How to Build a Spline Jig in an Afternoon (Free Plans Included)

If you've ever looked at a beautifully crafted box or picture frame and noticed the small contrasting strips reinforcing the corners, you've spotted splines. They're one of the easiest ways to take a plain mitred joint and turn it into something that looks like real craftsmanship. The catch? You need a safe, accurate way to cut those slots on a table saw — and that's exactly what a spline jig is for.

In this post I'll walk you through how to build my spline jig from start to finish. It's a beginner-friendly project that uses a single offcut of plywood, a handful of screws, and an afternoon in the workshop. I've also made the full digital plans completely free, so you've got everything you need to follow along on the bench.

What is a spline jig and why do you need one?

A spline jig holds a mitred frame or box at 45° as you push it across the table saw blade. The blade cuts a clean, perfectly aligned slot across the corner. You glue in a thin strip of contrasting timber, trim it flush, and you've got a spline.

Splines do two important things:

• Strengthen the joint. End-grain glue surfaces in mitres are weak. Splines add long-grain to long-grain contact, which holds far better.
• Look brilliant. A walnut spline in a maple frame, or any contrasting hardwood in a softwood box, instantly elevates a project.

Without a jig, cutting splines is fiddly at best and dangerous at worst. With this jig, it takes a few seconds per cut and the results are repeatable every single time.

The finished jig

The jig has four parts: a flat base that runs along the table saw, two side pieces that hold everything square, and two angled pieces that form the V-shaped cradle for your workpiece.

Total dimensions:

• Height: 186mm
• Width: 150mm
• Length: 362mm

It's compact enough to store on a shelf and big enough to handle most picture frames and small boxes.

What you'll need

Materials

• One piece of 18mm plywood, 800mm × 320mm
• Wood glue
• 8 wood screws

That's genuinely it. If you've got offcuts of 18mm ply lying around the workshop, this is the perfect project to use them up. Mine was built almost entirely from scraps.

Tools

• Table saw (or mitre saw for the 45° cuts)
• Drill
• Right-angle clamps, or any way to hold pieces square
• Tape measure and pencil
• Square

Cut list

Part	Length	Width	Thickness	Quantity
Base	340mm	150mm	18mm	1
Angle piece	480mm	150mm	18mm	1
Side piece	290mm	150mm	18mm	1

Cut all three pieces from your single 800 × 320mm sheet of plywood. The angle and side pieces will each get cut in half later, which is why their lengths are roughly double what you see in the finished jig.

Step 1: Mark and cut the mitres

Both the angle piece and the side piece need to be split into two halves with mating 45° mitres on the inside ends.

For the angle piece (480mm long), find the centre at 240mm and add 9mm — mark your cut at 249mm.

For the side piece (290mm long), do the same thing — centre at 145mm plus 9mm — mark at 154mm.

The 9mm offset is half the thickness of your plywood. It keeps the geometry right when the two halves come back together to form the V, so the apex sits where it should rather than off to one side.

Now cut both pieces along your marks at 45°. I used a table saw, but a mitre saw will do exactly the same job. Take your time on these cuts — the accuracy of the V cradle depends entirely on these mitres being clean and consistent.

Step 2: Attach the side pieces to the base

Apply wood glue to the bottom flat edge of one side piece and stand it on the base, flush with one end. Hold it square with right-angle clamps.

Drill two screws up through the underside of the base into the side piece. The screws are temporary — they hold everything in place while the glue sets, but they come out at the end. You don't want screws sitting anywhere near the saw blade when you start cutting splines.

Repeat on the other end of the base. You should now have a low U-shape: base on the bottom, two side pieces standing tall on either end.

Step 3: Attach the angle pieces

Now for the V. Add glue to the top edges of the side pieces and to the mitred bottom ends of the angle pieces.

Stand the two angle pieces up so their mitred ends meet at the bottom centre of the base, forming an inverted V. The outer corners of the angle pieces should land flush with the top corners of the side pieces. Press everything down into the glue.

This is the critical alignment step: check that the inside angle where the two angle pieces meet is exactly 90°. Use a square. If it's even slightly off, every spline you cut with this jig will be off too. Make adjustments before the glue grabs.

Once you're happy with the angle, drill two screws through each angle piece into the side piece below it. Leave the whole assembly clamped or weighted down until the glue is fully cured — overnight is best.

Step 4: Remove the screws and finish up

Once the glue has properly dried, back out every single screw. The glue is doing all the work now, and you really don't want a hidden screw anywhere near a spinning blade.

If you want a smoother slide across your table saw, rub some beeswax or paste wax on the bottom of the base. Apply a coat, let it soak in for about 15 minutes, then buff off the excess. Let it dry completely before using the jig.

That's it. Your spline jig is ready to use.

Using your new spline jig

To cut splines on a finished frame or box:

1. Glue up your mitred frame or box and let the joints cure fully.
2. Sit the assembled frame in the V cradle of the jig, with one corner pointing down toward the saw blade.
3. Set your blade height to the spline depth you want — usually about two-thirds of the way through the corner.
4. Push the jig slowly and steadily across the blade.
5. Rotate the frame, drop the next corner into the V, and repeat for all four corners.
6. Glue in your spline strips, let dry, then trim flush with a flush-cut saw or chisel.

You can vary spline thickness by using different blades, or cut decorative double or triple splines by making side-by-side passes.

Download the free plans

If you'd rather work from a clean printed PDF on the bench than scroll on your phone, you can download the full plans free here. They include every measurement, diagram and step laid out the same way I work through them in the shop.

If you build one, I'd love to see it — tag me on Instagram @whiteravenwoodworking so I can share it.

Happy building.

— Josh