An STL of the rattleback has been requested in the past by members and I had emailed it to them (since the file size was just a little too large to upload). I went back and slightly reduced the number of vertices to reduce the file size to upload. This is a shape plotted in a program called Maple from a formula I made up.
This is the smooth version. If you want a pattern on the top, you will have to add it yourself.
I am still making these when I get an urge to do so, the last ones out of hickory which looked nice. Usually I just give them away. I am still carving them sideways with good results and should make a dedicated sled for doing that.
As far as going both directions, I've considered making one double-sided - flip it over and it spins the other way. Not sure how the balance will be but easy enough to try.
Dan, a quick 21 minute carve and 5 minutes sanding in oak. I get an average of three turns in the reverse direction and occasionally up to 4.5 turns with mine. Very cool design.
I have one of your rattlebacks on my desk and every so often I give it a twirl. Don't really know why it fascinates me so much. The other ones I've made I gave to friends with small kids.
But the spinning got me to thinking. These oddities can rattleback both in clockwise and counterclockwise directions.
So I am going to try my hand at making my own design. I made a quick model and then mirrored on axis to shift the center of balance. I am hoping since they are mirror images they will have opposite preferential spin direction.
I will throw into the machine tomorrow and see what happens.
Who need fancy math software when you can eyeball it. lol. Last words of really expensive experiments (hubble telescope, mars explorer, Solar Heliospheric Observatory, and many more).
Even if it doesn't work I now have a 3d model of a rattleback for cool rendered images.
I screwed up and re-shifted the center of mass back to the center when I was cleaning up the mesh without realizing it. So I pretty much carved a symmetrical pattern. It likes to spin clockwise perfectly and it likes to spin counter clockwise perfectly. If I put a twist on it, it would make a great propeller.
Back to the drawing board. This time I am not going to re-shift the center of mass.
Well, success. They work the way they should with off center of mass rotation. Preferable rotation is opposite of each other. After a sanding I applied some lacquer and waiting for them to dry.
Please let me know if anyone wants these patterns.
But honestly, Dan's pattern is more elegant and it is also a lot more aggressive than my version. You can get the same results from Dan's by mirroring the pattern so that you have one of each with different rotation preference if you want.
I think your patterns are too offset. The offset only needs be subtle. Just think, all the action happens in a small neighborhood of the contact point.
You are right. My thought was to go with a gross starting point to set the outer boundary.
Since I am eyeballing it, it would be a series of tweaks until I got it just right visually while retaining the toy's aspect.
It was a fun little experiment but I think I have what I want from it. A simple understanding of the physics associated with rotational behavior in asymmetrical mass bodies.
The right one is from your pattern (it rocks). The left ones are my experiment and they have a clockwise and counter clockwise preferential rotation.
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