*Note: These settings are for crown molding lying flat on the saw table.
Understanding Compound Miter Cuts
A compound miter cut involves two simultaneous angles: the miter angle (the horizontal rotation of the saw blade) and the bevel angle (the vertical tilt of the saw blade). This technique is essential for installing crown molding flat on a miter saw table rather than propped up against the fence.
Key Terms Explained
Corner Angle: This is the actual angle of the wall corner. While most corners are intended to be 90°, they are rarely perfect. Using a protractor to find the exact wall angle will yield better results.
Spring Angle: This is the angle at which the molding "springs" off the wall. Most common crown moldings use a 38° or 45° spring angle. You can find this by measuring the angle between the back of the molding and the wall it sits against.
How to Use These Calculations
Once you have the Miter and Bevel angles from the calculator above:
Lay the molding flat on the saw table with the finished face pointing up.
Set the saw's miter scale to the calculated Miter Angle.
Tilt the saw blade to the calculated Bevel Angle.
Ensure the top of the molding (the part that touches the ceiling) is against the saw fence if you are cutting for a "top-against-fence" setup, or consult your specific saw's manual for orientation.
Calculation Example
If you have a standard 90° corner and a 38° spring angle crown molding:
Corner Angle: 90°
Spring Angle: 38°
Miter Angle: 31.62°
Bevel Angle: 33.86°
These specific numbers are common presets marked on many high-end compound miter saws.
document.getElementById('springAngle').onchange = function() {
var customContainer = document.getElementById('customSpringContainer');
if (this.value === 'custom') {
customContainer.style.display = 'block';
} else {
customContainer.style.display = 'none';
}
};
function calculateMiter() {
var cornerDeg = parseFloat(document.getElementById('cornerAngle').value);
var springSelect = document.getElementById('springAngle').value;
var springDeg;
if (springSelect === 'custom') {
springDeg = parseFloat(document.getElementById('customSpring').value);
} else {
springDeg = parseFloat(springSelect);
}
if (isNaN(cornerDeg) || isNaN(springDeg)) {
alert("Please enter valid numbers for angles.");
return;
}
// Convert degrees to radians
var cornerRad = cornerDeg * (Math.PI / 180);
var springRad = springDeg * (Math.PI / 180);
// Compound Miter Formulas
// Miter Angle = atan(sin(Spring) / tan(Corner / 2))
var miterRad = Math.atan(Math.sin(springRad) / Math.tan(cornerRad / 2));
// Bevel Angle = asin(cos(Spring) * cos(Corner / 2))
var bevelRad = Math.asin(Math.cos(springRad) * Math.cos(cornerRad / 2));
// Convert back to degrees
var miterFinal = miterRad * (180 / Math.PI);
var bevelFinal = bevelRad * (180 / Math.PI);
// Display Results
document.getElementById('resMiter').innerText = miterFinal.toFixed(2) + "°";
document.getElementById('resBevel').innerText = bevelFinal.toFixed(2) + "°";
document.getElementById('miterResult').style.display = 'block';
}