Normally, the roll-to-roll distance of a sinebar is some simple value. 2.5, 5 and 10 inches are common values; the one shown in the picture is 2.5. But the roll distance can be any value SO LONG AS WE KNOW WHAT IT IS. If you make your own sinebar or inherit one of unknown accuracy, you'll want to measure the roll distance so it can be used when setting the sinebar to a desired angle.
One tool is needed - an accurate angle plate. They typically come in the form of 30-60-90 or 45-45-90 triangles. Every serious metalworking hobbyist should have one, preferably both.
If we replace the part shown in the picture with the known-to-be-accurate 30-60-90 triangle and adjust the stack height so the hypotenuse of the plate is parallel to the surface plate, we will know that the angle of the sinebar is 30 degrees to the same accuracy as the triangle.
The sinebar equation is:
sin(A) = S / L
A = angle
S = stack height
L = roll-to-roll distance
Solving for L then yields
L = S / sin(30) = S / (1/2) = 2 * S
Once the L value is calculated, it's worth writing it on the bottom of the sinebar with a Sharpie.
Speaking of Sharpies, they now make paint pens...
https://www.amazon.com/gp/product/B0...?ie=UTF8&psc=1
which make a more permanent mark than the ordinary Sharpie marker. Since they don't make an ordinary one with white ink, the white paint pen is a boon. It's especially useful when marking dark metals.
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