In joinery, a spline is a simple yet effective method for reinforcing and aligning two pieces of material joined edge-to-edge. A spline is a strip of wood, plywood, or other material (such as Masonite), inserted into matching grooves or plows, along the edges of two boards. This creates a strong, hidden joint suitable for various woodworking applications.
Understanding the Spline Joint
The spline joint works by cutting corresponding grooves (often called "plows" in older terminology) into the edges of the two boards being joined. These grooves are typically centered on the edge thickness of each board and run along the length of the joint. The spline, a thin strip cut to fit snugly into these grooves, is then coated with adhesive and inserted into the assembled joint.
How Splines Work
When the two grooved boards are brought together with the spline inserted and glue applied, the spline serves two main purposes:
- Alignment: It perfectly aligns the two board faces, making glue-up easier and ensuring a flat surface.
- Strength: It significantly increases the surface area for glue adhesion and provides internal structural support against racking or twisting forces that could otherwise cause the joint to fail.
Materials Used for Splines
Splines can be made from a variety of materials depending on the application and desired strength. Common materials include:
- Solid Wood: Often the same species as the main project for aesthetic consistency, though a harder species can sometimes be used for added strength.
- Plywood: Provides cross-grain strength which can be beneficial in certain situations.
- Masonite or MDF: Economical options, but generally less strong than wood or plywood, best suited for alignment rather than maximum strength.
Common Applications of Splines
Spline joints are versatile and can be found in numerous woodworking projects:
- Panel Glue-Ups: Reinforcing the edge-to-edge joints of boards used to create wider panels for table tops, cabinet sides, or shelves.
- Mitered Corners: Adding significant strength to miter joints in frames, boxes, or trim.
- Edge Joining: Connecting boards along their length for simple extensions or structural elements.
Benefits of Using Splines
Opting for a spline joint offers several advantages:
- Enhanced Strength: Provides considerably more strength compared to a simple butt joint glued edge-to-edge.
- Precise Alignment: Ensures accurate and consistent alignment during assembly, reducing the need for complex clamping setups.
- Relative Simplicity: Once the setup is made (often using a router or table saw with a dado blade), cutting the grooves and splines is relatively straightforward.
- Hidden Reinforcement: The spline is entirely concealed within the joint, maintaining the clean look of the joined pieces.
Practical Considerations
When using splines, ensure:
- The grooves are cut consistently on both pieces and are centered accurately on the edge.
- The spline fits snugly into the grooves – not too tight to prevent full insertion, and not too loose to cause play.
- Generous glue is applied to both the spline and the inside of the grooves for maximum adhesion.
Splines are a fundamental technique in joinery, providing a reliable way to create strong, well-aligned joints that are both durable and aesthetically pleasing.
