Splined Miter Joint: Strengthen Miters With a Decorative Touch

Miters make quick and easy box joints, but they have no mechanical strength. If the glue fails, so does the joint. This post will show you how to avoid that pitfall by adding decorative splines.

contrasting miter and splines
Photos via wooden-box-maker.com

Adding a spline to a standard miter joint increases the strength and can add a nice visual touch as well. A spline is essentially a shim that fits into a kerf cut through the miter.

To make a splined miter joint, cut your miters as you normally do. I like to cut miters on the table saw using a miter gauge and sacrificial fence. I start by adjusting the blade angle to 45 degrees. I use the gauge on the saw to get close to the angle then I cut a couple test pieces.

cutting miters

While it is possible to hold a 45-angle to the cut, I find it easier to check the angle by putting two miters together and seeing if they join together to make a 90-degree angle.

check miters for square

Once I have the correct angle, I set up a stop on my miter gauge and cut my boards to length. I then go ahead and make the box in the usual manner.

After the box is glued up, I’m ready to cut the kerfs through the miters. I use a handmade cradle that holds the wood at a 45-degree angle as I run it over the table saw.

spline jig spline cuttingjig

The cradle keeps the box at a consistent angle and allows you to set the height of the blade for the size of the spline. The higher the blade, the wider the spline.

I layout the splines on the wood beforehand. Once I have the look I want, I cut the splines using the cradle. The base for the box is pretty narrow, so make sure you hold the box steady on the jig as you go over the blade.

Now you need to make your spline material. A bandsaw and drum sander make quick work of this. Rip your spline material close to the thickness you need, then use the drum sander to bring it to the exact size of the kerf.

If you don’t have the tools to make your own spline stock you will need to purchase thin stock and alter the thickness of your spline cuts to fit your wood (by using a datto blade, or making multiple passes over the table saw). You want the splines to be tight enough to need a tap or two to seat them, but not so tight you have to pound them into place.

When choosing the stock for your splines think about what you want the finished box to look like. You can either make the splines disappear by using the same wood as the box, or you can use the splines as a decorative element.

invisible splines contrasting splines

Once you have the kerfs cut through the miters and your spline material is cut to thickness, all that is left to do is glue the pieces into place. I cut the splines oversized, making small triangles using a hand made miter box.

handmade miter box

I glue them in place and set the box aside until the glue is dry.

glue splines into place

Once the glue is dry I use a flush cut saw to sand off the waste. Give it all a final sanding, finish as usual and your beautiful box is done.

invisible splines

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