Do Nothing Grinder/Trammel of Archimedes Fidget

Turn the crank and the glides move back and forth in their channels, never colliding, but also doing nothing.

This fidget is based on the Trammel of Archimedes, which is a mechanism that traces the shape of an ellipse (which can easily be demonstrated by putting a pen through the hole in the crank). As a fidget it provides a simple repetitive motion with a mesmerizing dance of the shuttles as they move back and forth in sequence.

The ellipse function works with any number of glides, and can be visualized as a small circle rolling around the inside of a larger circle of double the size. Each connection point for a glide is on the rim of the inner circle. For models with more than 2 channels the channel-solid base and handle-ring handle visually demonstrate the larger and smaller circles.

Sizes are minimized to keep this pocket sized. If there's interest I can generate larger models.

Printing Instructions/Tips:

Pick a number of channels, then print that many glides (and pegs if you don't want to use screws), one style of channel, and one style of handle.

For best results use the seam painting function on your slicer to keep it from putting seams in the middle of the channels: In my test prints whenever the seams were in the middle I had to file them down a little to get snag-free motion. Snags at the very end of the channels are fine: the glide stops flush with the end and is beveled so it shouldn't catch once it's all put together.

The channels and pegs are designed for printing with a 0.4mm nozzle. Nothing should need supports.

Variable layer heights have worked well for my prints.

I've printed it in all-PLA and with a PETG base and PLA glides. PLA is a little louder/clackier as it spins but it functions well either way.

My printer (Prusa Mini) is only big enough to print up to the 4-channel trammel. If you print a bigger one please post pictures.

Tolerance testing was all done on my printer. If you try it and it doesn't move well leave a comment and I can post it loosened up a bit.

Assembly Instructions:

I prefer to use M3-10 socket head screws, which will thread directly into the plastic, but I provided a snap-in peg to make it fully printable.

If using screws: Apply some downward force while screwing them in to reduce the chance of the screw pulling the top layers of the glide apart. If you do split on the layer lines so long as it's above the barrel part of the glide it should still function well, it just doesn't look as good.

If using pegs: The pegs are intentionally hard to snap in, but if you can't get them to work try reducing their scale slightly. They're possible to pull back out but shouldn't back out unless you're trying. I used the flat side of a pair of pliers to press them in; really anything hard to spread the pressure on your finger will make it easier.

For 3 channels and higher: Make sure you don't center the handle when attaching the glides or it won't be able to turn. The first glide should be positioned at the center where all channels cross when you attach the second.

Support:

If you like the design and can't be bothered to print it (or don't have a printer) I'm selling it printed in PETG with PLA glides on my etsy shop.