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Author Topic:   Bending plywood- Woodworking 101
acseatsri posted 01-01-2005 02:04 PM ET (US)   Profile for acseatsri   Send Email to acseatsri  
How do I go about making a 90 degree bend around a 6" radius using 3/8" plywood? I've tried making saw cuts to break its back, but the wood just splintered and broke.

Hint- I'm rebuilding the rear seat in an 18' Outrage due to rot, so there may be others interested.

Jerry Townsend posted 01-01-2005 02:12 PM ET (US)     Profile for Jerry Townsend  Send Email to Jerry Townsend     
Years ago, I saw a "recipe" for the spacing of multiple saw cuts to make such bends. I can't put my hands on it - but it was in some woodworking type book. As I recall and would suspect, it would be based on the radius and thickness of the material. ---- Jerry/Idaho
flyfisher1771 posted 01-01-2005 03:10 PM ET (US)     Profile for flyfisher1771  Send Email to flyfisher1771     
Soaking the plywood in warm water should get the job done. You will have to use a marine grade plywood that has waterproof glue.

Another way to try cutting would be a v groove rather than just a blade kerf. Set your saw up at a 45 and run top to bottom then turn around and run bottom to top. That would allow more flexability in the back.

Good luck

Hugh

simonmeridew posted 01-01-2005 04:29 PM ET (US)     Profile for simonmeridew  Send Email to simonmeridew     
I helped my son, who is a stairbuilder and custom furniture builder, bend some 3/4 inch birch plywood on about a six inch radius for the riser on the bottom step of a staircase he was building. Many times on a stairs the newel post is set on the bottom tread which is maybe ten inches wider than the rest of the steps and the riser is often rounded back on itself rather than squared off. You see this on a lot of traditional stairs but I never knew how they did it.
What you do is cut a series of parallel kerfs, leaving wood about a kerf wide between cuts. The cuts are obviously made on the back of the plywood, through all but the surface ply. This will allow the bend to be made around the radius, giving a very smooth bend, without creases and ridges on the surface.
Another way to make such a piece is to get a piece of "Wacky Wood", so called, at the lumber yard. This is a specialty plywood, which bends with little or no resistance, without making any kerf cuts. It has no stiffness at all, pick it up at one end and it just bends like a wet noodle. Not sure how it works but it does just as I have described. The downside is that the surface ply is rough and unsanded, I think not appropriate for finish work at all, without a surface veneer being applied. Normally he would bent this in the shape he wanted, around a form and veneer a finish veneer like mahagony to the outside, for example, for some of the "Art Deco" furniture he makes. I don't think the "Wacky Wood" is particularly strong and I'm thinking it shouldn't be used unreinforced in a place where structural strength is necessary.
The third way he makes curves, very strong, is to buy sheets of 1/8 inch plywood, many times birch, bend them around a form, and glue a series of them up, to whatever thickness you need, in effect making your own plywood. The outside or surface veneer can be any of the exotic woods available as a veneer from specialty lumber dealers. This technic is probably beyond the usual hobby or non professional woodworker as he uses a vacuum bagging system. This is however how the veneers are placed on most of the high end custom furniture, even non curved pieces would be vacuum bag veneered.
hope this helps
simonmeridew
newt posted 01-01-2005 09:34 PM ET (US)     Profile for newt  Send Email to newt     
Simondrew, very nice post. The "wacky wood" you mentioned is actually quite strong perpendicular to the side that bends. You can buy the wood so that it bends in the 8 foot direction or the 4' direction but not both directions. I have seen it used often for concrete formwork.
andygere posted 01-01-2005 10:19 PM ET (US)     Profile for andygere  Send Email to andygere     
Use two thinner plys, and laminate them together with epoxy on a jig. Zero void okoume, often used in stitch and glue boat building, is ideal for this. I would then encapsulate the wood with a coat or two of epoxy so that it won't rot on you again.

Here's a good source for these supplies: http://www.clcboats.com/epoxy_plywood.php/

wwknapp posted 01-02-2005 02:54 AM ET (US)     Profile for wwknapp  Send Email to wwknapp     
acseatsri:

Are you trying to bend across the grain of the surface layer of along the grain? Sounds like across, which is pretty tricky. And generally results as you describe. Was the original wood you are replacing done like that? If so laminating from thinner material will probably be the way of choice.

If it's along the grain (where the grain bridges the cuts) then you are probably not cutting deep enough or enough cuts. Soaking or steaming the wood can help, but often is not necessary if the cuts are deep enough. And close enough together. The most common error people make here is too few cuts. Ideally you remove enough wood in the cuts so that the back side the cuts just close.

There is one other thing I've seen give trouble. If your saw blade cuts out the sides of the kerf leaving a thicker middle that can sometimes be a problem. I always use blades that cut a kerf that's flat across the bottom.

Of course if the wood is not going to show when done, either painted or whatever, andygere's method with epoxy, or even entirely fiberglass could be done.

Walt

JBCornwell posted 01-02-2005 09:44 AM ET (US)     Profile for JBCornwell  Send Email to JBCornwell     
A 90* arc with a radius of 6" will be about 9.42"

A 90* arc with a radius of 6.75" will be about 10.6", or about 1.2" longer.

Kerfs of your cuts in the short (9.42") side of the strip should add up to the 1.2" difference in length and be evenly spaced. Depth of the cuts shouls be at least 5/8".

You can make the bend smoother by soaking or steaming it before bending.

Let it dry completely and then saturate the short side with epoxy to bond all the cuts together.

Strong, even, curved piece of plywood.

This is how my Dad taught me to do it, except he used casien glue to "set" the curve instead of epoxy.

Good luck. :)

Red sky at night. . .
JB

JBCornwell posted 01-02-2005 09:50 AM ET (US)     Profile for JBCornwell  Send Email to JBCornwell     
RATZAKRATZA!!

Should have read you post again, acseatsri.

Change the above for 3/8" ply to:

Outside arc is about 10", so the difference becomes about .6".

Kerfs should total that .6" difference.

Everything else about the same.

Good luck. :)

Red sky at night. . .
JB

acseatsri posted 01-02-2005 12:17 PM ET (US)     Profile for acseatsri  Send Email to acseatsri     
Well, after weighing all the options, I've decided to use 5/8 plywood and create a radius with 1" wide strips glued together around a 6" radius, then set it with fiberglass cloth and resin. I can sand it afterward to smooth out the transition lines. I have a plastic bucket which fits the bill perfectly as a jig.

I don't know about others, but this seat is nearly completely rotted, with the exception of the mohogany structural members underneath the seat. The seat bottom cushion uses 1/4" plywood as a backer, again TOTALLY delaminated and rotted. The 5/8" plywood actual seat bottom is nearly rotted thru- I can poke my fingernail into it very easily. It's a wonder that it didn't break with people sitting on it. If anyone else has a seat this age (1988), I'd suggest checking it for these same conditions. Looks are deceiving- the upholstery is in near perfect condition, so much so that I'm re-using it so it will match the rest of the upholstery on the boat.

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