J,

Thanks for that helpful insight. I will keep it all in mind. Sice I do not
have a TIG machine or experience using one, and I do not have much
experience using a torch to weld with, would it be kosher to tack the pieces
together with a MIG and then have a professional welder finish the process
by using a torch? The local pro that I have used in the past for various
projects gave me the impression that he would rather weld with his a/o torch
than his TIG machine.

--
Have a good one!

Steve
www.americanspiritppc.com
"Flightdeck" wrote in message
ink.net...
A few years back, a manufacturer of 4130 tube frames for a popular
high-wing
kit used TIG because of production and training considerations over
oxy/acetylene. They had to call back fuselages because of cracking in the
4130. They modified their approach and used oxy/acetylene to pre-heat and
post-heat the weld. They ended up with nice welds, but why not just train
enough welders to the a real good job with oxy/acetylene?

4130 is very touchy about cracking from too little pre-heat and from too
fast cooling of the weld AND the metal around the joint. It should be
welded in absolutely calm air and the torch flame must be adjusted
correctly
to avoid a brittle failure from random hydrogen in the air.

Some of the race car guys who were using 4130 found that they could get
very
strong joints (with less high-heat problems)by using a brazing technique
rather than true welding. In fact, the technique seemed to provide better
joints due to difficult clusters and different levels of expertise. There
are some very good brazing alloys out there now with very high PSI
ratings.
As with a good 4130 "weld" the joints must be a very good fit (NO gap
filling with the rod) and clean, clean, clean. You can shape various
grinding wheels, or you can use something like the "joint jigger" with
high
quality hole saws to form the various "fish mouth" shapes. The best jigs
permit adjustment of the vise angle and the drill angle. The best quality
of the hole saws will do an entire frame before sharpening or replacement
is
required.

Finding a full selection of tubing wall/id/od combinations is difficult
today. We ran into a source problem a couple of years ago when we needed
to
find some sections that would telescope over an existing tube during some
airframe repair. This was a case where replacing the entire length of the
structural member would have required a re-cover of the fuselage. If you
are working on a definate project and have too much trouble finding the
size
you need from one of the three aircraft suppliers that still stock a
decent
selection of 4130 tube, send me an email and I will pass along a couple of
sources that don't generally advertise.

J

"Steve Thomas" wrote in message
nk.net...
Has anyone on this list ever welded a thinwall 4130 frame with a MIG
welder?

What is a good source of 4130 seamless and welded seam tubing?

Thanks,

--
Have a good one!

Steve
www.americanspiritppc.com