This project was built and designed by David from dk builds. It’s a compact, easy-to-make jig that helps woodworkers cut precise spline slots to reinforce mitered joints.
Miter joints look clean and modern, but they’re known to be structurally weak unless reinforced. This jig makes it simple to cut spline slots accurately and repeatably, strengthening frames and boxes while adding visual interest with contrasting wood inserts.
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Why Use a Spline Jig?
Adding splines is a smart way to improve both strength and style. A spline reinforces the joint by adding material across the weakest part of the miter, helping to prevent splitting or separation over time.
On top of that, splines offer a decorative bonus. Using a contrasting wood like walnut or maple can highlight the corners and give your project a refined, custom look without extra effort.
Planning the Jig Dimensions
David starts by measuring his table saw fence to figure out the size of the jig. The key is to use 3/4-inch plywood that will ride securely along the fence without rocking or binding.
While the exact length of the jig can vary, around 14 inches is a solid choice for handling most small to medium-sized projects. The size gives you control without making the jig bulky or awkward to store.
Cutting and Assembling the Jig Frame
The base structure is made from cutoffs of plywood, rough cut with a circular saw and refined with a table saw for clean edges. The jig includes two side pieces that ride along the fence and a front face angled at 45 degrees.
David temporarily secures the pieces with brad nails to hold everything in place during assembly. Once aligned and squared up, he drives in screws for long-term durability and clamps everything down to ensure tight joints.
Building the Angled Front Support
The most important part of the jig is the 45-degree face that holds the workpiece at the right angle for cutting splines. David marks the angle carefully using a speed square and drafting triangle, then attaches the face with glue and brad nails.
Cleanup is essential at this stage, especially any glue that might interfere with how the workpiece sits or slides. The result is a stable platform that keeps picture frames and boxes in the perfect position for cutting spline slots.
Fine-Tuning for Better Performance
To make the jig slide more smoothly across the saw table, David applies a bit of paste wax to the bottom and sides. This reduces friction and helps avoid jerky or uneven movement during use.
He also makes a few passes on the saw to trim any uneven edges from the bottom of the support pieces. These adjustments ensure the jig sits flat and functions predictably every time it’s used.
Testing the Jig with a Scrap Frame
David tests the jig by making a basic picture frame from scrap wood. Holding the frame securely against the 45-degree face, he runs it through the saw to cut the spline slots.
For the splines themselves, he uses leftover thin pieces like shims, which are both affordable and effective. Once glued in place and trimmed flush, the splines add strength and a polished look to the frame.
Versatile and Beginner-Friendly
This jig is a great addition to any shop, especially for those who work with mitered corners. It’s compact, easy to build, and doesn’t require fancy tools—just plywood, glue, nails, and some careful measuring.
Even beginner woodworkers can follow the process and end up with a functional jig that pays off across multiple projects. Whether you’re building frames, boxes, or small cabinets, this jig is worth the time.
Final Thoughts
David’s design focuses on simplicity and functionality. It’s not overbuilt, and every part has a clear purpose, making it ideal for repeat use.
The jig proves that thoughtful design doesn’t have to be complex. With just a few offcuts and a careful layout, you can improve your joinery and add strength and beauty to your projects.
Please support David by visiting his Etsy Store and buying his plans.