Yamaguchi Framebuilding School - Day 2
Day two of the Yamaguchi framebuilding class, and it’s time to start with the metalwork. The metal in question, as selected by the Koichi, is a Columbus Zona tubeset. These tubes are fabricated from Columbus’ patented ‘Nivachrome’ alloy, so-named for the addition of Niobium, Vanadium, and Chromium. The Ni and Va serve to produce precipitates which prevent grain growth within the heat-affected welding zones, mitigating the loss of mechanical properties in these areas. Otherwise, the Nivachrome alloy is largely simmilar to 25CrMo4, which when translated to american is AISI 4130. The steel’s as-produced yield strength is ~1.03 GPa, ultimate tensile strength 1.2 GPa, and elongation at failure is 12%.
The tubes are seamless, and formed by hot rolling followed by cold drawing over a mandrel. They are also ‘double butted,’ indicating that the tube wall is thicker at the ends (0.9mm) than it is in the middle (0.5mm). The butting serves to increase the strength of the welded joints. Overall, the Zona tubes represent a good compromise of properties. They are not the strongest or lightest, but in the thicker double-butted sections they are fairly stiff. The alloy is also relatively forgiving with respect to welding, which is ideal for a frame welded by a total novice!
With that knowledge in hand, the first task is to cut the tubes to length, and then miter their ends so that they fit together properly. I had anticipated that we would perform this task using a milling machine - but no! Koichi informs me that the most sensible way to cut and miter the tubes is by hand, using only a hacksaw and handheld files. This runs totally contrary to my intuition, but I am here to learn after all!
We begin by cutting tubes to length using a hacksaw. This tool differs slightly from the one you may be familiar with in two respects. First, the saw frame holds the blade in a high state of tension, to keep it very straight and prevent the cut wandering. Second, the blade itself is manufactured from high-speed steel (HSS) and has very fine teeth (32 per inch). The blades are made by the L.S. Starret company, who are better known to me for their machinist’s tools. After a few practice cuts, I am surprised to find how rapidly the tough alloy tubing can be cut, while maintaining excellent squareness and dimensional tolerances.
Once the tubes have been cut approximately to length, they must be mitered in order to fit together properly. This requires cutting a smooth and uniform compound curve in the tube end - something that seemed extremely difficult to do by hand! However, Koichi demonstrated a remarkable method to accomplish this quickly and easily. First, a flat file is used to cut two planes tangent to the desired contour. The intersection of these planes with the tube forms a pair of conic sections - parabolas in this case. A half-round file can then be used to slightly reshape these parabolas into the required form. Just as Koichi said, with a little practice a miter can be cut in this fashion in less time than it takes to set up a mill to do the same job!
In the future, I will try to make an animation or video demonstrating the mitering technique. Maybe I am just the last person on earth to find out about it, but it really knocked my socks off. Another future project is a simple program to automatically generate high-accuracy templates based on the actual (not nominal) tube diameter measurements.
After a lunch break, my instruction in brazing begins. Since I've never done this before, we begin with an introduction to the equipment and materials. An small oxygen-acetylene torch is used to provide heat, and a "gasflux" system is used to mix a fluxing compound into the fuel gas stream. This reduces the need to manually apply flux beforehand in order to prevent oxidation during brazing. It also has the pleasing side effect of turning the welding flame an incredible Wizard-of-Oz green!
The material that we will be using to perform the frame brazing is Gasflux C-04 Nickel Bronze. Koichi tells me that this is the most widely used material in the US for brazing frames. It has a tensile strength of 430 MPa, and a liquidus of 882 degC. Much like the tube steel, it is a good compromise between mechanical strength, and ease of welding.
That's all for today, I've got hours and hours of brazing practice before I'm ready to start on the frame. More exciting new material tomorrow, I’m sure!
