4. BENDING WOOD FOR THE NET FRAME

       Wood bends because the cells that make up the structure of the wood can be crushed or deformed when they contain moisture, or they are plasticized by heat, and sufficient force is applied to one side of a piece of lumber causing it to deflect in opposite direction. The cells don’t really stretch an appreciable amount in proportion to their ability to compress, therefore, lumber will often fracture and split on the outside face when it is bent, a problem which sometimes can be reduced by using a flexible strap, say made of spring steel, that is kept under tension in contact with the outer surface of the wood so that the outer face stays in compression. This technique is frequently used by boat builders when steam bending the thick Oak frames for wooden planked vessels. Although clear vertical grain lumber is usually desired, it is possible to bend wood with irregular grain patterns and occasionally small, tight knots. “Green” wood, or freshly milled wood from a live tree, that has not had a chance to cure or dry, can also be very flexible, but will it probably twist and distort while drying, unless it is kept in a form (sometimes for several months). Most kiln dried lumber available from hardwood dealers will typically be in the range of 12%-18% MC (moisture content) which is still suitable for most bending operations. Most dry woods (6%-12% MC) will bend pretty easily by hand, if cut thin enough (1/32”-1/8”) (1mm-3mm), and will easily form gentle curves. Thicker sections can be also be bent by applying gradual force (gravity, bungee cord, tourniquet) over a period of time, like hanging a weight on the ends of a piece of wood for several days/weeks/months, depending on the section thickness, amount of weight, and the amount of bend needed. A ¼” (6mm-7mm) piece of Ash supported in the middle, with 1-5 lb. (.5kg-2kg) weights taped on the ends would form a typical teardrop shape, in a week or so. The limitation to these methods is that they are slow, need constant adjustment, lack precision, and you can only do the simplest of curves. Soaking thin sheets of veneer in water is sometimes done by furniture builders, and model makers will soak small thin pieces of balsa wood for some projects, but you would probably get poor results from just soaking the wood on anything thicker than 1/8”- 3/16” (3mm-4mm) thick without the addition of heat for most hardwood species.

       The addition of heat can accelerate the bending process by plasticizing the hemi-cellulose after the wood has dried. Leaving the aforementioned piece of 1/4” (6mm-7mm) Ash in the hot sun (yes! “solar bending”) would take only a day or two to bend into a u-shape suitable to make a frame with (although you would need more plies for the landing net frame). Using more heat offers the ability to make tighter bends and more complex shapes, but higher temperatures can be hazardous. Reasonable precautions need to be taken, like wearing gloves, arm, face, and eye protection, adequate ventilation, and so on; but using higher temperatures does allow the possibility to bend more difficult woods, and make bends with tighter and more complex turns. Heat can be applied using a hair dryer or heat gun for very small sections at a time, and bent with hand pressure, usually with mixed results depending on wood species. Luthiers (guitar and fiddle makers) will often use dry heat to shape the instrument sides by carefully rubbing and forming the sections of a thin piece of wood over a heated metal pipe, and applying hand pressure at the ends until the wood deforms, taking care to not scorch their hands, or the wood by constantly moving it and sometimes “spritzing” it with a fine spray of water. Coopers (barrel makers) also use dry heat by burning wood shavings inside the partially formed barrels, and when the heat from the fire softens the wood enough, they can hammer and force the steel bands into the proper position to bring the barrel staves together at the ends (this may also have the advantage of leaving the barrel insides coated with charcoal, which improves the taste of liquids stored inside, like water and whisky). Boiling water will also soften wood, but is impractical to do long lengths or larger sections and can be dangerous to handle. I have seen boiling water poured on to wood wrapped with rags to hold the heat and get small sections soft enough to deform. I have used hot tap water in a sink, to soften and bend some thin wood sheets that are 1/32” - 1/16” (1mm-2mm) thick with some success.

       I primarily use steam to heat the thicker wood strips for bending. The advantage to using steam is because it’s the easiest, most practical method to heat the entire piece of wood all at the same time, so that a larger area or long length of wood can be softened enough to bend around a form. The moisture in the steam has little effect on softening the wood other than to act as a medium to transfer heat to the wood. Only the outer few layers of cells would possibly re-absorb any water in such a short time, so only very thin veneer like sections realize any benefit from trying to increase the moisture content of the wood. I have tried pre-soaking the strips but it seems to offer little advantage when wood is that thin, and it’s being steamed. Steam will also darken many woods, and will raise the grain, requiring more smoothing before gluing and finishing.

       Using steam typically requires fabricating some type of enclosure, or “steam box” to contain and direct the steam around the piece to be bent. Pressure is not necessary, and can be extremely dangerous! A steam box can be as simple as a 2”-4” (50mm-200mm) section of ABS plastic pipe (PVC tends to soften and deform), with a towel draped over each end, and a length of automobile heater hose attached to a thrift store teapot, and with a camping stove to supply heat to boil the water. My current steam box is a 120” (300cm) long by 10” (25cm) square box made of Poplar, fastened with coated self tapping screws about every 4” (10cm), and has hinged door flaps on each end, with a removable divider to shorten the box when needed, and is epoxy coated (not necessary, but I had lots of older epoxy I needed to use up). I have removable Poplar racks, also coated with epoxy to make them more durable. There are two inlets for steam in the bottom of the steambox, and it has a stainless steel kitchen thermometer in one end to monitor the temperature. Steam is supplied by a new (not used) 2.5 gallon (10L) metal gas can, over a  propane “lobster pot” or “crab cooker” style burner (available from many marine and outdoor retailers), and uses 1” (25mm) marine exhaust hose, and bronze fittings to connect to the box. This holds a little over 2 gallons (8 liters) of water inside, and will produce steam in less than 10 minutes, and run for about 45-60 minutes before needing to refill the water. This steam box has the ability to be divided into shorter lengths and re-route the steam from the dual inlets, to a single side, for a little more efficiency, when doing shorter sections of wood, and is set up with a slight angle so the hot water which has condensed from the steam can drain out and not puddle inside the box. It’s ok to have steam escaping from the box to help insure good flow through of the hot steam.

       The wood should be supported in the steam box so the heat can penetrate all sides as thoroughly as possible. Metal supports or racks can stain some woods, steel will rust of course, but you may be able to adapt some re-cycled stainless steel or plastic coated restaurant equipment, or fabricate something suitable. Many woods will leave a stain also when coming into contact with other woods. I use Poplar, which seems pretty neutral, is cheap, strong and readily available, or Alder and Maple should be OK. Dowels inserted through holes in the sides of the steam box are a very simple solution for supporting wood to be steamed.

        A general rule of thumb for heating wood with steam is that it takes about one hour per inch of thickness to heat the wood enough to get sufficient bending. Thin strips may take less time, and you can over steam wood. You can use several short sections of the same thickness about a foot long or so, as test pieces. You will be able to tell when they are ready by simply hand bending them around the sharpest corner. Before putting the strips in the steam box I also mark the center of the strip to help locate it at the apex of the form, and have equal lengths on both sides of the frame form and handle. Once the wood is hot enough, and removed from the heat source, it needs to be formed very quickly before it cools, which is only seconds with thin material. See “Steam Bending Pics”. Having another person available makes forming and clamping the quickly cooling wood much easier. Although the wood will cool in less than an hour and can be removed from the form, there will inevitably be some “spring back” or tendency for the wood to try and return to it’s original shape. This isn’t too much of a problem with thin material, but unless I am in a rush and plan to glue and re-clamp the wood around a form right away, I’ll leave them on the form at least overnight, and frequently longer to maintain their shape until they are ready to use (I have a lot of extra jigs). Thick sections will typically have as much as 25%-30% spring back, so you may have to make two forms; one for “over bending” and one for gluing when working with thicker sections of wood like those used in architectural or furniture components. Aligning the dry bent wood to the gluing form becomes even more complicated if you have shapes with sharp corners and reverse curves. I will bend all the laminations for a single net around the same form, in the same order that I plan to glue them. If the laminations are thin enough (less than 3/16” (3mm)), I can steam and bend them all together at the same time, but for thicker material (1/4” - 1/2” thick (6mm-13mm)) it’s easier to steam them all together, and bend them one at a time in sequence, on the form. The most reliable technique seems to be to first bend the entire strip into a large loop by bringing the ends together, and then use spring clamps to attach the loop of wood to the landing net form, starting at the top of the frame and working quickly toward the handle (which should be temporarily clamped to the form), with clamps spaced just before and after each bend. To help prevent some of the highly figured woods like Curly Maple, or Bubinga from breaking, I will sometimes use another piece of thin wood on the outside when I make the bends, This extra step will act somewhat like how a “compression strap” is used with thicker wood to help prevent the outer surface of the wood from cracking or tearing apart; this strip can later be used as one of the inner laminations which are usually Ash, or some other straight grained wood which bends easily. This also helps when steam bending thick one piece frames. The compression strap can be also be made from fiberglass or metal (old 12”-14” bandsaw blades that are1/2”- 3/4” wide, wrapped in tape so they don’t rust or stain the wood, work great). Steaming wood may also darken the color of wood, and the bent laminations that are not used right away seem to become less flexible and more brittle after drying for several weeks and months, so they need to be stored in a jig or form, to hold their shape until needed for gluing. Sometimes the laminations will develop a slight crack in one edge or partially through the strip, but they can possibly still be used in the middle or as the inside lamination if the wood is not completely separated. It might be possible to glue the crack before it is used as a lamination, but if the break is not too severe, then during the process of gluing the frame, it will generally

       More information on steam bending  and steam box construction can also be found at this Lee Valley Tools website, the Tai Kobo Workshop site for general wood bending information, Foner Books, the WCHA FAQ’s, Woodweb forums 1, Woodweb forums 2, LMI for musical instruments, and other online sources. Printed information is available from Fine Woodworking’s publication “On Bending Wood” which is available from the Taunton Press, and the “Wood Bending Handbook” from Woodcraft. A PDF file from the Forest Products Laboratory 1999 Wood handbook chapter 19, gives detailed information on wood bending in general.

       Although it’s beyond the capabilities and needs of most do it yourselfers, it is also possible to make some extremely complex bends in wood using anhydrous ammonia, (not the common household cleaning products), but it is extremely dangerous, and requires specialized equipment. It also darkens many woods. You can get more information about this process from “Understanding Wood” by R. Bruce Hoadley at your library, or from most online  bookstores, etc… Also from Fine Woodworking, issue number 30 dated 1981 Sept/Oct., article by Bill Keenan. Experiments with ammonia bending have been conducted at the University of Wisconsin in plasticizing  (making pliable) wood, via immersion in gaseous anhydrous ammonia… the theory being that the ammonia is used as a solvent, and diffuses into the cell wall structure and disassembles the existing microscopic cell components producing a more pliable wood...as the solvent  diffuses out of the wood the wood cell components bond in new positions and retain that shape. Steam does the same job but ammonia plasticizes more completely and quicker. The key is the word anhydrous (anhydrous means without water), so the ammonia being referred to is chemically pure ammonia… NH3 in  gaseous form (and it boils at –28 degrees Fahrenheit). Keenan notes that household ammonia is a dilute solution of ammonia gas in water and will not bend wood; however, although I have not tried it, a stronger concentration of ammonia is available from suppliers for use in blueprinting processes. Keenan and the U of Wisconsin procedure is done in a treatment chamber (autoclave) for introducing ammonia gas into woods...at 130 psi into a stainless steel container welded to withstand 800 psi of pressure. A combination of gaseous ammonia and steam is fed into a cylindrical container that the wood to be bent is placed in, exposing the wood for about 45 min. and it comes out like limp spaghetti. Working time to bend and shape plasticized wood is now about 15 minutes.

           

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66" Steam box for making custom wood fly fishing landing nets104" Steam box for making custom wood fly fishing landing nets

Old steam box - 66” (165cm) long

New steam box - 120”  (300cm) long

How to build a custom wood fly fishing landing net

Steam bending wood picture

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