固执的海龟（魔法与物理模型）

Looking at each other, deeply in love, they easily turn in either direction when you let them spin.

When they sit next to each other facing the same direction, they can also easily turn in both directions.

Even if they are angry with each other and both have their backs turned to each other, turning in either direction is no problem.

But there is a glitch:

When they both look (over the long axis of the stone) in opposite directions, they only want one thing: stubbornly chasing each other! This desire is so strong that even if you spin them the other way (so they go backwards), they are able to reverse the direction of rotation and still chase each other again!

And there is more:

The desire to chase each other is so strong that a small tap on one of the long ends of the stone is enough to start chasing each other (rotating) again!

A small tap on one short side of the stone creates a rolling movement like on a ship (moving from left to right). And just as we humans often get seasick from this movement, they also get upset and start turning backwards!

Printing all parts is very staightforward and easy. No supports are required. Use you favorite settings. Advise however is to use a nozzle of max 0.4 and a maximal layerheigt of 0.2.

For the Stone i supplied 2 versions: without the text and with the text imbedded in the model for easy 2 color printing.

The turtles I manualy painted in the slicer with a total of 4 colors.

The pins should turn with friction, not freely. Otherwise they will turn themselves when spinning the stone. The pins are dimensioned that way, but adjust them by slightly scaling in XY if you don't get the perfect fit with your printer.

The stone is prefered to print in PLA because you need to smooth it by sanding (see below)

Important!

For a smooth and proper functioning of the "stone", the curved underside must be smooth! Do this by sanding the underside.

I successively used waterproof sandpaper with grit 180/240 and finally 600. I have provided 2 rings on which you can place the stone upside down or use it as a grip for sanding. Use them together with the pins.

There are two methods of sanding:

• Place the waterproof sandpaper flat on one hand and make a cup with that hand. Now rub the stone through the bowl with small circular movements.
• Place the stone upside down on the surface and follow the outer contour with the sandpaper.

Make sure that all layer thickness steps are neatly leveled and smooth after sanding! Let the sanding paper follow the printed curves (don't press too hard and do not use a hard sanding block). The more detailed you printed, less sanding is required. But even with 0.4 nozzle and 0.2 layer thickness it doesn't take long.

Use it on a smooth surface. I also supplied a disc of Ø150. When printed on a flat bed, you create the perfect flat surface. Use above story in your own way to present the magic to others. It is so much more appealing and fun than showing a normal (and dull) rattleback without the turtles. With this model it is possible to create both a normal rotation and the effect of a rattleback, while with a rattleback alone a normal rotation in both directions is impossible.

Many thanks to Squinn on Printables for sharing his model Cutie Baby Turtle and allowing remixes. They were a perfect candidate for this toy.
Also thanks to Professor Tadashi Tokieda for his enthusiasm for physics toys and special physical phenomena. His explanation in his Toy Models lecture on "LMS Popular Lectures 2008" on YouTube fueled the story and thus the experience of this physical phenomenon.

The rattleback, also known as a celt or wobblestone, is a type of mechanical toy with the curious property of spin asymmetry. Nowadays, there are different varieties of rattlebacks. This version is made with an old Russian rattleback toy in mind, called stubborn tortoise (because they are with 2 I named them Stubborn Turtles). When the rattleback is spun in the clockwise direction with the turtles both turned that way, it continues to spin clockwise until it slows to a stop. However, when the rattleback is spun in the anticlockwise direction, a self induced oscillation occurs, and the spin slows down and eventually its direction is reversed. If you turn the turtles anticlockwise, the prefered direction of spinning becomes also anti-clockwise..

For more info see the WIKI about it!