Steam bending timber

[Kittyhawk]

I need to put a fairly sharp bend in a number of lengths of 3mm thick x 7mm wide pieces of timber.
An online search says a ballpark one hour steaming for 25mm thick. Mathematically 3mm timber would require 8 minutes in the steambox but I dont think it works that way.
What would be an 'about' number of minutes to tightly bend my 3mm wood?

 

[Sgian Dubh]

That time sounds about right. In any case, you can steam for ten minutes or so, take the piece out of the steamer and gently try a test bend. If it's not flexible enough, put it back in the steamer.

Alternatively, you might consider not using steam at all to bend such a thin piece. A well established method is to fix a piece of pipe to a vise, or similar, and use a blow torch with the flame directed down the centre of the pipe to heat it. Once the pipe's hot gently rub the 7 mm wide part of the piece of wood you want to bend backwards and forwards around the pipe's circumference until it becomes hot and flexible and execute your bend.

The method I've described is quite a common one for thin sections, and much used by, for example, luthiers as well as furniture makers for bending stringing and the like, see an example below.

I'd say it will be worth your while to at least investigate this hot pipe bending method before you commit to steaming. Slainte.

PS. There's also the possibility that your pieces are thin enough that simply dunking them in hot water kept close to boiling for a ten or so minutes will make them soft enough to bend easily. Whether or not that method will work will depend upon the size (length) of the pieces, the continuity of the grain, and the suitability of the wood species for bending: some wood species are not well suited for bending.

 

[Peter Sefton]

Sound advice from Richard, I would try my bending iron first, this video shows a home made version.



Cheers

Peter

 

[profchris]

I've bent a fair amount on a hot pipe/bending iron. Much depends on how tight your bends need to be and what species of wood.

A bend similar to the curve at the waist of a guitar is quite hard to achieve, even at 2mm thick. For tighter bends I'm looking at 1.5mm or less. The three main failures are creasing, cracking across the grain, and separation due to runout.

A metal backing strip on the outside of the bends helps. So does bending over a damp rag which produces a little steam to speed up heat penetration. Don't force it, you can feel when the wood wants to move. And once bent, it will ease out of the bend unless you force it to stay there while it cools. Some wood requires overbending to allow for springback.

Oak usually bends well, so do walnut and ash. Mahoganies often fight back, ebony is brittle.

Good luck! Make more strips than you need to cover failures.

 

[stuckinthemud]

Wet wood for wet heat (steam or boil green or pre-soaked timber). Dry heat for dry wood

 

[baldkev]

I used to occasionally bend a wooden spatula when cooking pasta, about 5mm thick, 25mm wide handle boil it up and put pressure on it.

 

[Kittyhawk]

Pretty keen on trying the heart gun method, presumably you could get straight onto gluing after bending.
But it will have to wait for a few days - obligatory visit to the inlaws coming up☹

 

[aet1]

Take the wood, hot gun and metal with you - much better than sitting trying to think of something to say.

 

[MikeJhn]

Ohh a new use for my model turbine engine.

 

[COWS]

What wood are you using?

I've recently been steam bending maple @ 3.3mm x 35mm and it formed well. Laminating 6 pieces together for thicker finish but my glue up was unsuccessful so shelved the idea for now.

Thread link

 

[profchris]

If you are laminating the pieces, some kind of former is almost essential. Hand bending produces similar, but not usually identical, shapes.

It would help to know wood species, how sharp the bend, and what the next steps are.

 

[Kittyhawk]

Managed to break a lot of wood trying to steam bend and also tried the hot pipe dry method but without success. But now I have a system which works very well, so if the following helps anyone with similar problems, that's good.
Firstly, the timber I like to bend is NZ Rimu, a conifer which I understand is not ideally suited to steam bending. I am trying to bend the last 10cm of 60cm long planks, 2.8mm thick and tapering over the last 10cm from 7mm wide down to 3mm wide.
The first issue was with the steam box which wafted steam over the timber ok but thought it would be better if a bit pressurised so welded up a new steamer out of some offcuts of schedule 40 pipe. This is easily done in a variety of ways to suit what material you have available but you need to make sure that you have some sort of pressure release on it. You want a steamer, not a bomb.
In 15 minutes the timber was pretty bendy but was getting cracking on the convex side of the curve. The wood fibres on the concave side of the curve compressed without problems.
The problem appeared to be with my little bending jig.

A & B are bits of 9mm plywood. The steamed timber C is placed between A & B which are then clamped together. I think clamping like this places stresses on the wood causing cracks.
The solution was this:

B is the same bit of 9mm plywood, A is a strip of 0.6mm X 9mm wide stainless steel cut from the back panel of an old microwave from the box of stuff that you have no use for but are too loathe to throw away. Put B into the vice, jam the bit of steamed timber C up there between A & B and with a pair of pliers pull A tight and slowly down and clamp it all together.
It would seem that applying pressure simultaneously over the whole length causes cracking whereas applying pressure sequentially avoids this.
Works for me.

 

[Sgian Dubh]

I'm pleased you seem to have achieved what you were aiming for. With steam bending it's a common practice (not always, e.g., in free bending) to incorporate a metal strap with stops at both ends to trap the length of wood that's to be bent: the strap follows the convex curve. In steam bending the inside of the curve (concave side) experiences compression so there's a tendency for the wood cells to crumple - wood can cope with this stress quite well, although it's often possible to see at least some crumpling on the concave side of the bend. What wood can't cope well with is the tension stress (stretching) on the outside of the curve resulting in the grain tending to break apart due to longitudinal shear. A well made stop-ended strap, as I described above, prevents/resists the effects of longitudinal shear thus eliminating, or at least largely preventing grain separation.

Ercol, the furniture maker, have a well oiled system and method for bending multiples of the same parts for some of their furniture, see below for a couple of snaps, the lower image being of parts that have compound bends resting in their bending jigs whilst they cool in order to retain their shape.

It looks like you've advantageously discovered much of what I've briefly alluded to above on your own. Slainte.