Bending plywood Kerfing style, suggestions.

["Wild Bad Bob"]

I have a shop project where I need to bend some 3/8 thick plywood in about a 3"R, 75*, the piece will be 3" wide. I am familiar with kerfing but never done it b4. Any suggestions on how deep to make the kerf. I am thinking 2/3rd-3/4% deep. I read about the procedure and suggestions in the SS book, just looking for someone who actually did it in a project.

[Gene Howe]

The kerf should almost reach the face veneer and be 1/4" apart. I use a full width blade on the table saw. You can set up a box joint like jig to insure equal spacing.

[charlese]

It is truly a joy to read about Shopsmither's trying out different procedures. Best Wishes!

It's all a learning process!

[john_001]

There was an article in Fine Woodworking about kerf bending, but I can't remember the issue. As Gene Howe says, the kerf runs almost all the way through. In the article, he made the kerfs the width of the saw blade (1/8"). He recommended tests to find the right number. He filled the kerfs with tinted epoxy resin then sanded off the remaining attached layer. The epoxy fill might be a good idea for strength, but the sanding was just for looks.

[edit] Oops, sorry, it was June, 1992 - time flies when you're having fun ...

[JPG]

For a 3" radius, at 90°, the circumference(arc) would be about 4 3/4".

At a 1/4" deep kerf, 1/8" wide, the max possible bend is 30°.

I believe a 75° arc with 3" radius would be almost 4".

Since that 30° bend is theoretical and ignores the ability of the remaining wood to bend, something far smaller is realistically obtained per kerf.


Now how smooth a curve is desired? The further apart(fewer over the arc) the kerfs are the less smooth the curve is, since it will be a series of tangential segments.


Also what depth will result in only the face ply(hopefully the face will have the grain oriented along the arc) being bent?



Final strength will be determined by how much air(missing wood) is in the radius.



IIWM, I would try 8 cuts over 4" spaced at 1/2" apart starting and ending 1/4" from each end on the curved segment. I would glue liberally in the 'air'.


Assuming this is desired, I would bend in on a flat surface to preclude any inadvertent twist. A form would help assure an equal bend at each kerf.


I have not tried a 3" radius, but the above 'thinking' has worked with larger ones.


BTW how 'wide', and how long beyond the radius each way?

[reible]

If that doesn't work out for you then you can buy "bendable plywood".

Ed

["Wild Bad Bob"]

Thanks guys, but in the course of the day and the project, I came up with a better idea for it. I am going to use a piece of sheet metal internally and bend that instead of bending the whole frame to make my curve. It is going to be a planer dust collector frame and attach my DC hose to it.