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First and foremost, it helps to use a compound miter saw to cut crown molding. This saw can be adjusted to make clean, perfect cuts of multiple angles at once. Because the molding is installed at an angle to both walls and ceilings — a task complicated by the fact that individual pieces also meet at other intersecting angles in both inside and outside corners — you need a tool that can handle these multiple-axis or compound cuts.
Many carpenters use a simple trick to form compound angles — they cut the molding upside-down and backwards. By placing the molding’s bottom (or wall-facing) edge against the saw’s vertical fence and the molding’s top (or ceiling) edge flat on the saw table, you can make a standard 45-degree cut (Image 1) that will fit most intersections when the molding is placed right-side-up. Place a piece of painter's tape on one side of the molding to make it easy to remember which is top and bottom. If you need, make a few practice cuts using scrap pieces of molding.
Inside corners are best handled with “coped” cuts. This method creates a profile on one piece of molding that closely matches and overlays on top of another molding length.
To make a coped inside corner, start by cutting a square 90-degree angle (Image 2) on the end of the first molding piece, then install this end tight against the corner. On the second piece of molding, make a 45-degree-angle cut so that the exposed cut faces outward. Then use a thin-blade coping saw to back-cut at an angle along this face, following the molding’s profile (Image 3). Use a wood rasp or sandpaper to trim the cut, if necessary. The resulting profile should fit tightly over the intersecting molding when installed.
Another method for fitting together inside corners is to cut both pieces at opposing 45-degree angles with the molding placed upside down on the saw. If the intersection isn't tight, you will have to caulk the gap and paint over it.
Most outside corners can be joined with mating 45-degree-angle miter cuts. If the wall angle is slightly off, adjust the cuts on both molding faces equally by one degree at a time until you have a close-fitting joint (Image 4). This is easily done on a power miter saw, which can be adjusted to cut across a range of fractional angles.
Remember, on outside corners, the point of the molding will be towards the ceiling. On inside corners, the long point is going to be towards the floor.