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Aircraft Welding and Steel Tube Fabrication Part 2
A Primer for the Novice Aircraft Builder
By Budd Davisson (originally published in EAA Experimenter, September 1987)
To most people a hammer is a hammer-you drive nails with one end and pull 'em with the other. But, once you get deeper into carpentry, you realize that there are specialized framing hammers, sheetrocking hammers, tack hammers and jackhammers. For a given job there is a given hammer. The same thing is true in aircraft welding. You don't just run out and buy an oxy-acetylene torch without surveying the market to make sure you get exactly the torch you need for the job. However, the torch isn't all there is to it. Connected to the torch are hoses and regulators, tanks, carts, welding rods, goggles, etc., etc. Do a little planning at the beginning and you'll find you will save a little money at the beginning and a lot of money at the other end.
You wouldn't expect a painter to use just any old brush, and common sense would tell you that a sculptor requires very specific chisels and mallets to work his magic; the same is very true in aircraft welding. Look at the average steel tube fuselage and stop to think how many nooks and crannies there are nestled down in that spider-web confusion of tubing. You have to get in there to weld those tubes, and there may be two hundred areas that require you to start welding on one side and work your way through a crevice around to the other side. And probably a third of these are upside down. You will find yourself becoming a contortionist with a finger of flame that is working against you, your work space, and the structure, to build integrity into a haystack of steel tubing; the last thing that you need is a tool that gets in your way. Van Gogh didn't use a roller, so don't back yourself into the same corner when selecting your torch.
The average light airplane fuselage and related steelwork never contains pieces much more than 3/16ths of an inch thick, and 99.9% of them are in the .028" to .095" category (1/32nd to 3/32nds). An aircraft welder has very little in common with a bridge builder or shipwright and doesn't need a torch that they would use. (See Figure 1)
Fortunately most of the welding equipment manufacturers make at least one lightweight, small torch that is applicable to working on small aircraft, and at least one manufacturer makes a torch specifically designed for that purpose. Now, don't just go in and ask for the smallest torch a welding store has to offer, because you need to give more specifics than that. In the first place, many welding companies and welding stores consider the torches used by automotive body men to be the smallest torch that they should carry; thousands of aircraft have been built and repaired using the bodyshop-size torch. However, the smaller, so-called "airline type" torch is much lighter and easier to handle. For one thing, the smaller torches use 1/4" hose rather than 5/16", which means that the weight and stiffness of the hose coming up to your hand is much less, something which is of great importance when it comes to fatigue (after you have been welding for four or five hours, you'll know what we mean).
The smaller hoses also aid flexibility in getting into awkward areas. At least one of the small aircraft-type torches available has the oxygen and acetylene knobs on the front part of the torch body instead of the traditional position at the rear, which makes it easier to make the many small adjustments necessary when welding. Forward placed valves also move a little bit of weight forward on the torch, which helps balance off some of the hose weight, an almost insignificant difference, possibly, but still a difference.
The welding tips, or more correctly, the blowpipes, that screw into the front of the torch body come with different size holes in the end which determine the size of the flame and the amount of heat available. This, in turn, determines the thickness of the material you can weld with a given tip. Every manufacturer numbers or letters its tips. Unfortunately, each manufacturer uses a different designation system. In general, however, you'll need four tips which, using the most common designation, would include a 0, 1, 3 and 5. These will allow you to bridge the gap from .028 up to .187 (1/32 to 3/16). You don't have much choice in the type or style of tips, because you have to take what the manufacturer recommends for the particular torch handle and for the gauges of metal to be welded.
One additional item you should pick up is a bending tip. This is a specialized tip with a multi-hole end on it which puts out a much larger area of concentrated flame. This tip is extremely useful when you are bending tubing or plate. Be advised, however, when you light it up, use the techniques outlined in later chapters, or you'll wind up with soot all over the ceiling.
(See Figure 4)
Two regulators are required for gas welding set up; one for the oxygen tank and one for the acetylene tank. They are almost identical in appearance, except they are calibrated slightly differently, and the acetylene regulators, as with all other acetylene connections, use a left-hand thread so you can't confuse them with the oxygen line.
The two dials on the regulator indicate the pressure that's in the tank (how much gas is left), and the pressure that's in the line going to the torch can be adjusted by means of a knob, or "T," handle on the face of the regulator which moves a diaphragm inside the regulator in and out, thereby setting the line pressure.
The only thing to be known about these regulators, when purchasing them, is that if they're used, they must be inspected and rebuilt by an accredited welding repair shop; and you should know that they come in two different types - singe and double stage. The two-stage regulator is a little more expensive and the reason for that is that its diaphragms are set up so that no matter how much gas you take out of the tanks, the line pressure always remains the same. This is a real advantage when doing a lot of welding because you don't have to keep readjusting the torch to make up for lost pressure in the tank. The single-stage regulator doesn't compensate for that pressure loss, but it can be 30 to 50 percent less expensive than the two-stage type. In actual practice, however, there are very few times when you'll be making a weld of such length that doesn't require you to reset the torch, anyway, for changing metal thickness or to shut the torch off altogether to make some minor adjustment in the tubing. Therefore, 90 percent of the time a single-stage regulator will fulfill your needs nicely.
Oxygen and acetylene tanks come in a variety of sizes, from tiny little units that look like scuba tanks up to gigantic industrial jobs that look like hot water heaters. However, the most common and most usable for the amateur aircraft builder is a 40 cubic foot acetylene tank and a 60 cubic foot oxygen tank. These sizes of tanks allow you to do plenty of work without continually running to the re-filling station (you should be able to do a normal-sized fuselage on one filling, not including the tail), and they aren't so big that you need a forklift to move them around the shop.
Another decision about getting tanks is whether to buy or rent them. The Federal Rules concerning the ownership of high-pressure vessels like welding tanks has changed a number of times and is likely to do so again, so the question of ownership could be a moot point, if the regulations don't allow it. Either way, however, owning or renting, you'll never see the same set of tanks twice, because the re-fill stations generally work on an exchange basis-you bring in your empties and they give you a set of full ones. So, unless you're willing to stand around and wait until they fill your own tank, you may give them a set of shiny, new tanks and take home a set that's 40 or 50 years old and looks it. Of course, none of this makes any difference at all from a functional standpoint. Incidentally, when you're buying the tanks, don't forget to get a cart for them, too, because an airplane is a long, skinny object, and if you fasten tanks to the wall, you'll always find they're in the wrong place, and they're a major problem to drag around the floor without a cart. (See Figure 7)
There are probably a dozen different styles of goggles, and the only thing they have in common is that they are uncomfortable. The important thing to know about them, however, is that the shade of the glass in them comes in different densities, and you should have the right density for the type of welding you are doing, otherwise it can cause long-term damage to the retinas of your eyes. The Welding Institute of America recommends that shade No. 4 be used for acetylene welding of 4130 aircraft steel and any other similar type material. Do not use sunglasses and do not use the commonly available "visitor's glasses" that look like very dark sunglasses and are sold by welding shops. These are for occasional use only by a visitor coming into the shop who wants to observe your welding in progress. They are not meant to be used on a daily basis. (See Figure 9)
The important thing about goggles is to first make sure they are the right density, and secondly to find a pair that you find comfortable, because you will spend between 40 and 80 hours hunched over, making welds, and if they pinch your nose or grab at your temples, they are going to be distracting.
There are a lot of different styles of lighters, or strikers, ranging from the old-fashioned wire and cup type, that is most common, up to some new, exotic pistol-shaped affairs with a trigger on them. None of the new-fangled kind have been able to improve on the basic usability of the original wire-type lighter. In the first place, the wire and cup lighters don't cost much, and if you're buying new equipment, the welding shop should throw in two or three free ones, or you should buy your equipment from someone who isn't such a cheapskate. Also, they fit very nicely into a back pocket, so that you can light the torch, flip it into your back pocket, and flip it out enough to build several airplanes; but you'll need two or three of them, anyway, because you'll always be walking out of the shop with them in your pocket and leaving them upstairs on the dresser, which means you always need a couple of spares laying around the shop.
First and foremost amongst safety equipment is not the fire extinguisher as you might expect. Rather, it's a couple of innocuous-looking little pieces of brass tubing called "flashback check valves." These screw into the outlet of each regulator and the hose, in turn, screws into the check valve. These check valves stop the possibility of a flame "backflashing" through the lines and into the regulator, or, worse yet, into the tank itself. Although it's a highly uncommon accident when a flashback goes into a tank, everybody in the neighborhood is likely to remember it, and a few dollars spent on the check valve is worth it for the peace of mind alone.
Now come the fire extinguishers - and you'll notice it's extinguishers plural, not extinguisher singular. Three or four 5 lb. extinguishers scattered around the shop are far better than a single larger extinguisher, because their convenience makes it easier to snuff a fire as soon as it starts instead of having to run across the shop and lug a fire extinguisher back. Of course, proper shop safety techniques will eliminate the fire possibility, but some dry powder extinguishers are a good investment in safety, nonetheless. You will need them around when you're working with dope and fabric later on in the construction of the airplane, anyway, so you might as well buy them now.
Then you need chains, or a healthy rope, or a couple of safety belts - anything that will secure your tank to your cart, or the wall if you are not using a cart. The reason for these tie downs is that you don't know what real excitement is until you've knocked over a high-pressure oxygen cylinder and broken the regulator or the valve off . . . they turn into high-speed and very destructive rockets. Again, it's a long shot, but why take the chance.