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Build a Jig for Hidden Spline Joinery
Tools and Materials
We've covered the use of exposed splines in box construction before, but what happens when you don't want to interrupt a nice continuous grain pattern around all four corners of a box or small cabinet project with visible splines?
The answer is to embed splines within the miter joint, where nobody can see them.
This jig's design comes to us from Arkansas box-maker Doug Stowe. It's simple to construct and use, and is very effective at strengthening miter joints that might otherwise be a bit on the weak side. It's designed to hold workpieces with mitered ends in such a manner that the miter rests flush against your router table's top. The jig is then guided paste a straight cutting bit, which cuts long spline slots in the end grain of each miter that are perfectly registered with one another when building boxes or small cabinets.
A Simple Plywood Base with a Scrap Wood Fence
The jig is constructed by cutting a narrow (in this case approximately 9-in.) plywood scrap in half, with the tablesaw's blade angled at 45-degrees. One of these pieces is then flipped over and the face of this piece is then glued to the opposing miter. Once that's out of the way, it's simply a matter of attaching a simiple fence to one side of the jig and you're ready to roll.
Using the Jig
With a small straight-cutting router bit installed in your router table (1/8-in. to 3/16-in.) the bit height is adjusted so as not to protrude all the way through the work piece. After that, it's just a matter of installing two stop blocks to define the start and stop points of your slot.
For even more information on box-building techniques, be sure to catch Doug Stowe's video series on Basic Box-Making, available at our sister site, FineWoodworking.com. In this 14-part series, Stowe covers this technique and many others.
How to Make
Cut the Jig Base Components: Using a simple tablesaw sled for cutting 45-degree miters, cut a small piece of 3/4-in. plywood (the size depends on the work pieces you wish to join together using the jig) in half. These two pieces will later mate with one another to form the basic jig.
Cut Material for a Fence: While you've got your tablesaw set up for making 45-degree cuts, cut a small piece of scrap about 6-in. in length. It should have one end mitered to 45-degrees.
Glue and Fasten the Jig Base: Glue the two pieces of plywood together to form a "V." You can use tape to help secure the joint while you fasten it with staples or brads. Take note of the orientation of the two pieces.
Fasten the Fence: Glue and fasten the small fence piece cut earlier, so that it's mitered end will rest flush with the surface on which it is set.
Adjust the Bit Height: For my spline slots, I used a 1/8-in. straight-cutting bit. You could also beef it up a bit and use a 3/16-in. bit. To set the bit height, position your workpiece in the jig and rest it beside the bit. Adjust the height so that you've got enought glue surface to glue in the spline, but not so high that the pit protrudes through the workpiece. This is also a good time to adjust the fence on your router table (seen in the background).
Position and Clamp Stop Blocks: In using this joint for a box or similar project, you want to produce a "stopped cut" at the router table. This means that the slot should begin about 1/4-in to 1/2-in. in from either end of the workpiece. Use stop blocks on either end of the jig, clamped to the table, in order to start and stop your cuts for accuracty and repeatability.
Cut the Slots: With the workpiece held firmly to the jig, and against its fence, back the jig up against the right-hand stop block and slowly lower the jig and workpiece onto the spinning bit. Now run the jig from right to left until you reach the left-hand stop block. That's it. Your first slot is now cut. Repeat the process for all the other slots in your project.
Ready for Glue-Up: In a perfect world, this is what you'll be left with: slots that are perfectly matched and ready for splines and glue-up.
Glue-Up: With your splines ready to go, now it's just a matter of applying glue, inserting a spline, and clamping your project together. I cut my splines at the tablesaw. They're cut to match the width of the slot (1/8-in. thick in this case).
Job Done: Owing to the use of the jig and a couple of stop blocks clamped to the router table, these slots are perfectly registered to one another.