Kerfs
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Shop estimate. Measure your stock and cut a test on scrap.
How to use it
- Enter the inside radius and the angle of the bend. 90° is a quarter circle.
- Enter the stock thickness and the skin you will leave uncut. The cut depth is the difference.
- Enter the kerf your blade actually leaves.
- Space that many kerfs evenly along the inside arc. The drawing puts them on the inside face.
- Bend a scrap from the same board around the form before you cut the real piece.
The formula
Cut depth = thickness − skin. Kerfs = ceiling of (angle in radians ÷ (kerf ÷ cut depth)).
- Skin. The uncut web. Thinner bends easier and breaks easier. The page does not pick it for you.
- Ideal spacing. Inside radius × kerf ÷ cut depth. The even spacing is a little tighter because the count is a whole number.
- Inside arc. Radius × angle in radians. That is the length the kerfs are spread across.
Worked example
Three-quarter stock, a ⅛ inch skin, and a ⅛ inch kerf around a 6 inch inside radius for 90° needs 8 kerfs. Each kerf can close 11.46°. The inside arc is 9.425 inches, so the even spacing is 1.178 inches.
The unrounded spacing would be 1.2 inches. Rounding the count up from 7.85 to 8 puts the cuts a little closer. Kerfs go on the inside face. Bend a scrap before the real piece.
What it will not decide
- Not for a load, a handrail, or anything that has to keep its shape alone. The page will not call it strong.
- Real boards spring back, and knots split. The count is geometry with the kerf fully able to close.
- The radius is the inside. If you measured the outside, subtract the thickness before you type it.
Questions
Which face gets the kerfs?
The inside of the curve, so the cuts close as the board bends. The uncut face is the one you see.
What is the 6 inch, 90 degree example?
Three-quarter stock, a 1/8 inch skin, and a 1/8 inch kerf. Each kerf can close 0.2 radians, about 11.5°. A 90° bend needs 8 kerfs. The inside arc is about 9.42 inches, so the even spacing is about 1.18 inches.
Why is the ideal spacing different?
Ideal spacing assumes a fractional kerf. You cannot cut 7.85 kerfs. Rounding up to 8 puts them a little closer, and each one closes a little less than fully.