Lee Valley Tools Woodworking Newsletter
Vol. 2, Issue 6
July 2008
The Easy Curve

OK, get ready for that math. Here's the equation (www.woodworking.org):

R = (L/2)² + H²
  R = Radius
H = Height of arc
L = Length of arc

I'll begin calculating the numerator (top number). To figure out the length of the radius of the circle I need to draw to obtain the proper arc, I take the length of the arc and divide it in half (24/2), leaving me with 12". Then, I have to square that number (12x12), giving me 144. I take the height of the arc and multiply that by itself (2x2), giving me 4. I next have to add those two numbers together (144+4) for a total of 148.

To calculate the denominator (bottom number), I take the height and multiply it by 2 to get 4.

Finally, I divide the numerator by the denominator to come up with 37". That's the radius of the circle I can use to draw the arc that fits the mark perfectly.

I set a trammel compass to 37" and get to work. I carefully align my work board and a second board that is the same thickness as the workpiece. I place the pencil point on the maximum height mark of the project board, and the other point gets placed exactly 37" from there, perpendicular to the base of the arc.

Now, simply draw the arc. It will fit exactly.

Setting the trammel
Set the trammel to 37".

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