This compound mitre helper calculates the saw settings — blade bevel and mitre angle — for cuts that slope in two planes at once, such as crown moulding, sloped boxes, and angled frames. It converts the joint angle and the work's slope into the numbers you dial into the saw.
Why compound angles are tricky
A simple mitre tilts in one plane; a compound mitre tilts in two — the piece meets its neighbour at an angle and also leans back from vertical. Crown moulding is the classic case: it sits at a spring angle against the wall and ceiling, so a corner that is 90° in plan needs both a mitre and a bevel setting that are not the obvious 45°. Working those out by trial and error wastes expensive moulding.
The helper takes the corner angle (for example a 90° wall corner or the angle of a many-sided box) and the spring or slope angle of the piece, and returns the exact bevel and mitre to set. It can handle both flat-nested and sprung crown, and polygons with any number of sides for tapered boxes and planters.
Getting clean joints
Even with the right numbers, consistency wins: keep the stock the same way up and against the same fence for every cut, label the faces, and test on offcuts first. Small saw inaccuracies show up badly on compound joints, so cut a sample corner before committing to the real moulding.
Cutting standard 38°-spring crown moulding for a square (90°) inside corner.
- The corner is 90° in plan, so each piece turns the corner by half.
- For 38° spring crown, the compound settings are about 31.6° mitre and 33.9° bevel.
- Set the saw to those angles and cut a test corner on offcuts first.
Use roughly a 31.6° mitre and 33.9° bevel for a square-corner 38° crown — verify on scrap.
