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Vertical splines are a great way to joint mitered corners, but for good strength they need to have the grain in the splines running horizontally (across the joint). Locating the spline cuts more toward the inside of the joint angle also helps with joint strength. This is a very strong corner joint that is a bit of a challenge to master, but the end result is very pleasing.

I use my Unisaw to cut the spline slots in the mitered ends of the box sides at 90 degrees to the box end miters. Trying to get spline material that equals the kerf width of the blade used, and making them with the grain running across their width is a bit of a challenge.

I make the cross grained splines using a tenon jig on my Unisaw, setting it up so that the waste from making the tenon actually becomes the spline. With the spline donor board vertical in the jig and the jig to blade distance being the dimension needed for spline thickness, which needs to be a few thousandths thickness less than the kerf width of your spline cuts in the ends of your work pieces to allow for glue. Once set for correct spline thickness, you can easily cut a spline, then flip the board over and make a second spline from the opposite surface. Then repeat the process to make two more splines from the opposite end of the donor board. I then set up my miter saw at 90 deg with a stop to allow cutting the splines at the needed width to fit the spline cuts in the work pieces. I chop them free of the donor board and then go back and cut more splines on the table saw from the spline donor board so that I have spares. The resulting tenons made become the waste, since you only need the thin splines. With the grain running across the narrow dimension of the splines they will break easily, but they will produce a very strong joint when in use. Even broken pieces of these splines stacked in the joint with glue will make a very strong joint.

I'm attaching a photo of one of my spline joint projects where this method was used. The project was burial urns for cats and dogs. It's important for the grain in the splines to run across their width and not top to bottom. Notice also that I located the spline cuts toward the inside of the corners. Too close to the outside of the corner can result in breakage between the spline joint and the outer surfaces of the box side. This type of joint is quite strong when made correctly. It also helps keep the joint from mis-aligning during glue-up.

I make a lot of boxes, but usually make them with box jointed corners, like seen in the third and fourth photos. I use the same Unisaw, but with a special box joint cutting blade set for 1/4 and 3/8" box joints, or a FTG Ripping blade for 1/8" box joints, and an Incra I-Box jig. This is yet another very good joint to master. It's faster to make than dovetail joints, and I think stronger too.



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