Selecting a Welder....

[Sam Buchanan]

Originally Posted by akschu

5. For building a fuse, I think gas welding is better. Tig looks nice, but requires much tighter gaps, doesn't fully heat a cluster, and can make the area around a joint brittle. So even if you tig, you should use a torch to normalize the metal after the weld.

If you want a great little gas rig, or just want more info, look no further than Tinman Technologies:

http://tinmantech.com/

Here is the meco torch I use:
http://tinmantech.com/html/meco_midget_torch.php

schu

Yep, the Mecco Midget is what I use.

Originally Posted by wjnmd

What are you building Sam? I'm building a Marquart Charger.

Oxyacetylene welding is traditionally preferred for steel tube fuselage but TIG is rapidly taking over. I've done both and I prefer TIG

Bill Near

Just a little plunker for low-n-slow fun flying:

http://home.hiwaay.net/~sbuc/legaleagleXL/

[videobobk]

I have to side with Sam here. While it takes some time to master, and it is often slower, the old-school stuff really shines when top-quality is considered. TIG can be brittle if not properly heat-treated. 75 year old oxy welds are as good today as when they were done. And while TIG and MIG have many uses, I use oxy for everything from jewelry work to cutting up I-beams. Everything has its place; pick what you feel comfortable with and learn to use it well. No one system will do it all.

My father was an aircraft welder for about 50 years. I wish I were that good...

Bob Kelly

[DanH]

Originally Posted by Phil

So the question is which one do I want to buy? I'd like the opportunity to build a tube and fabric airplane at some point in the future (if it presents itself). How large of a welder would I need to get full penetration of a tube airframe?

Specific to your question, size (or more precisely, amperage) will not be an issue in welding a tube airframe. Even the smallest units will do fine with thinwall steel. Power can quickly become an issue with thickwall (more than 0.125") aluminum.

The Lincoln 175 mentioned by Bill has been very popular with the biplane clan. I bought a Miller Econotig circa 1995, back when it was the first "home hobby use" unit in the marketplace. Today most would consider it a cripple, but I still get a huge amount of work out of it. At the other end of the scale
you can get as fancy as your wallet will allow. Regardless of your choice, you won't regret adding welding to your shop skills.

Gas vs TIG; learn both. Each will do welds the other won't. I usually suggest learning gas first and TIG later It's not critical, but a guy who has learned gas can pick up a TIG and weld well in minutes.

TIG is far safer for use in the typical attached garage home shop....no sparks.

Ignore 90% of what you hear about normalizing, crystalized metal, brittle welds, etc.

Postscript:
Had a dim memory of specs for the Lincoln 175 so I looked it up. Were I to buy a new Lincoln I'd want the Precision TIG 275 to get the tiny 2 amp arc. Yes, although more power is nice I've often wished for less. Both the Lincoln 175 and my old Miller only go down to 10 amps, a bit too much for some fine detail work with thin steel. Same is true for Miller....need to move up to a Dynasty unit to get amperage as low as 1 amp.

[wilddog]

I'm with Sam on gas welding airplanes. I have both TIG and gas and use gas on almost all aircraft tube welds. If you use TIG you need a helmit (auto-dark is best) . Try welding in an awarkard position with the helmit banging around on the tubes and reaching for the pedal while working the torch and rod all at the same time, that is when you really like gas! TIG will make good welds, but I don't find it to be fun.

[wjnmd]

I have both TIG and gas welders. After reading the discussions posted here, I went out to my hangar and played with both. I can see that both are worthwhile and require skill. While I don't profess to be a welding expert by any stretch of the imagination, what I have read and heard in welding seminars is that both are equally applicable in welding aircraft steel tube fuselages. However, based on this thread,and since I have access to both I plan on practicing both to hone my skills since I see that both require a similar set of skills.

Once again, whatever method you pick, welding is still a gas, so to speak.

Bill Near

[Bill Wightman]

For welding at higher power - like about 30 amps or more - I'd really recommend a water cooled torch.

Messing around with TIG is really fun, and you can do some really amazing things with the technology available today. Get someone who really knows how to weld to show you the ropes. I've thoroughly enjoyed my Syncrowave 250.

[akschu]

In the bearhawk manual there is a picture of two identical T shaped joints where the leg of the T was welded to the top. One was welded with gas and the other with tig. To test the joints, the leg of the T was bent back and forth until the tubing failed. The tig sample broke right at the joint after 2 bends back and forth, and the gas sample was very distorted but in tact after 6 bends.

The truth of the matter is that gas is the stronger weld in aircraft construction, however tig is without a doubt plenty strong enough.

schu

[DanH]

Originally Posted by akschu

In the bearhawk manual there is a picture of two identical T shaped joints where the leg of the T was welded to the top. One was welded with gas and the other with tig. To test the joints, the leg of the T was bent back and forth until the tubing failed. The tig sample broke right at the joint after 2 bends back and forth, and the gas sample was very distorted but in tact after 6 bends. The truth of the matter is that gas is the stronger weld in aircraft construction, however tig is without a doubt plenty strong enough. schu

An incomplete picture leading to an improper conclusion.

The welding process itself has little effect on strength. Given identical joints in 4130 but different weld processes, strength is mostly a function of postweld cooling rate. 4130 is an air-hardening steel; it becomes both harder and stronger when cooled more quickly.

Welding with gas tends to warm the tube mass for a considerable distance from the weld zone. In the case of that demonstration tee, all the tubing would quite hot, if not glowing. Postweld cooling is slow and even. The result is a more ductile assembly....but not greater strength. In truth it will reach its plastic region at a low stress level.

TIG tends to quickly raise temperature in the immediate weld zone without raising it a lot in the adjacent steel. Much of the tubing mass would remain cool (or not very warm), in particular if the operator was quick. When the weld is ended and the arc stopped, the adjacent cool sections serve to quench (rapidly cool) the weld area. The weld area becomes harder, less ductile, and stronger; it will withstand higher stress before reaching the plastic region, but the plastic region will be more narrow.

So, back to the picture. Have you ever actually done this demonstration for yourself? You should, but let's do it in the mind's eye right now.

First clamp the top of the gas welded tee in a vice, stem up. Attach a pound scale to the end of the stem and pull, recording how much force it takes to put a moderate permanent bend in the tube. Now repeat the same test using the TIG welded tee. You'll likely find the TIG sample takes more force to install the same permanent bend.....as much as double the force, depending on quench rate. The TIG sample is stronger.

Now lets pull the stem of the both tees back the other way so as to form similar permanent bends in the opposite direction. Then reverse again, and again if necessary. The gas-welded sample will indeed withstand multiple excursions into the plastic region of its stress vs elongation plot, but not the harder, stronger quenched TIG sample. It may only last for a few reversals before cracking. The TIG sample is less ductile.

You can make the demonstration more extreme.....just use a 4130 filler rod instead of something with lower hardenability like ER70S-2. It will sure enough pop right by the weld bead.

Now just for fun gas weld another tee. Even do your best postweld "stress relief".....but then toss it into a bucket of water while still very hot. Fish it out and do the bend test. Despite the weld process, you'll find it to be very strong but brittle. It may never take a permanent bend, instead just flexing like a spring.....and breaking with a snap when you force it too far.

It's all about temperatures and rate of change, not flame vs electric arc.

Yes, I've simplified a lot. So shoot me

[akschu]

Originally Posted by DanH

An incomplete picture leading to an improper conclusion.

Originally Posted by DanH

The welding process itself has little effect on strength. Given identical joints in 4130 but different weld processes, strength is mostly a function of postweld cooling rate. 4130 is an air-hardening steel; it becomes both harder and stronger when cooled more quickly.

Sure, however each welding process does produce a different postweld cooling rate and the concentration of heat is in different areas. This certainly can cause the finished product to have different modes of failure as illustrated by the test I mentioned.

Originally Posted by DanH

Welding with gas tends to warm the tube mass for a considerable distance from the weld zone. In the case of that demonstration tee, all the tubing would quite hot, if not glowing. Postweld cooling is slow and even. The result is a more ductile assembly....but not greater strength. In truth it will reach its plastic region at a low stress level.

Correct, but even more important is that the tensile strength is more uniform throughout the cluster. The problem isn't how ductile or brittle the metal is per se, but rather a ductile region next to a brittle region.

Originally Posted by DanH

TIG tends to quickly raise temperature in the immediate weld zone without raising it a lot in the adjacent steel. Much of the tubing mass would remain cool (or not very warm), in particular if the operator was quick. When the weld is ended and the arc stopped, the adjacent cool sections serve to quench (rapidly cool) the weld area. The weld area becomes harder, less ductile, and stronger; it will withstand higher stress before reaching the plastic region, but the plastic region will be more narrow.

Not only will the adjacent tubing cool the weld, but an air cooled tig torch is also cooled by the insulation gas which can cause the weld to quench faster than a normal in air quench, especially if the torch is left over the weld for any period of time which is taught to most tig operators as a way of keeping the weld clean.

Originally Posted by DanH

So, back to the picture. Have you ever actually done this demonstration for yourself? You should, but let's do it in the mind's eye right now.

No, I didn't see the need since it was done by people I trust, and it makes perfect sense. Anyway, I'm always game for a thought experiment.

Originally Posted by DanH

First clamp the top of the gas welded tee in a vice, stem up. Attach a pound scale to the end of the stem and pull, recording how much force it takes to put a moderate permanent bend in the tube. Now repeat the same test using the TIG welded tee. You'll likely find the TIG sample takes more force to install the same permanent bend.....as much as double the force, depending on quench rate. The TIG sample is stronger.

Now lets pull the stem of the both tees back the other way so as to form similar permanent bends in the opposite direction. Then reverse again, and again if necessary. The gas-welded sample will indeed withstand multiple excursions into the plastic region of its stress vs elongation plot, but not the harder, stronger quenched TIG sample. It may only last for a few reversals before cracking. The TIG sample is less ductile.

You can make the demonstration more extreme.....just use a 4130 filler rod instead of something with lower hardenability like ER70S-2. It will sure enough pop right by the weld bead.

Now just for fun gas weld another tee. Even do your best postweld "stress relief".....but then toss it into a bucket of water while still very hot. Fish it out and do the bend test. Despite the weld process, you'll find it to be very strong but brittle. It may never take a permanent bend, instead just flexing like a spring.....and breaking with a snap when you force it too far.

It's all about temperatures and rate of change, not flame vs electric arc.

Yes, I've simplified a lot. So shoot me

Your argument that tig is stronger isn't in question, and I agree with everything you say in your thought experiment, however I don't think you really addressed the real issue which is different levels of hardness in adjacent structures which are subject to flexing and vibration. Ductile on an airplane can be a good thing as it tends to survive flexing and vibration without cracking better than something that is brittle, but especially better than something that has ductile right next to brittle.

The key to any metal work is to get the right material (including hardness) assembled in such a way where the forces placed on that metal do not exceed its yield strength. (Yes, I'm simplifying it a lot.)

In my opinion, (and in the opinion of a lot of other aircraft welders), tig welding causes an inconsistency in the hardness of the metal around the joint which probably won't ever cause anyone a problem, but why do it that way when the torch head is so much smaller, the head gear is so much smaller, and the cost of the equipment is so much less?

All of these things said, I wouldn't hesitate to fly in a tig welded airplane as long as I knew that the welder knew what he was doing.

schu

[scard]

A tour of the Air Tractor factory was a great demonstration of welding process for me. A whole lot of TIG tubing in process there.