How to Make Compound Mitre Crown Molding Cuts

Set up your miter saw station such that long pieces of molding will be well supported. This also involves extending the miter saw's fence to prevent the molding from twisting. Secure the miter saw to the saw station using clamps or screws.

Lay the crown molding upside down on the miter saw's table top. Make sure the molding extends past each end of the fence, because you will be using the molding as a guide to mark the full length of the fence. Position the molding just as you would on the wall except for the fact that it's upside down. Thus the surface that would be touching the ceiling will now be downward on the saw's table top, and the surface that would be touching the wall will now be against the fence. The molding should be at an angle just as it would be if mounted on the wall. Twist the molding to adjust this angle until the surface meeting the fence is fully flat against it. When it's at the proper angle, the surface against the table top will also be fully flat against it, because these two surfaces are cut at a 90-degree angle to one another.

Run your pencil along the top of the molding in order to mark the elevation on the fence from one end to the other. This mark will serve as a guideline so each piece of molding can be consistently positioned on the miter saw with the same angle. To make this guideline easier to see, run a strip of masking tape along the fence with the bottom edge of the tape flush with the pencil line. By using this system of cutting the molding in a sloping position, you can avoid using compound angle charts and complex calculations.

Cut two 16-inch pieces of molding for testing your angles. Release the saw's table so that you can rotate it counter-clockwise to align the indicator to the 45-degree mark on your right side and lock it into place. Position the first test piece on the right side of the blade, upside-down as before and with the edge that meets the fence in line with the guideline you made. Holding it firmly in place with your hand a safe distance from the blade, cut off the very end of the molding just enough so that the blade passes through the full profile of the molding. Do the same with the other test piece, but this time the piece will be on the left side, and the table aligned to the 45-degree mark on the left.

Use the layout protractor to measure the angle between the walls in the corner where you wish to start. Set the angle on the miter saw to half this value, which in many cases will be 45 degrees. Keep in mind that when the molding is upside down on the miter saw, the end that you need to cut is reversed from how it will appear on the wall. For example, if you need to cut for a corner that is on the left side of the molding, the saw blade will be on the right.

Make your first cut as you did with the test pieces -- upside down and flush with your guideline. Before nailing it into place, use the appropriate test piece to see how the joint will appear. You may have to cut a custom test piece if your molding was cut to anything other than 45 degrees. For example, if you made a 46-degree left-hand cut, you will need a 46-degree right-hand test piece.

Butt the test piece to the molding in the corner where it will be installed. Small discrepancies can often be overcome by slightly twisting the molding. With the test piece in one hand and the end of the molding in the other, rotate them in unison to see if a small adjustment will close the gap. This is similar to how you located the guideline on the fence by adjusting the angle until the surfaces were perfectly flat, but now the goal is not to have it perfectly flat but to close the gap of your miter joint. If the gap closes, nail the molding into place. Otherwise you will have to re-cut it to get a better fit.

Continue this process as you work your way around the room, carefully fitting each joint. With your longer pieces, leave the last three feet unnailed until you have successfully cut the piece that follows. This will allow you to twist the molding a little if you need to make adjustments. Remember to apply a thin layer of wood glue to each of the joints. Immediately wipe up any that oozes out.