Splined Miter. A Simple Corner Joint that also Simplifies Glue-up of a Box or Cabinet.

Spined Miter Corner

This is the second of four different corner joints it is useful to know about.  Best cut using a  table saw with a miter gauge to add some safety to the cuts.  It helps if the table saw blade is close to the same thickness of 3mm thick plywood as plywood's alternating layers make for a strong spline.  You can use wood as the spline if you make sure the grain runs across the joint rather than parallel to it.  On long mitered sides it is best to use the same material for the spline as the sides. Wood expansion/contraction  as humidity changes can create a shearing force that will lead to failure of the joint if the spline won't expand with the sides.

Material used was 5/8" thick.  The spline slot cut was 1/8" wide.

You can use a router table to cut the spline slot on small parts, but it requires a jig to hold the sides up at 45 degrees.  That or a router table that can tilt up 45 degrees. 

The initial miter cut can be cut on the table saw, or on a compound angle miter saw standing up or laying flat.  The height of the sides limits the use of the miter saw.   I have close to 6 inches of cut height on my Bosch glide saw, and 14 inches or so of cut length.  I get better result standing the sides up to cut so my limit is 6" of side height to cut them on the miter saw.  

Table saw blades might leave a V bottom in their cuts.  If you can find a blade that leaves a flat bottom it will make a flat bottom spline slot. The blades in a dado set are usually flat tipped so it might be worth changing to a single 1/8" or 3mm wide dado set blade.  A simple test slot cut using the table saw will reveal how flat of a bottom it leaves the slot.

To leave the joint strong it is best to have the spline slot closer to the inside of the corner, and not cut any deeper than half way through the sides.  When setting up the table saw to cut this slot I marked where I wanted it and transferred the position to the top/outside of the board. This helped me align it with the slot already cut in the miter gauge face.

Slots Cut

Using a miter gauge on the table saw to push the board through the saw blade is best done and easiest to align if you add a fresh face board to the miter gauge for the cuts.  Some friction tape on the front of the face board will help keep boards from sliding left or right when being pushed through the saw blade.  

Slot in Gauge Face Board

A stop block clamped to the miter gauge board can ease alignment for the cuts.  It is important that the slot position matches on mating sides.  Many miter gauges have a built-in sliding stop block, and you may be tempted to use it.  On a simpler set up a board clamped to the face board will do.    Best place for a stop block is at the tip of the mitered corner. 

Stop Block Position

Is a splined miter any stronger than a simple miter with no spline?  For simple boxes, it doesn't matter as there normally won't be any forces trying to pull apart or warp the corner.  The advantage is in gluing up the box.  The spline keeps the sides aligned when clamping together.   The addition of glue surface area should offset any weakness introduced from the  slot cut.

Questions or Comments welcomed!

4D