It is amazing how with a little heat in the correct locations can move a fuselage around and correct for not having it in a jig when welding it up.
It is amazing how with a little heat in the correct locations can move a fuselage around and correct for not having it in a jig when welding it up.It's not that 1020 or 1025 isn't heat treat-able...in fact it is...1500-1600 degrees and water quenched...it just doesnt get much for the effort, as you state correctly, 4130 annealled is overkill, strength-wise. Really, you are Carburizing the 1020(adding Carbon) whereas for 4130 you have quite a few selections of what you'd like to heat treat to, for the physicals you wanted.
The point is you'd have a ton of warpage if you heated an entire fuselage to 1550 and dunked it...which would never make sense and wasn't at all necessary when you gas welded 1020. The Pre-Heat, Weld and stress relieve (post heat) was all done, one cluster at a time. One worker with a cherry bomb for pre and post heat and another doing the actual welding. 4130 is mostly the same. post heating is cheap insurance to slow the bead shrinkage by cooling it more slowly.
Pretty efficient back then.
I believe they did away with 1020 simply for the fact that the steel mills quit producing in the thin walls and 4130 is still produced in thin wall tubing today, so it was just a simple, stability of supply issue.
And much of the structure is under compression rather than tension. The weld quality is only a factor if the joint is in tension.MIG welding 4130 is completely acceptable, as long as proper pre-heating AND controlled cooling after welding is done.
We aren't going to the moon in these little airships, I think it's a natural tendency to overthink the welding process. A cub guy most of you know, or at least have heard of once told me that a super cub fuselage could be tack welded together and could probably be flown safely until it needed new fabric. There is so much redundancy in strength built into the design of a super cub fuselage, it would be pretty hard to break something under 'nirmal'operating conditions.
By the way, I'm not advocating for anyone to fly a tack welded fuselage ...
When you think about the design of the fuselage, it's pretty remarkable that, basically, engineers used the same principal that 16th century builders used when they found triangles to be the strongest geometric shape.And much of the structure is under compression rather than tension. The weld quality is only a factor if the joint is in tension.
Many years ago I talked with Jim Soares in Bozeman who had gotten his PMA for PA-18 fuselages. He told me the airframe stretched 3/8” when it was finished welded. His jig had to accommodate that to exactly match Piper’s finished fuselage. Gas welded, if I remember correctly. All the tubes were fit perfectly by chucking a cutter into his lathe and advancing the tubing at the precise angle. He had drawers full of fittings already made. Jim’s principal business was making cabinets for gaming machines. He could load a sheet of steel onto a table, and a computer controlled punch cut all the parts. When the sheet came off the table, an employee would shake it, and all the parts fell away. Virtually no waste. Then he had special machines to bend the parts very precisely. Pretty sure he sold the PA-18 PMA to Univair. He was a really nice guy; flew a beautiful Interstate.In 1970 Piper started TIG welding, the fuselages stretched 9/16" when TIG welded.
Looked at a wrecked Kitfox, lots of broken tubes just outside of the heat effected zone, MIG welded. Don't see that in Super Cubs, oxy acetylene or TIG.
my frame from jim was tiged, i think there still building slot machine parts, i gotta check, spent 2 days in his shop, it was a lot of fun.Many years ago I talked with Jim Soares in Bozeman who had gotten his PMA for PA-18 fuselages. He told me the airframe stretched 3/8” when it was finished welded. His jig had to accommodate that to exactly match Piper’s finished fuselage. Gas welded, if I remember correctly. All the tubes were fit perfectly by chucking a cutter into his lathe and advancing the tubing at the precise angle. He had drawers full of fittings already made. Jim’s principal business was making cabinets for gaming machines. He could load a sheet of steel onto a table, and a computer controlled punch cut all the parts. When the sheet came off the table, an employee would shake it, and all the parts fell away. Virtually no waste. Then he had special machines to bend the parts very precisely. Pretty sure he sold the PA-18 PMA to Univair. He was a really nice guy; flew a beautiful Interstate.
