A spline joint is created when a wooden spline is inserted and glued into a slot or groove that has been cut in another woodworking joint, usually a butt, edge or mitered joint.
The spline serves to reinforce the joint and help keep the two sections aligned with each other. This small enhancement adds considerable strength to whatever joint it's applied to.
The spline itself can be made from plywood, hardwood or the same material as the joint being strengthened. For the ultimate in strength, the grain of natural woods should be oriented so that it's running across the joint in the workpiece.
Splines should never be forced into the grooves, which could cause them to distort or split. Rather, they should slide in easily, but without any side play, to allow enough room for glue to ensure a solid joint.
The width of the spline when used between two boards, whether miter or edge joint, should be slightly less than the width of both slots, to ensure the joint will close fully.
The miter spline is great for reinforcing picture frames, mirror frames and even cabinet face frames that utilize mitered corners. Small decorative boxes can use contrasting colored splines with mitered corners for visual effect, as well as reinforcing the joints.
Spline between miters.
Spline in slot cut in corner.
Both styles of miter spline above can be made using a table saw or router table to cut the slots. A simple jig can be made to cut the slots on the table saw, while a slotting bit can be used with the router table.
The splines are generally not more than a third the total thickness of the workpieces. It's best to cut the spline oversize and then trim and sand smooth once the glue has dried. A dry fit before glue up is also recommended.
Miter spline joints look great when using contrasting colors of wood. A light colored spline set in a darker wood will draw the eye and highlight the joint. An ebony or black walnut spline set in a lighter colored wood will do the same.
Edge splines are often used to make larger panels out of several narrower boards. Although an edge to edge glue up is already fairly strong because of the long grain to long grain gluing surfaces, splines offer an extra measure of support as well as aiding in alignment of the joint.
An edge spline running the length of two boards.
The most common edge spline joint has the groove and spline running the full length of the boards. The spline is visible when the joint is viewed from the ends.
If appearance is not important, plywood makes an excellent spline for this purpose. If appearance is important, the spline can be made from solid wood to either blend in or contrast.
The stopped edge spline is much like a stopped dado joint in that the groove is stopped short of the end of the board, although on both ends in this case.
A cleaner look with a stopped groove.
Mainly used for the tops of hardwood tables and other fine furniture, where the craftsman would like the extra strength from the spline, but without it being visible.
Although the groove can be cut on the table saw, it is probably easier and less work to rout it out on a router table with a slotting bit.
The corners of the slot can either be chiseled square to accept the spline or the spline's ends could be rounded over with a sander. A plywood spline can be used without it affecting the appearance of the joint.
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