Spline-cutting Fence for your Router Table
Here's a handy method for beefing up long, mitered joints in jewelry boxes and the like. You can make slots for hidden splines with a straight bit, two stopblocks, and a simple support block.
Set your 1/8" straight bit to project 1/4" above the router table. Clamp an auxiliary fence to your router table fence, so that your workpiece won't slide into the bit-clearance notch. Set this fence the same distance from the bit's center as the thickness of your stock, or slightly farther. In the drawing below, we're cutting spline slots in 1/2"-thick pieces.
Miter-cut your box sides to length. Take the two ends of the box, or the front and the back, place them face-to-face, align the edges, and join them with cloth-backed, double-faced tape.
Bevel-rip a scrap piece at 45° to make a support board. Hold the taped-together assembly in the corner formed by the router table and fence, and use it to place the support board parallel to the fence. Clamp both ends of the support board to the router table.
Now, mark the ends of the planned slot on the workpiece. Use those marks, matched with the cutting edges of the bit, to set stopblocks on the fence to the left and right of the bit.
Turn on the router, hold the workpiece firmly against the fence, and lower it onto the spinning bit, as shown in the photo above. Keep the right side of the workpiece against the right-hand stopblock. Carefully slide the workpiece across the table to the left-hand stopblock, and raise it straight up the fence.
After cutting eight slots for a rectangular box, cut matching splines. Hardboard and plywood work great for this, or you can cut splines from the same wood used for the box. In that case, the grain of the splines should run in the same direction as the sides, to avoid problems with the wood splitting along grain lines.