HandyAtLeast wrote:Thoght I'd throw in some thoghts on bending foam sheets with kerfs. With a little math you can figure out the number and size of kerfs to cut in foam to get a desired radius.

Example:

Lets say you have a sheet of foam that is 1 inch thick and you want to make a 90 degree bend with a 6.4 inch radius. This is a very tight bend for this kind of foam, but with some kerfs cut in it, you can do it.

First some math. Illustrated above you see a 90 degree bend in 1 inch foam with a radius of 6.4 inches. We need to find out the arc length of the outside and inside curves of the foam. This will help use figure out how wide the kerfs need to be.

The formula for finding the arc length is:

arc length = (angle of bend / 360) x 2 ( pie x radius)

so we have:

10 inches = (90 degrees / 360) x 2 (3.14 x 6.4 inches)

The arc length of the outside curve is 10 inches. Then with the same formula we find that the arc length of the inside curve is 8.4 inches.

When we subtract the two we have a difference of 1.6 inches. This is how much material we need to remove from the inside curve with kerfs.

So, if we decide to make 5 kerfs on the inside of the foam, they will have a spacing of about 2 inches apart and 1 inch from the end.

Divide the 1.6 inches by 5 and you get 0.32 inches. This is how wide each kerf needs to be. The kerfs should be cut to a depth of about 75% of the 1 inch foam thickness in a triangular shape. the widest part of the kerf will be 0.32 inches. This can be done with a straight edge and a sharp utility knife.

OR if you want to make the kerfs with a circular saw and more kerfs, then you would divide the 1.6 inches by the thickness of the saw blade (also called the kerf of the blade.) With a 1/8 inch thick blade you would need to make about 13 kerf cuts spaced evenly apart. ( 1.6 / 0.125 = 12.8 )

Apply some glue inside the kerf cuts and bend the foam to the needed radius. It should hold its shape fairly well on its own after the glue dries.

HandyAtLeast wrote:Okay, I found a scrap piece of 1.5" foam. Its 15"x21" and I'm going to put a 12" radius in it to 90 degrees.

I calculated a differece of about 2.3". So, using a table saw, I cut 18 kerfs spaced 1" apart. Tomorrow I will glue it up with Gorilla glue and tape it in place until it dries. Just bending it seems to make a perfect 12" radius to 90 degrees. The glue will fill the voids at the bottom of the kerf cuts also.