Im making a 1/2 of cylinder 3' outside diamter - 2'7" inside diameter to be
used as a base of a table Im making.
I tried using bending ply w/ veneer - but I can get the color to match w/
what is on the rest of the project (1/2" oak ply)
So im going to use the 1/2" oak ply - kerf cut it and bend it around the
staves (or ribs or what ever you want to call them)
Is there some general rule of thumb on how far about the kerf cuts want to
be and how deep?
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Distance between cuts is dependent of how much you want to bend the piece.
I make the depth of my cuts about 1/8" shy of all the way through. The best
is to try this on a scrap piece the same length and start with the cuts
about 1" apart. If the piece does not bend enough add cuts between the
previous cuts. For the smoothest curve you do not want the kerf gaps to
close on the back side. If the kerf gaps close on the back side the front
side may end up looking like a curve made up of a series of short straight
I just did something similar for a piece that I am making for my daughter,
but my radius was quite a bit tighter than the 3 feet you are doing. My
radius was only 10" and I was using the cheapest plywood that Home Depot had
since it would all be covered.
Before I went to HD, though, I had a scrap piece of 1/2" (and it was 1/2")
mahogany plywood that I used to see if this could even be done and still
look ok. I cut a few kerfs that were about 1/2" apart, then a few that were
only about 3/8" apart, and finally some that were only 1/4" apart. I did as
a previous poster suggested and made the depth 3/8" which would leave about
1/8" thickness. This was not deep enough and the veneer started to crack.
I recut all the kerfs leaving only about 1/16" (barely more than the
mahogany veneer itself), and the pieces of plywood where the kerfs were 3/8"
and 1/4" apart wrapped arount the curve very nicely. The part where the
kerfs were 1/2" apart looked a little bit like narrow strips. You had to
look for it and I could have used that spacing in this particular project,
but I went with the 3/8".
When I got the cheap plywood from Home Depot, it was the "new" 3/8" which is
actually 11/32". I went with the thinner plywood because I was thinking
that I needed to keep the weight down. I am going to have to lug this piece
up a flight of stairs when I get it done. But this stuff was so dry and so
brittle that it still cracked a little with the kerfs as deep as I dared
make them without cutting all the way through the wood. I happened to see
the front of the piece with the sun shining through the back and you can
really see the kerfs.
So what I am trying to say with all these words is that if there is a
general rule, it would have to be modified for many factors including the
thickness of the piece you are kerfing, probably it's moisture content, type
of wood (and veneer), radius of the bend, thickness of the kerf (I was using
a standard width blade so mine were 1/8" wide), and there are probably more
factors. If you do this often, you will probably come up with your own rule
of thumb and then you can share it with us.
As for your case with a 3' diameter, I would think that the kerfs placed
1/2" apart, and leaving 1/8" thickness should be good. A trial piece will
tell you fer sure, dude.
Sorry for being so wordy.
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To actually remove enough wood in a half circle of 1/2' ply, you need
pi / 2 * 8 = 1.6" inches or so of wood removed by the kerfs. But
because that's only 13 * 1/8" cuts or so that will make a lot of
straight bits. So if you are cutting 1/8" cuts, basically I would
practice on scrap first, starting with that number and going between
until it's smooth.
Cut nearly through. Make sure you are working on a very flat, stable
surface, as you are working right on the edge of _cutting_ right
through. You want the ply to be held down flat on a flat surface.
Have you noticed that people always run from what
they _need_ toward what they want?????
Make a kerf in the center of the board to within about an eighth inch
of the face veneer.
Clamp one of the edges that is parallel to the kerf, to the bench,
with the open side of the kerf up.
Make a mark at a point equal to the length of the radius away from the
Lift the board up until the kerf just closes. Measure the distance
from the bench top to the top of the board, at the point indicating
the length of the radius.
This gives you the spacing between kerfs.
"People funny. Life a funny thing." Sonny Liston
Thomas J.Watson - Cabinetmaker (ret.)
tjwatson1ATcomcastDOTnet (real email)
It seems like your project would be much stronger if you laminated
1/8" or maybe 1/4" strips together around a form.
If you go with the kerf-cut method, you might want to steam or
otherwise moisten the outside veneer to keep it from cracking.
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