How to Cut Compound Miters and Miter Joints

Step-by-step demonstration of how to set up a miter saw to cut accurate compound angles for crown molding.

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Transcript:

Crown molding has a way of adding elegance to just about any room, although it can appear massive, it's really an illusion that comes from spanning the ceiling wall joint with a strip of decorative wood set on an angle.

One of the trickiest things about crown molding is cutting the correct angles at the corners. Each corner requires two angles, a miter angle which is cut across the face of the molding and a bevel angle that's formed across the end or edge. When working with crown molding, both of these angles have to be made at the same time, and that's why they're called compound angles. Accuracy is critical, an error of even one degree or less can result in an ill fitting joint. The tool to make this cut is the power Miter saw. One way to make a combination miter is by cutting the molding while it's held on an angle. When using this technique though, it's important to make sure the back surfaces of the molding are fully in contact with the fence and saw bit. While you can handhold the molding in place there's always the chance the workpiece can move during cutting. Some saws like this DeWALT model have available as an accessory, these adjustable crown molding stops.

Another way to make certain the molding does not shift while being cut is to make a simple wooden jig to hold it firmly in place. A jig can be easily made by attaching together two pieces of half inch plywood, making certain that they form an accurate right angle. After the glue is dry, the jig is set in place on the miter saw bed and attached with screws inserted through pre-drilled holes in the metal fence.

With the jig secured cuts are made at 45 degrees and zero degrees. Then a sample piece of molding is set into the jig, wooden stops are then put in place next to the edge and stapled in place. No glue should be used here since the stop would need to be relocated if a different size molding were being cut. The jig holds the crown securely in just the right position each time a cut is made, no adjusting, no slipping.

What does have to be adjusted though, is the swing on the miter saw. That's because even though they may look square, many wall corners do not form perfect 90 degree angles. So each corner needs to be measured individually and that measurement must be accurate to get a tight fitting miter.

Now, the old way of doing this was to take a bevel gauge, adjusted to fit the corner, lay the bevel gauge on top of a protractor, read the angle on the protractor, subtract that reading from 180, then divide the result by two, that provided the miter saw setting.

Well, fortunately, there's a much faster, simpler and more accurate way. This 505P protractor from Starrett Tools is designed just for this job. Place it in the corner and open the wings until they come into contact with the wall. Now, and this is the slick part, simply read the miter saw setting directly from the tool. No calculating, no transferring measurements and practically no chance for error.

Now there are two types of crown miter joints, inside corners and outside corners. Here are the steps for making each.

In most cases, the crown molding bottom is the edge that has the most detail. One thing is common to both inside and outside corners and that is the molding must be placed into the saw with the bottom edge up. In other words, the bottom edge will always be against the miter saw fence. It's also important to keep in mind whether you're cutting the left hand or right hand piece of molding. OK, here we go. When cutting the left hand piece of an inside corner, swing the miter arm to the right and place the workpiece on the right side of the blade. To cut the right hand section of an inside corner swing the miter arm to the left and place the workpiece to the left of the blade.