Bending 1/8 plywood tighter

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Bending 1/8 plywood tighter

Postby dancam » Wed Aug 23, 2017 7:53 pm

So i am making a tongue box for my trailer, trying to keep it light and aerodynamic. I made a plywood frame/skeliton and planned to bend 1/8 plywood around it.
Problem is that it doesnt bend as easy as i had hoped. The 4 side pieces dont want to bend nicely. Its not waterproof glue, i tried steaming it and it just seperated.
Tried to cut some kerfs but 1/8 is so thin already and its hard to hold the plywood down on the saw evenly the whole time.
Any tips or tricks to bending? This is just rotary mohogany, i could get baltic birch if its possible to bend with water/heat/steam, but it seems less flexable dry than this mohogany.
There is 4x8 sheets of 'bendable plywood' available but its $60 a sheet and im a bit worried about its strength and warpage.
Anyway, any help is appreciated, what should I do?

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Re: Bending 1/8 plywood tighter

Postby pchast » Wed Aug 23, 2017 9:01 pm

Ask at a local commercial lumber supplier for bending plywood.
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Re: Bending 1/8 plywood tighter

Postby greygoos » Wed Aug 23, 2017 9:48 pm

Dont use plywood. Use material that is meant to bend such as FRP
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Re: Bending 1/8 plywood tighter

Postby dancam » Thu Aug 24, 2017 12:32 am

pchast wrote:Ask at a local commercial lumber supplier for bending plywood.

I can get bending plywood like i said, its just $120 for the 2 sheets i would want rather than free for the plywood i already cut or $40 for baltic burch. Ill do that if i need to, just hoping there is a cheaper way.
greygoos wrote:Dont use plywood. Use material that is meant to bend such as FRP

Hmm, ok. I thought frp was fairly stuff and didnt bend well. I will have to find some and check it out though, not sure where its sold locally.

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Re: Bending 1/8 plywood tighter

Postby Woodbutcher » Thu Aug 24, 2017 7:08 am

Bending plywood has very little strength. So while it bends nicely it will not be very durable, unless you cover it with something else, which works against your weight issue. Since you already have the 1/8th inch, I would add in cross supports between your arched ribs. The more surface behind the bending piece the better. Then work slowly. I covered the front and back radius with 8.00 1/8" plywood. But I am covering the plywood with metal so I could use lots of fasteners as I went along. The glue I used was just panel adhesive. I started by stapling the edge all the way along. Use a wide crown stapler to keep the head from pulling through the wood. Some screws may be necessary also. This is where more cross bracing will help. Work slowly and steadily work forward. You can also roll the panel up slightly and put a couple band clamps on it to slowly relax the panel. Each day, just tighten the band a little. I'm sure you have noticed that when you lean wood against a wall after awhile it has a bend in it even under only it's own weight. If the fasteners showing are a problem, you can cover the finished box with Marine Vinyl material. Like a vinyl roof on a car.

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Here is a about a 4" radius from a past build. It was a tight bend but it worked.

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Re: Bending 1/8 plywood tighter

Postby kramergwt » Thu Aug 24, 2017 9:06 am

You could woodstrip part or all of it like a kayak/canoe. There will be zero spring back to fight. If you Fiberglas it, it will be strong and beautiful.


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Re: Bending 1/8 plywood tighter

Postby dancam » Thu Aug 24, 2017 9:47 am

Woodbutcher wrote:Bending plywood has very little strength. So while it bends nicely it will not be very durable, unless you cover it with something else, which works against your weight issue. Since you already have the 1/8th inch, I would add in cross supports between your arched ribs. The more surface behind the bending piece the better. Then work slowly. I covered the front and back radius with 8.00 1/8" plywood. But I am covering the plywood with metal so I could use lots of fasteners as I went along. The glue I used was just panel adhesive. I started by stapling the edge all the way along. Use a wide crown stapler to keep the head from pulling through the wood. Some screws may be necessary also. This is where more cross bracing will help. Work slowly and steadily work forward. You can also roll the panel up slightly and put a couple band clamps on it to slowly relax the panel. Each day, just tighten the band a little. I'm sure you have noticed that when you lean wood against a wall after awhile it has a bend in it even under only it's own weight. If the fasteners showing are a problem, you can cover the finished box with Marine Vinyl material. Like a vinyl roof on a car.

Image

Here is a about a 4" radius from a past build. It was a tight bend but it worked.

Image

Image


Thanks for all that information! I had wanted to use as few fasteners as possible so water has less places to get in. I was planning to soak it all in exterior urethane which will add some rigidity but i am not sure how much. The band clamps are a good idea and i could use ratchet straps to hold some parts on while the glue is drying, hadnt thought of that. Thats quite an impressive bend you made in the last photo! Is that 3/8 plywood?
kramergwt wrote:You could woodstrip part or all of it like a kayak/canoe. There will be zero spring back to fight. If you Fiberglas it, it will be strong and beautiful.


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What is woodstripping? I tried to look it up on google but only got how to remove finishes from wood...
Thanks

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Re: Bending 1/8 plywood tighter

Postby kramergwt » Thu Aug 24, 2017 9:56 am

Try googling "strip built kayak" and check out images. I have a book that will come up in that search "The Strip Built Sea Kayak" by Nick Schade. It opens your mind to alternate construction techniques. I also think Advisory Circular 43.13 is good too, for learning. That was the FAA spec for wooden airplanes in the early years of aviation.


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Re: Bending 1/8 plywood tighter

Postby tony.latham » Thu Aug 24, 2017 10:14 am

or $40 for baltic burch


Ouch. Somebody is ripping you off. I'm on the edge of nowhere, in Salmon, Idaho. It's a one-lumberyard town and three hours from the nearest box-store town. When I need 5x5 sheets of 1/8" Baltic, it's a custom order and costs $14 per sheet.

Check the price from a different supplier. 1/8" Baltic likes to bend and it's glue is waterproof. But remember, it only likes to bend in one direction.

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Re: Bending 1/8 plywood tighter

Postby aggie79 » Thu Aug 24, 2017 11:51 am

"Russian Birch" 3mm (1/8"), 5' x 5' ply is available near me for $19 per sheet.

I used the 3mm to skin my teardrop. The lower front radius is 6". The lower rear hatch radius is 10".

You aren't trying to to a compound bend (in more than 1 direction) are you?
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Re: Bending 1/8 plywood tighter

Postby drhill » Thu Aug 24, 2017 7:13 pm

I think he means $40 cdn for 2 sheets. Windsor plywood has 1/8" Baltic Birch and it was $19.99 3 yrs ago. I haven't bought any since. If you look at the picture of the tongue box on my trailer, that is 1/8" baltic birch and the bend radius is 16". It was not wetted before installation and was glued in place with no fasteners as I like a clear finish.

Pretty sure he is not trying to do a compound bend, that won 't even work with paper.
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Re: Bending 1/8 plywood tighter

Postby dancam » Thu Aug 24, 2017 7:38 pm

kramergwt wrote:Try googling "strip built kayak" and check out images. I have a book that will come up in that search "The Strip Built Sea Kayak" by Nick Schade. It opens your mind to alternate construction techniques. I also think Advisory Circular 43.13 is good too, for learning. That was the FAA spec for wooden airplanes in the early years of aviation.


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Thanks, that looks cool! I had originally intended to do this with pmf but changed my mind to wood so it would go quicker and be tougher against rocks. Doing this in strips would work everywhere except where the 2 sides of the lid meet the top of the lid. Any sort of a brace there for the wood to rest on would have to be a compound curve. This is the spot i am having trouble getting the 1/8th to do.
Thanks for the info, i like the strip built wood look and idea, i think i want to try that on somethjng in the future.

tony.latham wrote:
or $40 for baltic burch


Ouch. Somebody is ripping you off. I'm on the edge of nowhere, in Salmon, Idaho. It's a one-lumberyard town and three hours from the nearest box-store town. When I need 5x5 sheets of 1/8" Baltic, it's a custom order and costs $14 per sheet.

Check the price from a different supplier. 1/8" Baltic likes to bend and it's glue is waterproof. But remember, it only likes to bend in one direction.

Tony


$20 CAD per sheet, after conversion thats like $1/sheet more than you pay.

aggie79 wrote:"Russian Birch" 3mm (1/8"), 5' x 5' ply is available near me for $19 per sheet.

I used the 3mm to skin my teardrop. The lower front radius is 6". The lower rear hatch radius is 10".

You aren't trying to to a compound bend (in more than 1 direction) are you?


I am not trying to get the wood to bend in a compound curve, but one joint is a compound curve so there is no rib there.
Its where the 2 sides of the lid meet the top of the lid.


drhill wrote:I think he means $40 cdn for 2 sheets. Windsor plywood has 1/8" Baltic Birch and it was $19.99 3 yrs ago. I haven't bought any since. If you look at the picture of the tongue box on my trailer, that is 1/8" baltic birch and the bend radius is 16". It was not wetted before installation and was glued in place with no fasteners as I like a clear finish.

Pretty sure he is not trying to do a compound bend, that won 't even work with paper.


Yes! Windsor plywood is the place im going to.
Not sure what my bend radius is. The one photo shows the bend i need it to go around well but im not sure on the measurement.


So i got a sample piece of frp and its super flexible. I would have to make some kind of support behind all of it, not sure if high density foam would be strong enough. Or does frp come in different thicknesses?

Then i went to windsor plywood to buy 1/8th bendable plywood. The only sheet they had that wast flexable the 4ft way was broken up and not strong to begin with. Bought a sheet that bends the 8ft way but its hardly more flexable than regular plywood. The 1/4in bendable ply was far more flexable than it. I figured it must have been the wrong stuff so i drove to another windsor plywood to see theirs and it was all the same. Except they had a sheet of bendable ply the 4ft length that was in good shape. Was gonna buy it but they wanted $70/sheet. It wouldnt have fit in my car nice anyway so i told them i would have to decide between baltic birch and the bendable ply tonight.
Im sort of leaning towards trying the baltic birch first.
Has anyone here steam bent plywood?
It will be fine without on the bottom half of the tongie box. Part im worried about is on the top half where the sides and top meet. No brace there.




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Re: Bending 1/8 plywood tighter

Postby drhill » Thu Aug 24, 2017 8:08 pm

those bends don't look too tight for the 1/8" baltic birch. But it will be rather tricky to get the sides and front/top to meet in a nice even line. Somebody with some good CAD skills could pull that off.

So are you planning to cover this with canvas and paint afterward? I would think that is the best plan as that joint will be hard to do a good enough job that you would want it visible with a clear finish,

By the way, my tongue box has a clear finish but after 3 years and about 26,000 km it is showing the beating from all the sandblasting it gets behind the tow vehicle. I lightly sand and recoat it each year, but I know at some point I will be building another box. It will be exactly the same as I am happy with the size and look.
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Re: Bending 1/8 plywood tighter

Postby kramergwt » Thu Aug 24, 2017 8:21 pm

dancam wrote:I have a I had originally intended to do this with pmf but changed my mind to wood so it would go quicker and be tougher against rocks.


Kayaks, especially sea kayaks, are made to slam into jagged rocks. The strip building process depends on Fiberglas on both sides for strength. It holds up better than aluminum and can be replaced by heating, removing, sanding and re-fiberglassing. Just info I read in that book. I want build a kayak when I finish the teardrop.


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Re: Bending 1/8 plywood tighter

Postby dancam » Thu Aug 24, 2017 9:08 pm

drhill wrote:those bends don't look too tight for the 1/8" baltic birch. But it will be rather tricky to get the sides and front/top to meet in a nice even line. Somebody with some good CAD skills could pull that off.

So are you planning to cover this with canvas and paint afterward? I would think that is the best plan as that joint will be hard to do a good enough job that you would want it visible with a clear finish,

By the way, my tongue box has a clear finish but after 3 years and about 26,000 km it is showing the beating from all the sandblasting it gets behind the tow vehicle. I lightly sand and recoat it each year, but I know at some point I will be building another box. It will be exactly the same as I am happy with the size and look.


Ok.
Yes, its all been drawn up and i cut it on a cnc router, the issue is that when bending it this far the unsupported ends want to bend a bit funky and not exactly the same as where its been clamped and glued onto the ribs. I have some plans if thats an issue, none of them good, but until i get wood that i can bend the corner with i dont really know what will happen.
I wasnt planning to. I was thinking the paint would chip off the canvass and the canvas might mould? But i was gonna soak the wood with outdoor polyurethane and then paint over that. Will have to see what it looks like when its togeather, i was hoping to use mostly glue and very few fasteners.
Good to hear that you made something similar and its holding up.

kramergwt wrote:
dancam wrote:I have a I had originally intended to do this with pmf but changed my mind to wood so it would go quicker and be tougher against rocks.


Kayaks, especially sea kayaks, are made to slam into jagged rocks. The strip building process depends on Fiberglas on both sides for strength. It holds up better than aluminum and can be replaced by heating, removing, sanding and re-fiberglassing. Just info I read in that book. I want build a kayak when I finish the teardrop.


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I know fibreglass would be very strong against rocks, pmf (with canvass and paint) isnt.
Fibreglass>latex paint.
I dont think im prepared to learn how to do fibreglass on wood right now, i just want this done...



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