Quick and Dirty Sanding Ball

This instructable is based on my quick and dirty solution to the problem of sanding a concave surface while carving a wooden bowl. Though not the most elegant tool, I like to think that the beauty comes though in the ease of obtaining all of the materials needed to make the sander. I am admittedly not someone who likes to travel farther than my front door to solve a problem.

We will begin by collecting the following supplies:

1. One screw of any sort, as long as it is greater than 2.5" in length
2. One well used sponge, dry as a bone
3. Super Glue
4. Duct tape
5. One 2" scrap of 1/8" plywood 
6. Sandpaper of any grit desired
7. Heavy Duty Shears (not pictured)

Using your shears, proceed to cut your dry old sponge into three squares, a 2" square, a 1.5" square, and a 1" square, as pictured. Trim the corners of the squares off to make 3 octagons, then do your best to continue rounding off the corners to make 3 circles of 2", 1.5", and 1" in diameter.

The next step will be to screw through the center of the 1.5" sponge circle, then the 2" sponge circle so they stack up as shown in the picture.
With the screw backed out of the sponge disks about .75", apply super glue to the shaft of the screw where it passed through the 1.5" disk, then drive the screw through the sponge before the glue dries. This will carry  the superglue into the sponge on the shaft of the screw so the sponge and the screw get well bonded together. This will be important to prevent the screw from spinning inside the sanding ball during use. Add some more super glue to the shaft of the screw where it passes through the 2" sponge disk for additional insurance.
Now super glue the 1" sponge disk over the head of the screw as pictured. Covering this over with the last sponge disk will prevent the head of the screw from wearing through the sandpaper during use. 
The final form will look like a stepped sponge cone with a screw sticking out of the back.

Using the duct tape, wrap the sponge ball up tight. Pulling tight on the tip of the cone with the tape will give the ball more of a rounded hemisphere shape. Squish the tape down into your palm to round the form as it all sticks together.
It took me a couple of layers as the tape had a hard time sticking to the scouring pad part of the sponge. 

From your 1/8" scrap plywood, cut out a circle that is just under 2" in diameter. I was able to use my shears for this, but I image most plywood in the world is a bit stronger than the scraps I have laying around, so you may need to use a hacksaw for this step. 
Mark the center of the plywood circle and screw the sanding ball assembly through. Again, a pilot hole may be necessary for a stronger piece of scrap, but I was able to push this through like it was a cracker.
Don't screw the wood circle all the way down onto the sponge ball yet as it will ultimately be used to clamp the sandpaper onto the sanding ball. Leaving 1/8" or so will give some room to slip the sand paper between the sponge ball and the ply wood before clamping it down.

Next step will be to prepare your sandpaper. These will get worn out and need changing, so making multiples will pay off down the line.
Mark a circle on the sandpaper about 4" in diameter (I used my duct tape as a template) and cut it out. 
Next, fold the circle into eighths and mark these lines with a pencil. Draw a circle in the center of the sandpaper that is about 1" in diameter. Cut the previously drawn lines down to the edge of the 1" circle to create a pinwheel shape. (see images for clarity)
This sandpaper is now ready for action.

Next, place the sanding ball down on the center of the sandpaper pinwheel as pictured. Fold the sandpaper up and tuck it under the wooden disk. At this point, do some massaging of the sandpaper into your palm to round it up and to push any loose sandpaper tight under the wood disk. Now screw the disk down onto the sandpaper to clamp it against the top of the sponge ball. You are now ready to sand.

Lock your new sanding ball into your favorite power drill and sand away!