Learning to fillet braze

Bit of a photo essay for you. This isn't so much a project, more a technique I'm planning to use in one.

Last week I did the Fillet Brazing masterclass down at the Bicycle Academy in Frome. I've done a bit of brazing before, but never fillet work, so really I'm a novice.

I thought it might be interesting for people thinking of building their own frame. I'm a great believer in professional education, especially when dealing with techniques that are used in safety-critical applications.

We started with basic safety and some theory, then got down to the practical business of laying down braze. The aim initially is to get nice and consistent with the braze. You can see here that I'm a bit blobby and wobbly in places, but over time I get to the point where I can lay down a row of "stacked coins" which is what I'm aiming for.

First Practice

This is a close up of the look I'm after

Stacked Coins

Then it was onto joining two pieces, learning to tack pieces in place, then how to bias the position of the flame to help the fillet adhere evenly to both pieces.

First Joint

Once that was done, I moved onto the kind of joints you make at the head tube on a frame, so the geometry is more complex, with deeper valleys and flat sections to contend with. This is plain-gauge 2mm steel, as it's a bit more forgiving of potential overheating.

Mitred Joints

After getting these mitred joints done, we stepped up the difficulty with thinner steel (1.4mm) and overlapping brazes, such as you'd find on a bottom bracket. What you want is for the brazes to integrate into each other. You can see how they blend together right in the centre of the shot.

We also did a bit of root brazing, which is where you run a very narrow braze right around the joint before you lay the fillet. That's a belt & braces approach that boosts the strength, although a standard fillet is very strong in the first place.

Overlaps

After the creation, time for some destruction. We cut open the 1.4mm practice piece to assess the braze integrity. You can see there are no voids at the joints, so these are nice and strong.

Opened Up

Now for some pretty tricky stuff; narrow tubes with 1mm walls. The low mass makes them prone to overheating, which damages the strength and integrity of the steel. I did a narrow join, and a fillet. The fillet uses loads of rod, as it's a deep vee you're filling. You feel like you're pouring it in.

Narrow

Fill

The last piece of work was joining the sort of tubing used in Moulton frames. That's a bit like joining drinking straws with acute bends, but that went fine. No pictures, unfortunately.

We'd been using gas fluxing, which minimises deposits and clean up. At the end of the course I had a play with laying fillet using paste fluxes. Feels quite different, but not too bad once you get the knack.

I brought the pieces home, and did a rough file and smoothing of one of them to see how they look if you want that smooth transition from tube to tube. Here's a joint after about 15 mins of filing and sanding.

Smoothed

So over the space of a day I went from never having laid a fillet, to being capable of producing decent joints across a range of tube types. Next step is to transfer these skills over to a frame.

If you're thinking of building a frame it's the sort of course I'd recommend without hesitation. I had a great time, and walked away with an entirely new set of skills.

If you have any questions, fire away.

fantastic! The raw fillets look awesome. For my fillet frames I had to file them as the edges were pretty ragged.

I just signed up for the course myself Mike. I explained that I had Fillet Brazed before but with Paste, not Gas Flux. Andrew explained that although they use gas flux, it is mainly to allow the students to read the pool; the same technique is usable with flux paste. (I wouldn't need to invest in a ~£900 gas fluxer after the course, my current setup will work fine). I am really looking forward to it.