Biscuit joiner spline jig

Create strong, attractive mitre joints with splines.

Mitred corner joints, such as those on small boxes or picture frames present a poor surface for gluing; this is because you end up gluing end grain to end grain. Adding splines (feathers) on a mitred corner is a simple way to add some face grain glue surface to strengthen the joint. An added bonus is that the splines provide a decorative corner treatment, particularly if you use a different species of wood.

Making the Jig

• Cut all of the pieces in the material list to size, leaving tool guides (E) a little long for now. Left over ¾” ply is a good choice for this jig. Sand all parts before finishing.

• Locate the short fence (B) 12″ from one edge of the base (A) and the long fence (C) 8 ⅞” from the adjacent edge. Screw two gussets (D) to each fence. Drill ⅜” dowel holes on the bottom of the fences and gussets, and then, using dowel centers, transfer the dowel holes to the base plate.

• Drill dowels holes in the base and then dowel and glue the fence assemblies to the base.

• Cut a 45º bevel on the front of one of the tool guides (E) so they can butt up against the gussets. Place a square against the two fences and mark the intersection of the two fences (B, C). Place your biscuit joiner on the base at a 45º angle with the center mark on the tool at the point of intersection of the two fences and snug the tool guides against the side of the biscuit joiner. As the biscuit joiner is placed into the holder from the top, you may need to cut some slots and rabbets in the tool guides to allow for the various bits that project from the front of the tool. In my case, using a PC 557, a piece of ½” Baltic birch ply served as a temporary spacer between the two fences (B) and the biscuit joiner fence.

• Mark the location of the tool guides, remove the biscuit joiner and permanently mount the tool guides to the base using screws or dowels.

• Place the biscuit joiner on the jig and mark the front edge of the tool’s face on the tool guides. Attach the tool stops (F) to the front edge of the tool guides at this line to provide a positive stop for the joiner.

• Cut some ½” spacers from Baltic birch plywood to place under the tool to act as spacers when cutting slots up the sides of a box. Size them to fit between the tool guides to suit your tool.

Using the Jig

Begin by cutting the mitre joints for your project. Before you apply the glue to the mitre joints, ‘size’ the mitre edges. Mix some wood glue with a little water and then paint this on the exposed end grain to seal the surface. When the sizing is dry, apply glue as you would normally and clamp the work piece together using a band clamp. After the glue has dried, place the mitred work piece in the jig and then use the joiner to cut slots for splines in the corners. To create a row up the side of the work piece, raise the joiner using the spacers that you prepared earlier. When you reach the halfway mark, turn the work piece over and continue from the other edge. Use a thickness planer to prepare the feathers to fit the slots. Glue them in place (contrasting woods add visual appeal) and then plane or sand them flush when the glue is dry.