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Can I use veneer to add strength to kerf bent plywood?

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rocketmagnet

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I would like to add a nice bent wood feature to my next project, and I figured that kerf bending is the best way to go. Mostly, the kerfs are made using a circular saw, but I saw a video where someone used a tapered router bit to cut them:

1632340070757.png


This way he was able to close up the gaps very nicely, which should make them stronger.

1632340196911.png


I would like to do the same thing, but make it even stronger. Presumably, when overloaded, it's most likely to fail at the glue join. But I wondered if it would make sense to glue a veneer over those joins to help stop them breaking.

What I would do is:
  • sand the inside of the bend so there are no flats, just a nice smooth curve.
  • steam bend the veneer, at right angles to the grain direction.
  • glue the veneer to the inside of the kerf bend, with the grain running at right angles to the kerfs
  • vacuum bag it while the glue sets to make sure I get good adhesion.
Question: Is this likely to make much of a difference to the strength of the bend?
 

TheTiddles

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I’d think not, unless you are aiming for several mm of veneer.
How much load are you putting on it? If it’s taking enough that you are concerned about it breaking, it’s probably the wrong thing to use, but it’s still a pretty strong thing, but I’d not make a cantilever chair frame that way, for example.
 

Jameshow

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How about putting a layer of fibreglass cloth and epoxy on the inside if it isn't seen. Then the plywood becomes more of a core in compression?

Veneer on the outside would help too.

Cheers James
 

rocketmagnet

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Thanks for your replies guys. Here's what I'd like to make:

1632342911368.png


It's a bunk bed for my son. It's supported by being anchored to the brick walls on two sides, and by the ladder in the corner (Didn't CAD the rungs yet).

My main concern is that some of the weight is being taken through this bend (and similarly at the other wall):

1632343049001.png


I'm intending to use three sheets of 18mm plywood, to make a total of 54mm thick ply. So those bends are pretty thick wood, but I'd really like to be sure, since I don't want the bed to fall on anyone.

James: Thanks for your suggestion. Actually, that was an idea I had. I wondered if I could actually embed some dry fibreglass or carbon fibre between the pieces of plywood, then use the vacuum resin injection method to fill it with resin after the layers of ply were glued together.
 

TheTiddles

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Nice design.
I’d avoid having to make the bends take the majority of the load at all.
Make a truss, probably a triangle out of box steel and anchor that to the wall so the bed then sits on top, adjust the base so it’s deep enough to cover the truss so it’s still floating. The ladder is then only taking the out of balance loads.
I’d check your calculations and boundary conditions pretty well, give what could happen if it goes wrong.
 

Inspector

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If you are kerfing 3 sheets of plywood and laminating them together those corners are going to be strong enough to party on. If you have doubts kerf a 200mm or 300mm piece of whatever you have, rip it in three and glue them together. When the glue is dry see how strong they feel. Bet your face will turn red and the veins in your neck will pop before you can break it.

Pete
 

Cabinetman

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I agree with Pete, but if you want a bit more strength you could intersperse some bendy ply— I think that’s what it’s called. Ian
 

Inspector

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I’ve been looking at bending plywood for other purposes and the minimum bend radius is in the 8” to 12” (200mm to 300mm) range. To big for the needs of the OP.

Pete
 

Droogs

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you could use trhe RTC modellers trick of inserting strips of carbon fiber that have been stuck with resin

the idea below

 
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rocketmagnet

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Thanks for all your commnts guys,

Pete, I'll try that and I'll let you know the results.
Droogs, if that test isn't strong enough, I'll try the carbon fibre.
 

Jacob

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........
Question: Is this likely to make much of a difference to the strength of the bend?
Yes definitely. It's "stressed skin" construction. Without it it resists bending in one direction, with it it resists in both. The core matters less than the skin either side.
 

rocketmagnet

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Just in case anyone's interested, I've made up a test piece, and it's almost ready to load up.

The tapered router bit did a really good job, and the kerfs close up beautifully. Using Solidworks, I calculated the number of kerfs and the spacing I needed to create the three bend radii.

I added a piece of oak veneer, which isn't glued perfectly at the ends, but was clamped pretty tightly at the bend, so it should be strong there. The wood is 18mm ply, 100mm wide.

Anyone want to take bets on how much load it will take at a distance of 400mm from the bend?

IMG_20211001_205538.jpg


IMG_20211006_094506.jpg


IMG_20211006_094452.jpg
 

Woody2Shoes

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There's an article in the current (#292) issue of Fine Woodworking using this - fairly well-established - kind of technique. The example shown used a tabe saw (what else, for a North American publication?!) to cut the kerfs, rather than a router, and slightly thicker, home-made, veneer (which would be my preference).
 

sometimewoodworker

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Thanks for all your commnts guys,

Pete, I'll try that and I'll let you know the results.
Droogs, if that test isn't strong enough, I'll try the carbon fibre.
If you do try carbon fibre I suggest that you are careful of the orientation of the fibre related to the load, as while the video is interesting the load was in a silly orientation, had the carbon fibre been on the underside (resisting stretch) it’s very likely that the breaking load would have been much higher.
 

rocketmagnet

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OK, I load tested it, and it started to make splitting noises at about 25kg, 40cm from the bend. So I figure I should take 10kg as a safe number. That makes that side of the bed good for 160kg. That's probably enough.

I used a tapered router bit from an eBay supplier called Lionmount Hauling

1634029124374.png


I'm not sure this kind of bit is really intended for cutting wood, but I went slow, about 100mm/min, and 4mm depth, and it seemed to be OK, but had to clean the dust out of the channel after each pass.
 
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