6 Steps
This started out as an attempt to make a Gyrocompass from 3 hard drives; and ended being a Dead Reckoning Navigation Device (keeps pointing at its initial heading).

A gyrocompass is designed to find and point to true north, however in this case I think there is too much friction, and not enough rotational inertia for it to work.

I'll run through the theory and build anyway, and maybe somebody will have a suggestion to get it working? or will be able to build one better!

What it does now, is look really cool, spin up to around 12,000 rpm, and maintain its current heading. Because it maintains its heading while the base is moved, the degree markings can be used to obtain the new heading with respect to the original. In this way it can be used as a dead reckoning navigator. It is also very educational, I already knew theoretically how a gyro worked, but this puts it into perspective a lot better.

What you need:
2 hard drives
Assorted materials to create brackets/gimbals, I used brass and aluminium
Depending on your hard drive circuitry, you may also need a BLDC motor controller.

Equipment required:
Hand tools for dissasembly/assembly (hard drive take torx head screwdriver bits)
Metal working equipment to make brackets and gimbals
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Step 1: The Theory

Once you get your head around precession, a gyrocompass is really a fairly simple device.

I found an article which had a very good explanation, and the figures are taken from that.
http://www.maritime.org/fleetsub/elect/chap17.htm

Firstly, a gyro works by using a spinning disc of high rotational inertia, gained by having large mass towards the edge of a disc spinning at high RPM.

The first figure shows a free gyro, with three axis of freedom, two are on pivots while the third is the gyro wheel's spin axis. A free axis gyro will keep a directionally constant spin axis as there are no forces acting on it.

Precession is how a gyro resists the change in direction. When a torque is applied to a gyro, it applies a precession torque perpendicular. One of the more practical instances of this is countersteering on a motorcycle, as the rider aggressively twists the bars (by pushing on the inside bar), the front wheel generates a torque to lean the bike into the corner, allowing the motorcyclist to change direction very quickly.

The second figure shows the type of gyro I have made; a Pendulous Gyro. This is just a free gyro, but with a weight at the bottom. This weight applies a torque to ensure the spin axis is always horizontal. Because this torque is applied to a gyro, it in turn applies a precession torque, which rotates the gyro around its vertical axis.

The third figure shows how this is used to create a gyro compass. As the earth spins, the horizontal heading changes unless the gyro is pointing along the same spin axis as earth. The gyro attempts to follow the horizontal heading due to the torque from the pendulous weight. Once the gyro spin axis is aligned with the earth's spin axis there is no applied torque to the gyro, and its heading stabilizes.
Tomohawk says: May 13, 2012. 2:06 AM
Beautiful work! I'm sorry to hear it doesn't work quite as you wanted, but it looks totally amazing!
andrew.spencer.2 (author) says: Apr 30, 2012. 3:54 AM
I have since talked to a guy who services shipboard nav gyros, he said the rotors they use on similar RPM units weigh upwards of 10kg, and are larger diameter. So my gyro is probably getting about 5% of the rotational inertia that the commercial units use.
rachel says: Apr 30, 2012. 1:17 AM
Love this! No idea why it doesn't do what you expected, did you wait for like a day with it running? I'd assume it would need that general range of time to observe the effect, since despite how fast the earth's surface moves, the change of angle with respect to north would go pretty slowly. But I don't really know what I'm talking about, this is just a guess.
andrew.spencer.2 (author) in reply to rachelApr 30, 2012. 3:52 AM
The article said about 80 minutes for a cycle. The earth rotates at 15 degrees an hour, and applying just a minuscule force (and less than one degree) to the pendulum (to change its orientation as the earths spin would) causes the heading indicator to move. So I would expect the 4 hours I left it going for should have generated some perceivable movement.

But on the plus side, it can hold it's current heading for at least four hours!