Releasing wood tension by heat / Wood bending?

[o_LuCaS_o]

Hi!

Today I watched this clip on Youtube:

http://www.youtube.com/watch?v=AgNVCOpYojA&feature=BFa&list=PLB0C475ACB9DF4E79

At (03:20) the Craftsman checks the board for bow and twist and then heats up the wood over an electric heater/stove and starts "de-bowing" and "un-twisting" the board before jointing. The boards length is around 1,20-1,30m so it's rather long and I think that by this procedure he wants to reduce the amount of deflection from straight to save as much thickness as he can. The clip is narrated in Japanese with no subtitles, so unfortunately, I don't have the slightest clue what is being said

Has any body done or uses this procedure before milling stock? Does this procedure "calm" the board down and reduce the amount of movement that may occur after milling?

Kindest regards,
Lukasz.

 

[Giff]

He looks as though he knows what he is doing. It would be interesting to know WHAT he's doing.! Geoff

 

[AndyT]

Lukas, I think your guess sounds right, but I don't have any other knowledge of what he's doing. However,

- it's beautifully straight-grained wood
- it's cut radially so there won't be much distortion at all
- from the way he handles the glued-up board, it's very light weight - maybe like cedar?

Also, do you think the boards would stay straighter if they weren't stored leaning against the wall like that?

Fascinating videos though. I especially liked the different approach to dovetails at the start of part two - make deep knife cuts and snap the waste off by hammering it!

Edit: another video from the same source shows a more dangerous version of the same technique - just start an open fire in your workshop and pass the wood over it!!!

http://www.youtube.com/watch?v=YQvURVFpqSI

 

[o_LuCaS_o]

Hi!

The clip You linked to shows a lot of great stuff. Choosing of the log and cutting to the exact requirements of the Shokunin, the hydraulic press to remove cup...no need for fore planes huh

You mentioned the positioning of the wood. This is another movie, in which I see the vertical/diagonal way of positioning boards. Totally opposite to the European method. I wonder, if this saves some bow induced by storage?

Regards,
Lukasz.

 

[o0dunk0o]

Edit: another video from the same source shows a more dangerous version of the same technique - just start an open fire in your workshop and pass the wood over it!!!

http://www.youtube.com/watch?v=YQvURVFpqSI

Traditionally the gondola builders in venice use an open fire to bend the hull planking, I've got a picture somewhere showing a 30 odd foot long plank propped up over an open fire with weights strategically placed to induce bend and twist.

 

[AndyT]

Fascinating about the gondolas. At least they have water nearby; I was just a bit shocked at the size of that fire and where it was!

Thinking about that video some more, it would be nice to understand the context. Presumably even in Japan they don't make all their furniture that way. The blend of hand and machine was interesting too. Optimising for use of that lovely straight grained timber means that they used a hydraulic press to flatten boards which can't have been a traditional method like the planing and jointing were.

Interesting to see that the 'polissoir' was not only known to the French, to be rediscovered by the Americans.

 

[marcus]

I love watching Japanese woodworkers except that it always makes me feel totally incompetent!

When building wooden ships with really substantial timbers they would actually light one half of the timber and let it burn for a while to heat it up before bending...

 

[o0dunk0o]

When building wooden ships with really substantial timbers they would actually light one half of the timber and let it burn for a while to heat it up before bending...

Which culture does this?

I know Chinese Junk builders would dry heat (over a fire) the hull timbers so much that the wood would char but they wouldn't actually set light to it.

Setting light to the timber could damage the plank edges and make it hard to get a watertight seal :?

 

[o0dunk0o]

The blend of hand and machine was interesting too. Optimising for use of that lovely straight grained timber means that they used a hydraulic press to flatten boards which can't have been a traditional method like the planing and jointing were.

I cant see quite how that press would work, unless they leave the boards in the press for a week or the plattens are heated slightly just pressing and releasing the boards wouldn't have much effect :?

 

[marcus]

Which culture does this?

Any culture that built really big wooden ships, including our own — not for the planking though, but for more substantial timbers - frames, stem and apron etc. Remember that in a ship these could be huge. After the piece was bent into place the charred timber would be adzed off.

 

[o0dunk0o]

Ship frames, stems and aprons were never bent, they were sawn from crooks.

frames in a ship can be made up from several futtocks (smaller curved pieces) to create the appropriate frame shape.

I was asking as I'm a time served wooden boatbuilder and my whole family has a history with wooden boats so I was interested to hear of a technique that I've ever come across before ;-)

 

[marcus]

I was told of this technique by Nat West who owns and is head of training at the International Boatbuilding Training College. I'm quite willing to be wrong, if I am, but he knows his stuff!

It was a long time ago that we had the conversation, but I got the impression that it was not something which was done very often, but was one of the tools in the box that was available and used sometimes.

I know what a futtock is :wink:

 

[o0dunk0o]

I was told of this technique by Nat West who owns and is head of training at the International Boatbuilding Training College. I'm quite willing to be wrong, if I am, but he knows his stuff!

It was a long time ago that we had the conversation, but I got the impression that it was not something which was done very often, but was one of the tools in the box that was available and used sometimes.

I'd be interested to know any references he has to this as neither I or my Dad, who's interest in historical techniques is greater than mine, have heard of this.

There are references to substantial timbers such as gunwales being fire bent (held over an open fire) or bent by stoving (timbers heated in wet sand)

The main problem I could see with bending timbers for a stem or apron is that the timbers would be so massive they would have to be held in shape for months after the firing to prevent spring back, especially if it was made significantly over size then cut down.

I know what a futtock is :wink:

yer when writing that post I hadn't realised you were a Ibtc graduate

 

[marcus]

Think you're probably right re stem and apron. Hadn't given it that much thought as it isn't a technique I'm ever likely to use! I'll ask him about it if I see him - I pop in from ocassinally when I'm in Suffolk.

 

[Bigdanny]

I've read somewhere that a lot of clothes chests like those in the video were / are made out of Paulownia wood. It grows very fast and has good lightweight properties and has strong straight grain. Its also known as foxglove tree. But the fire bending is very interesting.
http://paulownia.co.uk/

Danny