Instructables
Picture of Spline Jig, yep I made it at TechShop
What you need

3/4 inch plywood
13.5 x 9.5
6.25 X 13.5
7.25 x 13.5
7.25 x 9.5
Clamps (TechShop)
Table Saw  (TechShop)
Chop Saw (TechShop)
wood  Glue
Brad Nailer (with nails)
square
pencil
Cordless drill with 2 screws
Tape Measure
Scrape Piece of Plywood

I make a fair amount of frames, I don’t like to nail the frames and like the look of the miter corners, I always add splines. The reason you need to add spline to the miter corners, is end grain to end grain glue up is not a strong joint, it will brake over time. Having the splines you are adding glue to the long grain to long grain which gives you more surface area. I made this out of large pieces of scrap that I had around the shop.

Step 1: Ripping Down the Boards

Picture of Ripping Down the Boards
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You are going to have four pieces of ply wood cut out. The sizes that I used are:
Outside
13 1/2x 9 1/2
6 1/4 x 13 1/4
Inside
7 1/4 x 13 1/2
7 1/4 x 9 1/2

To do this you are going to rip your ply-wood down to size. I ripped the two middle pieces the middle pieces first to 7 1/2 inches on the table saw. Move the fence over to 13 1/2 and cut down the next piece, and the last one to 6 1/2. Moving over to the chop saw, I cut the boards to the desire length.

On the inside two pieces you have to cut 45 on the bottoms. The two inside pieces are 7 1/4 x 13 1/2 and 7 1/4 x 9 1/2. You don’t have to cut 45 on the top, I do. Using the table saw, set up the sliding miter with a scrap piece of ply-wood or MDF ripped down to about 3 inches high attach. Make sure that the sliding miter is square to the blade. With two screws and cordless screw gun attach the ply-wood to the miter fence. Move the blade to a 45 degree angle and cut the sides off the inside pieces.
 
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