Saturday, April 16, 2005

Fillets this Time.

I had some time today, so I decided to try another fillet joint and then cut it up - pictures are below.

Before going farther, lets describe fillet and lugged joints, in one paragraph, for those who may not know the difference. There are two ways of brazing tubes together to make bicycle frame. Brazing, by the way, is essentially a process of gluing together a couple of tubes where the glue is a metal that melts at a lower temprature than that of the tubes - the idea is to avoid overheating the tubes and thereby changing their mechanical properties. Many brazed frames are built with lugs. A lug is a metal piece that has a circular opening for each of the tubes being joined. It completely covers the area where the tubes meet, and covers some amount of additional real estate on each tube. The lug itself doesn't hold the tubes together (that's done with the brazing material), however it may impart some additional strength and/or stiffenss to the joint. Lugs can be big or small (click on the link to the left for Strawberry Cycles and see how small a lug Andy Newlands is able to produce). Lugs are often decorated either by carving (shaping) them in pleasing way (including putting holes of different shapes through them), or by painting them, or (in the case of stainless steel lugs) by polishing them to a chrome-like luster. The alternative is to work without a lug. In this case a fillet (pronounced fill-it) of bronze is built up between the two tubes. This is filed to a pleasing radius (which may be big or small depending on a host of variables). In the process, a fillet should be formed on the inside of the tubes as well. A fine line of bronze runs between the two tubes (where they meet) and bridges the inner and outer fillets. In this way, the joining tube is encapsulated with bronze for a strong joint.

Working with bronze, I seem to fear the heat some; which was part of my focus today. Starting with the biggest flame available on the 153 tip at 10lbs O2 and 7 lbs propane, I also adjusted it more towards neutral than carborizing. This seems like the right way to get things hot enough for the bronze. I also explored using the side of the flame some of the time. Instead of pointing the flame directly at the rod, I'd run the flame almost parallel to the tubes. This way it was possible to to get a large area very hot (after pre-heating) and edge the hottest part into the rod without seeming to burn the flux so badly. Mostly this worked well where I could reach the work this way.

One thing that isn't clear to me is how useful or even important it is to have the joint suspended in a way that it can be turned upside down and spun from right to left while building the fillet joint. I'm working from an aluminum armature held in my vice to which the two tubes are clamped. As such, its hard to move things around. I had to do some moving because the vice is connected to the corner of the workbench. Certain angles just aren't accessible to work the flame. Anyhow, I may try clamping the armature into my bike stand next and see if the added flexibility helps.

I did one dumb thing with todays joint. I mitered a larger diameter tube to a smaller diameter tube. This results in a pair of ears that extend beyond the outer circumference of the smaller tube. The difference in tube diameters is small (0.125"), but this is enough to make the joint much harder to execute well as you can see in the pictures below. I was also using a strong rod, but one which tends to be more liquid when melted and consequently forms a narrower fillet. This tends to excacerbate the problems caused by the tube sizes I chose.

Looking below, there are a couple of key findings: 1) the inner fillet is large and encircles the joint giving strength all the way around (I used the trick of placing a circle of rod inside the tube before the joint is assembled); 2) where the miter touches the main tube, a nice fine fillet formed which files to a pleasing radius; 3) where the miter gaps from the tube, the fillet only forms a nice transition between the tube, leading to a less strong joint.

My summary, I'm getting closer to being able to create functional joints either with lugs of filleted, but still need some improvement. I'm going to try a couple more head-lugs and then move to practicing on a BB. These will all get cut up to determine if that improvement is occuring, or if I need to keep at it.


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