Join Date: Jan 2019
Location: Central North Carolina
If you go with splines as shown in the photo in post #2, look carefully at the grain direction of the spline. For joint strength, the spline needs it's grain running across it's width, not end to end (look closely at grain direction in the picture). If you make these splines with end to end grain, the joint won't be much stronger than if you just glued the two mitered pieces together without a spline. For strength, the grain direction in the spline needs to run across the narrow width of the spline.
I use my tenon jig on my table saw to make these. When I set the jig up I set it so the resulting spline is the distance between the jig and the saw blade. The donor board is then stood up on end and clamped in the tenon jig as you would for cutting a tenon and then run it through the saw. I can then flip the board over and make an identical thickness spline from the other face of the board. If I need more splines, I repeat the process on the other end of the donor board. Then I go to my miter saw and set a stop to determine the width of the spline. Once set carefully (should be 2X the depth of the spline slots in the mitered joint) I then place the donor board against the stop and cut the splines free. The tenon between these two splines gets cut off and becomes waste. If I'm making additional splines I hen swap the ends of the donor board and cut the two more splines from that end. This results in 2 or 4 perfect thickness splines with their grain running across the narrow width of the splines. If the length of these splines isn't enough for the full length of the mitered spline slots, or if the cross grain spline should break, you can still use the pieces end to end to fill the joint and the joint will still be very strong if glued properly.
Actually, biscuit joints can also be used as splines for mitered joints. With both the cross grained splines or the biscuits, I like to locate them closer to the inside of the resulting corner, rather than putting them in the center of the joint. Having more material left on the outside of the corner further reduces the chances of the corner breaking and keeps you from cutting the biscuit or spline slot too deep and coming too close to the outer surface of the corner.