This tutorial will show you how to make a disk sander for practically free. I creat mine only with salvage parts.
It's for 125mm sand disk and I find it very useful for small woodworking and 3D printing.

Step 1: Materials

The main part is a cheap cordless drill with a dead battery that a friend gave me. For the base I use some mdf wood. I power the drill with an ATX power supply from an old PC. The rest of the parts are a switch, some copper wires, washers and some screws.
You will just need to buy some round sand paper with a sticky face (3$).

Step 2: Cut the Drill

A friend gave me this drill because the battery was dead. I didn't want to change it because it doesn't worth it: the drill cost 15$ new. In the dead battery some accumulators were dead but 5 of them were not so I take them for a future project.
First disassemble the drill and take off the motor and the switch. Reassemble the 2 plastics parts and cut the handle. Cut the wires of the motor and solder 2 longer wires to it. Put back the motor into the plastics parts and drill 2 holes to pass the wires. Screw back the plastics parts and then remove the chuck.

Step 3: Make the Holder for the Drill

To raise the drill I use a big piece of wood. To hold the drill in place on it I use a large piece of aluminum, I cut it, and screw it at the end of the drill. I then screw the piece of aluminum on the wood piece with 3 screws. I screw 2 more screws on each side and attach a copper wire around the drill to avoid any movement.

Step 4: Make the Base

For the base I use only small pieces of mdf wood. I first cut the bottom part. I cut 3 more little boards for the stand, glue them on the bottom piece (use some clamps to hold them tight).

Step 5: Make the Disk

This disk is where the sand disk will be stick. It has to be attached to the shaft of the drill. I cut it with my dremel. Because it wasn't perfectly round, I turn on the drill and sand the edges of the disk while turning. This makes the disk round and creates less vibrations.

Step 6: Finish the Base

Drill 4 holes under the base for fixing the holder of the drill. Then cut the stand and glue it in place. The end of the stand must be as close as possible with the sand disk. It has to be perpendicular too.

Step 7: Connect the Motor

The last step is to connect the motor with a switch. Make a hole for the switch on left side of the stand. Connect to it one of the motor wires. Connect the other motor wires and the switch wires to a screw terminal. It makes the connection with a power supply easy.
The drill works at 12v and 3 amps but the start peak current is about 10 amps. So I power it with an atx power supply. If you want to change the direction of rotation, inverse the polarity.

Step 8: Enjoy!

You have now your own disk sander! Just need to buy extra round sand paper. You can easily sand wood, plastics parts and even aluminum with it. And I hope you like it!