Framebuilding with Dave Bohm: Day 6

On Saturday I spent all day preparing the parts that will allow me to build a frame with bilaminate joints. Bilaminate construction is essentially a half lug silver brazed on one tube and fillet brazed to its mating tube. It offers the sleek, seamless look of a fillet with the traditional refined edge of a lug. And yes, it’s a hell of a lot of work to do it.

Since I had already built some frames coming into this class, Dave suggested that I try a bilaminate bike for an added challenge. He must have been reading my mind because I had already dreamt of making such a bike. It was great to know that he thought id be up for the challenge.

And challenging it was on Saturday. Since the lugged portion of bilaminate construction is not an off the shelf part, I first had to come up with an idea of what I wanted the lugs, shaped sleeves really, to look like. Now I could have taken this opportunity to make some elaborate design the bike geek world has never seen, but in the interest of time I thought it best to go with something simple. The bike geeks will have to save their drool for my next creation.

To come up with a design, I drew a grid the width of the circumference of one of the sleeves and the length of the tip of the sleeve’s point. Then I used some French curves to sketch out a flattened version of the sleeve. It was hard to previsualize this, so I borrowed a spare lug to use as a reference. Once I was done, I could cut out the design and adhere it to the sleeve to use as a template for carving into the final product.

Here are the sleeves after being thinned on the lathe and mitred on one end at the proper sizes and angles. The sleeves on the left and right will slip over the top tube and connect to the seat and head tunes. The middle sleeve will slip over the down tube and connect at the head tube.
Here are the sleeves after being thinned on the lathe and mitred on one end at the proper sizes and angles. The sleeves on the left and right will slip over the top tube and connect to the seat and head tunes. The middle sleeve will slip over the down tube and connect at the head tube.
Here are the sleeves ready for action. The sleeve on the far left will slip over the seat tube for added strength.
Here are the sleeves ready for action. The sleeve on the far left will slip over the seat tube for added strength.

Before I could start carving though, I needed to create a basic sleeve. The rule of thumb is that you can use a tube as a sleeve if it is one-eighth larger than the main tube that it will slip over as long as the potential sleeve has a .58-inch wall thickness. This is a common wall thickness for 4130 steel tubes for a good reason. Even though it’s easy to source as sleeve, it still doesn’t mean it will make a good lug. The best lugs are far thinner than .58-inches. For reasons I don’t quite understand, a thinner sleeve will prevent stress risers from occurring at the edge of the brazed sleeve. Those paper-thin lugs you see from some of the top framebuilders are both aesthetically and structurally awesome.

So instead of cracking out the files to manually take the .58-inch-thick sleeve to maybe .30-inches-thick, we powered on Dave’s South Bend lathe to do the grunt work. I’d only ever used Doug’s lathe about eight years prior, so Dave had to show me the ropes. The lathe is a wonderful machine. I wish I could fit one in my micro garage, if only to stare at it in awe. The lathe is all business. It’s a deceptively simple machine that can turn metal into pretty much anything you want, if you can learn all of its secrets. I was happy enough to just make some simple sleeves.

After producing the raw sleeves, I glued on the templates and began the dark art of shaping metal with a hacksaw, sandpaper, and hand files, oh and Dave’s kickass belt grinder. The belt grinder is typically used by knife makers, but it works well for shaping curves in metal tubing too.

Although the work was challenging and stimulating, it was a relief to call it a day. I didn’t quite finish all of the sleeves. I saved one for Sunday morning, our official day of rest before five more fun-filled days of completing our steel frames. Will Cooper and I finish in time? Stay tuned to find out.

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