Thread: Air dried timber for steam bending

I have tried to steam bend some kiln dried Vic Ash and it broke, wasn't a real tight bend either, whether air dried Vic Ash would bend OK at this point I don't know, as for the air dried Jarrah maybe put in a call in the small milling forum.



Pete

Very good question Trav. I will look forward to hearing some answers.

Regards
Paul

The perils of kiln drying..MM

Keith Bootle's book "Wood in Australia" would be a good reference source for this info. Not easy to find but the bigger / good public libraries will have a copy.

Kiln dried timbers have already been modified by controlled pre-steaming / pre-drying / "holding" / reconditioning / final drying treatment so not so suitable for steam bending. The schedules vary considerable for different species. Jarrah has been kiln dried since about 1919 and is very well understood so do a Google search on "kiln drying eucalypts" or "kiln drying jarrah" and wade through them. Jarrah is pre-steamed up to 95C.

These general links may also help you
http://www.vccaq.org/BendingTimber.pdf
http://www.dpi.nsw.gov.au/__data/ass...-of-Timber.pdf

This one is more specific,
http://www.ipef.br/publicacoes/semin...aria/cap12.pdf

Generally speaking... very generally speaking...

The rule of thumb is to try and have wood at around 20% moisture content for steam bending purposes. Differing species bend differently so thats why I say "generally speaking". Bend the timber first then dry it... not dry it then bend it.

If your moisture content is too high you'll have hydrostatic stress issues around the inside of tighter bends.... comes off like wrinkles in the timber.

If your moisture content is too low your timber will need to spend a long time in the steam box. Moisture and/or the lack thereof whether from air seasoning or from kiln drying has no effect on the lignin. However the moisture in the wood is what is carrying the heat from the steam through the wood to soften it. Lack of moisture in the wood means the wood fibres themselves have to transfer that heat and well... wood is a very effective insulator and doesnt transfer heat well.

If you must use wood at low MC for your bending project you will have better results using gaseous anhydrous ammonia rather then running a steam box.

Thanks all. Interesting stuff. Perhaps I should be looking at laminating a bend, rather than steam bending. It is for handles though, and I was hoping to avoid the look of laminations.

Alternatively, I could just give it a go and see if it works. What's the worst that can happen? (Famous last words... It's been nice knowing you all)

Trav

There's a very good article on steam bending in the current Fine Woodworking mag - by Michael Fortune.

Note his use of metal straps on the outside of the curve and the way he increases leverage on the piece to be bent. Unlike the guy who tried to teach me steam bending at Sturt, Mr Fortune does actually know what he is doing.....

I've got that FWW article - it's really good and it's where I became concerned about using kiln dried timber. I particularly liked the idea of a metal strap to manage the stretching on the outside of the bend.

Trav

Trav - Michael Fortune did quite a bit of steam bending when he used to visit Sturt, and his "failures" are hung up outside one of the workshops for all to see. He did comment on the difficulty in finding air-dried timber here, and that several local species just don't steam bend very well.

John.G is correct - too much moisture can be just as bad as too little. Blackwood is actually quite a good timber for steaming, but the only stuff we could find was very green. We got the "wrinkled" effect every time....

After seeing all the things that can go wrong with steam bending, I'm afraid to say I'd go laminating every time. More predictable, less dependant on moisture content, and you can get almost invisible glue lines by tinting the epoxy. Less springback too, so you end up with the shape you wanted.

An interesting approach (which I've only seen demonstrated) is to laminate commercial thickness veneers from a pack (each probably 0.6-0.8mm thick). You need to use a slow curing thin epoxy which can be rolled onto the veneeers, giving time to get the entire stack glued before epoxy goes off. Very easy to bend in a tight radius, and due to each layer being so thin the overall effect is very like solid timber.

Ask around about finding someone who is familiar with using gaseous anhydrous ammonia for bending. It's not particularly hard or complex ( a bit smelly maybe) but it's one of those things where it's easier to learn from being shown, rather then explanations and/or books etc unless you have the time for repeated trial and error attempts.

I have been steam bending Blackwood for Windsor chairs for some years and have found green wood is best I use metal straps to hold the curve . I got my straps from a band saw blade which had no teeth I use a winch to bend 30mm by 30mm as I can't do it on my own. If you use air dried (and I have) I soaked it in water for a week first but it does change the colour a bit. I use a wallpaper steamer to make the steam and I work on the 25mm by 25mm takes one hour steaming.
Once you have a really good bend you will want to do it again and again I have a large number of bends waiting for chairs.

Although I'm no steam bending expert, I love it as a topic. Below are some points that contribute to my understanding of the process;

* Different timber species have differing cell structures and lignin levels. This is why some species are more suitable than others for bending. Different species will also have differing ideal MC's for steam bending. To complicate things further, different species encounter their own specific variations to the 'one hour per inch of thickness rule'.

* Lignin is a protein that binds or glues the cellulose structure of the timber together, and it is suggested that as wood dries it loses lignin, not moisture. Over time, lignin can also be broken down by oxygen and UV radiation. This is evident in what we would regard as heavily weathered timber. Sealing or finishing dried timber is what slows or prevents it from breaking down any further.

* When steam bending, the cells in the timber's fibres are softened or plasticised and are then able to be compressed/tensioned. Lignin is also softened during the process (similar to reactivating certain glues) and then re-bond the compressed/tensioned cells as the timbers cools or sets to it's form.

* 'Wrinkles' on the compression face (inside) of the bend, are normally caused when the timber is too green. If the wood is too dry, you are likely to get fractures on the tension face (outside) of the bend.

* Metal bending straps help eliminate fracturing that can occur even when the timber MC is ideal, as well as help to ensure the piece to be bent has even pressure applied to the curve of the form over it's entire length.

* Timber that is slightly too green is better than timber that is too dry. If the timber is too dry, then the lignin content of the timber will be too low. Soaking dry or over dried timber is somewhat of a futile activity as it is not possible to replace that which has been lost...lignin. Soaking it will however contribute to the steaming process as the introduced moisture vaporises once heat is introduced.

* Due to the differences in the cell structures between species, some timber will experience more spring back once removed from a form as it's cells are more resistant to the level of compression/tension trying to be achieved. Most forms are built beyond the required radius to accommodate this.

* Generally speaking, dry timber is better suited to cold forming. This is where a series of strips or laminations are created, and glue applied between the layers before placing them in the form. This method is usually more successful than steam bending because modern glues (if correctly selected for the task) are stronger and have more rigidity than timber lignin.

Ultimately, it's something that is to be experimented with even if you think you've done your homework.