Splined Miter Joints

Greater strength, easier assembly, and an interesting look — three good reasons to add a spline to a miter joint.

Mitered face

Spline adds mechanical and gluing strength

Tightly mitered corners look great and are a real woodworking feat. But mitered corners have a weakness — literally. While a miter joint hides the end grain of the mating pieces, it also relies on end grain for its gluing surface. In solid wood, this doesn't produce a very strong glue joint. This, on top of the hassle of assembling this "non-locking" joint, can make using miter joints a bit of a challenge.

ADD A SPLINE. But there's an easy fix to this problem. I use a time-tested technique to add a spline across

Crossgrain spline won't split across joint spline

Grain direction of spline

NOTE: Spline slot and spline can be cut on table saw the miter joint, as shown in the photo above. A spline is simply a thin piece of wood fit into slots cut into the mitered faces.

This operation kills two birds with one stone. It adds mechanical strength and gluing surface, and when assembling the miters, the spline helps keep them aligned. An added bonus is that both parts of the job can be done on the table saw—creating the spline slots and cutting the splines to fit them.

SOME BASIC INFO. Before getting into the technique involved, it's helpful to know a bit about the "mechanics" of the joint. The drawing at left illustrates this basic information.

First, notice that the slot is cut near the heel of the mitered face. This allows you to cut a deeper slot to hold a longer spline. A good rule of thumb is to place the slot about Vg" from the edge of the heel.

The depth of the slot can vary depending on the thickness of the material. The goal is to maximize the length of the spline without weakening the workpieces. Most often I cut a slot that extends through about one half to two thirds the thickness of the workpiece.

Finally, the spline itself should be made with the grain running across the joint. This has two benefits. It keeps the grain direction of the spline and the workpieces consistent so wood movement won't be a problem. And more importantly, a crossgrain spline creates the strongest joint (see margin drawings on the opposite page).

SET UP THE SAW. Once the miters are cut on all the pieces, the next step is to cut the slots in the mitered

Splined Miter JointSplined Miter Joint

Finished width of

Mitre Joint Cutter

rip fence

Spline Joints With Table Saw

Weak Spline.

A long grain spline can split along its length.

Blank is slightly wider than lengthy of splines-

Push block

Thickness of spline

Note grain direction

Blank wakes four splines faces. When cutting the slots, you want to focus on two goals. They should be perfectly aligned across the joint and square to the mitered faces. The drawing on the right side of the opposite page shows how to set up the saw for the job.

The first step is to tilt the saw blade to 45°, if it isn't already. When you pass the workpiece across it with the mitered face oriented opposite to the tilt of the blade, you'll get a perpendicular slot.

Now you're ready to set up the rip fence. It's positioned alongside the blade to locate the slots accurately and ensure they line up. The tip of the miter simply runs along the fence as you make the cut. For this to work, the blade has to tilt away from the fence. So depending on the tilt direction of your saw, you may have to move the fence to the other side of the blade.

You can position the fence by using a mitered workpiece. I mark the location of the slot on one of the miters and then line up the marks with the saw blade. Just nudge the fence over until it touches the tip of the miter and lock it down.

You still need to adjust the blade height, but the only reliable way to do this is with test cuts. So at this point, just make sure the blade isn't too high before starting the cuts.

MAKE THE CUTS. There are two ways to feed the workpiece, depending on its size and shape. If the mitered edge is long, and the piece is narrow, I simply run it across the blade using only the rip fence as my guide. Otherwise, I always use the miter gauge.

Just be sure to keep the mitered tip tight against the fence and the workpiece flat against the saw table. A backup piece or auxiliary miter gauge fence will control chipout at the end of the spline slot.

This is the method I use most often. For a different take, check out the jig in the box below.

THE SPLINES. Once the slots have been cut, the final step is to make the crossgrain splines to fit them.

The spline is the workhorse of this joint, so sizing it correctly is important. It should slide easily, but snugly, into the slot. And if the ends of the spline will be exposed, you want them to fill the width of the slot completely when the two mitered pieces are assembled.

The two drawings below show you how to make the thin splines safely and accurately. The trick is to start with a short (along the grain) blank that's a bit wider than the length of the slots the splines need to fill. First, you'll cut the splines to thickness, as shown in Figure 1. You can get four splines from the blank by making a cut at each corner.

Next, I mark the length of the splines, reposition the rip fence, and cut them from the blank (Figure 2). Just be sure to cut them loose to the outside of the saw blade.

After checking the fit of the splines, you're ready for assembly. Once the glue dries and the splines are trimmed flush, the joints will surely stand the test of time. H

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