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# Equation for accurate bending (kerfing) of plywood

I am planning out my first Instructable. It will be a bent (kerfed) plywood coffee table,

There have been a growing number of projects out in the 'makeosphere,' where people are cutting slits or kerfs that go almost completely through a thicker piece of wood, allowing it to be easily bent. I am on a super-tight budget, and cannot afford to ruin my $22 sheet of plywood, so I was hoping that someone might be able to help me with some measurements for cutting the kerf-slits in the plywood. Here is the idea all sketched out. I need help with the measurements that are in bold.

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Stability is more important to me than curve radius. I don't know if spacing cuts close together for a small curve radius is more or less stable than placing the cuts further apart for a larger radius. I've never done this, but I have seen that there is an equation for this. I haven't been able to find the equation actually expressed, I've just heard that there is one. Does anyone know about this area of woodworking/physics?

There have been a growing number of projects out in the 'makeosphere,' where people are cutting slits or kerfs that go almost completely through a thicker piece of wood, allowing it to be easily bent. I am on a super-tight budget, and cannot afford to ruin my $22 sheet of plywood, so I was hoping that someone might be able to help me with some measurements for cutting the kerf-slits in the plywood. Here is the idea all sketched out. I need help with the measurements that are in bold.

* * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * *

**PROJECT DESCRIPTION:****Materials:**- 1x | 4 ft. x 2 ft. 3/4 in birch plywood
- 2x | 3 ft. 5/8 in threaded metal rods
- 8x | flat washers
- 4x | standard nuts
- 4x | "end nuts"

**Steps:**- On the sheet of plywood, measure in
**X inches**and mark**X number**of cross cut lines,**X inches**apart. - Measure in 2 inches from each end of the board and draw a line.
- Measure in 2 inches each each side of the board, and draw another line.
- Drill out a 5/8 inch hold at all 4 of these crossed lines.
- Thread a standard nut onto each end of the threaded rod and screw it on 1-2 inches roughly, then slip a flat washer over the threaded rod.
- Bend each end of the board up to create almost 90 degree angles.
- Insert each end of the threaded rod into the holes, slip a flat washer over the threaded rod and screw on each of the four "end nuts."
- Use the standard nuts, and the end nuts to fine tune the bends into 90 degree angles.
- Flip the table over, and enjoy.

Stability is more important to me than curve radius. I don't know if spacing cuts close together for a small curve radius is more or less stable than placing the cuts further apart for a larger radius. I've never done this, but I have seen that there is an equation for this. I haven't been able to find the equation actually expressed, I've just heard that there is one. Does anyone know about this area of woodworking/physics?

## Discussions

3 years ago

http://stusshed.com/2007/07/17/kerfing-bending-woo...

Woodworking is an art. You probably have to experiment with some scrap to see what actually works. It sounds like you are building a big tray with low sides and flipped over to become a table. The table will have a nice curved edge all around but you will be hiding the beauty of the kerfed bend. Maybe you should build a lesser wood frame and veneer with a thinner luan plywood - easier to just score/kerf bend. It would be cheaper but you can get thinner plywood in better choice of woods. Good luck.

Reply 3 years ago

Thanks caitlinsdad... I agree completely. My piece of plywood is actually 2 ft wide, but I plan to rip off 6 inches to that I can make the table more narrow, and have a 6 inch wide piece to experiment on. I am a huge perfectionist, and I was hoping to have a mathematical equation to work from. I will still test, and I will free-hand it if no one can help me come up with an equation. But I still wanted to ask.

Reply 3 years ago

Oh man... thanks! I didn't see the link. Thank you!

Reply 3 years ago

The formula was in the comments of that link. Still a lot of variables to account for such as your machinery and the wood. The posts I have seen by people who use lasercutters to kerf it still do it by trial and error, hoping for the best.