Steam Bending

UKworkshop.co.uk

Help Support UKworkshop.co.uk:

This site may earn a commission from merchant affiliate links, including eBay, Amazon, and others.

niall Y

Established Member
Joined
1 Nov 2018
Messages
1,255
Reaction score
988
Location
CARDIGAN
A quick question for the folks out there - Is there a preferred orientation of the grain, when bending wood? In other words is it best to bend the wood parallel to the annual rings, or at right angles to them?

I note that bow makers have the bows bending parallel to the annual rings. The obvious difference of course is that I wish my wood to stay in position when bent. :giggle:

An additional question arises from this, which is - Is there any advantage in having, what would have been the outside of the tree, on the outside of the bend?
 
Wood will bend either way, although it tends to bend easier parallel to the growth rings.

I don't believe there is any major advantage to having the "outside" of the tree on the outside of the bend, although in some species, you might find such wood is more porous and therfore absorbs the stream more readily.

George
 
It doesn’t really bend both ways. parallel is easier and less likely to spring back or buckle. . Think of a ruler. In my limited experience, bending wood sideways is possible but you get very limited movement. If you only need a few degrees of bend then it’s fine but otherwise go parallel.

Thickness is the biggest factor. Go over 10mm at your peril
 
Last edited:
I did a course last week on making Windsor chairs which included steam bending and was told the main thing is to ensure the wood is air dried and that the grain is straight and runs parallel to the edge of the piece with no knots or defects.

Two hours steaming and the pieces were pretty pliable. The arms are 40mm thick.

Sorry it’s not a full answer but there was no mention of where the wood came from on the tree as being important.


IMG_7734.jpeg


IMG_7733.jpeg
 
I may have been a bit florid, but, the time needed in the steamer rises steadily with increasing thickness, below 10mm things happen relatively quickly and with very simple equipment, even a kettle spout or foil lid over a saucepan with the lath laying across the pan will work, thicker than that needs some form of steam source. I like boil in the bag off a wallpaper stripper. Wrap the bag in a few towels to insulate things a bit can help. After that a steam box made of 2” insulation board Is extremely effective. You can steam really thick boards with that sort of setup
 
I did a course last week on making Windsor chairs which included steam bending and was told the main thing is to ensure the wood is air dried and that the grain is straight and runs parallel to the edge of the piece with no knots or defects.

Two hours steaming and the pieces were pretty pliable. The arms are 40mm thick.

Sorry it’s not a full answer but there was no mention of where the wood came from on the tree as being important.


View attachment 175375

View attachment 175376
Ah! Now I was always told that it was best to have fairly green wood for steam bending as it's got a head start on being pliable. The little I've done was always with green wood and seemed to work fine, with the orientation of the grain not making any difference. I suspect it's one of those things where if you ask 5 people, you'll get 5 opinions...
 
Ah! Now I was always told that it was best to have fairly green wood for steam bending as it's got a head start on being pliable. The little I've done was always with green wood and seemed to work fine, with the orientation of the grain not making any difference. I suspect it's one of those things where if you ask 5 people, you'll get 5 opinions...
Yes that’s pretty much exactly my experience.
 
There is very sound reasoning why green timber will bend more readily.
The moisture content is higher, and all that contained water will also
heat up, including to a greater depth. Dry timber will struggle to absorb
the steam deeply, and so be more difficult to manipulate.
 
Thanks for the many useful replies. As a bit of background - I have some ash in the workshop, that has been split from a long narrow trunk. This has given me four lengths that are roughly a quadrant in cross section. I'll be using them shortly, to make the arms of some chairs. So , I was curious about how best to take the rectangular cross section I need, out of the stock . My thoughts based on the replies, is that I should bend parallel to the annual rings, though I might experiment with some spare stock.

I do have an insulated box for steam bending, that I have used quite successfully in the past. It was given to me, by a friend, and is made from a 12" diameter length of galvanised, steel ducting, with lids to both ends. Wrapped in an old, ratty, sleeping bag it has all the appearance - as the friend who made it remarked - of a down-and-out sleeping in a shop doorway. :)
 
Whilst it's correct that green wood bends most easily, my preference for steam bending is material that's been dried to roughly 20%±2% MC. This is because if very wet wood is bent, i.e., wood well above FSP with significant quantities of liquid in the cell cavities, the bending can cause bursting of the cell walls as any fully charged cells compress and disrupt. Such disruption leads to unnecessary loss of strength in the wood, which is undesirable. There’s less cell wall damage caused during the bending process if there’s ‘wiggle’ room inside the cell itself due to it being empty—the condition found in wood below FSP.

In addition, compared to green wood, wood dried to ±20% MC has had chance to reveal some of the distortion, checking, splitting and other faults the drying process can cause. Selecting fault free air dried parts for bending means they are likely to go through the heating, steaming and wetting process, and the subsequent re-drying and cooling after bending with less risk of those faults showing up in the finished bend.

As to the question of best grain orientation for bending it's much more common for growth rings parallel with the bend rather than perpendicular to it. I think the reason for that is prosaic rather than for technical reasons in that tangentially sawn material is much more common than radially (quarter) sawn stuff. There are times when sapwood should be on the convex side of a bend. One example is the yew English longbow which, when drawn to shoot, i.e., temporarily bent, where the sapwood has good ability to cope with tension better than the wood's heartwood which copes well with compression (the belly side) and thus faces the archer in use. Apart from that specific case I can't think of other circumstances where the bark side or the pith side should be on the concave or convex side of a bend, which only means what I say, i.e., I don't know, and there may be circumstances where it matters. Slainte.
 
Last edited:
I did a course last week on making Windsor chairs which included steam bending and was told the main thing is to ensure the wood is air dried and that the grain is straight and runs parallel to the edge of the piece with no knots or defects.

Two hours steaming and the pieces were pretty pliable. The arms are 40mm thick.

Sorry it’s not a full answer but there was no mention of where the wood came from on the tree as being important.


View attachment 175375

View attachment 175376
Thanks for the reply, your top photo is roughly the jig I will be using to perform the same operation.
However, I am curious about how much the wood springs back when it has dried and been released from the clamp The sides of the jig you show, look to be parallel. So, I'm interested to know just how much one would need to overbend, for the finished piece to conform to the shape of the jig, when dry.
 
Thanks for the reply, your top photo is roughly the jig I will be using to perform the same operation.
However, I am curious about how much the wood springs back when it has dried and been released from the clamp The sides of the jig you show, look to be parallel. So, I'm interested to know just how much one would need to overbend, for the finished piece to conform to the shape of the jig, when dry.
They are left for a couple of days in the former which is the shape required and when they come out are stable. Steve (the guy who runs the course) stores them like this with a piece of twine keeping a bit of pressure. When the twine comes off they don't move.

My knowledge on this is limited to what I saw and was told but it definitely worked.
 

Attachments

  • IMG_7654 Large.jpeg
    IMG_7654 Large.jpeg
    282.4 KB · Views: 0
"You should expect any radius to increase by about 30% after it cools but has not dried (for outside work), and if you also dry the wood below about 12% moisture content you can expect the radius to increase (spring back) by between 1% and 4% depending on the exact characteristics of the wood."
http://learnsteambending.com/steam-bending
That page has some useful info. Hopefully helps
 
Green wood will still have a relatively high moisture content after steaming and will need seasoning where dried timber will only be wet and need drying, a much shorter time period
 
Having steamed a lot of ribs into wooden boats, i would make the following observations, bending with the grain flat is easier, flatter sections bend easier than square ones. As said thin stuff steams quick with thicker taking longer. Best thing is to test every ten minutes, there is a sweet spot when its just right. If you go too long you overcook the wood & its far easier to break! Always have spares on hand cos you will break a few.
If using dry timber I used to machine the ribs sand them up & tie the bundle up in the creek for a week or so to soak. Always sand a slight radius on what will be the outer side of the bend, Any nick in the grain will encourage a break when you bend it.
 
That pre-soak for seasoned timber is really important - the water in the wood transports the heat through the timber very effectively but you do want the wood to be wet right to the centre of the lath - thinner the lath, the less time it needs to soak. I actually prefer to "boil" pieces - placing them in boiling water instead of steam - but finding a pan big enough is an issue - I have seen videos of large drum boilers (40 or 55 gallons) used for this
 
For steam bending big jobs i used a beer keg with gas ring under it fueled by propane bottle, A wooden box or aluminium mast section was good. On small jobs or on site i often used a 3" steel pipe with wooden plug hammered in the end. Stick the plug end on the ground & open end up on crossed sticks , build a small fire under the low end or if you were somewhere you couldnt do that I used a gallon oil can with holes cut in each end for the pipe. Take the cap off & stick a blowtorch in the hole with a few vent holes. That contained the heat nicely. Last time i did it i finished at high tide & kicked the steamer over to empty it. A gallon or so of inky blue black water from the tannic acid in the oak poured into the creek & left a huge blue black stain!
 
Back
Top