Wood Bending

the bending of whole or laminated (glued-together) stocks of wood and the fixing of the bent form given to them. Wood bending is based on the plastic properties of wood. Wood of hard deciduous types—oak, ash, and especially beech—is many times more plastic than the wood of conifers and soft deciduous-types (aspen, alder) and is more suited to bending. Wood bending is used in building ships and railroad cars and in making trusses, arches, frames, vaults, profiles, piping, furniture, skis, barrels, wheel rims, and other bent-laminated products. There are hot and cold wood-bending processes.

Hot wood bending is useful for obtaining bent parts from whole and also glued multilayered stocks. It is based on the fact that wood has greatly increased plasticity when heated to 80°-120° C if the moisture content of the wood at that temperature is close to maximum hygroscopicity (25-35 percent).

In hot wood bending, the wood is plasticized, usually by steaming it through or by placing it in boiling water. The plasticized stocks are bent to a template, fastened in presses, dried until they have a moisture content of 10-15 percent, and cooled. During bending, the convex side of the stock is stretched and the concave side is compressed. The maximum value of shrinkage for plasticized wood of hard deciduous species is equal to 25-30 percent of the original length of the stock, and for coniferous and soft deciduous types it is 5-7 percent. Maximum values for stretching are 2-3 percent (for beech, as much as 5-6 percent) and 1-1.5 percent, respectively.

The main index for wood bending is the relationship of the thickness of the stock to the radius of the bend. For a thickness/radius ratio of less than 0.05, hot wood bending can be done without special equipment; for a ratio of greater than 0.05, the wood may tear on the stretched side of the stock. In order to prevent this, a metal band 0.5 to 2 mm thick is placed on the stock and fastened to the template. During bending, this band takes part of the stretching forces onto itself through its end stops. One of the stops is adjustable so that the tension on the band can be regulated. Use of such a band makes it possible to bend hard deciduous types up to a thickness/radius ratio of 0.3. Stocks up to 20-30 mm thick can be bent by hand, but thicker stocks are bent in bending machines.

In order to bend coniferous and soft deciduous types with low plasticity, hot bending is used simultaneously with pressing across the grain, usually with the help of a pressing roller. The surface of the template is rippled for bending with pressing. Use of the rippled roller prevents the stock from shifting on the template and makes its concave side slightly wavy and, with the lateral compression that develops during pressing and the lengthwise shifting of layers, prevents it from buckling. Stock will shrink laterally only where the older growth rings occur. Simultaneous bending and pressing cause the thickness of the stock to be reduced by 10-30 percent and the density and firmness of the wood to increase.

Cold wood bending is a widespread means of obtaining multilayered bent-laminated products. It is based on the natural flexibility of wood. For a part of required form and cross section, a specified amount of glue-smeared dry (moisture content of 7-12 percent) wood sheets (boards, planks, sheets, or strips of plywood or veneer) is placed in the form of a lamination into a press form, squeezed, and held until all the glue adheres. The process whereby the glue hardens can be speeded up by heating the lamination. This also helps remove the excess moisture brought into the wood with the glue. The shape of parts produced by cold wood-bending is preserved better than that of parts made by the hot method. The more sheets in the glued lamination, and therefore the thinner each sheet, the more stable the form. The thinness of the layers in a lamination also makes it possible to obtain bent and glued parts with large cross sections but quite small radii of curvature. The allowable thickness/radius relationship in cold bending of layered laminations made of thin sheets (2-5 mm) reaches 0.05 and even 0.2. This becomes possible because this relationship of the thickness of the individual bent sheet to the radius of the bend is very small and does not exceed 0.02 to 0.01.