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ptscho
12-30-2018, 07:25 PM
TLDR - I've had trouble finding steel bar long enough (12 feet) to make a compression strap for steam bending, so I was wondering - Is it viable to use chain with angle iron linked to the ends instead? If so, what load rating do you think is necessary to safely assist some 6/4 white oak around a moderate curve?

Full context: http://forum.woodenboat.com/showthread.php?246581-Fixing-unfair-steam-bent-keel

Favorite
12-30-2018, 07:59 PM
TLDR - I've had trouble finding steel bar long enough (12 feet) to make a compression strap ...
Try a steel supplier. 12 feet is the standard length. 20 feet will cost you extra.

Ian McColgin
12-30-2018, 08:25 PM
Why steel? Lay out your curve with a batten on something strong enough, like 1" plywood bolted to some planks under. Then set up stations on the inside of the curve that are more closely spaced than the frame stations. Like 1' or even 6". I'd make these stations with steel L braces and wooden blocks. Beats trying to get a fair bend in a continuous bit of steel. You'll need to buy more clamps, but more clamps is always a good thing.

This is frames, not stem, but you get the idea.

https://scontent.fbos1-1.fna.fbcdn.net/v/t1.0-9/10406831_10203323057292297_5652897002400975848_n.j pg?_nc_cat=101&_nc_ht=scontent.fbos1-1.fna&oh=b8be55e970b8586eae2f01d473feff1c&oe=5CC86FD7

Paul Schweiss
12-30-2018, 08:44 PM
I believe you are needing spring steel, not steel bar, it’s purpose is to keep the outer fibers from rupturing during bending, and the blocks bolted to the ends of the spring steel also assist in making some rather dramatic bends.
The spring steel will not retain the bent curve,but come back straight, unlike steel bar which is going to add to the bending force required and also not spring back flat for the next piece coming out of the steam box.

navydog
12-30-2018, 09:43 PM
As Ian commented the issue is providing a proper mold shape to bend the keel over. Your problem is exacerbated due to the prior steaming, the wood is now dried considerably.

Canoeyawl
12-30-2018, 09:47 PM
Steel bar stock around here is sold in 20' lengths. But I think anything from the steel yard will be a pita.
Look at steel strapping.
https://www.thepackaginggroup.com/products/1-25-x-029-signode-magnus-steel-strap?variant=12694905946182&gclid=EAIaIQobChMIppuw5oXJ3wIVDttkCh1QIwNpEAQYBCAB EgJrV_D_BwE

Ian McColgin
12-30-2018, 09:56 PM
If the grain on the outside is breaking and assuming correct steaming, you're either putting too thick a piece through too tight an angle or you have a badly cut out piece of wood. In either case, try bending two pieces that are smaller for later glueing or if the problem bend is near an end, just kerf it.

Sometimes a whole block just does not work. Again, these are frames that were steamed just fine but broke on installation.

https://scontent.fbos1-1.fna.fbcdn.net/v/t1.0-9/1235964_10203303992135680_4047325021959985663_n.jp g?_nc_cat=101&_nc_ht=scontent.fbos1-1.fna&oh=30aeeb2d81a2939d4fb67337d1a2166f&oe=5C965313

If you look closely just in front of the lowest head you can see the dark kerf line that allowed this frame to take the turning sharper curve lower down.

https://scontent.fbos1-1.fna.fbcdn.net/v/t1.0-9/1653428_10203323057652306_7917974819449123221_n.jp g?_nc_cat=100&_nc_ht=scontent.fbos1-1.fna&oh=cc5e98e6315750450aaec8c89ba9a28d&oe=5CD390DC

nedL
12-31-2018, 09:35 AM
If this is still for your keel I should think you could use just a strip of 1/2” plywood to back up while bending. A “compression strap” is mostly to keep the grain from tearing out. It will not make an unfair curve into a fair curve, that is what your form and clamps are for.
For the slight curve of your keel you shouldn’t have a problem with grain tear out.
These rib sections are 4/4 x 1 3/4 and were all bent without any kind of strap (and the oak wasn’t all the best.)

Peerie Maa
12-31-2018, 09:42 AM
^What he said. Or if you have enough stock, use another steamed piece of timber for your compression strap.

MN Dave
12-31-2018, 10:48 AM
Free steel strapping might take a little dumpster diving, but it is easier to just ask. Lumberyards and lots of industrial places get a lot of large bundles tied down with steel strapping. They cut it and throw it away.

StevenBauer
12-31-2018, 12:27 PM
I’ve had good luck using polycarbonate (Lexan) as a compression strap for steam bending a tight radius. This was for a toboggan project with a 6” radius in some kiln dried Ash. I had plenty of failures before I learned to use the compression straps.