Hi
As a furniture maker I have successfully bent timber after heating to around 100 degrees centigrade – steaming or indeed sometimes boiling in water.
However I am struggling with my current project of making a classical guitar. I am trying to bend the sides using the traditional tool of a heated pipe and hand pressure with a small amount of water. So far with let’s just say poor results!
I have searched the literature and it is clear that professional guitar makers use bending machines based on the ‘Fox side bender’. This involves squeezing the side around a mould using a strong heat source. The interesting thing is that the temperatures used seem quite high despite a timber thickness of only around 2 mm.
My question then is this: Is there some hard information about what temperature is required to bend timber of this sort of thickness to these sort of radii. Also to what extent is water required.
Cheers Dewi
Replies
Two successfully bend wood two things must occur. The wood must reach 180 degrees F (82.22 degrees C)or higher (hopefully higher)and have 20% moisture content (which happens as a result of the steaming or poaching). My notion is that your are not getting the wood hot enough. Once hot you only have a short time to do the bend or the wood cools sufficiently to loose its plasticity. In short it takes a strong heat source to bring the water to just under boiling but the wood is not left in contact with the heat once bent.
With best regards,
Ernie Conover
Ernie
Thanks for the reply. I particularly take your point about bending quickly. In a furniture context this would be about getting your component out of the steam chest or whatever and clamped into the mould. Not always an easy task to do before the thing has cooled off.
You suggest some particular values for temperature and moisture content. Without doubting your figures I wonder if there is some published work on this? In my reading of the literature - including the musical instrument people - this seems to be treated as an art with few hard facts. Bruce Hoadley gives a good description of the bending theory but isn't precise about temps and MC.
Any further info would be appreciated.
Cheers Dave M
Dewi,
I built boats for a spell. I learned that heating is essential, and it just happens that the easiest way is to use steam or boiling water - I prefer steam. My experience shows that moisture isn't necessary, but starting with material at its fiber-saturation point helps (see Bruce Hoadley). Consider that Chinese shipbuilders were known to use the dry heat of fire to bend timbers of 10-inch section. My experience also proves that no tools or high tech devices are necessary to bend wood, and that experimenting will give you the experience and confidence you need succesfully form the outside of a guitar. As Ernie says, you just need heat.
John Gardner, whose books on boatbuilding are available through Mystic Seaport Museum - http://www.mysticseaport.org/, presented that we bend wood by heating and plasticizing the lignin which binds its fibers. Lignin, like Titebond glue, becomes more elastic when hot and allows adjacent fibers to "slide." It's Nature's hot glue.
A boatbuilder's rule of thumb is to steam for 1-hour per inch of thickness, which suggests that you may need to steam for only a few minutes the thin material for guitar sides. It should be hot enough to require you to wear gloves when removing it from your steam box. Then you'll have to work VERY quickly to bend it over your form, because your thin material will lose heat VERY fast. Its not like a 1-in-thick boat frame which would cool slowly enough to give you a few minutes to bend and clamp it over a form.
The thought occurs to me that you might try a hot iron, like the one you use to iron your shirts. I might start with one end of the wood clamped to the form (at the neck?) and then move the iron slowly as I form the material over the form's curves. An iron with a convex sole might work better than one with a flat bottom (I've belt sanded them to shape them). Like the Chinese boatbuilders method, the iron's high heat will plasticize the lignin, and experimenting will teach you how fast to move the iron. Think of it like cooking: the variables are temperature and time, or how hot the iron and how long to leave it in one place. Idealy, you've got to heat the wood to just below the point of burning it. Then, a second pair of hands would help to clamp your piece to the form.
I've seen instrument makers sandwich their thin material between a wooden form and a springy, wide, metal band with matching wooden cauls used to clamp the material into the concave, hollows in the form. The metal band forms it around the convex shape of the form without matching outside cauls. I think it'd help to pre-heat the metal band to prevent the wood from cooling to quickly.
Well, that's my 2-cents worth. I'd just like to know how this works.
Gary W
gwwoodworking.com
Edited 11/28/2006 11:26 pm by GaryW
Edited 11/28/2006 11:28 pm by GaryW
Gary thanks a lot for your thoughtful reply.
Yes the instrument makers do use the sort of device you describe. For the guitar it is the 'Fox Bender' and I am sure it works fine, but a) I am an amateur builder aiming for one or two instruments, and b) I would really like to emulate the traditional methods of building so I would like to crack this use of the heated pipe.
I am also curious on an intellectual level as well as the craft as to 'how and why' the thing works and your input is appreciated.
Ernie thanks as well - Joyce is always worth a look, and he does give some hard data.
I think we may be boring people now so will close this discussion - unless I really feel in my practice sessions that I learn something worth reporting.
I would just add that for anyone who hasn't tried it bending timber it is a quite a thrilling addition to the normal techniques of woodworking. Give it a go!
Thanks to both Dave M
Try pages 250 to 252 of Ernest Joyce's Encyclopedia of Furniture Making.With Best Regards,
Ernie Conover
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