Steam bending oak. ??

[Teckel]

Hello
I'm wondering those of you who are experienced in bending wood by steam. Does it bend better when steamed or actually putting the wood down in the boiling water??
Thanks in advance

 

[Richard T]

I assume that steam is best, as I have seen elaborate chambers built connected to boilers where it would have been much simpler to just boil it if boiling were just as good....?
Not tried it myself though - it's one of those things on the looong list.

 

[Andrewf]

Not sure if steaming is better , but is easier than trying to boil a large length of wood. Finding a bigger enough container would be a problem and the heat source would need to be large. While for steaming you could use a box knocked up out of some planks and an old wallpaper stripper as the steam source. Boiling water works well for bending a localized areas, you wrap the area in old sacks and pour boiling water over the sacks.
The ribs and planks for boats are always steamed. You have to work quickly as it cools very fast. The oak must'n be too dry when steaming. Also the grain direction within the cross section of the piece being bent can be critical. Seem to remember that it needs to be quarter sawn . Also need plenty of spares as not unusual to break several during the operation.

 

[custard]

Good question.

There's a growing body of thought that says it's not the steam that's important, it's the heat, at specific temperatures the wood becomes plastic. It's just that steam is a particularly convenient way of hitting that precise temperature without scorching.

I've tried three different wood bending methods.

1. Hot pipe bending, I've used this for inlay and small components such as handles. Works great on materials up to about 6-9mm thick, beyond that the outer wood scorches before the inner wood reaches the critical temperature. What's really interesting about this method is you can feel the wood suddenly start to give after about a minute of heating. It happens quickly and goes from rigid to full elasticity in just a few seconds, that's why I'm convinced the wood has to hit a precise critical temperature. A few degrees under and there's zero bending effect, a few degrees over and the wood starts to scorch.

2. Immersion in boiling water, tried this for making Shaker boxes. Works great for that application, medium length thin pieces, bit of a faff for anything else. I found boiling time was too long for anything thicker than about 6mm. Also, even in boiling water, kiln dried stock is no where near as good as air dried stock. Maybe kilning doesn't just effect moisture content, maybe it changes the internal structure of the wood? In any event, bending kiln dried stock is an exercise in frustration. For bent kilned wood I'll only ever laminate.

3. Steam, I've used this for chair components. Cheap, versatile, reliable, quick. This is my preferred route, especially for larger and thicker pieces. I try and keep the workpiece out of the condensed water that gathers if the steaming pipe is horizontal by using bearers, I'm guessing but I think the condensed water isn't quite hot enough.

By the way, cleft wet oak (or at least air dried) bends better than anything else I've ever tried.

 

[custard]

Not sure if steaming is better , but is easier than trying to boil a large length of wood. Finding a bigger enough container would be a problem and the heat source would need to be large. While for steaming you could use a box knocked up out of some planks and an old wallpaper stripper as the steam source. Boiling water works well for bending a localized areas, you wrap the area in old sacks and pour boiling water over the sacks.
The ribs and planks for boats are always steamed. You have to work quickly as it cools very fast. The oak must'n be too dry when steaming. Also the grain direction within the cross section of the piece being bent can be critical. Seem to remember that it needs to be quarter sawn . Also need plenty of spares as not unusual to break several during the operation.

Hello Andrew, are you a boat builder? I used to have a boat moored on the Medway. Only ever sailed plastic boats I'm afraid, but there were so many beautiful wooden boats around the Thames estuary. I sail in the Solent nowadays and you just don't see many classics at all.

 

[Phil Pascoe]

If God meant us to have plastic boats, He'd have given us plastic trees.

 

[Tom K]

Hi Teckel, good example shown in "Hands" he soaks his timber and then steams it.
http://www.youtube.com/watch?v=XthOkO-wsK4

Though you probably watched this one already

 

[Teckel]

Hi Teckel, good example shown in "Hands" he soaks his timber and then steams it.
http://www.youtube.com/watch?v=XthOkO-wsK4

Though you probably watched this one already

I did Tom. That's why I asked the question. He had it down in the water but I've seen it before done with the steam.

 

[Jacob]

Don't know anything about it except that steam is hotter than boiling water, which maybe makes the difference.

 

[Tom K]

Hi Teckel, good example shown in "Hands" he soaks his timber and then steams it.
http://www.youtube.com/watch?v=XthOkO-wsK4

Though you probably watched this one already

I did Tom. That's why I asked the question. He had it down in the water but I've seen it before done with the steam.

:? He leaves it to soak in the loch (don't know if he said how long) and then steams it I thought? The idea being the water in the timber heats up and the steam get deeper into the fibres. No?

 

[Teckel]

Hi Teckel, good example shown in "Hands" he soaks his timber and then steams it.
http://www.youtube.com/watch?v=XthOkO-wsK4

Though you probably watched this one already

I did Tom. That's why I asked the question. He had it down in the water but I've seen it before done with the steam.

:? He leaves it to soak in the loch (don't know if he said how long) and then steams it I thought? The idea being the water in the timber heats up and the steam get deeper into the fibres. No?

I would say your right but custard would probably know better. He seems to be well up on this subject.

 

[dickm]

Don't know anything about it except that steam is hotter than boiling water, which maybe makes the difference.

Sorry, not correct, Jacob. The temperature of the steam will be the same as the boiling water from which it is generated, unless you have a separate heat source that just heats the steam. INDUSTRIAL steam IS hotter, because it can be held under pressure so the water doesn't boil until a higher temperature, but the steam that comes off at atmospheric pressure will by definition, be at boiling point, i.e.100C.

Don't think many woodworking steamers (steaming woodworkers?) would have pressurised equipment.

 

[Jacob]

Don't know anything about it except that steam is hotter than boiling water, which maybe makes the difference.

Sorry, not correct, Jacob. The temperature of the steam will be the same as the boiling water from which it is generated, unless you have a separate heat source that just heats the steam. INDUSTRIAL steam IS hotter, because it can be held under pressure so the water doesn't boil until a higher temperature, but the steam that comes off at atmospheric pressure will by definition, be at boiling point, i.e.100C.

Don't think many woodworking steamers (steaming woodworkers?) would have pressurised equipment.

Correct actually. The temperature can be from 100ºC upwards, depending on how fierce the heat source - as it forms at 100º it can be heated beyond 100º before it escapes.

 

[Tony Spear]

You're definitely wrong this time Jacob!
Dick's generally correct, except that you don't neccessarily need a separate heat source, as long as the one you've got is capable of generating the heat you need.

If you heat water in a container open to atmospheric pressure, it will boil and generate steam at 100 C. Regardless of whether your heat source is 100 C. or 1000 C.
To get the steam any hotter than that means that you have to prevent it escaping from the container until it reaches the temperature you require.
This produces "superheated steam". As you heat it, it expands and therefore increases the pressure inside the container.
The more you heat it, the more it expands and up goes the pressure until the container explodes! :shock:

This is the principle on which boilers (and pressure cookers for that matter) work .

Boyle's Laws of Gases. :wink:

P.S. I have seen this done for wood using a steel pipe sealed at one end, a screw on cap at the other and a pressure relief valve. God knows why they were doing it, maybe it's faster that way? :?

 

[Tony Spear]

God knows why they were doing it, maybe it's faster that way? :?

Thinking about it, I could be right there. Pressure cookers were invented to cook stuff (such as vegetables) quicker, and a piece of wood is basically only a chunk of dead vegetable matter!

 

[Jacob]

You're definitely wrong this time Jacob!
Dick's generally correct, except that you don't neccessarily need a separate heat source, as long as the one you've got is capable of generating the heat you need.

If you heat water in a container open to atmospheric pressure, it will boil and generate steam at 100 C. Regardless of whether your heat source is 100 C. or 1000 C.

Yes

To get the steam any hotter than that means that you have to prevent it escaping from the container until it reaches the temperature you require.
This produces "superheated steam". As you heat it, it expands and therefore increases the pressure inside the container.
The more you heat it, the more it expands and up goes the pressure until the container explodes! :shock:

Almost, but in fact the water vapour can continue to be heated by the sides of the container (e.g. in a metal kettle) even though it stays at atmospheric pressure and escapes. It can even get heated in the bubble after formation at 100º if the bubble is in contact with a hot enough surface.
This obeys Boyle's law :wink: - the volume increase with temperature but unless constrained the pressure is constant, whereas in a sealed container the volume is constant.
I wonder if the various arrangements of metal pipes are doing just that - producing water vapour at higher temperatures

This is the principle on which boilers (and pressure cookers for that matter) work .

Boyle's Laws of Gases. :wink:

P.S. I have seen this done for wood using a steel pipe sealed at one end, a screw on cap at the other and a pressure relief valve. God knows why they were doing it, maybe it's faster that way? :?

 

[jimi43]

You can bend small pieces...or thin pieces of wood using heat...this mandolin repair I did....

...needed the rosewood binding bent around some quite tight corners....

The Rio went easily around the gentle curves but there was no way it would go around the tight bend at the base without bending...so I bent it using an old soldering iron to get the right radius...

....which worked fine....

Steam bending is something I want to get into as well....but I think I will do thin strips first....laminating them. I kind of prefer laminated pieces for curved furniture...I want to make a rocking garden chair and that is what I will do to make the rockers.

Jim

 

[TheTiddles]

Don't know anything about it except that steam is hotter than boiling water, which maybe makes the difference.

ahahahahaha, that's one of the stupidest things you've ever said and let's be honest, you've come out with a lot.

Steam could be hotter if it's pressurised, but so would the water, e.g. a pressure cooker.

It probably is the temperature that makes the wood soften along with being damp but you have to have a method of transferring the heat from the source to the wood. Air isn't great at transferring heat, better than nothing obviously, but water has much more ability to absorb heat and therefore carry it from the source to the wood.

Aidan

 

[Jacob]

...
Steam could be hotter if it's pressurised,

Or heated at the same pressure. "Water vapour" that is - not visible "steam" which is water vapour condensing back to water droplets.
The relationship is PV/t = c

 

[TheTiddles]

...
Steam could be hotter if it's pressurised,

Or heated at the same pressure. "Water vapour" that is - not visible "steam" which is water vapour condensing back to water droplets.
The relationship is PV/t = c

Thank you, I appreciate your apology for being wrong and the diligence of actually providing the equation to show this.

Of course I jest, we all know it isn't possible for Jacob to be wrong or to ever learn from anyone, which is why he's so well regarded as a source of wisdom.

For those who might have a passing interest, the part of the equation he eludes to is the "ideal gas equation" which says:

PV=nRT

where:
P is the pressure
V is the volume
n is the amount of substance of the gas (in moles)
R is the gas constant (8.314 J·K−1mol-1)
T is the absolute temperature

Ok, suspend reality and any questions for just a second whilst I explain the first bit...

The amount of gas isn't changing and we know the value of "R" we need so we can call these a constant like, ermmm, c.

So now we have:

PV=cT or PV/T=c as our such learned chum says.

What this means is that if I increase T, either P or V has to increase, so that P x V still equals c x T. So to increase temperature the volume has to increase or pressure must. As we are talking about an un-sealed system, i.e. a steam bending oven, the volume can increase but then the steam is just going to go out the hole at the end into the atmosphere, thereby reducing the temperature of the steam in the oven. The only way to increase the temperature would be to keep that volume the same and increase the pressure (i.e. a pressure cooker)... that's that one dealt with.

Now, where I mentioned suspending reality earlier is because of this, steam isn't always an "ideal gas". In atmospheric pressure it's actually a mixture of gas and liquid normally (called "wet steam"). Steam is a clear fluid, the white stuff we call steam is condensed liquid water droplets in amongst the steam caused by contact with the colder air which is mixed amongst it all. So that equation, whist useful for showing why people shouldn't use formulas they don't understand, isn't actually very helpful here.

What the so highly educated Jacob may be confusing things with, is what is known as saturated steam which can then be heated up much hotter by a process called superheating. To do this, you take steam and take it AWAY from the source of more water and add heat, you have to remove the source of more water other wise the energy you want to heat the steam will just go into evaporating the water instead of heating the steam, this source of water may be other wet steam around your locally heated bit of steam. Now, if you do this (like you do in a steam driven locomotive, ship's boiler, nuclear power station etc...) then you can heat the steam to much higher temperatures than the water at the same pressure. Of course to do this in your steam bender for woodwork (returning to the post topic at last!) you'd need to heat the pipes going from the boiler to the chamber, or the chamber itself, as well an installing a little gadget I'll come onto in a minute, which you wouldn't, so the steam would only ever be at 100c and cooling rapidly as you move from the boiler to the chamber. Not exactly cutting edge science, they've known this for a good few hundred years which is why there is a component called a "steam regulator" which separates the boiler from the superheater in steam engines to stop the liquid water taking the heat that you want to go into your steam.

So there you go, I wonder what reaction there will be but I think I already know.

Aidan