What it works out
Enter the wall corner angle as you would measure it, say whether it is an inside or an outside corner, and give the spring angle of the molding. You get saw settings for two ways of cutting: lying flat, with a miter and a bevel, or nested against the fence, with a miter only. The left and right pieces use the same settings, mirrored. Test on scrap first.
You enter
- Wall corner angle
- Spring angle
The formula
- Bevel
- arcsin(cos(spring) × cos(corner ÷ 2))
- Miter
- arctan(sin(spring) ÷ tan(corner ÷ 2))
In words: Bevel = the arcsine of cosine of spring times cosine of corner divided by 2; miter = the arctangent of sine of spring divided by tangent of corner divided by 2
Worked example
A square 90° corner with 38° spring angle crown
- Wall angle 90°
- Spring angle 38°
- Bevel = arcsin(cos 38° × cos 45°) = 33.9°
- Miter = arctan(sin 38° ÷ tan 45°) = 31.6°
Lying flat, set the saw to those two angles. Test on scrap first.
Good to know
- Walls that are out of square need the true measured angle.
- Settings are a starting point: always make a test cut.
- A calculator, not a building-code check: confirm local rules with your building authority.
Questions
What is the spring angle?
The angle between the back of the molding and the wall (or ceiling) it sits against. 38° and 45° are common.
Do left and right pieces differ?
They use the same settings, mirrored.