools has a nice cart with a flat plate top. Certiflat is another more DIY option.
I’m excited about the potential for 3d printed parametric models and single use tooling. Done well, you could make plastic alignment tools that would allow just about any joint configuration to be held securely and welded with much lower costs than all of the tooling listed above. Validating the results would require a surface plate or other measuring devices, but those precise measurement tools could be reserved for measurement, keeping them nice and precise. Nothing ruins a cast iron surface plate faster than weld spatter.