Build a Table Saw Spline Jig for Mitered Boxes
Miter joints are a great choice for woodworking projects like keepsake boxes, small cabinets, and decorative cases. They hide the end grain and create clean, continuous corners that really show off the wood.
The downside is that a plain miter joint can benefit from a little extra strength. One of my favorite ways to reinforce it is by adding wooden splines, or keys, across the corners.
These splines add long-grain glue surfaces to the joint while creating an attractive decorative detail, especially when you use a contrasting hardwood.
The challenge is cutting accurate slots in an assembled box. You need to hold the box securely at 45° while passing the corner over the table saw blade.
That's where this simple table saw spline jig comes in handy.
How the Spline-Cutting Jig Works
The jig holds an assembled box at a 45° angle so you can cut slots across the mitered corners.
Its construction is straightforward. Two plywood sides support a carriage that holds the box in position during the cut.
The carriage keeps the workpiece at the correct angle, helping you make consistent slots from one corner to the next.
A replaceable hardboard backer provides support directly behind the cut. It acts like a zero-clearance insert, helping reduce tearout as the blade exits the wood.
This combination of support and zero-clearance backing is especially useful when working with small boxes or delicate hardwoods where a clean cut matters.
Add an Adjustable Toggle Clamp
Holding the box securely is one of the most important parts of cutting spline slots.
To keep the workpiece stable, I added a toggle clamp mounted on an adjustable base.
The adjustable mounting arrangement lets you position the clamp to suit the box you're working on.
Once the box is positioned in the carriage, the clamp holds it firmly in place while you guide the jig through the cut.
This provides better control than trying to hold the box by hand, especially when cutting multiple spline slots.
Cutting Slots for Miter Splines
With the jig assembled, cutting the slots is a straightforward process.
Start by setting the blade height to produce the desired slot depth without cutting through the inside corner of the box.
Position the assembled box in the carriage and secure it with the toggle clamp.
Then guide the jig through the blade to cut the first spline slot.
Repeat the process at the remaining corners, using the same setup to keep the slots consistent.
If you're adding multiple splines to each corner, lay out their positions carefully so the spacing is uniform.
After cutting the slots, fit matching hardwood splines, glue them in place, and trim them flush once the glue dries.
WOODSMITH TIP: Use Contrasting Wood for Decorative Splines. Splines don't have to be purely functional. Using a contrasting hardwood can turn a reinforced miter into an attractive design feature. For example, walnut splines in a maple box or maple splines in a cherry box create a subtle detail that highlights the joinery.
WOODSMITH SAFETY TIP: Keep the Jig Under Control. Make sure the jig is guided securely during the cut and that the box is firmly clamped. Keep the clamp and other hardware clear of the blade path, and use a suitable guide arrangement rather than pushing an unsupported jig freehand over the blade.
Stronger Joints and Cleaner Corners
Adding splines is a simple way to improve both the strength and appearance of mitered box joints.
And with this shop-made jig, you can cut the slots more consistently while keeping the box supported and securely clamped.
The plywood carriage, adjustable toggle clamp, and replaceable hardboard backer make it a useful accessory for anyone who enjoys building decorative wooden boxes.
Add Decorative Dovetail Keys to Miter Joints
Want to take your box joinery a step further? Dovetail-shaped keys add strength and a distinctive decorative detail to mitered corners.
Learn how to use a router to cut the slots and install contrasting hardwood dovetail keys.
LEARN HOW TO MAKE DOVETAIL KEYS → A Different Way to Reinforce Mitered Corners
