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How to Bend Tubing Successfully
By Tony Bingelis (originally published in EAA Sport Aviation, August 1983)
FORTUNATELY, MOST OF our homebuilt projects do not require very much tube bending. However, I assure you that these few components are important and many of them are, for the most part, highly visible. Being so visible, their fabrication merits your very best effort. But giving it your best effort does not mean you have to make a big production out of this tube bending thing. That goes for bending steel tubes, copper tubes and aluminum tubes as well.
Anyone building a tube and fabric airplane will probably encounter more opportunities for developing his tube bending skills than he would if he were building a composite. These welded tube aircraft are ordinarily endowed with nicely curved tail surfaces made of small diameter steel tubing. Some of them also have tubular wing tip bows, fuselage formers and seat frames as well. Builders of other types of aircraft can add windshield and canopy bows, flap handles and sometimes "S" shaped control columns to the bent tube list.
Just about the only copper tubes used in homebuilts are the oil pressure line and the primer lines. Even these are falling into disuse as more builders switch to electrical gauges. These copper tubes are a mere 1/8" in diameter and may be easily bent by hand or formed around an empty tin can or jar of the proper diameter. No serious tube bending problem here.
Aluminum tubing is not only used for fuel lines, it is also used for windshield and canopy bows and wing tip bows, too. These being tubes of a larger diameter present bending problems similar to those experienced with steel tubes. Ultralight builders utilize a lot of pre-bent aluminum tubing in structural elements but these components are, ordinarily, purchased pre-cut and pre-bent from the factory or supplier. Aluminum tubes are important in fuel systems and so are the bends you make in them. After all, why use a fitting where a bend in the tubing will do as well? Tubing bends are much lighter, cheaper and, unlike fittings, can't leak.
Do not be lulled into thinking that simply because 3/8" aluminum fuel lines are rather ductile you can easily form them by hand. You can, of course, but hand formed ends often become flattened and have a poor irregular appearance. Such wavy flattened bends are the usual result of attempting to bend thin wall tubing to small radii without the aid of a tube bender. A flattened fuel line can restrict the flow of fuel and could, ultimately, fail in service.
What Happens When A Tube Is Bent?
To make a bend in the middle of a piece of tubing, you would grasp it at each end, right? Then you would place it over some hard curved surface and push down hard on both ends of the tube . . . and the tube will begin to bend. If the radius of the underlying object (form) over which you are making the bend is large, the bend will be large and gentle and the tube will retain its round cross section. If, on the other hand, the surface beneath has a small radius, the curvature developing in the tube will be localized. For the first few degrees of bend, the bend will develop nicely. Then, you will notice that the tube is beginning to flatten. The top side of the bend is now under increasing tension and has to stretch. The tubing, however, is reluctant to do so and takes a short cut around the bend causing that unwanted flattening in the tube. But, what about the bottom side that is jammed against the "bending form"? It is being severely compressed and although the tubing resists this crowding it is beginning to show signs of buckling across the tube diameter. If you continue the bending, the flattening on the top side will become more pronounced while at the same time, the crowded metal on the bottom side relieves the pressure by developing more wrinkles. Ultimately, the tube will suddenly bend sharply and fracture.
Thin wall tubing is more difficult to bend successfully because it is very quick to flatten and buckle.
The larger the tube diameter, the greater the pressure you must exert to make it bend. While you can easily form a 90° bend in a 1/8" diameter copper line that is only 6" long, you simply cannot do the same with a similar length of 3/4" tubing. Not even if you had a form to bend it around. Why is that?
You Need Leverage
No matter how you go about bending a length of tubing, you will find it much easier to do if the piece is long enough. You can then take advantage of the leverage it affords (you know the principle). A lack of sufficient leverage could make it almost impossible for you to form a bend near either end of a 3/4" diameter steel (or aluminum) tube. Always start with a long length of tubing when bending it. The larger the diameter of the tubing, the greater its excess length should be. Allow an extra 12" on each end for tubing up to 3/4" in diameter. Allow even more for larger diameters . . . you will need the added leverage it affords. If you find you need more leverage, you can always slip in a steel rod or slide a larger diameter tube over the end to serve as an extended handle of sorts.
You don't need much in the way of equipment or materials to make good uniform bends. These items are definitely needed.
1. You will need a large rugged vise. The effectiveness of almost any type bending device will be greatly improved by clamping it in a heavy duty vise mounted securely on a solid bench. In addition to freeing both hands (and in some cases, feet), it will enable you to more precisely apply the bending pressure to the tubing. A substitute for the vise would be a good solid immobilized bench to which you could bolt a bending device horizontally.
2. Some sort of bending device is essential for any bend exceeding, say 15° to 20° especially when that bend is concentrated around a small radius.
3. A filler material (sand, bending alloy or salt) for those hard to make bends.
4. Templates cut from plywood. How else can you check the bend you are making?
Tube Bending Devices
You can bend tubing successfully with just about any simple homemade tube bending device if it is properly made. You have a wide variety of types from which to choose.
Locally you might have access to some commercial tube bending machine. If so, great . . . just remember to bring your templates, too.
Others of you might be able to locate someone who has an Electrician's Conduit Bender and arrange for its use.
It should allow you to make simple bends with a high degree of success provided it can accommodate the diameter of the tubing you need to bend. Most of the tubing we use in homebuilts is either 1/2" or 3/4" in diameter (sometimes 5/8", too). If the tube bender you have access to is made for larger diameter tubing you probably shouldn't use it as it might cause your bends to flatten excessively. The design of an electrician's tube bender is simple enough that you could duplicate it for the size tubing you need to bend.
A simple plywood bending form nailed to a work bench is a good basic bending device for large bends in small diameter tubing. However, when cutting the form to shape you must make its curve sharper as the tubing will have a tendency to spring back. This sort of bending device is easy to use for uniform curves because you can fasten one end of the tubing and pull the free end around the form causing it to bend smoothly in one easy sweep.
An improved variation of the plywood bending jig (form) is one with its edges grooved to the diameter of the tube for which it is intended. The groove is important. It reduces the tendency for tubing to flatten and somewhat sharper curves or bends in the tubing are possible. An even greater improvement would be to make the routed groove somewhat deeper so that the tubing will nestle in it beyond its half diameter. This added depth permits the walls of the groove to exert a restraining effect against the tube's tendency to flatten. It is equally important, when making any grooved pulley or bending jig, for the grooved edges of the form to be strong enough to resist the flattening and widening of the tube. A wood form, particularly a plywood form, is rather weak in this respect so it should have considerable edge distance between the groove and the edge of the pulley, form, jig or whatever. A grooved aluminum "pulley" would be much stronger but is harder to make if you don't have a large hunk of aluminum plate or a metal working lathe. Figure 5 shows some options for making grooved tube bending devices, be they pulley types or the simple wood jig types. Figures 1 through 5 should give you enough ideas to help you devise a bending device of your own.
Tubing in aircraft work is usually bent cold. Contrary to what may seem logical and contrary to what you may have heard, attempting to bend tubing by heating it can turn out to be a lousy adventure. Most of us don't have the skill and patience to play with a hot bend and will usually get unhandsome results. The problem lies with localized uneven heating and poorly coordinated bending pressures . . . to say nothing of impatience. Pressing a hot tube against a bending form will surely cause it to flatten on the inside of the bend. Hand bending it without the aid of a form is likewise very difficult to do successfully. In short, bend it cold.
Heating tubing to a red hot condition does have a place in bending. Bending causes the metal to harden somewhat. So, by heating the tubing you can anneal it and continue the bending carefully after the tube cools. In other words, it is possible to make bends over a smaller radius if you anneal the tube once or twice as the bend progresses. This is a slow way but it can make an otherwise severe bend possible. There is another way to reduce the risk of making a poor bend. Use a filler material.
The Use Of Filler Materials
It is easy to see that packing a tube you want to bend with a solid material like sand or a molten bending alloy will greatly inhibit the flattening tendency.
Before you attempt to bend any large diameter tube (3/4" or larger), you should fill it with densely packed sand. (I understand that salt works as well although I have never tried it.) The sand has to be dry and well sifted to remove all foreign matter and the larger grains of sand. Plug the bottom end of the tube with a wood plug and pour in the sand. Tap the bottom end of the tube repeatedly against a solid surface (concrete floor). After the tube is full of sand, the continued tapping will cause it to settle and pack more densely. Add additional sand. After the sand shows no further sign of settling, drive a wood plug into the top end of the tube being sure it bottoms solidly against the sand. Your tube is now ready for bending. The risk of a flattened bend will have been considerably reduced.
A more effective filler, of course, is any of the commercial bending alloys, CERROBEND, for example. These alloys have a very low melting point, some as low as 165° F. When this bending alloy is heated to its melting point it can be poured into the tube you want bent . . . sure, plug the bottom first. Cooling the tube in water will solidify the alloy and you are ready for the bending effort. After completing the bend, the tube has to be reheated to melt out the alloy. The material can be reused any number of times. However, since it is unlikely that most of us could find an economical source for a bending alloy, the sand treatment is and will continue to be the one most used by homebuilders.
More About The Bending Process
You cannot hurry your tube bending chore. It will ordinarily take much longer than you would expect. Allow yourself plenty of time for bends that do not have a uniform radius.
Most any bending device you use, with the exception of a template-like bending jig, will have a rather small diameter grooved pulley, roller or disc over which the tube will be bent. Obviously, you cannot bend the tube very much in any one place if your bend requires a radius many times larger than the bending device pulley. That means the bending process may become quite long as you have to bend a bit, check the bend in the tube against a template, etc. As an aid to checking the accuracy of your bend as you proceed, you should mark a center line around the tube to use as a reference mark keyed to your template. (Use a black laundry marker on aluminum tubes and a silver lead pencil on steel tubes.)
Do not forget to check the tubing after the bending is completed to be sure that it is not twisted, as viewed from the ends. Lay it on a flat surface for a quick check. If warpage is present, a twisting pressure in the proper direction will take out the warp. Recheck the tube's bend against the template again before congratulating yourself on a job well done.