Formful Wood. Explorative Furniture

SEBASTIAN KRAFT
the material does not have to stretch or compact. By placing a metal bending rail
on the outer (pull) side, the tensile component can be completely removed from
the wood. Metal has a considerably higher tensile modulus (and a higher tensile
strength) than wood and takes over the tensile stress. As a result, the wood
“only” has to compress—on the inside now several times over. According to a rule
of thumb, hardwoods such as beech, oak, ash and elm can be bent by a minimum
radius of three to four times the thickness of the piece in question. Conifers, on
the other hand, only allow 18 times the radius. 1 [→ FIGURE 3]
DAMPNESS
In preparation, the air-dry wood is moistened with water. When moisture is supplied,
the cells swell and the properties of the cell building blocks change. E-modulus
and strength decrease. Moistening beyond the fibre saturation point does not
further soften the wood, contents are washed out if necessary and cell cavities
filled with water can lead to breaking outwards on the pressure side during bending.
TEMPERATURE
Temperature has a great influence on the mechanical properties of the wood.
In combination with wood moisture, an increase in temperature leads to a loosening
of the fiber composition and to plastic deformation (in contrast to elastic
deformation, in which the object returns to its original state after loading/bending).
Elongation now causes much less stress than with dry, unheated wood. The
deformability of the cell walls allows for strong compression. Wood should be
continuously warmed to 80° C and moistened up to the fiber saturation point.
GROWTH RINGS
A load in the tangential direction is advantageous, i.e. the cross-section to be bent
should preferably have standing growth rings (90° to the bending direction). Grain
orientation should be parallel to the longitudinal axis. Cut fibers break outwards
during bending, the piece has a reduced load-bearing capacity.
METHODS
The wood is steamed or boiled in a closed container, then bent and fixed as quickly
as possible over a press mold or several points. After cooling it is loosened and
can be further processed. Since the wood springs back a little after forming, the
bending form must be 10–20 % beyond the target geometry. Additional fixation
over several days or weeks—e. g. with a clamping set or nailed strips—prevents
slow resetting further. Each wooden part takes on an individual shape after bending,
therefore it is important for parts that are in a series production that they are
fastened at several points (at least three) when installed.
167