96 Australian Wood Review
TIMBERW
ood is a complex material consisting of
naturally produced compounds that include
water, carbohydrates (cellulose and non-cellulosic)
and aromatic compounds (lignin). These are organised
as mostly long, slender, and hollow cells with tightly
wound fibrils and fillers. For the most part, lignins
cement the cells walls together but are also found
within cell walls.Wood bending is an age old art used for making
furniture, boats and ships, agricultural implements, tool
handles, cooperage, coachwork, wheel rims, sporting
goods and musical instruments. Knowledge of species
properties and processes guide the techniques used.For strength
Straight, solid wood cut in curves will eventually produce
weak cross grain areas. Bending carefully selected
straight grain sections of wood can virtually eliminate
this weakness. In other words, material thickness can be
reduced while maintaining strength.Boat building and cooperage use gradual bends, however more
severe bends require more care and a better understanding of
the process in order to obviate bending losses through failures.The selection of species and wood sections is important, as
is an understanding of the process of plasticising wood.Plastic and elastic
To get a straight piece of wood to remain bent you have to
work in the ‘plastic’ range of its properties otherwise in its
elastic range the bend will not remain permanent. Creep
can occur after plasticity and before failure, and must be
avoided as it may produce unacceptable fractures.Steam power
The simplest way to make tight bends is by plasticising the
wood with steam at relatively high moisture. In the presence
of moisture, the heat from steaming will soften the lignin
and make the wood pliable. Just heating the timber with dry
air is unsatisfactory because that will lower the moisture
content (MC) and not plasticise the timber. (This is a similarThe Technicalities of
Bending Wood
Why, when and how wood can bend. Story by Dr Jugo Ilic.