Keegan ReillyThis test was a hack in every sense of the word. I just wanted to try spinning something for fun, so didn't bother taking any video (it wasn't very exciting in the end). I will try to describe what I saw, there were some interesting things to ponder.
I began with a crudely cut disc of regular paper. It was crudely cut because I couldn't find my protractor or compass right away, so just folded it diagonally about 5 time then cut the corner off with scissors. It was pretty close to circular, but had little lobes on the edge. I don't think it mattered as much as I thought it would. About 8.5" in diameter. I duct taped this to a cutting wheel on my Dremel and turned it on.
First test at 5k RPM it spun beautifully. Despite the folds and creases and poor cutting job it flattened out instantly and felt very smooth. Looking at it edge on it formed a nice thin edge, the forces flattened and stiffened the disc.
Then I gradually increased the speed and it got much worse about 10-15k RPM. The disc was wobbling or warping or something. Too fast to see, but looking at it edge on, the blur had widened to about 1/4", and I could feel some turbulence.
From just this info it's hard to tell whether this was due to my poor mounting system of duct tape or due to the inherent physics of the system. On subsequent tests with the same paper, I was never able to reproduce the smooth flat spin of the 1st test.
Once the turbulence started I hit another snag, the Dremel maxed out it's power at only about 15-20k RPM. I tried running it faster but it refused to go. I didn't have an RPM gauge hooked up (I said this was a quick hack!), but I could hear the RPM change quite clearly in the lower ranges. Beyond 20k on the dial and the motor didn't change its tune.
This was pretty dissapointing, I had really wanted to make something explode. I really thought paper wouldn't take much at all. Oh well. I have an old unused vacuum cleaner I may take apart and scavenge, I hear those have pretty fast motors. I'll find my compass and cut a proper smooth disc too.
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Regarding vacuum cleaner motors, we tried using those for a CD exploder. We went through several of them before finding one that barely gets up to the necessary speed (~30000 RPM, IIRC). We thought smaller vacuum cleaners would have faster motors (to make up for smaller rotors), and tried to get Dyson motors, but didn't end up finding a small one that worked.
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Wow, good to know! My current plan is to get a small rough vacuum chamber built to do burst speed tests, to eliminate the aerodynamic effects. This should mean I can get away with a much less powerful motor. We'll see...
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BTW! It's been many years ago, when I measured how well the RPM setting matches actual rotation speed of dremel. I did not test it under load, though - it was free-running.
The result was - at lowest setting of 5k, the actual speed was 10k. It was spot on in range 22k-30k, and refused to spin faster than 30k even though the setting goes up to 33k.
The Dremel was almost new back then. Now that it's worn out substantially, it may behave a bit different. It can also be affected by mains voltage (since there is no direct rotation sensing - it is done somehow through power terminals of the motor).
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Human hearing only reaches up to around 20 kHz, perhaps you cannot hear the increase in RPM above 20k because of this? Also while paper is not very strong, it is also not very heavy so the loading may still not be enough to cause it to fail. Maybe consider doing your first test again but with a rough outer case and a hole at the centre similar to what you intend in the final product? I am no specialist in fluid dynamics either but I'd bet that a disk spinning in free air behaves differently to one which only encounters free air at its centre.
Anyway, good start. Good luck for the rest!
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That's a great point about human hearing! I hadn't thought of that, but it really didn't "feel" like it was accelerating at all. I know that's not a very scientific data point though :) I think I'll devote some time to (first cleaning up my garage) and building a proper test stand. Thanks for reading!
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20k RPM has nothing to do with 20 kHz. RPM = revolutions per minute, Hz = cycles per second. So, 20k RPM == 333 Hz rotation rate, perfectly hearable.
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