How To Make A Spline Joint

Learn how to make a spline joint, just one of the nine essential joints you should consider for your project. WOOD magazine's Craig Ruegsegger shows you not only how to make the joint, but why this joint might be the best choice for your project.

You can take this same concept of splines for other types of joinery as well. For example, here's two boards joined end to end. There's a slot in each, and we've used a hardboard spline in this case. You can take two boards, fasten them end to end. Here's an edge-to-edge joint where you're taking narrower boards, making a wider panel by cutting matching slots and putting a spline in between. Spline joints can also be used when you're building cabinets. If you have, for example, a butt joint like this where a shelf would join a cabinet side, or the top of a cabinet would join the side, you can again slot these pieces and add a spline to reinforce the joint. Here, where you're putting end grain to face grain, a spline would be a good idea to add some face grain gluing surface to that joint. 

To do a spline joint like this, you need to cut matching slots in each piece. And there are several ways to do that. You can get a variety of slot-cutting bits. These come in different widths. You can use these either in a handheld router or at a router table. I'll show you how to do each of those. You can also cut those slots on the table saw. And we'll discuss that too. 

First, let's go with the handheld router. I’ve set up a router mat here to keep the pieces from sliding around. Take a couple of these boards, and we'll cut matching slots in the edges so that we can edge-join them like we did these two. All you have to do is put the piece on the bench, extending a little bit over the edge, so the router bit is going to clear. I've got the router bit adjusted so it cuts a slot that is just slightly off center of the thickness of this piece. First, rout a slot on one face, flip it over, and do the same. You end up with a centered slot. Let's see if that fits our spline stock. Terrific. That fits like a glove. 

Now cut the slots in the mating piece. There we go. Nice friction fit there. The mating piece goes on. You can see how nicely that's aligned on both faces. Can't even feel that seam across there. 

Working with a slot cutter bit and a handheld router works especially well with larger pieces. My preference though is to mount the slot cutter bit in the router table. And I'll show you how to do that.

I've positioned the router table fence so it aligns with the bearing of the bit. That way the piece moves smoothly past the bit, while the bit cuts at full depth. A push pad grips the stock and moves it safely past the bit and keeps your fingers a safe distance away. Flipping the piece over for a second pass centers the slot on the thickness. 

Slots can also be cut on the table saw. First, set the blade height. Then position the rip fence to cut just slightly off center. Then make a pass with each face against the rip fence to create the centered slot.

Now if you want to groove the end of a workpiece at the table saw, you want to stand it on end. This can be a little dicey. The tall piece can rock as you move it along the rip fence. So instead, make a saddle jig like this one. This is just some MDF. It's real simple to make. The first thing you do is cut a piece that's exactly as wide as your rip fence. Cut two pieces that go on either side, and then glue them together right around your rip fence so that you have a nice, sliding fit. Then glue an upright on the back here. Make sure this is 90 degrees to the saw table. Now this extra height here, and this stop back here, gives you extra support so that when you're pushing this across the saw table, you're going to have a much stabler workpiece. 

But you do have to consider the thickness of the jig and reset the fence to make your slot cut. There. We've got a nice groove. And the end of that workpiece made it safely with this jig. It kept things from rocking back and forth. 

To slot the end of a workpiece at the router table, back it up with a wide scrap. This provides a broad stable surface against the fence so the piece doesn't wobble, and it also prevents tear-out on the back edge where the bit exits.