How to Make Stronger Miter Joints with Splines
A clean, tightly fitted miter joint is an attractive way to join the corners of a box, cabinet, or other woodworking project. The angled joint conceals the end grain and creates a continuous appearance around the corner.
But miter joints have a couple of drawbacks.
First, the angled faces can slip out of alignment when you apply clamping pressure. And second, the end-grain surfaces don't provide the strongest glue joint on their own.
Fortunately, there's a simple solution: add a spline to the miter joint.
A thin spline fits into matching grooves cut in the two mitered faces. Even a spline that's only as thick as a saw kerf helps keep the parts aligned during glueup while providing additional long-grain gluing surfaces.
Best of all, you can cut the grooves and make the splines with your table saw.
Why Add a Spline to a Miter Joint?
A spline is a thin strip of wood that fits into matching slots cut in the mating pieces.
When the joint is assembled, the spline bridges the two halves of the miter.
This provides two important benefits.
Better alignment. The spline helps prevent the angled faces from sliding out of position while you're applying glue and clamps.
A stronger glue joint. The spline adds long-grain gluing surfaces, reinforcing the miter and helping the corner resist separation.
The spline can also become a decorative feature if it's left exposed or made from contrasting wood.
For this technique, however, the spline is concealed within the mitered faces.
Step 1: Cut the Miters and Lay Out the Spline Slots
Start by cutting the mating 45° miters on your workpieces.
Before cutting the spline grooves, decide where the spline will be positioned and how deep the slots should be.
I like to locate the groove near the heel of the miter, where there's more material to support the cut.
For 3/4"-thick stock, I position the groove about 1/8" from the edge, as shown in the illustration.
Locating the groove near the heel allows you to use a wider spline without weakening the thin outside edge of the miter.
As a general rule, avoid cutting the slot more than about halfway through the thickness of the workpiece.
The goal is to create a deep enough groove for a strong spline while leaving plenty of surrounding wood.
Step 2: Cut Matching Spline Grooves at the Table Saw
With the miters complete, set up the table saw to cut the spline grooves.
The rip fence establishes the location of the slot.
Adjust the distance between the blade and fence so the tip of the miter rides against the fence while the blade cuts the groove in the desired position.
Once you've established the setting, lock the rip fence in place.
Support Narrow Workpieces with a Miter Gauge
For narrow workpieces, I use the miter gauge equipped with an auxiliary fence.
The auxiliary fence supports the workpiece and backs up the cut, helping reduce tearout.
As you make each cut, keep the workpiece flat against the saw table and the tip of the miter in contact with the rip fence.
Consistent positioning is essential because the grooves in the two mating pieces need to align perfectly.
Use a suitable hold-down or push block to keep your hands clear of the blade. Check that the fence and miter gauge arrangement allows the workpiece to move freely without binding or trapping an offcut.
Once the grooves are cut, you're ready to make the spline.
Step 3: Make a Crossgrain Hardwood Spline
For narrow hardwood assemblies, I prefer using solid-wood crossgrain splines.
You could make a spline by simply ripping a thin strip from the edge of a board. But a long-grain strip can create a problem.
If the finished joint is placed under stress, a thin spline with grain running lengthwise along the joint may split along its grain.
A crossgrain spline avoids that weakness.
With the grain oriented across the joint, the spline is better able to resist splitting in the direction that could cause the miter to fail.
There's also an appearance benefit. If the spline is visible at the edge of the joint, its end grain more closely matches the surrounding mitered pieces.
Cut the Spline from a Hardwood Blank
The illustration below shows how to make crossgrain splines from a short hardwood blank.
Prepare the blank according to the drawing, keeping the grain direction oriented across the finished spline.
Use the table saw to cut thin spline pieces to fit the grooves.
Make a test spline first and check its fit in the slots.
The spline should slide into the grooves without excessive force, but it shouldn't be loose.
If necessary, lightly sand or plane the spline to achieve the correct thickness.
Step 4: Glue and Assemble the Splined Miter Joint
Before applying glue, dry-fit the mitered pieces with the spline installed.
Make sure the corners close tightly and the spline doesn't bottom out in either groove.
A little clearance at the bottom of the slots allows room for excess glue.
Apply glue to the mating mitered faces and the spline grooves, then insert the spline and bring the joint together.
The spline should help keep the two halves aligned as you apply clamping pressure.
Use enough pressure to close the joint without forcing the parts out of alignment.
After the glue cures, remove any squeeze-out and trim exposed spline material flush if needed.
Tips for Stronger Splined Miter Joints
A few details make a big difference when cutting and assembling these joints.
- Cut accurate miters first. A spline won't compensate for poorly fitted 45° cuts.
- Keep the grooves aligned. Use the same fence setting for both mating pieces.
- Locate the groove near the heel. This leaves more material around the spline.
- Don't cut too deeply. Avoid weakening the workpiece with an oversized groove.
- Use crossgrain hardwood splines. Proper grain orientation helps prevent the spline from splitting under stress.
- Test the fit before glueup. The spline should fit snugly without preventing the miters from closing.
A Simple Way to Build Stronger Mitered Corners
Miter joints are an attractive choice for boxes, cabinets, and furniture projects, but they can be frustrating to assemble and aren't especially strong on their own.
Adding a spline solves both problems.
The spline helps keep the mating pieces aligned during glueup and adds gluing surface to reinforce the corner.
And because the slots and splines can be made at the table saw, the technique doesn't require complicated equipment.
The next time you're building a mitered box or case, try adding a spline. It's a small extra step that can make a big difference in the finished joint.
