Splined Miters

Create stronger corners by partnering your tablesaw or router with one of these simple splining jigs


Providing registration and mechanical reinforcement to mitered corners in frames, small boxes, and large cases.

The trouble with a basic miter joint is that it’s more beautiful than brawny. On its own, an end grain-to-end grain glue joint is inherently weak. It needs backup.

Enter the spline. Inserting a strip of wood, plywood, or other material (such as hardboard), into slots cut on the miter’s opposing faces reinforces the joint and helps keep parts aligned during assembly.

These days, the biscuit joiner is the go-to tool for splining, but there are better ways to skin (or spline) the cat. By using your tablesaw or router, you can make full-width splines that can add strength and detail to your design. Investing a little time to make the trio of jigs shown here will expand your joint-making repertoire by enabling you to produce perfect spline slots with either machine.

Making the Cut – Blade or Bit?

Spline slots can be cut with either a blade or bit, but a flat-bottomed cut is essential for a clean-looking joint. Here’s what you’ll need to get started.

Tablesaw: Several manufacturers offer special flat-topped, joinery-grade FTG blades for around $150, but a good ATBR (Alternating Top Bevel with Raker) blade can get the job done for about $70.

Router: Slot-cutting bits range from $30 for a single, fixed-width cutter, to $80 for a multi-cutter set. Cutters are available in widths as thin as 1⁄16", an advantage for small projects. Bearing-guided bits permit you to take the tool to the work, but even with larger bits, the slot depth will max out at 1⁄2".

Twin-Faced Tablesaw Jig for Splining Frames

When cutting spline slots, a standard tenoning jig is ineffective because it demands that half the slots must be cut with the workpiece’s show face against the jig, and half with it facing away. If the slot isn’t perfectly centered, the spline joint will be misaligned.

Designed to straddle the rip fence, this twin-faced jig ensures perfect slot alignment, even when making offset slots. A 4 × 7" hold-down board keeps the workpiece from sliding and serves as a backer when slotting the right-hand miters.

Set up the first slot. Position the right-hand miter against the face panel show face out, and clamp the hold-down to the jig. Slot all of your right-hand miters before changing the setup. (To eliminate the chance of overcutting the slot on the return stroke, remove the workpiece before retracting the jig.)

Block prevents blowout. To set the jig to make the left-hand miter slots, simply turn the jig around on the fence. Positioning the hold-down so that it covers the top end of the slot ensures a clean exit.

Aligned splines. Again with the show face out, cut the slots on the remaining (left-hand) miters. After cutting the slots, cut the spline to fit.

A Simple Sled for Table-Routed Frames

A router table splining sled may not be as versatile as its tablesaw counterpart, but this jig is well suited for smaller workpieces, and easy to knock together when the need arises. Superior to a simple angled pushblock, the plywood base registers against both fences, eliminating the chance of tipping your workpiece into the bit. In addition, the fence-mounted clamp fixes the work against the fence to prevent tearout.

Use a bar to set the bit. Setup bars offer an accurate means of setting the bit without squinting to read numbers. After setting the height, position a brass block as shown, and adjust the fence so that the bit’s carbide tip grazes the test block.
Rout the rights. To assemble the jig, register the edge of the base and the end of a sacrificial fence against your router table’s fence and tape the two together. To rout the right-hand slots, slide the sled past the bit.

Finish with your left. To ready the jig to rout the left-hand miter slots, rotate the sled and reposition the toggle clamp. The clamp’s vertical locking handle allows you to put clamping pressure closer to the cut.

A Multipurpose Jig for Case Miters

When cutting spline slots on wide case pieces at the tablesaw, you may be able to set the saw blade perpendicular to the face of the miter, lay the work flat on the table, and push the panel over the blade. For smaller pieces however, a jig can be a big help.

What makes this jig worth building is that it can be outfitted to work at the tablesaw or router table. In either operation, the jig registers the workpiece solidly on the face of the miter, instead of on its crushable tip.

Mitering and splining at the tablesaw.

One big advantage to using this jig at the tablesaw is that it enables you to cut both miters and spline slots without fussing with your blade’s bevel angle.

After mitering, simply flip the stock so that the opposite face rests on the ramp, lower the blade and adjust the rip fence. For strength, position the slot close to the inside edge, or root, of the miter.

Mighty miterer. Guided by the rip fence, the angled base enables you to cut perfect miters without blade adjustments. The toggle clamps keep the stock secure and fingers safe.
All set to spline. To cut the slots, flip the stock, and adjust the fence and blade height.

Two ways to rout splines.

With minor modifications, this jig can rout spline slots in both large and small case pieces on the router table. Attaching a stop to the ramp turns the jig into an angled sled. Adding an auxiliary panel to the ramp and clamping the base to the router table creates a chute that can help keep long mitered edges registered against the router table’s fence.

Slide the jig. For smaller case pieces, attach a stop to the ramp, then hold or clamp the stock in place and feed the ramp past the bit.
Or slide the stock. For longer splines, attach an auxiliary face to the ramp and clamp the base to the router table. 

Splines–Simple or Strong?

Woodworkers will agree that a properly-sized spline should fit snugly, while allowing the joint to close completely. How to cut the strips seems to be a source of some debate.

For maximum strength, a spline’s face grain should run perpendicular to the face of the miter. To do this, use a tenoning jig and slice the strips from a piece of stock that’s been dressed flat and squared on both ends. Then, saw them to length using a miter gauge and stopblock.

To simplify spline making, some settle with ripped strips. Ripped strips offer registration, but they are more likely to snap if roughly handled. If your project requires long splines and you’re pressed for time, consider sizing the slot to fit a plywood spline. If the joint is visible, cap the ends of the slots with solid wood.

Super-solid splines. Using a tenoning jig to hold the workpiece on end, slice the splines from the outer faces. By rotating the board and flipping it end for end, you can quickly cut four splines to the same thickness.
Quicker but weaker. Ripping offers an efficient means of producing long splines, but the installed strips can split along the grain line.
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