I've recently bought a TIG welder from Kowax and was facing a challenge of grinding the tungsten electrodes.The grinding is very common task when you start with TIG welding as maintaining a precise distance of a millimeter or two between the electrode and the welded surface is a challenging task for a beginner.
There are many professional grinders available for very professional (high) prices. I was briefly considering buying a small handheld grinder from ebay for about 100 bucks, but then i saw a cheap diamond grinding wheel for fraction of the price in a local welding store and decided build the grinder myself. I bet every IT guy has a stock of old hard drives lying in a drawer, so why not the join the old HDD together with the grinding wheel?
Grinding tungsten electrodes with thorium is dangerous. A radioactive dust is released and you risk a lung cancer and polluting your workshop with a slightly radioactive dust. Buy non-thorium electrodes or get a professional grinder with water system to catch this dust.
The good old rotary hard drives contain a nice BLDC motor. These motors need a driver that generates a three phase signal driving the engine. There's a bunch of cheap BLDC drivers for RC model motors, but these generally require a servo signal for setting the RPM, you could simply use a well-known 555 circuit or program a small MCU to generate the signal for you, but that adds an unnecessary complexity.
There a bunch of nice Brushless motor controllers on your favorite Chinese supplier webpage, just pick one with 4 outputs (U, V, W and COM/ground) and reasonable input voltage (around 12 V is OK).
Get rid of the PCB on the bottom of your HDD (backup your data before :D ), solder the 4 (you might encounter motors with only 3 leads) to the motor pads (some motors unfortunately don't have the solderable pads, only the flex cable that is impossible to solder on - these are not suitable for conversion) and try spinning it. I would recommend to take off the HDD top cover and try braking the spinning drive to see if the torque is enough, if not, you might not get the order of the BLDC wires correct, just re-connect it another way and try again until it the engine stops stalling under light load (or you are out of combinations, in this case, get a different HDD or controller).
The HDD platters are fragile, take them out and throw away, DO NOT use them!
The platter material is fragile although it's not obvious on the first sigh, hit them hard and they shatter as glass with all these sharp pieces flying around. The motor can spin them pretty fast, that's not a good combination. I've made a simple grinder from HDD before simply by printing a plate of needed thickness and diameter on my 3D printer and gluing a grinding paper on it, that's much safer than using the original platters. Always use the eye-protection anyway....
The diamond grinding wheel I bought had a smaller hole diameter then the HDD engine shaft, I had to mount it to a lathe and increase the hole diameter. The wheel material is extremely though, it was pain to make a reasonable chips out of it, even with very small feed rates. If you are faced with the same issue, take your time to make the hole as concentric as possible, even a small error will make your grinder to shatter and jump all over the table.
I have a bunch of 12 V wall adapters lying around, so finishing the project was mostly about soldering a switch and a correct power jack to the BLDC controller. I managed to utilize the original HDD case by cutting it to half, exposing the grinder wheel inside while hiding the controller together with the necessary wiring inside the case. A small piece of aluminum sheet glued to the HDD base shields the controller from the pieces of the grunt electrode material.
Due to a small weight imbalance created by not-perfect machining setup for enlarging the grinder wheel hole the grinder shakes a bit when powered on, but few rubber stands and ziptying to the welding table solved the issue.
Use an eye protection! The tungsten electrode is brittle and might shatter. The grinder runs on a high RPM, it could catch the electrode and throw it to your face. The grinding produces a dust that is not very healthy to inhale - use a breathing mask or dust extraction system.
The HDD motor is not very strong and tends to loose the torque when loaded too much during grinding, but with a smooth touch of the tungsten electrode it holds the RPM very nice and it's a pleasure to use it. The diamond grinding wheel holds pretty well, after maybe two hundred tungsten sharpened, there's almost no sign of wear. I even use the wheel side to cut the end of tungsten when a huge ball of material happens to appear on the sharp end accidentally (yeah, yeah, it's always fun to soak your electrode in the weld pool or touch it with the filler wire).