Oooh, glad I stopped by this thread. I'm just putting together a photo storage box and it is to have splines. I knew I'd seen one somewhere but couldn't remember the original thread, now here it is. Thanks Tom, so timely.
This was a lot of fun to put together. It is very precise and I did use a right angle triangle to make sure it was dead on 90 degrees. But I started by cutting two 18 inch wide pieces of 3/4 ply in half. One about 1 ft x 18 inches, the other about 20 x 18 inches. What made it so precise was cutting it on the table saw, not quite down the center, after using a Wixey to set the blade angle at precisely 45 degrees (the picture shows it at a different angle).
Next step was setting up the base, another 18 inch long by about 12 inches wide. I set it square to the fence after placing 3/4 x 3/8ths strips in the miter slots, raised slightly on pennies then lined the base up with the fence (also in perfect adjustment) so the base is perpendicular to the blade. Take your time doing this because everything else can then line up perfectly parallel or perpendecular to the blade.
Once the base was set up, I then used an engineer's square to mark the alighment for the first vertical support (the shorter set of 45 degree cut stock. Lined the first one up square across the base and tacked it in place and glued it.
Once dried, I took the front and back piece, and the other short vertical piece and hand held and fiddled them in place so the front and back 45 cuts were down and touching the base. Then I worked the last short vertical piece in place and checked with the draftsman's square to make sure the angle was 90 and the short vertical piece held everything in place. Tacked and glued the short vertical piece in place. and once dry, refit the two supporting pieces to the 90, marked where the back vertical piece intersected the back support piece and struck a line where I then cut a dado on the surface for the blue aluminum T track strip. Then pre drilled holes through the T track holes to connect this strip and the back support. Glue on the 45 degree surfaces and screws hold all that in place. Then pretty much repeat the process for the front support, taking great care to create a perfect 90.
The vertical support is of 1/2 baltic birch, and was cut to be perfectly square. What keeps it square are the two 90 degree supports. The horizoneal surfaces are connected with simple butt joint, the vertical is attached as is obvious in the picture. Two trapezoids hold the vertical fence at 90 to the back and fron support surfaces. A knob and T bolt let me lock it down for perfect repeatability. I could have put marks every half inch or so to set the vertical support, but its just as easy to use a tape measure.
This is a really great jig and will probably last my remaining lifetime. As mentioned I can use a custom blade that cuts 1/4 and 3/8ths dados as well.
The side view of the the jig shows you how the supports, back and front fit together.