Cutting Miter Joints

Learn how to properly make miter joints, one of three joints used most often to install trim.

Intro

The miter joint, where the ends of the pieces to be joined are cut at an angle (usually 45 degrees), is one of three joints used most often to install trim. The others are the coped joint and the butt joint, where two square faces meet. Miter joints are used for outside corners, while coped joints handle most of the inside ones. Both allow molding profiles to continue around a corner without interruption.

To cut miters quickly and accurately, use a power miter saw, commonly called a chop saw, or a handsaw and a miter box. Most professionals use a chop saw for its speed and accuracy. A miter box can be just as accurate; however, it takes longer to make a cut by hand. If your budget is tight and you plan to do a limited amount of trimwork, opt for a miter box. You can buy a professional-quality integrated miter box and miter saw for the price of an entry-level chop saw.

Practice making miter cuts on scraps, both to improve your skills and to check the accuracy of your tools.

Step 1

Set your saw to cut a 45-degree angle with the blade angled to the left. Make a test cut in a scrap piece of molding. Swing the saw to cut a 45-degree angle with the blade angled to the right. Make a matching cut in a second scrap piece held on the other side of the blade.

Step 2

Check to see if a saw makes accurate 45-degree cuts by holding the test pieces together against a framing square. There should be no gap in the joint. If the saw's stops are not set correctly, consult the owner's manual to learn how to adjust the saw.

Step 3

Place the test pieces in position. If the miter is open on the outside, adjust the saw to slightly less than 45 degrees. Make test cuts until the angle mates perfectly with one of the original test pieces. Cut one of the pieces you'll install to the adjusted angle, the other to exactly 45 degrees.

Step 4

If the test pieces are open at the back when you place them against the wall, set the saw at slightly more than 45 degrees. Cut scrap until you find the correct angle, then cut the angle on one piece you'll install and an exact 45-degree angle on another piece.

Step 5

If you have to make very subtle changes to the miter angle, you might find it easier to insert shims behind the molding rather than trying to shift the blade a slight amount. Plastic-coated playing cards work well for this job; keep a deck in your toolbox.

What If... The Corner Isn't 90 Degrees?: Step 1

With corners that are slightly off 90 degrees, you can achieve a close fit for miter joints by cutting one molding piece to exactly 45 degrees and then making minor adjustments to the saw to "sneak up" on the matching angle for the other piece.

For corners that are intentionally far from square, you'll have to reset the saw to split the angle in half. Fortunately this is easy to do without math calculations. You just need a T-bevel and a compass.

Yet another method involves a protractor that accurately measures the angle.

Set a T-bevel against the corner to set the angle. Transfer the angle you want to a board.

Step 2

Starting at the apex, mark an equal distance along each leg of the angle with a compass. From these new points, draw two intersecting arcs with the same compass setting.

Step 3

Draw a line through the apex of the angle and the point where the arcs intersect. This line bisects the original angle. Transfer the new angle to the saw via the T-bevel.


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