Steam bending with a strap

1

121

Active Member
Joined
12 Feb 2010
Messages
61
Visit site
Does anybody have practical experience of this technique to produce very tight bends in Oak?

I need to replace a horrible timber that was badly broken in 4 places (presumably through collision damage and extreme age - and maybe not enough wedge support at the base...).

All attempts to bend this curve in situ have failed - unsurprisingly; however I have read about a process that converts the tensile stresses in the external radius of the bend into compression, by tightly constricting the timber in this area with a spring steel strip and then bending over a former.

Anybody have any advice (other than lamination), please?
 
Oak is green & straight grained, stored under cover in a cold barn and not more than 3 weeks old. Stock is machined down from 1 1/4" x 1" to 1" x 3/4" prior to steaming. Inside edges are chambfered and the piece is coated with linseed oil before steaming to preserve moisture content. The timber is cooked for 45mins in atmospheric pressure steam.
 
Used this technique recently to steam timbers into the tuck of a new boat. Used an old band saw blade (about 1 1/4 inch wide). Drilled a few staggered holes at each end of a section long enough to cover the extreme portion of the bend. Attached with pozi screws and steamed as usual. Rather than use a former we actually steamed directly into the boat, waited for the timber to cool and took it out again to remove the strap before reinstating and fastening. Was amazed at how effective the technique is.
 
For a 1" x 3/4" timber 45 minutes is way too long, i would normally steam a timber that size for no more than 15 minutes or so, cooking it longer dries it out & makes it far more prone to breaking.
 
Not to sure about Quote:-the the piece is coated with linseed oil before steaming to preserve moisture content.
All our ribs were kept submerged in a brine tank and we found that the ribs that required a much tighter bend worked better with double steaming, ie steam for 20mins or so remove dunk it back in the brine tank for maybe 2 to 3 mins and then re-steam and you can then tie them in knots.
We only had one split and that was a rib with a poor grain structure.
It also helps if you have a steamer that produces copious quantities of "wet" steam.
 
121 said:
Does anybody have practical experience of this technique to produce very tight bends in Oak?

I need to replace a horrible timber that was badly broken in 4 places (presumably through collision damage and extreme age - and maybe not enough wedge support at the base...).

All attempts to bend this curve in situ have failed - unsurprisingly; however I have read about a process that converts the tensile stresses in the external radius of the bend into compression, by tightly constricting the timber in this area with a spring steel strip and then bending over a former.

Anybody have any advice (other than lamination), please?
Click to expand...

The technique you mention has been used successfully to steam oxbows to shape - the u-shaped collars for ox yokes - and they are probably a tighter curve than you have there. In this case the timber is loaded inside something looking like a crossbow, on the inside of the bow, and then lines to the outside ends of the bow are pulled back using a windlass. (They use a former for this because, I guess, oxen don't take kindly to having steaming hot timbers wrapped around their necks.)

A bit off piste but on the same planet, just!
 
For a 1" x 3/4" timber 45 minutes is way too long
Click to expand...

So when do you apply the "1 hour for every inch" rule?

Not to sure about Quote:-the the piece is coated with linseed oil
Click to expand...

Harvested from "Clinker Boatbuilding" by John Leather - I'll try anything to achieve this, even witch doctor remedies like that!!

Double steaming hasn't been attempted, would be interested in what the chemistry would be behind that approach...

This is about 160 degree bend over about 6".

Steam generational apparatus is an integral woodburner / boiler made from 47 Kg gas cylinder; boils 3 gallons in 20 mins.
 
I would normally advocate "steaming in a bag", by putting the wood inside a polythene tube and bending in situ with a wallpaper steamer tube, then ripping the plastic off.

But in this case I can see that steaming the whole length in one go it is going to be impossible to stop it bending too far in the middle and snapping before the more gentler curves on the sides get a chance.

I would make up a jig by putting pegs into a sheet of plywood, clamping one end, and then using a shorter polythene bag at the clamped end, moving it along as the wood bent into shape.

The advantage of steaming in a bag is that you give each section precisely the amount of steam it needs, by testing the flexibility, waiting a bit, then bending further, without risk of over-steaming..
 
Thought some of you might want an update.....made the former out of scrap ply and screwed to bench, screwed wedge blocks around the perimeter and steamed the timber with saw blade bolted directly to the timber with a couple of M6 bolts at each end. Nervous about how much compression required, I invoked a slight -ve bend in the stock before steaming to try and nullify any possible expansion of the steel blade. Steamed for 20 mins and let rip...

Hopefully pictures tell the story:-

Many thanks for the input - next step planking the foredeck.......
 
Ahh! no, they would be my indecently expensive Knipex wire cutters used to cut off the excess copper nail, flush with the rove to allow me to pein over and form the rivet.

Normal pincers just won't do it without huge quantities of grunting, bulging carotids and repetitive strain injury after about 4 nails.....
 
You must log in or register to reply here.

Share this page

Top