How a fillet-brazed custom steel frame is made [Behind a Custom Steel Bike]
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How a fillet-brazed custom steel frame is made [Behind a Custom Steel Bike]

October 26, 2019


Today I’m going to show you how we build a custom steel frame with fillet-brazed construction. You will discover the benefits of this fascinating technique. And you will see an Italian craftsman in action while he’s working on a fillet-brazed frame. I’m Alex, and we’re at the Battaglin bike workshop in Marostica, Italy. Let’s get started! You might have already heard about fillet-brazing as one of the most common techniques to build an exclusive custom steel frame. But how fillet-brazing works exactly? Fillet-brazing is a low-temperature process to join steel tubes on a bicycle frame. The frame builder melts a brass rod into the joint. Brass has a lower melting point than steel. And it joins the tubes by creating a metallurgical bond with them. Fillet-brazing is a process that requires an expert artisan. In Italy, we call such a skilled builder “Maestro”, which means “Master”. Becoming a “Maestro” is a steep learning curve. Getting sculpted joints as those you can see on this frame is high craftsmanship. It requires hard work and years of practice. But what are the benefits of fillet-brazing over other techniques? The first benefit of fillet-brazing is that it’s a low-temperature process. Brass, the filler metal, has a lower melting point than steel. So, the low temperature required for the fillet-brazing process doesn’t melt the steel, preserving the tubing’s properties. In fact, heat control is crucial when building a quality steel frame. Overheating today’s lightweight and thin-walled tubes can make them more brittle and lead to frame failure. At Battaglin, we use a special green flame to avoid overheating the tubing when melting the brass rod. The second benefit of fillet-brazing is that it gives you more choices in terms of geometry. You can build a custom steel bike both with a sloping top tube or with a horizontal top tube. At Battaglin, we offer our fillet-brazed models in both versions. It all depends on the cyclist’s preferences. The third benefit of fillet-brazing is… …the look of the joints. A skilled frame builder can create seamless joints that will make for contemporary and clean aesthetics. If you dream of a custom steel bike with a modern look and the latest components, oversized tubing and fillet-brazing are the way to go. Some people might even think it’s a carbon bike! I hope you enjoyed this video about fillet-brazing. Leave me a comment down below and let me know which part of the custom bike building process you’d like to see. I’ll see you in the next video!

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  1. Nice video, Alex. I would love to see some of the finish work that gives the fillets their final, smooth profile, especially around the head tube. Also, an overview of the polishing and chrome process in the cromovelato finish would be interesting… How about more info on your lug designs and a tour of where they are cast? Especially the lugs on the Portofino! Cheers!

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