hacking an ancient disc drive

[darryl]

I'm upgrading my spin casting capability so I can make some custom lenses from clear casting resin. I already have a spin casting table, but at maximum rpm it's too slow for the lenses I'm wanting to make right now. So- dig through the stockpile of 'junk' and I find an old disc drive motor. Old 5 1/4 inch drive- as is it runs at 300 rpm, and I need a range from about 100 to about 600 rpm. There is an adjustment pot on the circuit board, but it seems to be for minimizing the torque ripple and doesn't adjust the speed. I wonder if anyone has hacked these and found a way to alter the rpm?

There is nothing I can find on line about any of this, so I'm probably on my own to experiment, etc. I realize there must be several different motors that were used in these drives, so my question is more about whether the rpm spec is built in to the controller ics, or might there be some part I could change to alter the rpm. I have a couple of these motors, but they are so old that I can't find anything online about them.

Of course an option is to use an older servo capstan motor from a vcr, and study it until I can see how to make it turn at a selected speed. Or use a more common brush motor with a tach coil on it and come up with a pll circuit to lock it to a chosen rpm- this is how my original spin casting table is made.

[danlb]

As far as I remember, they are essentially brushless DC motors so a BLDC controller may be able to do the job.

Dan

[fixerdave]

Depending on the motor you have, this may work... designed for HD motors, not floppy, so the RPM might be off by an order of magnitude or 3.

https://www.ebay.ca/itm/5V-12V-DC-Brushless-Driver-Board-Controller-For-Hard-Drive-Motor-3-4-Wire-New/201881091822

David...

[darryl]

Ok, I did figure it out. The adjustment pot does control the rpm- but over a very small range, which is why I thought it probably had a different use. So it turns out I can change a few resistor values and get the range I want. I have another similar, but newer drive, but it doesn't have any pots on it. So this old drive is getting a new life spin casting.

Because I'm generating a fairly steep parabolic shape as a mold, the part that is being created in the mold does not want to come out easily. I discovered this in the previous experiments with a different motor. I'm using mold release of course, but there's no good way to grab onto the part to break it out of the mold. I destroyed one of the molds just getting the lens out. I have two more testers, so one I'll put in the freezer for awhile, then take it out and put it in hot water. That might make the mold pull away from the lens and allow it to come free without damage.

The first lens came out good, even though the mold is shot. It's just a little too steep, so my next mold will be a bit shallower.

While I'm playing with this I'm thinking about how to make the process easier. I thought of layering a heavier liquid into the mold to begin with, then topping that with the clear casting resin. Both layers will form into a parabolic shape as it's spun up, and the resulting resin part will be hollow. I could then fill this with more resin to the exact height I want, or if the lens is to be larger in diameter I could cap it with a glass plate and use a mineral oil fill. A major requirement here is that the 'heavy' liquid and the casting resin don't mix. First thing that comes to mind is mercury- my 'thought experiment' says that mercury won't mix with the resin, and it will form a smooth enough surface so the resin will take a smooth surface as well. Mercury being ill regarded these days, an alternative liquid might be gallium. Heated to 90F or so it will be liquid. Obviously I'd have to keep the temperature of the whole thing up to this level, but that's not so problematic. Another liquid is a chemical compound with low toxicity, used in separating rocks and minerals for identification. Whatever liquid I would use, it must be completely immiscible with the resin- two completely separate layers must result. Obviously with mercury, or gallium I would presume, you don't just wash it off the part like you could do with the chemical compound. This could be an interesting method to use- you would probably pressurize it slightly to pop the cured resin part out of the mold. The only part of the resin casting that would require breaking free would be the very edge where it touches the container. If the sides of the container were sloped, like a plastic shot glass, it should pop out nicely. A suitable liquid is key to this process.

[RichR]

Because I'm generating a fairly steep parabolic shape as a mold, the part that is being created in the mold does not want to come out easily.

Make a 2 piece mold with each piece forming half the circle.

[darryl]

I have the motor and base complete, on off sw and speed control mounted, tested- working fine. I'll turn up an interface piece to mount to the motors hub, which will carry the mold structure. Ready to go again.

Interesting to see in testing that as the water in the cup spreads outwards, climbing the walls of the container, the center at the bottom of the cup tends to pull the water downwards and creates a central hole in the water. Surface tension then makes that hole spread open and the accurate shape of the parabola is lost. Suddenly there's more than gravity at work now. When I spun up the test molds, the epoxy behaved the same way. If there was too little thickness of epoxy at the bottom of the cup, then the cup itself caused the epoxy to shrink away from the center point of the cup. I had it spinning at just the right speed, and I had the right amount of epoxy to cover the center pip in the cup. It ran for about a half hour with nothing changing. I went for a bite to eat. When I came back, each of the cups had overflowed. Drops of epoxy everywhere. I thought perhaps the speed had crept up, but then I noticed this effect start to happen again while doing another test. The cure is to have higher sides on the rotating mold and enough epoxy to completely prevent this effect from happening.

I'm having fun playing anyway. I'm going to try glycerine as a float medium, which is heavier than the casting resin. It will be interesting watching that spin.

[old mart]

I had a Quantum Bigfoot 5 1/4 drive back in 1997 and it ran at 4000 rpm, I think you should recheck yours as I don't think the heads would lift off at only 300 rpm.

Of course, if you are referring to a 5 1/4 floppy drive, then it's a different story.

CD and DVD drives have variable speed motors, they might be an alternative.