Last Friday we finished our brazed bicycles. If you’re confused, please read Stay & Finish.
But there was no break. First thing Saturday morning we started our Tungsten Inert Gas (TIG) welding seminar, helping us get familiar with that style of welding.
Quickly – brazing is similar to soldering, just at a higher temperature. Both use a dissimilar material (copper, silver, brass) to join the worked material, which in our case was steel bicycle tubing. Welding uses the same material as the worked material, so in our case we were using steel wire to join steel tubes. The electrical spark is hot enough to melt the two steel tubes, plus the filler wire, for a fraction of a second, after which it “freezes” to a solid piece.
Learning the technique is a bit like learning to rub your belly while scratching your head. And doing jumping jacks.
The dominant hand keeps the tungsten electrode hovering just about 1mm above the surface. The non-dominant hand holds the steel wire, ready to feed it into the molten pool. The dominant foot controls a pedal, which modulates the amount of electricity flowing out of the electrode. All of this behind a dark glass visor that makes everything difficult to see, impossible if you don’t have a spark.
As thin as the bicycle tubing is, a mistake in any one of those areas could lead to a hole blown through the tube wall. It happens faster than you would believe. It is impressive.
I made some pretty big holes in tubes this weekend.
While the technique will take some time to master, the process is simple.
Cut and miter the tubes to make the appropriate connections:
Then weld them together:
Not too shabby for just a couple of hours of practice. But a lot of room for improvement.
These were practice pieces, so we were using cheap tubing instead of actual bicycle tubing. Quite a few impurities, which caused the pitting in the welded seam on the right. That and a lot of technique on my part, since that area gets very tight to get everything into and stay functional. That tight area was were I blew through every single one of the holes that I made this weekend.
After a day and a half of practice, we were give actual bicycle tubing to simulate the joining of the seat tube to the bottom bracket. No tricky angles, but since the bottom bracket shell is quite a bit thicker than the seat tube, it meant a shift in technique to make sure we didn’t blow a hole in the seat tube. I didn’t, but my weld was nowhere near as nice looking as the one above.
But looks didn’t matter (much) for this exercise. We were concerned about the strength of the weld. And there’s only one way to check it – break it.
It took a 4 foot long steel bar inserted into the seat tube and a lot of body weight (good thing I have that) to take it to failure. It failed exactly as we were hoping for, by the seat tube tearing on the outside edge of the heat-affected zone next to the weld. If it failed in the weld, that meant that I wasn’t doing it correctly.
It was an interesting two days, full of frustration, exasperation and successes. I’m looking forward to next month, when we’ll use the technique to build our custom titanium bicycles.