Anyone attempting
to build a car body from Ash will sooner or later need to bend some timber to
shape, so this is a short article detailing how I made the two bent sections of
the “C” cab sides.
Some of you may
know that wood is basically a composite material like fibreglass,
in that it is made from
fibres held
together with natural resins/substances. A virtue of this is that we can weaken
the bonds artificially by steaming and bend a piece of wood and then set it
again allowing it to remain shaped/bent, much like a lady having a perm.
Ash can be bent
without steaming depending on the section size and how long the wood has been
dried for. The best way of bending Ash like this is to use air dried wood that
still has an amount of moisture left in it. Wood that is too dry or has been in
a kiln will be harder to bend and may require re-hydrating to some extent.
I chose instead to
steam my sections because I fancied a go at it! I could probably have bent them
without steaming although on saying that my wood was fairly dry as it had been
in my friends wood yard for some time.
THE
STEAMER
The steamer itself is constructed from a piece of pipe I found at work that
happened to be the right size. What you require is a box or pipe big enough to
fit the wood in, with some room around to let the steam circulate, you could
make one out of ply. I drilled the pipe and fitted two metal supports to rest
the wood on whilst steaming to allow all round contact with the steam, don’t use
wood for the supports, as if your steamer works it will bend! A wallpaper
stripper then has the square stripping end unscrewed and the steam supply pipe
is then inserted into one end of the steaming pipe and held in place with a
rag.
The wallpaper
stripper is then filled and allowed to heat up and generate steam. You MUST
allow copious amounts of steam to be flowing through your steam box/pipe before
you slide your wood sections in. This took some time in my case. Once the wood
is in the steamer, stuff a rag in the remaining open end and allow a vent hole
to keep the steam circulating. You need to see plenty of steam going through
and straight out again to get a good flow. Condensed water will run out as
well, which you could save and re-use seeing as it is so expensive.
THE
PROCESS You
need of course first to decide on the shape/form of the bend. A template of
some kind needs to be made, that can be transferred onto a bending jig. The jig
in my case was a sheet of thick ply with some wood blocks positioned were needed
so that the wood work piece could be bent around them. The jig needs to allow
for over bending as the wood will relax slightly when removed. This seems to be
were some skill comes into it! I guessed slightly wrong and my finished pieces
were not quite as curved as my original drawing. This is easily cured by making
another drawing afterwards to match the finished bend so it looks like you got
it spot on if anyone asks!
The aspect ratio
of the finished section should now be considered i.e. is it thicker than it is
wide? The piece should really be thinner than its width, in other words 2 by 1
timber would be bent so that the 2 inch sides would be the inside and outside of
the bend and the 1 inch sides would become the sides of the bend. If, as in my
case you want an aspect ratio that breaks this rule you must bend several pieces
and then laminate them together, taking care to remember that the radius will
differ as you build up the thickness. I bent three pieces at the same time on
the jig so I knew they would fit together correctly.
The
wood needs to be as straight grained as possible and free from knots or other
defects, otherwise it WILL break! The rule of thumb for steaming is one hour
for every inch of thickness, so my half inch thick pieces were steamed for half
an hour. Do not leave them in there too long as the resins in the wood will be
lost and the wood will snap and become brittle. Likewise the wood must be in
steam NOT boiling water, as the water can wash out the natural resins.
Gloves are needed
to remove the pieces and you MUST work quickly and have everything to hand.
Remove the wood from the steamer and place into/on your jig and clamp or wedge
one end. The wood can then be bent around to shape with a smooth even pressure
using one hand for the thin pieces I did. You will feel the wood bending and
here it sort of creaking as you go around, if it doesn’t feel like it is bending
it will probably be springing and wont take the shape or may break, in this case
more time in the steamer is needed. It will stop bending quite soon after
removal from the steamer and just spring instead so make sure you are ready to
go as soon as it is out! As an example I removed and bent my pieces in less
than 20 seconds after removal from the steam pipe.
The wood will be
clamped in position once on the jig and should be left for 96 hours before
removal, but NOT in a dry hot place. It will dry naturally in a garage with good
results.
The pictures show
the steam pipe with the wallpaper stripper visible on the floor with the steam
pipe leading into it, and the bending jig with a piece of wood in position to
demonstrate its use. The other picture is the finished article after sanding
and glueing
together the three laminates into one section.
This article, written by Steve Martin, originally appeared in CA7C Seven Focus
in Feb 2008 pp 16-18.
You
need of course first to decide on the shape/form of the bend. A template of
some kind needs to be made, that can be transferred onto a bending jig. The jig
in my case was a sheet of thick ply with some wood blocks positioned were needed
so that the wood work piece could be bent around them. The jig needs to allow
for over bending as the wood will relax slightly when removed. This seems to be
were some skill comes into it! I guessed slightly wrong and my finished pieces
were not quite as curved as my original drawing. This is easily cured by making
another drawing afterwards to match the finished bend so it looks like you got
it spot on if anyone asks! 