Hi,

I'm building a plywood camper, and need this group's expertise.

I need to butt-join (don't ask me to scarf - I don't have the time or
the talent!) two pieces of 1/4" Mahogany plywood. My question is: how
easily does the join bend? I need to bend the plywood fairly aggressively
(can't tell you the exact radius, but it's quite a bit) and if I do it the
way I want, I will have to bend the joined part.

My join will be a "standard" epoxy butt-join, with epoxy resin and tape
on both sides of the join.

Any ideas?

Lloyd Sumpter

Lloyd Sumpter wrote:
Hi,

I'm building a plywood camper, and need this group's expertise.

I need to butt-join (don't ask me to scarf - I don't have the time or
the talent!) two pieces of 1/4" Mahogany plywood. My question is: how
easily does the join bend? I need to bend the plywood fairly aggressively
(can't tell you the exact radius, but it's quite a bit) and if I do it the
way I want, I will have to bend the joined part.

My join will be a "standard" epoxy butt-join, with epoxy resin and tape
on both sides of the join.

Any ideas?

I know that you don't want to hear this, but if you want a smooth bend,
you're best off to scarf the plywood. Fiberglass on the joint will make
is substantially stiffer than the surrounding plywood.

Scarfing is not difficult, particularly with thin plywood. All you need
is a sharp block plane, a straightedge of some sort and some patience.
If you're only doing one joint, what's the big deal?

Lloyd Sumpter wrote:
Hi,

I'm building a plywood camper, and need this group's expertise.

I need to butt-join (don't ask me to scarf - I don't have the time
or
the talent!) two pieces of 1/4" Mahogany plywood. My question is: how
easily does the join bend? I need to bend the plywood fairly
aggressively
(can't tell you the exact radius, but it's quite a bit) and if I do
it the
way I want, I will have to bend the joined part.

My join will be a "standard" epoxy butt-join, with epoxy resin and
tape
on both sides of the join.

Any ideas?

Lloyd Sumpter

You won't get a smooth bend if you use glass and epoxy on both sides of
the joint unless you taper the ends of each side of the joint so that
the glass doesn't make the joint thicker than the rest of the panel.
That, however, would be more difficult to do than a scarf joint. One
other method which I have used successfully in areas where the bend is
not extream and strength is not a big issue is the double lap joint.
You need a portable router for this. I use a trim bit with a piece of
spine to even up the laps. It is much faster and easier to do than
either a scarf or dealing with tape and epoxy. If you manage to get
the lap depths and the grain just right, the joint is invisable and it
bends well.

Al

On Thu, 14 Apr 2005 08:40:56 -0700, a_gunther wrote:

Lloyd Sumpter wrote:
Hi,

I'm building a plywood camper, and need this group's expertise.

I need to butt-join (don't ask me to scarf - I don't have the time
or
the talent!) two pieces of 1/4" Mahogany plywood. My question is: how
easily does the join bend? I need to bend the plywood fairly
aggressively
(can't tell you the exact radius, but it's quite a bit) and if I do
it the
way I want, I will have to bend the joined part.

My join will be a "standard" epoxy butt-join, with epoxy resin and
tape
on both sides of the join.

Any ideas?

Lloyd Sumpter

You won't get a smooth bend if you use glass and epoxy on both sides of
the joint unless you taper the ends of each side of the joint so that
the glass doesn't make the joint thicker than the rest of the panel.
That, however, would be more difficult to do than a scarf joint. One
other method which I have used successfully in areas where the bend is
not extream and strength is not a big issue is the double lap joint.
You need a portable router for this. I use a trim bit with a piece of
spine to even up the laps. It is much faster and easier to do than
either a scarf or dealing with tape and epoxy. If you manage to get
the lap depths and the grain just right, the joint is invisable and it
bends well.


Thanks for the reply!

I used that lap-joint on the floor, when I discovered that 6 1/2ft is 78
inches, NOT 66 inches (DOH!!!). But since the bottom of the floor doesn't
show, I taped one side as well. And of course that part doesn't bend.

I think you have the image sideways, though: the seam is longitudinal.
Think of a jonboat-like structure, where I'd be bending the bottom up to
form a bow, and the seam is lenghwise down the keel.

But you're right - there's not a lot of strengh required here, so I might
just do the lap-joint, and possibly tape one side (to ensure
water-tightness!) How much do you lap (I used an inch on the floor)?

Lloyd

Lloyd Sumpter ) writes:

I will have to bend the joined part.

it all depends on the radius of curvature of the bend.

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What about making a backing block with the same curvature as your bend? If
you have a small gap afterwards - that's what filler is for...

--
Andrew Butchart



"William R. Watt" wrote in message
...

Lloyd Sumpter ) writes:

I will have to bend the joined part.

it all depends on the radius of curvature of the bend.

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network
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"Andrew Butchart" wrote in message
...
What about making a backing block with the same curvature as your bend?
If
you have a small gap afterwards - that's what filler is for...

But the problem then is: how you you glue the flat ply on that curved block?

Meindert

Lloyd Sumpter wrote:

I think you have the image sideways, though: the seam is longitudinal.
Think of a jonboat-like structure, where I'd be bending the bottom up to
form a bow, and the seam is lenghwise down the keel.

That's a very different animal than I was envisioning. Since the two
pieces are parallel to each other, why can't you bend them in place and
glass the joint afterward? Or, you could butt join them with glass only
on the inside. Since the bend is along the joint rather than across it,
that should be strong enough.

On Fri, 15 Apr 2005 12:09:12 +0000, Andrew Butchart wrote:

What about making a backing block with the same curvature as your bend? If
you have a small gap afterwards - that's what filler is for...

I don't think that would work in my case, but you gave me an idea: I could
do a simple join like a lap, bend the plywood into it's position, THEN
apply epoxy and tape! (As I said, this seam is longitudinal along the
curve) Can't see why that wouldn't work...

Thanks!
Lloyd

Lloyd Sumpter wrote:

I think you have the image sideways, though: the seam is
longitudinal.
Think of a jonboat-like structure, where I'd be bending the bottom up
to
form a bow, and the seam is lenghwise down the keel.

But you're right - there's not a lot of strengh required here, so I
might
just do the lap-joint, and possibly tape one side (to ensure
water-tightness!) How much do you lap (I used an inch on the floor)?

I've destructive tested full lap joints to find the minimum lap length
and with epoxy it always came out to 2.5 times thickness. For a half
lap it might be less, but for 1/4 inch, it would only be 5/8" anyway.
If you make the half lap joint first, let it cure and then make your
bend you would get a smooth bend and you could glass over that. If you
have doubts about the joint taking the bend, you can make a test of the
same material and bend it over the sharpest part of your mold before
cutting into the big sheets.

If the boat will be stored upside down in the sun after it is built,
that seam might get hot and the epoxy could soften from the heat and
move a little, possibly causing problems. To be on the safe side, I'd
want to put a heat lamp on the joint or set it in the sun so it gets a
good high temp cure and has a chance to cool down before doing the
bend. That, at least in theory, will make it more stable.

Al