I've got three mirror frames to build, two in cherry with a curly eucalyptus panel, and one in walnut with curly maple accents. The corners are mitered, an inherently weak joint that's mostly end grain to end grain. I like to spline mitres, a technique that gives the joint plenty of strength and is a nice design feature. I start by milling up the frame stock, and making the mitre cuts with a chopsaw and stop block. I use biscuits to help with alignment during clamp up, for which I use a band clamp (this one time, in band clamp...). From there the frame goes to the tablesaw for kerfing.
The spline jig is a simple one, little more than a vertical piece of plywood with a fence saddle. An old push stick was added for a little more control. This workholder is useful on the shaper as well, holding stock at angles impossible to maintain by hand. The jig gets a fence of scrap material screwed on at 45°, the frame is slid down it until the leading corner touches the saw table, then it's firmly clamped into place.
It is important to choose a saw blade with a flat top tooth configuration. Using an alternating top bevel will result in a kerf with a v-shaped bottom, and the spline will not sit cleanly in the groove.
The spline material is now milled to match the kerf. The dry fit should be a little loose, you should be able to bottom it out in the kerf without force. If it's tight at this stage it will be impossible to seat after the addition of glue. For maximum strength the grain should run perpendicular to the mitre cuts, the long grain spanning the joint. I make a clamp block from scrap, apply glue to the kerf, and insert the spline. The clamp draws the spline in, but go slowly and don't apply too much pressure, or the spline could break, or the clamp block could wedge the joint apart.
Then the joint is clamped across the spline, with as much pressure as possible. This ensures the spline is firmly glued to the frame, and eliminates the glue line. Clamp pads are used to avoid marring the stock. After these clamps are applied, the f-clamp and clamping block can be removed for use on the next corner.
After the glue has dried, the excess spline is sawed off close to the frame.
The joint is cleaned up, as it's end grain I am removing I choose a low angle block plane. The spline must be worked from the outside to the inside, working from the corner to the middle of the frame edge. Working toward the outside will result in the corner of the spline breaking off and ruining the nice clean work. I use a chisel, flush to the surface, to sever the fibres close to the edge to avoid tearout there.
The spline is then planed flush.
The result is a strong joint with plenty of long grain gluing surface, and a nice design feature that doesn't detract from the view from the front.
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