1. The wood must be plasticized.
Although wood can be plasticized chemically or even by microwaves when
in a green state, the most convenient way to plasticize wood is with steam.
Wood cells are held together
by a naturally occurring substance in the wood called lignin. Imagine
the wood fibers to be a bundle of rods with the space between them filled
with lignin. The strength of this lignin bond between the rods can be
decreased by subjecting the wood to steam. With unpressurized steam at
212? Fahrenheit, steaming for one hour per inch of thickness (regardless
of the width) will soften the bond enough for bending. Substantial oversteaming
may cause the wood to wrinkle on the concave face as the bend progresses.
2. Only air-dried wood of an
appropriate species should be used.
Kiln-dried wood must not be
used; the lignin in the wood has been permanently set during the hot,
dry kilning process. No amount of steaming or soaking will weaken the
lignin bond sufficiently for successful bending. The same applies to air-dried
wood that has been allowed to dry and stabilize below 10% moisture content;
the lignin will only partially plasticize with steam, not enough for successful
bending of anything beyond a shallow curve.
3. Wood must be kept under
compression during the bending process.
Because of this third requirement, the Veritas line of clamping
equipment for steam bending was developed. Wood fibers will stretch only
a small amount before they fail, usually less than 1/2 of 1%. If you think
of bending a stick over your knee, as the wood bends, it is the fibers
on the outside of the bend that start to separate and break first. The
drier the wood, the easier it will break. However, when well plasticized,
wood will compress to an amazing degree. It is these two properties of
wood that we avoid and exploit respectively in the steam-bending process.
To stop the wood from stretching on the outside face during the bending
process, it must be restrained at either end by stops securely attached
to a metal backing strap. The wood face in contact with the strap is not
allowed to stretch as the bend progresses; however, the wood face against
the form is subject to compression exerted by the end stops.
For example, a straight piece
of wood 1" thick and 18" long bent to 90? around a 4" radius will remain
18" along the outside (immediately next to the strap), but will have the
inside dimension reduced to almost 16". Nearly two inches have virtually
disappeared through compression along the inside face!
Species for Steam Bending
To avoid a lengthy description
of wood cell structure here, we are providing general guidelines regarding
appropriate woods for compressive steam bending using the Veritas Steam-Bending
System.
Two rules of thumb are:
- Exotic woods do not bend
well.
- Softwoods do not bend well
and should be avoided.
Some common domestic hardwoods
will bend with great success. Based on air-dried 1" thick stock at 25% moisture
content going into the steam box, the smallest bend radius you can achieve
without risk of failure is shown in the table below. You can get tighter
radii but at higher risks.
| Species |
Smallest Radii |
| Oak (red and white) |
2" |
| Hickory |
2" |
| Elm |
2" |
| Walnut |
3" |
| Ash |
4.5" |
| Cherry* |
6" |
| Maple** |
8" |
* Requires some experience
to bend flawlessly. It is prone to compression wrinkles on the inside
face. These can often be removed during shaping and sanding once the bend
is complete and cured.
** Can be difficult to bend.
It requires more leverage to put the blank into compression.
Bending Blanks
The stock itself should be the
straightest grained, knot-free material you can find. The ends should be
square-cut and all sides planed smooth. Generally, we recommend bending
pieces only slightly larger in cross section than the finished piece needed,
just enough to square up, sand, or scrape the surface down to clean wood.
The exception is if you are bending a wood that is notoriously difficult
or if you are bending to a very tight radius. For example, cherry is prone
to compression failure so bend a piece 1/8" or 1/4" thicker than needed
and then bandsaw away the inside face marks. If the exact curve is critical,
you have to remember to accommodate this difference on the bending form.
The discoloration caused by
the metal strap in contact with the wood may also require additional thickness.
Woods high in tannic acid such as red oak, white oak and walnut will develop
a 1/16" deep purple stain if the strap is left on as the piece dries on
the form. Most woods (ash, cherry, hickory, maple) do not develop this
stain to the same degree. Light stains can be removed by sanding or scraping.
Deep stains may be more easily removed by bandsawing a thin slice off
the stain area, followed by sanding.
When bending tight radii, some
reduction in stock thickness will occur as a result of the wood being
compressed between the strap and the form, particularly right at the corner
of an "L" or "U" bend. Bending a piece of 1" thick ash around a 2" radius
form may reduce the thickness as much as 3/16". Backing off the end stop
one turn, halfway around the bend, will decrease this amount.
The grain of the blanks should
be relatively straight. The grain should not "run out" in less than 15"
along the blank, and preferably not where the bend is to take place (see
Figure 2).
Knots should be
avoided. However, if there are small grain irregularities, then they should
be put next to the strap side of the blank; the wood remains relatively
neutral there, neither in tension nor compression. Our tests have not
revealed any advantage in bending wood with the growth rings of the blank's
plane, rift or quarter-sawn face against the form face. Never attempt
to bend a piece that is thicker than its width, as it is prone to collapsing.
To obtain such narrow pieces (e.g., a boomerang), bend a square cross
section, and then bandsaw it to the required width once it is dry (see
Figure 3).
When to Steam Bend
The ability to bend solid wood
can give your projects both a structural advantage and an aesthetic appeal.
For example, a sweeping curve on the back leg of a chair can be very weak
if it is cut from a wide solid board. A portion of the leg will inevitably
be short grain and prone to failure if it is subject to any stress. That
same leg, if steam bent, will retain virtually all of the strength of the
original straight piece of wood. The grain will also follow the curve and
visually reinforce the shape you have created.
Questions that you might want
to ask yourself before deciding to steam bend a curved part of your project
are:
- Is the curved part structurally
important?
- Is it visually important
that the grain follow the curved shape?
If both answers are "No", maybe
a bandsawn shape will do. From a design point of view, try to avoid steam-bent
parts that have unanchored ends that are not fastened down. Humidity changes
can cause the extended piece to "wave" a bit.
- Will I be shaping the piece
afterwards?
Laminating a curve from thin strips
of wood may cause problems when applying the finish. Any glue that is exposed
during shaping will not accept the final finish in the same way as the wood.
Also, the laminates are always under stress and, if some are cut away during
shaping, the curvature may change. Steam-bent parts, on the other hand,
have no memory of ever being any other shape, unless immersed in water.
Making Forms for Wood Bending
The best bending forms are made
of plywood stacked slightly higher than the width of the blank to be formed.
Particleboard can also be used, but is lower in tensile strength, so larger
cross sections are necessary. When bending to a tight radius (e.g., less
than 4"), it is best to use plywood for most of the form and insert a hardwood
nose.
If bending on a bench, screw
the form down to a larger piece of particleboard and in turn secure that
to your work surface. If you are using a bending table, you will have
to drill holes in the form to match table holes. Whenever possible, the
back of the bending form should be cut parallel to the face; it makes
it much easier to apply clamps as the bend progresses.
There are some exceptions to
this practice. To minimize clamp congestion, large holes (e.g., 1 1/4")
can be drilled in the form to accommodate clamp heads (see Figure 5).
If you are using particleboard
forms, it may be necessary to use more holes because of the greater width
of the form. It might even be simpler to leave the form in its basic rectangular
shape and rely completely on clamp holes and bar clamps.
The work surface should be
attached to the floor (or anchored to the wall or a post) if the blank
is thicker than 3/4". Whenever possible, cut the back of the bending form
parallel to the face (see Figure 6 and Figure 14) so that it is easy to
clamp the part as the bend progresses.
When bending a piece narrower
than the strap, it is best to still have the part run down the center
line of the strap. This helps to keep the forces on the strap and end
stops balanced, which in turn reduce the possibility of the workpiece
skewing perpendicular to the bending plane. Block the table up with spacer
blocks 2" apart along the face of the form so that the part can be tapped
down snugly to the blocks to keep it parallel to the table during the
bending process.
Springback, Close-In and Failure
It is difficult to give hard and
fast rules in wood bending because of the variability of the material. Not
only do you have to deal with differences between species but even within
species. There is even variation within a single board due to the uneven
grain. As with many things, your own experience becomes your best guide.
But there are general precautions you can take that will avoid many problems.
If all the main variables are
under control: air-dried domestic hardwood at the appropriate moisture
content, temperature in the box, steam time, bending radius not too tight,
correct end pressure (not so compressive as to create wrinkles on the
concave face nor so loose that fiber separation occurs on the outside
of the bend) and the form is cut to the exact shape you want, there are
still a few things to keep in mind.
Springback
Springback can occur when the
curve is so shallow that the lignin does not shift enough to hold the new
shape, the part is too dry (either from original moisture content too low
before steaming or insufficient steaming time), the end pressure was not
adequate, the part was not left on the bending form until fully cooled,
the part was allowed to straighten while being transferred to the drying
jig, or the part was not allowed to dry thoroughly (less than 10% M.C.).
Close-In
Close-in can occur for several
reasons. If the moisture content of the wood is too high when bending (in
excess of 30%), the displaced and compressed wood fibers continue to contract
as the bent wood dries out.
If the accumulated end pressure
is too great during the bending process, the overcompressed inner face
of the blank will contract as the wood dries out. On a "U" shape, it is
advisable to back off the thrust screw a couple of turns halfway around
the "U". (See Figure 19.) If a part is bent to a radius smaller than the
limiting radius for that species, again the overcompressed face will contract
on drying.
Failures