This jig was made to cut an accurate radius so concaves would be easier.
The box is contsructed from Baltic Birch plywood. Any stable plywood will work. The most important part on the jig is the curve of the sides. They have to be cut and placed exactly the same on the box so there won’t be any warping in the finished block.
The raw block is placed in the center of the box and held in place with a simple wedge. I use balsawood for the blocks, because it’s light in the hand, but any wood will do the job. Whatever works for you.
Once the block is tight, the “sled” is placed on top and the router does the rest. My sled is made so that my router base just brushes the edges, but moves freely. Furniture wax like Minwax helps for smoothness.
I also have another box built for sanding the convex curves on decks.
Once you build one these, it takes about 30 seconds to make a sanding block.
The sandpaper is glued on with spray adhesive.
I have less headaches now when I do concaves. I love this jig. Doug
