Miter with Muscle

A mitered corner is one of the weaker joints in woodworking because it relies on gluing end grain to end grain. But there are good reasons to make a mitered corner. For one thing, it hides unsightly end grain. And wood grain can be made to wrap continuously around a mitered corner. Here are some ideas to help you make mightier mitered corners.

End grain looks and acts like the end of a bundle of tubes—some solid areas among a lot of openings—so it sucks up glue. Improve your chances of making a strong mitered joint with woodworking glue by generously brushing glue sizing—one part glue mixed with three parts water—onto the joint faces to seal them. After the sizing dries, glue the joint, as usual.

Epoxy works well for gluing mitered joints and doesn't require sizing. Beware of quick-setting epoxy if you are gluing several joints at once; allow enough open time for assembly.

Whichever adhesive you choose, always clamp mitered joints for the full cure time recommended by the glue manufacturer. Remember that cool temperatures increase cure times.

Reinforce a mitered joint quickly and easily with a pneumatic brad nailer or pinner. Nailer reinforcement works equally well on frames or boxes. Short, thin (23-gauge) pins reinforce joints in thin stock with less risk of splitting. A brad nailer works well in heavier stock.

But beware one major drawback: The thin nail might follow the grain and break out through the surface of smaller pieces. Prevent that by avoiding overly long fasteners and driving them as far from an edge as practical. Pinners, which drive a smaller, headless fastener, are ideal for thin, narrow parts. For the most secure joint, drive a brad or pin into the joint from each side.

Another benefit: You can pop these fasteners into the joint while the glue is still wet so the brads or pins hold the joint together without the need for clamps. Still, allow the adhesive to cure fully before handling the assembly.

Splines add face-grain gluing surfaces as well as mechanical reinforcement to strengthen a mitered joint. Outside splines serve as decorative elements, too, especially if you use a contrasting wood species. The grain for inside or outside splines must run across the joint for maximum strength.

Outside splines
Add visible splines to a frame corner after assembling the joint and letting its glue cure. Cut the spline slots using a simple jig, like the one shown at right, with a tablesaw. Center the spline slots on the thickness of the frame, cutting them to a depth of about two-thirds of the frame-side width.

Apply glue to both sides of the spline, and slide it into the slot. Apply the clamp pads to the the top and bottom faces of the mitered corner, as shown in photo. Glue the spline in the slot with its grain running across the joint, and clamp. Trim the spline flush with the frame after the glue dries.

You can reinforce and decorate box corners with outside splines the same way, modifying the jig dimensions and spline locations as necessary.

Inside splines

Unlike outside splines, you cut slots for hidden splines in a frame before you assemble the joint. Start by setting your tablesaw fence to center the blade exactly on the thickness of the stock. That's vital because you will slot one side of each corner with the front face against the fence and the other side with the back against the fence. Adjust the cutting depth to about two-thirds the width of the part. Precisely center the slots on the mitered joint sides to ensure that both faces will be flush when you assemble the joint.

Cut each slot with the short edge of the miter cut against the saw table.

Make the length of the slot about equal to the width of the stock. Cut a spline from material the same thickness as the saw kerf. Jigsaw or scrollsaw a spline to fit the rounded kerf and test-fit before gluing. Plywood and hardboard are good for unseen splines. Assemble the joint, and glue in the spline with the grain running across the joint. Cut the spline flush after the glue dries.

Biscuits are as effective as splines in adding gluing surface and reinforcement to mitered joints. And, putting biscuits into a joint can be easier and quicker. However, the stock may be too narrow or thin to add them to some joints.

To make a gauge that will aid in choosing the right-size biscuit for a joint, cut biscuit slots of different sizes at the middle of each edge of a 6×6" square of MDF or scrapwood.

Label each slot for its biscuit size, and mark the centerline and ends for each slot on the face of the gauge. Hold the gauge against the joint parts to determine the biscuit sizes that will fit. Choose the largest biscuit that you can put in the joint.

Frame biscuits
Determine the biscuit size, using the slot gauge. Center the slot in the gauge on one face of each joint, and mark the biscuit centerline on the part. Hold the joint parts together, and transfer the biscuit centerline to the mating part of the joint.

Set the biscuit-joiner fence to 90°, and adjust it to center the slot on the edge of the workpiece. Plunge a slot at each biscuit location. Align the biscuit joiner on the biscuit location marked on the part. Cut the slots for both sides of the joint from the same face. Apply glue to the slots and miter faces, insert the biscuits, and clamp.

Box biscuits
Determine the biscuit size and location, using the slot gauge. Mark the biscuit centerlines on the short face of one joint side; then hold the joint together and transfer the marks to the other part. A wider joint may have room for more than one biscuit. Space the slots evenly across the face of the joint.

Adjust the biscuit-joiner fence to 45°, and set the distance from the fence to the center of the blade at one-third of the thickness of the part. Test-cut slots in miter-cut stock the same thickness as the joint parts to make sure the slots don't break through the face. The cutter plunges the slots straight into the face of the miter cut, allowing easy assembly and gluing of the biscuit joints. Then, plunge the slots, apply glue to the slots and joint face, insert the biscuits, and clamp.