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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.
 
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Step 1: Ripping Down the Boards

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.

Step 2: Marking the board

Picture of marking the board
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Take the larger outside board with the framing square, found the point the two boards are going to meet and mark. You are going to mark the line at a perfect 90 degree angle. my mark  was at 9 5/8. With the framing square mark your line. I marked the inside and the outside of wear the 3/4 inch plywood is going to stay. You are going to want to make sure that it is at 90, if it is not at 90 it will mess up your splines when you cut them on the table saw.

I added some band saw cut to the top of the board. this will help you grab on the the box or frame when you cutting.  Making out the the curve with a pencil, i did this freehand its doesn't have to be perfect. Cutting the curve out on the band saw along your line.

Step 3: Assemble

When you are putting this together you may need to have someone help you. If you don’t have anyone to help you can clamp the larger outside piece to fence of the table saw. You are going to want to start with the larger outside piece and one of the inside pieces, apply wood glue to the end of the inside board and put two nails into the top half of the board. You want to put them at the top so you know that you will never hit them with the saw blade. Repeat for the other inside piece. Make sure that the two inside pieces are at a 90 and that the jig is stetting flat. I like to use a machines square to check to make sure that the inside is at 90. Using the same order of operations put the last piece onto the jig. You will want to clamp the jig. We did not add any nails by the bottom of the jig, putting a clamp on the jig will help the wood glue set. Remove the clamps. Now you are ready to use the Spine jig. 

thanks