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HOUSE WORKS: Four tips for cutting mitre joints like a pro

This is a mitre saw and it makes mitre joints easier to cut accurately. Tips, practice and struggle are still necessary to develop the skills to use a mitre saw well.
This is a mitre saw and it makes mitre joints easier to cut accurately. Tips, practice and struggle are still necessary to develop the skills to use a mitre saw well. - Steve Maxwell

Whenever I teach online classes about woodworking and cabinetmaking, I often get questions about how to cut mitre joints accurately. When two pieces of trim, moulding or baseboard meet at a square corner, the ends need to be cut accurately to 45º so everything meets together as it should. This is a mitre joint. Sounds simple enough, but creating great, tight, gap-free mitre joints still comes down to a few subtle skills. Follow these four tips and you’ll be surprised how easy it is to get things right.

Tip 1: Mark All Cuts With a Knife

You can’t cut mitre joints accurately if you don’t know exactly where you need to cut your trim. This is where the sharp tip of a fresh utility knife blade comes in. It makes a nice thin, crisp mark on wood that’s easy to follow with a saw — better than a pencil. As you cut your trim, shine a strong light on the wood and sneak up to the knife mark with several shallow cuts. Mitre saws with a built-in light that shows where the blade cuts make it easier to home in on an accurate cut.

Tip 2: Shim the Trim

A mitre saw makes it so much easier to install trim than any other tool, so you shouldn’t attempt your trim job without one. Some of the best and handiest mitre saws these days are cordless, too. They work great for small and medium-size trim installations, but this doesn’t solve every challenge. Windows, doors and rooms aren’t always square, so it’s sometimes necessary to fudge the exact angle of trim that’s supposed to have two 45º angles meeting. Your trim needs something different than 45º if there’s a gap in the mitre joint when pieces come together, but how much different? In cases like these you could tweak the setting of the saw one way or the other, but sometimes it’s easier to simply shim one end of the trim by eye and leave the saw set for 45º. Cardboard or small wooden wedges placed against the mitre saw fence work great as shims.

A sharp knife is always better for accurately marking wood than a pencil. The crisp, sharp line is easier to follow when super-accurate cutting matters. - Steve Maxwell
A sharp knife is always better for accurately marking wood than a pencil. The crisp, sharp line is easier to follow when super-accurate cutting matters. - Steve Maxwell

Tip 3: Adjust Joints With a Block Plane On the Back

No matter how skilled you are with a mitre saw, sometimes a piece of trim is just a little too long. That’s better than too short, because a few strokes with a block plane can make everything perfect. One of the best ways to slightly shorten the effective length of an almost-right mitre joint is by planing the back surface of trim, not the angled end. If you need to remove more than just a paper-thickness of wood, plane the joint face itself, but only with the trim fully supported on a solid surface. You want all movement to be from the plane only, with the trim steady as a rock.

Tip 4: Secure Trim With Pins

In many cases, the best way to secure trim and moulding isn’t with nails, but with something called a 23-gauge pin nailer. The metal fasteners these air tools shoot are roughly the same diameter as a standard sewing needle, so they leave behind marks that virtually disappear after sanding and finishing. The best 23-gauge pinners for trim installation fire two-inch-long pins that can secure door and window trim as well as full-size crown moulding. For strongest results, apply a little glue on hidden back surfaces of trim before installation.

Installing trim is one of the most effective ways to make your house elegant and unique, while saving good money. Doing the work yourself also lets you connect with your home in a way that only hands-on investment can deliver.

Visit BaileyLineRoad.com/29147 for a 15 minute video tutorial on cutting mitre joints well.

Steve Maxwell is a syndicated home improvement and woodworking columnist who has shared his DIY tips, how-to videos and product reviews since 1988.

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