Splined Miters

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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.

Splined Miters


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

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.

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.

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.

Splined Miters

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.

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.

Splined Miters

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.


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