I was lucky enough to have as a TIG welding instructor, an aerospace
weldor. He knew lots of little tricks for dealing with thin wall (
0.028" and 0.035") tubing.

I was asked, off list, what some of these tricks might be. I know a
few of you have TIG welders so I'll post to the list.

The usual recommendation to TIG weld steel uphill, to slow the puddle
for better penetration, is a good one for heavier tube thicknesses,
such as used on motorcycles. But on lighter, 0.028" and 0.035",
tubes more appropriate to bicycles, you want to weld slightly
downhill to speed up the puddle.

Making loose fitting aluminum slugs to keep air away from the inside
of the joint, in lieu of an argon purge.
(I'm going to look into the possibility of using Superior #9 Welding
Flux on the inside of the tubes instead.)

Flowing argon onto the joint before striking an arc, by holding the
torch away from the work and tapping the pedal to get argon
flowing. Then moving the torch back to the joint, and starting the
arc while the argon is still flowing. I understand newer machines
now have a pre-flow setting, but I've never used one.

Putting a long taper on a 3/32" tungsten, instead of using the skinny
ones usually recommended for light gauge steel.

In some situations, (hard to describe which ones) holding 1/16" rod
in the root of the joint and consuming it with the advance of the puddle.

"Airtight" miters, but with a major difference from a good miter for
brazing. On the sides of the miter (especially if both tubes are the
same size) you get a wide surface between the ID and the OD. This
area is important for maximizing contact area for brazing. But for
welding you want to file that area away. It's hard to describe, so I
made a sketch.
<<http://bikesmithdesign.com>http://bikesmithdesign.com/brazing/braze_vs_weld.jp\
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If you weld with the brazing type miter the edge melts away
unpredictably and may interfere with full penetration. Also, with a
welding type miter it's easier to see the edge as you weld.

BTW I use 1/32" ER70S rod for 0.028" and 0.035" tubes, if they are
plain gauge 4130 or lesser steels.
For high grade bike tubes I use 1/32" Weldmold 880 rod. It's a high
ductility stainless that can be used to join just about any steels.


Mark Stonich;
Minneapolis Minnesota
http://mnhpva.org
http://bikesmithdesign.com