How To Make A Splined Joint

Two pieces of solid pine. Note the FSC certified label!

I had a customer request this week for a 2'x2' version of my "Great Oak Of Portmore". The only problem with that is I like to use solid wood for these projects. That means I'll have to join two solid pieces together to accommodate the request. Plywood is okay but will lack the character and look cheap when finished. Joining panels together in this way can produce a very weak joint if you simply glue and clamp them together (butt end joint). That's why I am choosing a "splined joint" for this project which will add much needed gluing surface area and strength to the finished panel.

Sketch of a "Splined Joint"

So a splined joint adds reinforcement by running another piece of material through the core of the joint. I threw together a quick illustration (left) of what this looks like. It has been recommended by some that you use a material for the spline that is man made like masonite because the lengths and widths are not as limited as with solid woods. However, since my project is an art piece as opposed to a table top or other utility design, I am opting for a solid wood spline. There is a good chance I will carve into the spline (exposing it) so it will need to stained and finished the same color.

To start I set up a box end bit into my router table. The depth I'm going with is around 5/16" to give the spine enough grab when it nests into the groove. This is of course completely optional and up to you the depth you feel is necessary. This is one of those things you can play with and find what works.

Next, I offset my fence to be "in the ball park of center" being perfect doesn't matter as long as you keep track of the up and down side of the panels. You can lay them flat on your workbench and look for an offset. If you see one just flip on of the two panels over so they align. Once you have run the two panels through they should look something like the image to the right. Note: Please don't disable your guard on your router table. I'm not your mom but we need more craft in our world. You'll contribute more if you keep all your fingers. I'm simply holding my guard back for the photo.

To make the spline itself run another piece of wood through your router setup but adjust you bit depth to twice the size. This should equal the width of the spline you need. For the part I chose a smaller 6" wide piece I had in my stock just to reduce waste and make it easier to handle. Set the fence and blade height to trim off the lower edge of the router cut (left). If done correctly, you should be left with a perfect spline insert (see inset on left). If not you may need to adjust and rerun the board but it should be very close the first time.

The last step is to glue it up and clamp it (see right). Run glue into the channels of both panels. I like to puddle up some glue on a scrap piece and use my fingers to butter some glue onto the surface areas that will be joined too. This will result in less glue waste and more importantly less glue seeping out of the joint onto the panel surface. Believe me, this will cut down on sanding time for you later. Clamp it along the width first (orange clamps) using bar clamps to pull the joint together. Next, use some lengthy scrap wood to clamp down the ends (blue clamps). This will keep the two panels parallel and flush during the drying. It is important to make sure no glue is seeping from the joint before this last step. Otherwise, you'll have an unwelcome additional wood block permanently glued to your new panel.

After 24 hours drying you'll have a super strong extra wide panel to do with what you will. I'll follow up later with what I did with my splined joint panel.