A limitation when working with that curved components may create areas of weak short grain, which could easily split or crack. A way to overcome this inherent weakness in wood is by steam bending, a traditional wet technique. It is used widely in making Windsor chairs and musical instruments, such as violins. Ash is favoured for chair-making; close-grained hardwoods, such as sycamore and maple, are used for instruments.
A length of wood is heated in a steam box for up to 1% hours, during which time it absorbs the steam and acquires a flexible, bendable nature. While still hot and flexible, the component is taken out of the steamer and very quickly bent around a former. It is cramped to the former and allowed to dry and cure overnight as it cools. This completes the first stage of drying. The component is removed from the former and kept under tension so that it does not spring back. It is then hung to dry out completely, this second stage sometimes taking several weeks.
Steam-bending boxes are not commercially available, and most equipment needs to be made in the workshop. You can make a steam box from any scrap wood for one-off bending, although exterior plywood will last longer for repeated use. Build it to suit the size of the piece of wood that you want to bend. For thin strips of wood, it is possible to use a length of plastic down pipe with a wooden bung in each end and a hose for the steam inserted through a hole in one of them. You can provide steam from a kettle, boiler or wallpaper stripper, with two bungs in the pipe and sections of tubing.
A traditional tool used by musical-instrument makers, a bending iron consists of an alloy block that is cylindrical or oval in shape. An electric element inside heats it up, and the temperature is controlled by a thermostat. Timber is planed typically to a thickness of 2.5 mm (7/64 in) and soaked in water before bending. The wood is held against the iron and carefully pulled and worked around it until a curve develops. During this process lignin particles in the wood become soft and pliable, allowing the wood to bend. A flexible metal strap is sometimes used to prevent thin wood from breaking.
Timber for bending should be smooth, straight-grained and free from defects. Soaking it in water for a few hours will help during bending.
Word Discription :
Former A mould shaped to the required curve, around which laminates are cramped while gluing. Often female and male moulds are used together, with laminates cramped in between.