How to Make Cuts for Inside Corners on a Mitre Saw
Installing inside corner trim usually isn't as simple as mitering two pieces of wood at 45-degree angles. Usually, inner corners are going to be out of square, more so than outer corners, because of the nature of wall building (the plaster or drywall compound piles up slightly in the inner corners). If the wall is dramatically out of square, you'll need to make a coping cut, which is an art of its own. But if it's mostly square, you can make that 45-degree miter cut, then plan on adjusting it a bit with a coping saw. Does this Spark an idea?
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Instructions
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Make your inside corner cut on your miter saw. Make it by standing the trim up on the saw platform (as it will be on the wall), swiveling the blade to 45 degrees inward toward the piece.
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Squeeze the saw's trigger. Bring the circular saw blade down in one smooth motion, then back up. Don't release the blade trigger until it's clear of the wood. (The cut will leave an area of exposed wood at one end of the piece, with the edge of the exposed wood mimicking the shape of the trim.)
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Repeat the process for the second piece of the trim, for the other side of the corner. Cut the trim from the opposite end, and swivel the saw in the opposite direction.
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Set the pieces together in the corner, as they will be installed, and assess the joint. Use your pencil to mark on the top of the pieces to show how they have to be filed. If there is a gap between the pieces in the front, it means the back side of the cuts have to be trimmed back using a coping saw; if the front joint is tight but there's a space in the back, visible from the top, then the front of the cuts have to be trimmed.
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Use your coping saw (looks like a little hacksaw, but with a very thin blade) to trim back the ends of the boards as needed to make the miter cut fit tightly. This process is called "coping." Only cut the bare wood that was exposed by the miter cut. Cope, re-check the fit, then cope again. (Note: It's better to cope too little than too much, as you can always cope more.) Continue until you get a tight fit. Install the trim.
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Tips & Warnings
Wear eye protection at all times when using your miter saw.
