Problems with steaming and bending a bow slider crossbar

[Chris]

Hello,
as I told you a few days ago, I´m trying to build a laminated crossbar like John Winters shows it on his homepage. It need a nearly 90° angle on a short way to fit. I made a pattern for it and tried to work out the crossbar. I used 3 pieces (ash/cherry/ash) of about 7mm thickness, put them into warm water over the hole night and steamed it like the stems but for nearly two hours. I tried it several times with differnet strips, but the strips are breaking any time. Is there a special trick to bend the wood if a small radius is needed?

Best wishes

Chris

[garypete]

You've done everything right as regards to soaking and steaming. If you're using kiln-dried ash, that may be what's preventing thr tight bend. Try air-dried ash if you can get some.

Sometimrs steaming too long inexplicably makes bending harder and pieces break. The general guideline for ash is 1 hour of steam time for each 1" of thickness. If you're steaming 7 mm for two hours, it's way too long according to the above rule of thumb.

Only thing left is to reduce the strip thickness. If steamed 7 mm breaks, two layers of steamed 4 mm probably won't.

[Patricks Dad]

Chris, I'm not expert on this but various wood species will bend to different minimum radii. Cherry has been 'quoted' as bending down to 6" radius. My personal experience is a bit better than this (~5" with 1/4" thick strips).

Some info at the following website might be helpful:
http://www.megspace.com/lifestyles/njmarine/Steam.html

The following are some general dimensions to which some wood can be bent:

Ash............4.5” radius
Cherry.......6” radius
Elm...........2” radius
Hickory.....2” radius
Maple.......8” radius
Oak...........2” radius
Walnut......3” radius

Perhaps you could try some different wood (air-dried rather than kiln dried).

Good luck.

[Glen Smith]

Hi Chris, your strips seem a bit too thick to make such a sharp bend. You might have better luck with half that thickness. Another method I have used for bending wood is the way the strips are cut from the board. If the board has face grain on the top and bottom, cut off a slice that is the width of the strips you want. Then turn that piece 90 degrees and cut your strips. This way your strips will have the face grain on the top and bottom.

I was making a kayak cockpit coaming lip with walnut and it was always breaking. I cut some new strips with the method I described and the strips bent very easily.

[hoz]

I agree the strips are too thick. For a 90 degree bend you should use 1/8 (3mm) thick strips.

You could try using a metal backer strip on the outside of the bend to hold the grain in, instead of splitting out.

[Jeff in Pembroke]

Hi Chris,

I second Hoz's advice. Try using either a metal band or a piece of webbing strap on the outside of the strips as you bend them. It is helpful to attach small blocks to the metal or webbing. spaced the same length as the strips. This way, when you bend the wood, the fibers on the inside surface of the bend are compressed and those on the outside of the bend remain unchanged in length. If you don't do this, on a tight bend the fibers on the inside of the curve remain unchanged in length, and those on the outside need to stretch as the outside of the curve is longer than the inside because of the width of the strip. This leads to splitting and tearing of the wood on the outside of the curve.
I expect if you try again and support the outside of the strip so that all the bending takes place by compressing the wood fibers on the inside of the curve you will probably be successful.

Cheers,

Jeff

[Chris]

Thanks a lot for your advice. I have just cut a few pieces of ash, laong grained, about 3mm thick and they`re resting in the bathtub for the night, waiting for their bending...

Hope I can tell you about sucess the next days!

Chris

[Markham_David]

Hi Chris,

I agree with the previous advice re needing a backer strip to keep the wood in compression as it is being bent. One other factor is steam: you need lots of it, and it must be hot.

I realize that statement seems redundant, but it is based on experiece. My first attempt at bending strips for stems failed because I used a camp stove to heat the water, and it could not supply enough heat to generate a sufficient head of steam. I switched to the kitchen stove, large burner on high heat, and had no trouble bending wood after that.

The largest pieces I have bent were 1-1/2 inches square (37mm) air dried white oak. They were used as replacement stems for an old Peterborough canoe. I steamed each of them for 1-1/2 hours, with no pre-soaking in a 3 inch ABS plastic drain pipe. The end of the pipe was partially blocked with a rag to ensure both that the pipe filled completely with steam, and that fresh steam, and therefore heat, was continually moving through the pipe.

When ready, they were placed in a compression clamp formed from metal banding with a block of wood bolted to one end and half of a pipe clamp (the adjustable end, attatched to a pipe nipple bolted to the banding with toilet bolts) bolted to the other. The steamed wood was quickly clamped in this jig, just enough to put it in compression, and the whole was clamped to the bending form, then bent. I bent three pieces this way, and two were successful. One faild because the compression clamp slipped halfway around the bend, causing the wood to snap, which emitted a report like a gunshot!

Lee Valley Tools (www.leevalley.com) offers a free booklet on steaming and bending wood. This was the source of my success.

Anyway, good luck with your bending, and if you want more details on my setup, I'll be happy to oblige.

Cheers,

Dave

[Chris]

Hey,

today I started a new try to steam and bend the crossbar. 6 Strips with long grain, cit from 2 parallel strips of the same board of ash, 3mm thick, overnight in water and steamed for more than 1 hour - and it worked! I made a much heavier pattern than before, screwed it on a plywoodboard and fied the bend wood with more than 20 clamps - it really hard work to do this before the wood get cold.... I had to use some power to get the strips round the small edge and of course this time I added a piece of iron on the top to keep the splinters inside th wood :big grin

I´m going to leave it this night on the form - do you think I should let it dry on the form as well or will it better dry when I put it of?

Thanks again and best regards

Chris

[Glen Smith]

Chris, after all the hard work you have put into this boat, why take chances now? I would leave them on the form for at least a few days. Maybe you could put a small fan blowing air on it to speed up the drying.

[Chris]

Maybe I should go paddling with my good old Mad River Duckhunter???? Weather is fine and I should get that special "rubbermaid" feeling

Chris

[Markham_David]

Chris,

I agree with Glen. Ideally you should leave it on the form, with the metal removed to prevent staining from rust, until the wood is really dry, around 6 to 8 percent moisture content. Then you know it will retain its shape.

Congratulations!

Dave