Need help curving plywood

[Margaret]

Hi. I make simple things like my work tables and chicken house, but am not an accomplished woodworker by any means. I have the chance to paint an exterior mural on a 4 ft diameter round kiosk and plan to use a plywood backing, but I need to bend the plywood. The ply I have is nominal 1/4" (actually closer to 3/8") and the two panels will be 6 1/2 feet long. We plan to wet the panels with hot water and bungee them to the surface, dousing with hot/warm water as we go until dry. I thought I'd make a frame to hold the panels in shape as I paint. Does this sound as if it would work?

Kerfing such a thin board seemed like it might not hold. The surface may get a lot of wear, as it's atop a bulletin board at the downtown library. I imagined a flyer being stapled, the kerf popping and the youngster doing the stapling flying into the street.

I'd appreciate ANY suggestions at all. Thanks so much.

 

[zapdafish]

I don't have any suggestions on how to bend the plywood but this might be an alternative.

http://hardwoodstore.com/hardwood-plywood

2nd from the bottom is bending plywood. You might be able to find a source closer to you.

 

[cpw]

Margaret,

If you want to use ply as a substrate, kerfing is definitely the way to go. I did the piece below as an experiment with cheap 1/4" ply and SYP in 1990. It was 4' in diameter and I made the kerfs on the table saw very methodically. If I remember correctly the space between kerfs varied from 1/2" on center to 1" on center at the middle. This was done for visual effect, not for any structural reason. Basically the tighter the curve you need the closer the kerfs will need to be. The depth of the kerfs were half of the thickness of the ply. It was disassembled and moved several times with no problems. You just need a solid structure to mount the ply to. Sorry the photo isn't better.

Cheers,
Charles

 

[Margaret]

Thanks for your reply, Charles!

That looks lovely. I notice that you have a frame supporting the outer edge. I won't be able to do that, as the surface will be a mural. I hope kerfing w/o the outer frame will be sturdy enough.

I'm mounting it to a sturdy round tower made of 2x4ish slats. Should be sturdy enough. I only worry about the kerf's 'tearing through' in the future.

Do you think I can cut the kerfs less than half the depth? I'm hoping the 1" would be okay all the way around, as the curve is even. My coworkers has a circular/table saw.

The bungee/steam idea is beginning to sound a little more like vaudeville than good sense.

Thanks so much for your help.

 

[Charles Lent]

I would go with bending plywood. It works very well for a 4' radius.It has all of the veneer layers running in almost the same direction, so it will bend very easily in one direction.

Here's a story about "How Not To Do It"

I was once involved in making a giant gum ball machine for an exhibit. The ball was about 8' in diameter, the base about 5' diameter and the whole machine stood over 12' high. The base was made from 3/4" cabinet birch plywood cut in arcs and glued together to make 5-6' circles, which were then stacked and glued to make the base rings and upper ring portions below the ball. Between these were straight vertical pieces every 30 deg. or so to support the plywood covering. It was initially planned to use 1/4" bending plywood, but somebody decided that 1/4" Masonite would work better. This decision almost created a disaster, as the schedule was too tight to allow waiting for the special plywood to arrive.

We ended up soaking the Masonite completely through from both sides to soften it and get it to bend enough to even try to wrap it around. We then screwed one end of the Masonite to the birch and attempted to wrap the cylinder. Only after trying this several times, ganging our efforts, and improving our methods between each try, did we manage to get the Masonite fully installed. It took latex based construction adhesive (the Masonite was wet) and screws installed every 6 " along the top, bottom, and at each vertical support, while 2 of us pulled and held the Masonite in a tighter and tighter arc, to allow the adding each row of screws to succeed. My arms hurt for 2 days after this.

Several band clamps were installed around the cylinder to keep it all together "until the glue dried". The next day the clamps were removed, followed by careful removal of the screws, and we were surprised that it actually held together with only the glue. The screw holes were filled with Bondo , sanded smooth, and the whole gum ball machine was painted with several coats of exterior enamel to make it weatherproof (it had to last for 3 months outside). Some of you Charlotteans may remember it sitting on the sidewalk at the corner of 6th and Trade streets about 5 years ago.

This would have been a very easy job if bending plywood had been used. I've used 1/4" bendy plywood around 8" radius bends without significant difficulty.

Charley

 

[ehpoole]

Thanks for your reply, Charles!

That looks lovely. I notice that you have a frame supporting the outer edge. I won't be able to do that, as the surface will be a mural. I hope kerfing w/o the outer frame will be sturdy enough.

The outer frame is not necessary, just an inner frame to secure the plywood to and maintain its curvature.

I'm mounting it to a sturdy round tower made of 2x4ish slats. Should be sturdy enough. I only worry about the kerf's 'tearing through' in the future.

Do you think I can cut the kerfs less than half the depth? I'm hoping the 1" would be okay all the way around, as the curve is even. My coworkers has a circular/table saw.

The bending plywood suggestion is by far the best way to go. However, if you wish/need to use kerfing, keep in mind that the spacing of the kerfs as well as the depth and width of each kerf govern how tight a radius you can make and how closely it will emulate a perfect radius. In other words, you will want to experiment. The kerfing basically converts a circle into a series of short straight sections between each kerf (though the stretching of the wood helps to moderate and improve the illusion of 'roundness'). A half-depth kerf the width of a standard saw blade is a good starting point as is the 1" per kerf. Do some trial runs with narrow strips (of good length) bent to the same radius to get a feel for the effect created by those parameters. If it looks too much like a series of straight segments rather than a perfect radius, then you will need to adjust the spacing of the kerfs. If the radius is causing the kerfs to close up before you complete the radius, then you may need to cut the kerfs wider. If it is too hard to bend to the radius then you may need to cut the kerfs deeper, but this may also require spacing the kerfs closer together since the wood will want to bend more sharply at the kerf and this may make the bend along each kerf more noticable on the face side.

HTH

 

[JackLeg]

Margaret: Is wood your only choice? Laminate will bend easily. :icon_scra

 

[ehpoole]

Margaret: Is wood your only choice? Laminate will bend easily. :icon_scra

Have you ever tried stapling bulletins to laminate? Not much holding power, assuming it does not shatter. :nah:

 

[Margaret]

Wow. This is obviously the place for good advice. You guys are great.

The panel will be 7 ft up and hopefully no one will staple on it (that was a worst case/paranoid scenario). Would laminate be something I could prime the heck out of, paint and seal from the weather? By laminate, do you mean indoor paneling - just worried it wouldn't last or accept paint/varnish/polyurethane.

I've called most lumber places in town and no one is willing to order small amts of bendy ply. The kerfing sounds possible but scary.

Thanks so much you guys.

 

[ehpoole]

I've called most lumber places in town and no one is willing to order small amts of bendy ply. The kerfing sounds possible but scary.

You would likely have to drive a bit to get to one of the specialty wood suppliers to pick up a small quantity of bending plywood (unless you just happen to have a local supplier).

There really is not anything scary in kerfing -- you just have not done it before so it requires a little practice on some identical scrap (or just an extra sheet purchased for experimentation).

You can use either a tablesaw or a circular saw (coupled with a straightedge) to cut the kerfs in the plywood. You don't say what sort of shop equipment you have, but even most beginner shops have a circular saw so you probably already have everything you need.

 

[ehpoole]

Just an FYI, but when selecting your plywood, choose an exterior rated plywood if you are planning to place this outdoors in the weather. The framing lumber should also be pressure treated or a weather resistant variety. You may even wish to use screws over nails or staples if it is likely to get wet while exposed to freezing conditions (which can cause nails to back out). If it is meant to be very permanent, an exterior rated glue would also be a good backup fastener.

HTH

 

[zapdafish]

Did you try http://www.ashevillehardware.com/ ? They are one of our sponsers to the right.

 

[JackLeg]

Wow. This is obviously the place for good advice. You guys are great.

The panel will be 7 ft up and hopefully no one will staple on it (that was a worst case/paranoid scenario). Would laminate be something I could prime the heck out of, paint and seal from the weather? By laminate, do you mean indoor paneling - just worried it wouldn't last or accept paint/varnish/polyurethane.

I've called most lumber places in town and no one is willing to order small amts of bendy ply. The kerfing sounds possible but scary.

Thanks so much you guys.

I was thinking more along the lines of an inexpensive Formica type product that you could paint on. :dontknow: Comes in 4 X 8 sheets and bigger. Pick a grey or neutral color. Check the salvage houses or specials at the Borg's.

 

[James Davis]

If you cut the plywood the short way, meaning that the pieces are cut so that they are standing up when installed, they will bend better than if you try to wrap a 8' piece around your form. the strongest fibers in the plywood run the 8' way so cutting them the short way just allows it to bend more readily.

James

 

[sushinutnc]

Along the lines of the laminate/formica idea--

HD and Lowes sell white plastic sheets used for shower/tub surrounds. I believe they call it shower board or something similar. WAY cheaper than formica and laminates. Like $10 a sheet. They are usually stacked horizontally near the floor in the area where they sell complete tub surrounds. They may have several kinds. The one I'm talking about has glossy bumps all over one side. The other side has a matte, smooth side. Funny, but they sell it for the bumpy side... but you want the back side. It takes primer and paint VERY well. The only think I'm not sure about is how flexible it would be for the radius of curvature you need. You'd have to test that at the store.

If you see something that looks white and glossy, but on a paper/wood substrate, that's not the right stuff. And it's not a melamine or "tileboard." This is a solid/molded plastic with bumps all over one side.

It's totally weather resistant (made for showers!). I've made several exterior signs with it.

 

[Drew Roy]

Margaret, Someone as a lumer supply house should carry pre kerfed plywood, check ashville or the hardwood store Stock or one of those places are not going to have it. As to the idea of dousing with hot water, well you could soak the sheets in a swimming pool and I dont see you getting the result you are wanting, lets just say I've tried that same scenario.

 

[cyclopentadiene]

As you can see from my screen photo, my other hobby is skateboarding. I have constructed many skateboard ramps in my life and bending plywood is a must. I have been able to develop skateboarding ramps with transitions as small as 4' radius. The approach is to space 2 x 4 or if you do not need to support weight 2 x 2's on 8 inch center around a 3/4" plywood frame. I generally secure with 2" wood screws through the plywood into the skeleton (2 per 2x4) . !/2 inch plywood will easily bend without soaking in water to a 6" radius, either on an exoskeleton for a skateboarding roll in or endoskeleton for a transition ramp. Start on one end of the plywood and use 1" sheetrock screws 8" apart on center with the skaletonand secure the end. move to the next 2 x 4 and repeat. Continue through the plywood. at 6" radius, you will need to make the curve 4' wide as 8' is more challenging. At 8' radius, either way for plywood placement works fine. Soaking the plywood in water for about 3-4 hours before applying to the skeleton works even easier. Dry you will hear cracks etc. but this is usually not a problem (skateramps have 2 layers 1/2 inch and 1 layer surface at 1/4 thockness. If you choose 1/4, it bends easier. 3/4 material will not bend even if soaked.
Masonite is often used on skateboard ramps as the top surface as well. it is very flexible and can bend to a 2" radius if you are careful.
They also sell a bendable plywood (hard to find but not much more expensive than regular) in which the frain all runs in the same direction for all ply. a 1/2 inch piece can easily do a 2' radius and a 1/4 piece can bend to a 1" radius without soaking.
If you need support, 2 layers with the seams alternating are extremely strong and hold up for several years in a skateboard park with significant abuse. the surface does wear out if it is masonite and the better skateboard parks use skatelite ($150 per 4x8 sheet) which is essentially trex type material. It lasts 8-10 years outdoors with no problems. in strong weather conditions, such as at the beach it degrades in about 5 years.

This is long but may be useful to others on the site that want to bend plywood, build skateboard ramps for their kids or themselves.

FYI, nothing beats concrete skateboard parks, the photo with my screen name is Charlotte NC Grayson Skatepark on the 10' (9' radius) wall deep end of the bowl.

 

[ehpoole]

!/2 inch plywood will easily bend without soaking in water to a 6" radius, either on an exoskeleton for a skateboarding roll in or endoskeleton for a transition ramp

Buddy, if you can bend 1/2" plywood sheets, dry and un-kerfed, to a 6-inch radius (12" diameter) without splitting the plywood -- or yourself -- in two, then you are one **** of alot more talented than I. :rotflm: