Often we are bending sheet goods into tight corners for parts of consoles or cockpit or forward seating. How do we get the pretty curves with the nice corner radius? There are typically two materials used:
- Structural foam such as Divinycell
- Marine Plywood
With structual foam, two different techniques can be used:
- Kerfing
- Heat
Bending marine plywood requires the first technique - Kerfing. We'll cover that first.
Kerfing: This technique involves removing material from the inside of the curve to allow the panel to bend. The key is knowing how much material to remove. The material is typically removed by sawing slots (kerfs) across the material on the same axis as the bend you are trying to achieve. Let's, for the sake of this discussion, assume that the material we are working with is 3/4 (19mm) thick. In that case, the slots should be about 5/8" to 11/16" deep. How many slots? How far apart?
There are some relatively simple formulae for calculating the answers to these very important questions. The ideal bend will allow the inner surfaces to exactly come together when exactly the desired radius is achieved.
Do you remember your geometry in school? Specifically, circumference = pi x diameter. Diameter = 2 x radius. The correct number of kerfs to allow the inner surfaces to exactly come together is calculated by dividing the difference between the inner and outer circumferences by the saw blade thickness. We also need to know the amount of bend we are trying to achieve as a fraction of a full circle. For example, a 90º corner is 1/4th of a circle.
Now for an example:
Let's start with what we know:
- We are trying to achieve a 6 inch outer radius.
- Our material is 3/4" inch thick (.75).
- Our saw blade thickness is 1/8" (.125).
- Our bend should end up 90º.
Calculations:
Outer perimeter = 6 inch * 2 *pi /4 = 9.424777961 inches Note: you divide by 4 because 90º is one-fourth of a circle.
Inner perimeter = 5.25 inches * 2 * pi / 4 = 8.246680716 inches
Difference = 1.178097245 inches (the amount of material that must be removed via the kerfing)
Number of kerfs = 1.178097245 (Difference) / .125 (Blade width) = 9,42 (round up) = 10
Distance between kerfs = 9.424777971 (Outer perimeter) / 10 (Number of kerfs) = .942 or about 15/16"
The cuts are always made on the inside of the bend. For a perfect bend, you should bend and clamp around a form. Protect the form from glue, fill the kerfs with thickended epoxy, bend, clamp and let it kick.
In the illustration below, you see the simple progression from a flat piece of foam to a nicely bent 6" radius.
Click on the picture above to download a handy spreadsheet that calculates everything for you!