The landing gear toggle assembly is connected to the airframe by very thin box steel trusses. Approximately forty feet of .035" x 3/4" 4130 steel s used in the construction of them. (Taking practice pieces into account) Welding of this thin metal requires good control of welding variables to prevent burnthrough and other defects. Thicker metals are relatively easy to weld because you can fudge(to some degree) your variables and still end up with an acceptable weld.
Here is a list of things that I found handy in producing nice TIG welds in this thin material. I am assuming that the reader has little welding experience. The following views are my opinions and should not be regarded as fact until verified! This information as with all other information given in this website is for consideration NOT instruction. Recieve proper instruction in use of any equipment before attempting to use it.
Perfect fit of parts. Anything less than "daylight tight" fitup and the metal easily vaporizes at it's edge. The metal needs to conduct heat away from where it's being applied and can only do so by touching something. You can get away with a little gap but you'll have no inert gas shielding the backside of your molten puddle. (Atmosphere mixing in the weld pool produces a weak weld)
Where to start the arc. It helps greatly to direct your initial arc starting on the object with more mass. For example, on a tee butt weld you would direct the arc at the bottom of the tee. With this thin material eveything happens so fast (even for TIG) that you have little time to respond when things get out of control.
Jigs Have some means of holdng your parts rigidly. Parts will bend quite a bit as the weld contracts during cooling. You should tack weld both sides before laying any beads down.
Experimentation Each material has it's own welding characteristics. Make several passes on scrap material of the same composition and thickness and inspect after each pass. Change your variables such as speed, amperage, amount of filler rod added to each puddle to find out what produces the best weld. Bend test your good welds. There is a "right " combination of variables for each situation. On this thin material penetration is not the issue, excessive dropthrough is.
Everything properly pre-cleaned. Any life supporting structure has to have the best welds. Welds with contamination will fail readily. Acetone wipe everything that will be in the weld zone.
Glass bead blasting NOT sandblasting. Sandblasting cleans well BUT cuts the metal and erodes it. For the same reason mechanized sanding should not be used. Glass shot (fine or extra fine) should be used to assure that metal thickness in the weld area is the same as the rest of the metal.
Sharp tungesten Arc starting becomes easier when your tungsten is sharp. Using 45 -55 amps, a 1/16" electrode and a gas lens worked well on .035" 4130. An .040 electrode would have started easier but tungsten inclusion in the weld would have been higher therefore unacceptable.
Contaminates that you don't want include...
Rusty or dirty filler rod. Run your rods through a new scotch brite (plastic type from welding store) pad and acetone wipe prior to use. Be aware that wherever your hands touch the clean rod is contaminated and needs to be wiped before it enters the weld pool.
Mill scale from factory or welding. When you get 4130 it is covered with a gray film of scale that protects it from rusting. This scale needs to be sanded or ideally glass bead blasted off of the material you are about to weld. This same scale is also formed during welding as you move the shielding gas away from areas that are still hot. Holding your TIG torch over the end of the weld and allowing the post-purge Argon to flow until the weld cools prevents this scale from forming. One place that this may give a little trouble is on a square where the shielding gas flow may not wrap around the next side. Remember if you start a weld with a contaminated bead the rest of the weld is contaminated as well.
Lint from rags Your metal should be de-burred and perfectly smooth to prevent your acetone dipped rag from leaving behind snagged fibers. These fibers will of course contaminate your weld.
I found that glass bead blasting did a fine job of de-burring the edges of my material while not removing too much material.
Contaminated tungesten. Anytime your tungsten electrode touches anything while welding it becomes contaminated. Consult a weldors textbook for proper breaking and resharpening procedures.
