I love splines. Relatively simple. If you have a long spline, you have to use short sections with the proper grain orientation. It's also important to get both sides of the joint straight and the same size, and to put both pieces through the router with the same (usually the front) side down. If you do one up and the other down, you generally wind up having to sand, sand, sand because it's almost impossible to get a bit perfectly centered.
If you cut the groove on both pieces face down, they will automatically line up when glued up. It will take a minimum of sanding or scraping to get a perfectly flat surface. Fingers can feel a mismatch of just a couple of thousandths
The following is just background info. The flatter your stock, the better your joints will turn out. Which is the job of the jointer and planer. But if you flatten stock and let it sit for awhile, it may warp or twist or resume its slightly-off surface. So you want to use your prepared material fairly soon after working it.
Sometimes you use an exotic or different color stock for the spline, which then becomes a decorative element. For example, I often use purpleheart for splines on the corners of frames or boxes because it really looks great against a lighter wood.
Splines should be very close to the exact width as the groove as possible. Too tight stresses the joint and leaves no room for glue. Too narrow and the joint will be weaker. This is where setting your saws and tools up carefully becomes important. If your table saw blade is not a perfect 90 to the table, your splines will be off as well. That's where a Wixey angle gauge come in handy. They're about $30 on Amazon.
The more I do, the less I accomplish.
Last edited by DesertRatTom; 08-29-2018 at 12:01 PM.