Many people have an old hard drive laying around. In this project we will put it to good use by turning it into a powerful disk sander! It's a cheap and easy project, but it has already proven to be very useful when sharpening tools and sanding though materials.

Let's get building!

Step 1: Parts & Tools

Parts

Old harddrive (although a new one would also work :p)
Electronic Speed Controller (about $4)
Servo tester (about $1.5)
Sandpaper
2 Banana terminals

Total cost: less than $10!

Tools

Torx screwdrivers
Soldering iron
Dremel / rotary tool
Drill
Multimeter

WARNING

Make sure your harddrive has aluminium platters, and not glass ones. The glass platter might shatter and cause serious damage! How to check this?

Take a strong neodymium magnet and go over the platter. You should feel some resistance if it's an aluminium one (due to eddy currents)
Look at the sun though the harddrive. If you can slightly see the sun, it's glass.
To be completely sure: hold it in a vice, wear safety equipment (especially safety glasses!) and give it a slight hit with a hammer. If it shatters you shouldn't (and can't at this point :p) use it.

Step 2: Disassembly

The first step of the build is disassembling the harddrive. Simply remove all the screws; there will probably be one under the sticker as well.

Next, we'll need to take out the guts. Undo all the screws you see and remove the components.
You might have to use some force to remove the magnets, but these are worth keeping!

Also remove the hard drives and spacers themselves, since we'll have to restack them.

Small bonus project

These strong magnets are ideal to make a magnetic parts tray. You can remove the protruding metal bits with a pair of pliers. They will break off quite easily, since they are quite brittle.

I had a lid of an old metal pen case, to which I attached the magnets; et voila!

Step 3: Wiring the Motor

The crucial step of this project is controlling the motor of the harddrive.

The motor inside a harddrive is a brushless DC motor (BLDC). They come in 2 variants: triangle and star topology. For the full explanation, take a look at the Wikipedia page, I will focus on the wiring.

A BLDC motor can easily be controlled by an Electronic Speed Controller (ESC), and while we could have made this ourselves, it's far easier and cheaper to buy a speed controller. Furthermore, they can easily be controlled by a servo tester; as the name implies, it is used to test servo motors. However, the type of control signal for a ESC and a servo are the same: it's a simple PWM signal. In case of the servo, this corresponds to a motor position; for the ESC it corresponds to a speed.

Let's focus on the wiring of the motor, depending on your harddrive you will have 1 of 2 types:

3 Pins

If your motor has 3 pins, it's triangle topology: there is a coil between every pair of pins. This is the easy case: just solder the 3 wires of the ESC (the blue ones) to the 3 pins of the motor (a,b,c on the diagram) and call it a day.

4 Pins

In case there are 4 pins on the motor, you have a star topology. We will need to connect the ESC to points a,b,c on the diagram. To do this, we have to identify to which 3 of the 4 pins they correspond.
To achieve this, we'll have to bust out the multimeter and measure the resistance between the pins. As we can see on the picture, 3 combinations give a resistance of 1 Ohm, while 3 others give 2 Ohms. 1 Ohm is the resistance of 1 coil (1 of the Z's on the diagram), while the 2 Ohms is the resistance of 2 coils in series.
Conclusion: the combinations where we measured 2 Ohms are the ones we need (a,b,c on the diagram)!

After soldering the ESC, mount it inside the hard drive case where the reading head was. Every harddrive is different, so try to find a way of attaching it securely; I just some scraps of aluminium, although glue would have also worked.

Attach 2 banana terminals to the red an black wires of the ESC. Drill 2 holes in the enclosure to mount them. You could also use a barrel jack if you want to use it with a 12 V wall adapter instead of a bench powersupply.

You can already connect the servo tester to the 3 pin connector of the ESC, plug the banana jacks into 12 V and give the motor a try. That's the electronics basically done!

Step 4: Enclosure

Now that our motor spins, we can turn it into a sander!

Start by taking a piece of sandpaper and cut out a circle that fits your hard drive platter. Attach it to the platter with double sided tape. This will allow us to easily replace it in the future.

My harddrive had 2 platters with some spacers in between. Put the spacers down first and then add the 2 platters on top of each other for extra strength. Screw it tightly in place, this bracket will also keep the sandpaper firmly in place (that's why we didn't glue the sandpaper).

The last thing is to cover up and protect the ESC and to make it more usable. Therefore, we will reinstall the original lid, with a cutout for the harddisk. Since the cover is very hard and reasonably thick steel, it is preferable to use a thin cutoff wheel on your rotary tool. Wear some kind of face protection and a breathing mask while doing this: it produces dust and the cutoff wheel can easily break and fly away!
I also added an upright piece out of thin aluminium that seals off the side and glued it in place.

As a last step, attach the servo tester to the 3 pin connector of the ESC. I decided to leave it external, since there was not enough place inside the enclosure, and since it allows me to use it to test actual servo's when I'm not sanding.

Step 5: Test & Enjoy

We're done! The only thing left to do is to test our new hard drive sander.
Connect the servo tester and plug it into a 12 V supply which can deliver about 1 A (see how I made one from an old ATX powersupply here, or a fancy digital one here). Make sure the servo tester is in manual mode and turn the knob, the sander should start spinning!

I hope you liked the project and have found a good use for that unused old hard drive!

Feel free to check out my other instructables: https://www.instructables.com/member/ThomasVDD/