One of the most difficult joints to assemble is the cross grain mitre joint. This joint always tends to slip out of alignment when the glue is applied.
Whether you are using solid wood or plywood veneered stock, one of the easiest ways to stop the slippage is to insert a spline. Another way to stop such slippage is to use a biscuit, but that is not as decorative. When you use a spline, you can actually accent the joint with a different coloured wood.
The spline jig is a very popular jig that I have designed for the sliding compound mitre saw. It was featured on the television show “In the Workshop” on HGTV. Since the show, there has been a lot of interest in knowing how to make and use the spline jig.
The jigs in my shop may not look fancy, but they do work! I have actually tried to re-make some of them in an effort to make them look better. But for some reason, they just don’t work as well.
MAKING THE SPLINE JIG
Select some 1/4″ plywood that is 1-1/2″ wider than the table of your mitre saw.
Glue a 3/4″ x 3/4″ locator cleat on each side of the plywood to stop any side to side movement.
Fasten an auxiliary fence to the plywood. That will allow for the radius (of the back of the blade) so that when you cut down, you will start cutting at full depth. I used a 2” fence for my DeWalt DW 708 saw.
Yours could be different, depending on where the blade is in relation to your fence.
Your saw needs a depth stop. Flip the stop down and adjust the cutting depth with the saw beveled at 45 degrees and the mitre angle at 0 degrees.
Find the location in the mitre that you want to cut and mark the tip of your workpiece. Glue a small stop block at this location to ensure repeat accuracy.
With the jig removed cut your mitres at 45 degrees.
Place the jig on you saw and don’t forget to flip your depth stop down or you will cut right through your new jig.
Now cut your spline grooves in both pieces.
Make and install your splines to match your groove widths, but be careful – there are many different blade widths on the market. You will have to adjust accordingly.
There you have it. Now you have a jig to help you make a cross grain mitre joint.