Rotary Tumbler

Project: Late 2018

Writeup: April 2021

Also, if you turn it upside down it's a little car

This was a prototype that turned out so ROCK SOLID™ that it's become one of my most-used devices and hasn't missed a beat.

It's just a jar which you fill with some kind of media and your parts. It then rotates at a speed you set in order to gently grind / polish whatever you put inside it. I use it a lot for small parts (eg. knife bolsters) and disassembled tools, the only limit is what you can physically fit in the jar.

I just wanted an excuse to play with some casting epoxy...

The cylinder itself is constructed from a cheap screw-top plastic food container (from the legendary Khan's Bargain, Peckham. They've got everything...). I chose this one because of it's hourglass shape which I thought might help it self-center itself on the rollers. It turned out to actually make it much worse, but two little guides on either end keep it in place. Internally, I added some wooden baffles to help with agitating the media. Then the whole thing is covered in a hard casting epoxy which is slowly worn away and can be topped up with a new pour if needed (I've only done this once after two years of use). All seems to work fine. I considered glass jars, but I'd be nervous about heavier parts hitting that 1-in-10000 ultra-bounce and smashing the whole thing...

I've mostly used it with specialised ceramic media and water, with a splash of detergent (the foam that forms helps to quiet the whole thing down). I did buy a bottle of the expensive solution you're supposed to use with the media but never found it to be needed. Maybe it prolongs the life of the media or something, but it's not like I use it enough to notice.

For a very fine matte polish, I've also found a mixture of stainless-steel pins, metal polish and WD40 to work surprisingly well. Much messier than the water though.

I tend to set it up on the sofa to prevent vibrations being transferred into the ceiling downstairs (sorry No. 75....). This coupled with an ad-hoc isolation booth made of cushions and throws means you can't even hear it in the next room. Fine to leave running all day.

Aside from it's surprising robustness, this monstrosity is only notable because it was one of my early forays into PCB design. I happened to have the stepper motor to use, so decided to make a control board for it based around a jellybean A4988 driver. These drivers just take a PWM signal to tell the motor to step so abstract away all the complexity of microstepping etc. In my case the variable-frequency PWM signal is provided by a 555 in an astable configuration. Don't laugh at the whole buck converter just for the 555, this was before I knew about voltage regulators. Obviously a 7805 would be fine in place of that.

Can you tell I'm not an electrical engineer?

This was my second PCB, after the relative success of the Pumpmaster 5000. All done via EasyEDA because JLCPCB was the service I'd seen advertised most. For a complete amateur, it was remarkably easy to convert this simple schematic into a board layout and then into a physical board! The hardest part was getting all of the footprints for the specific boards I had correct. But we got there in the end.

All in all, two revisions isn't bad for a first timer!

If I was to do another revision, I'd have dip switches for all of the configuration pins on the driver so that you could modify the microstep parameters etc. on the fly. And even add another 555 assembly to feed a slow PWM signal into the step direction pin! For a kinda back-and-forwards washing machine motion which would help even out the wear in the drum automatically.

The next hardware revision will get a bigger drum, bigger motor, actual bearings on the casters, an actual enclosure for the driver and an actual knob for the potentiometer.