Box Spline Jig
Most woodworkers will tell you that adding splines to miter joints not only make the joints stronger, but they are also a beautiful visual element that gives the project a sense of refinement. Along the same lines, a lot of woodworkers struggle with making good spline cuts. This article and the accompanying video explains what I do to get perfect splines for my wood boxes.
Make a Jig
The first step to getting great looking splines is to make a jig that holds your wood box or tray. I recommend using sheet goods such as plywood or particle board verses solid wood because solid wood can move over the seasons and you want your jig to be stable and predictable. I used particle board to make the majority of the parts of my jig.
Base of the Jig
Start with making the foundation of your jig, which includes the runners and the base plate of the jig. Runners allow your jig to slide across the surface of your table saw by utilizing the T slots on the saw. Runners can be made from scrap wood or even plastic. Old plastic cutting boards are great for runners! I prefer to use a hard wood because they are hard enough to withstand the toll of being slid back and forth across the surface of the saw. Runners are thin and narrow enough that wood movement isn’t much of an issue in most woodworking shops, unlike if you made the entire jig out of wood.
Since the T slots on a table saw can vary from manufacturer to manufacturer, I can’t tell you how thick and narrow to make your runners. But I can say that you want them so that, when placed in the T slots, they sit just below the surface of your saw. They should be narrow enough to fit into the slots, but not wiggle around. You want them loose enough to slide forward and backward, but not wiggle side to side. I cut my runners to approximately 11 inches long.
The base of my jig is made from 3/4 inch particle board. I cut the particle board into a rectangle that’s approximately 11 inches wide and 16 inches long. After placing my runners into the T slots, I aligned them and the particle board along the back edge of my table saw. I also used my table saw fence as another reference point. This will help to get the foundation of your jig square, but don’t worry about making it perfect. That step will come later. Once everything is lined up, glue and/or screw your particle board to the runners.
Wings of the Jig
The next step is to make two wings, which are the parts that hold the box while making your cuts. I used 1/2 inch particle board for mine, but 3/4 inch thick board would have worked too. I cut my boards to approximately 6 inches wide and 16 inches long.
The wings sit at a 45 degree angle to the surface of the table saw. To hold them in place, I made 8 wedges (4 for each wing). The wedges are basically triangles cut to 45 degrees. I glued the wedges to the underside of the wings. The point of the wedge was aligned to the bottom edge of the wing, and I used a square to ensure the wedge was straight. The wedges are glued evenly across the wings, ensuring that the center of the wing is left open. You don’t want a wedge there because that’s where the blade cuts. I glued the wedges to the wings and let both components dry before attaching them to the base of the jig.
Assembling the Jig
After the glue on the wings was dry, I attached them to the base of my jig. This is the point where we align everything up. I turned on my saw and pushed my jig base into the blade, but I didn’t cut all the way through the jig. You DON’T want to cut your jig in half; only cut it to a little past the center of the jig. Turn the saw off. Since the blade is sticking out of the top of the jig, it’s now possible to line up the back wing (the one closest to you) with the saw blade using a square. I put glue on the bottom of the wedges, lined up the wing to the blade, and let the glue dry. I didn’t bother clamping the jig while doing any of the glue-ups.
Attaching the second wing is really simple. Add glue to the bottom of the wedges and press the second wing up to the first one so that there is no gap where they meet. Congratulations, you have a jig! You can apply finish or wax to it if you want. You can round over the edges or add toggle clamps. This is your jig so modify it so it meets your needs.
Secrets to Perfect Splines
Building this jig alone won’t ensure that your spline cuts are perfect, but applying these two additional secrets will absolutely help you to improve your woodworking game.
Flat Tooth Blade
Perhaps the most common mistake that I see woodworkers make when cutting splines is using the wrong blade. Woodworkers typically fall into one of two camps: 1) They use a combination blade in their saw. A combination blade lets them make both ripping and cross cuts relatively smoothly, or 2) They switch between ripping and cross cut blades depending on the type of cut they want to make. Many combination and cross cut blades have angled teeth. If you use a blade with angled teeth to cut the notches for your splines then the cuts will have a V cut in them. That means you will have a gap when inserting the spline. You want to use a flat tooth blade such as a ripping blade. I use my 24 tooth count ripping blade to cut my notches and they come out perfectly crisp every time. And if you are hesitate to buy a special blade just for splines, know that I use my ripping blade for cutting grooves and for joinery such as rabbets. My ripping blade gets just as much use as any combination blade in my shop.
Zero Clearance Insert
Another common issue that woodworkers face when cutting notches for splines is tear out. The blade does a good job of cutting into their box, but when it exits the backside it also splinters the wood. This is caused when there’s not enough support behind the box. If you make a cut with your saw blade raised high and then you lower the blade back down, you are left with a kerf (the cut out area) that is bigger than the current height of your blade. That area is where your box will splinter when attempting to make spline cuts. To avoid splintering, stick a sacrificial board onto both wings of your jig. Now you have a clean surface to use. This eliminates the oversized kerf and eliminates the odds of splintering your box.
I hope that this blog helps you to make some killer spline joints in your next project. If so, leave a comment and let me know how they turned out. Getting perfect splines doesn’t have to be difficult. By making this spline jig for your table saw and applying my two tricks, you’ll be cranking out boxes that will make everyone envious of your skills.
