For cutting crown molding I use a fixture made from an ordinary wooden miter box and two 1x stops. First, find the horizontal dimension (X in the drawing) of the crown molding, then screw the bottom stop in place so that its rear edge is that distance from the back of the miter box, as shown. Cut the other 1x wide enough to support the upper edge of the molding. Screw it in place, making sure that the screws are out of the saw’s path.
Cut outside or inside mitered corners by setting the molding against the stops, wall edge down. Unlike Harry Stangel (see Crown-molding fixture), I think it’s more accurate to cope inside corners to the molding’s actual profile than to a penciled line, so after I cut the inside 45°, I cope that profile as shown in the drawing.
Peter Sacks, Somerville, MA