A miter joint without any reinforcement is not very strong because the grain is cut diagonally. Incorporating a spline adds tremendous strength to a miter because now there are long grain fibers running across the joint line.
This is a relatively easy joint to cut once you are set up for it. Everything can be cut at the table saw with two jigs. I prefer to use a miter sled at the table saw to cut the miters and I have a spline jig, which Iíll explain later, to cut the slots for the splines.
The horizontal walnut frames for this six foot tall display stand are made from stock that has been jointed and planed to three quarters of an inch thick. Milling all of the parts to the same dimensions and being careful to check that everything is square will pay off when you cut the miters.
My miter sled consists of two runners on the bottom which ride in the two slots in the table saw top. The bed is made from half inch thick aluminum and has two fences meeting at a 90 degree angle. The idea is to cut one miter on the left side and the matching miter on the right side and as long as your fence meets at 90 degrees, your two miter joints will add up to 90 degrees.
After cutting all the miters, glue up the frame. Apply glue to the miters, let it soak in a minute, and then add some more glue. I like to use a band clamp to pull the four corners together and then add some bar clamps. The bar clamps will exert the necessary force to pull the miters tight.
After the glue has dried, clean up the squeeze out with a sharp scraper. Now we are ready to cut the slots for the splines. The jig is pretty straightforward. Take a two inch thick piece of wood that is about ten inches long by five inches high. Lay it on its side, mark the center and make a point approximately one quarter inch from the bottom. Now take a square and draw a ninety degree corner from that point, after that, take it to the band saw and cut it out. At this point you will have a cradle that will hold one corner of the frame. Take the jig and place it against the table saw fence, set the frame in it and raise the blade and set the fence so that it will cut a slot through the miter.
Itís a good idea to use a square tooth ripping blade, otherwise most saw blades will leave little dog ear cuts on either side of the kerfs. The splines can be milled separately, just make sure that the long grain is oriented so that it runs perpendicular to the joint line for strength. After the splines have been glued, clamped and cured you can trim the excess on the band saw and use a block plane to flush them smooth. The end result is a strong joint and the exposed spline is quite attractive. Have fun experimenting with different colored woods to create contrasting colored splines.