What does a Rock tumbler do?
A rock tumbler, also known as a stone grinder, is used to give rough stones a glossy shine by grinding and polishing. Silicon carbide is used as the abrasive in this process. There are essentially two different types of stone renewal machines.
The first type is the rotating drum that picks up the stones. These machines are particularly suitable for grinding rough stones into a round shape.
The second type of grinder is the vibratory stone grinder. These machines are perfect for finishing work. In other words, to polish the stones to a high gloss. It works on a completely different principle, in that the stones are set in motion by vibration and rub against each other. Most vibratory stone grinders use a motor that drives an unbalanced weight to generate the vibration in the container. One disadvantage of this design principle is that there are moving parts that can wear out quickly.
A very attractive design alternative is to generate the vibration using a electromagnet rather than a motor. The magnet is driven by a frequency generator and attracts a metal anchor plate on which the container with the stones is located.
My tumbler consists mainly of 3D printed parts, an electromagnet and two compression and two extension springs. The construction is very simple and can be made with any 3D printer and commercially available parts.
You can download everything you need to build it here: Etsy, Cults3D
The function:
The stones are placed in the upper section of the black container shown, which is closed by a fitted lid. This container is attached to the swivel bracket by means of a strap. A vertical spring can be seen on the left-hand side of the picture, while a horizontal curved spring can be seen on the right-hand side. The electromagnet, and the compression and extension springs are located in the middle of the picture.
The vertical spring gives the container one degree of freedom in the horizontal direction, so that it can only move horizontally in this area. In contrast, the horizontal, curved spring allows the container to move both vertically and slightly horizontally. When the electromagnet is activated and the iron plate is attracted, the container undergoes a horizontal back and forth movement on the left side, while a slight circular movement takes place on the right side. This circular movement results in a minimal throwing motion inside the container.
This continuous circular movement of the container, which oscillates at around 30 Hertz (i.e. 30 tiny throwing movements per second), causes the stones to be thrown slightly further with each swing. Due to the circular inner structure of the container, the stones inside also move in a circular motion. This causes the stones to rub against each other extremely quickly, with the silicon carbide creating the abrasive effect.
The stones behave differently depending on the grain size of the silicon carbide. It is therefore crucial to be able to precisely adjust the frequency and vibration amplitude of the container. These settings are influenced on the one hand by the fixed frequency of the magnet and on the other hand by the pre-tensioning of the tension and compression springs. These parameters allow the machine to be varied from a gentle to an aggressive setting.
Every body has a specific resonance frequency at which it starts to vibrate and reaches its maximum amplitude. In structures such as bridges, this frequency range can be dangerous and even lead to collapse. The situation is similar with the 3D-printed rock tumbler – it is important not to operate it at its resonant frequency. This is easy to recognize when you approach it: The iron plate starts to vibrate and bumps against the magnet
It is therefore advisable to operate the machine in a frequency range slightly above the resonance frequency. As the frequency increases, the amplitude of the container decreases.
Another decisive factor that influences the amplitude of the tumbler is the pre-tension of the tension and compression springs. A higher tension leads to a stronger damping of the system, which reduces the amplitude, i.e. the vibration amplitude of the container. It is important to find a balanced middle ground that meets the respective requirements. I will explain the exact instructions for setting the machine correctly in more detail later.
3D printing:
The components are characterized by their simple shape, which enables printing without support structures. Due to the high stress caused by vibrations, there are some considerations to take into account when 3D printing. All parts should ideally be printed from PETG, although PLA will also work but could potentially limit the life of the parts.
Vertical spring: The spring is printed flat on the print bed and care should be taken to ensure that the perimeters run from bottom to top (i.e. in a vertical direction). Transverse perimeters could cause the nib to break along the lines under load. An infill of around 25-30% is recommended.
Curved horizontal spring: This spring is subjected to the highest load as it has to move both horizontally and vertically. It is printed upright with a honeycomb infill pattern with 23% infill and two outer perimeters.
Stone container: The black container in the picture above was successfully printed using an SLA printer and ANYCUBIC Tough 3D printer resin. Alternatively, it can also be produced with a conventional FDM printer, whereby the number of outer perimeters should be increased to about 6 to ensure watertightness. I have printed containers made of PETG and TPU.
TPU is a plastic that has rubber-like properties. It is very abrasion-resistant and is therefore very suitable as a container material. The PETG container wears out relatively quickly. However, it works very well for several months.
Another very convenient option is to have the container printed by JLC3DP , which offers an excellent and affordable service.
Magnet bracket: The bracket to which the magnet is attached can initially be made using a 3D printer. However, for improved vibration properties, it is recommended to use an 8-10 mm thick metal plate. Alternatively, JLC3DP can also be recommended here for printing the metal bracket. I had the company 3D print the metal magnetic console 3D printed.
Container cover: The container cover is printed in TPU, but is optional. A cling film and rubber rings can be used instead to easily seal the container while allowing a good view of the stones.
I had the pleasure of trying out JLC3DP and am very enthusiastic about the service they offer. JLC3DP not only offers plastic 3D printing processes such as
SLA, FDM, SLS and some more, but also high quality metal 3D prints at a really reasonable price. Furthermore JLC3DP offers turning and milling services
using a 5-axis milling machine and complete production of circuit boards. I am convinced that I will be using this service very often in the future.
JLC3DP 3D Printing Starts at $0.3, Sign Up Get $60 coupons: https://jlc3dp.com/?from=Fraens
Assembling the vibration tumbler
It would be advisable to watch my video on Youtube as it shows the assembly of the machine in great detail. If you decide to build the Rock Tumbler yourself and download the necessary files, you will receive an assembly drawing with a parts list in which all the required parts are linked. The download package also includes the circuit diagram, the Arduino code and, of course, all .stl files. You will also find a complete 3D model of the machine in .step format.
The most important thing is the precise adjustment between the magnet and the metal plate as well as the preload of the tension and compression spring. With these parameters, the tumbler can be adapted very precisely to the respective requirements. Other important parameters are the switch-on and switch-off times of the solenoid. I will go into this in more detail later.
In order to provide the screws with a secure and long-lasting hold, it is recommended to press fusible nuts into the 3D-printed parts using a soldering iron. This method has proven to be the most durable and stable solution. It has worked perfectly for me for several months. Alternatively, it is also possible to connect the parts with nuts.
The point where the semicircular spring is connected to the reservoir is the point where the greatest forces occur. I repeatedly had spring breakages at precisely this connection point. This problem was solved by using another semicircular part. Since then I have not had any more broken springs.
The vibrations generated by the tumbler are quite considerable. It is therefore essential to create a solid foundation. For this purpose, I used a foundation stone, which I also weighted down with iron weights. The heavier the better. The entire tumbler should be connected to the foundation using rubber buffers to ensure optimum damping.
Electronics and Arduino code
The circuit diagrams and electronic information presented on this website are for illustrative purposes only and are based on my own experience and procedures. I assume no responsibility for the accuracy, completeness or timeliness of the content provided. Any use or implementation of the information presented here is at the user’s own risk. I expressly exclude any liability for damages that may arise from the use of this information. It is recommended that you consult a specialist before implementing any electronic projects and take appropriate safety precautions.
The electrical installation is relatively straightforward. A 24V voltage source, an L298N H-bridge, a 12V electromagnet, an Arduino Uno and two potentiometers are required. The wiring has been designed in such a way that even beginners in electronics should have no problems with it.
It should be noted that the 12V electromagnet in my setup is operated with a voltage of 24V. This gives the solenoid considerably more power and generates a much stronger vibration. This combination is only possible because the solenoid is operated at a frequency of around 30Hz. In this way, the magnet has enough time to cool down between cycles. In addition, the Arduino program switches the tumbler off completely for 10 minutes every hour to allow the magnet a further cooling phase. The machine thus runs for 50 minutes and then cools down for a further 10 minutes. This system has been working faultlessly for several months. However, it should be noted that use is at your own risk as the magnet is operated outside its specifications. Initially I had doubts as to whether this would work, but after about 4 months it has proved to be possible without any problems.
Two potentiometers are used for control. The first potentiometer can be used to set how long the magnet remains switched on, while the second is used to set the duration of the switch-off time. This enables extremely precise adjustment of the oscillation amplitude, allowing the tumbler to be customized to suit individual needs. This has proven to be one of the most important and best functions that can be integrated into a tumbler. The ability to precisely adjust the tumbler using both the spring system and the magnet’s on/off time means that particularly precise adjustment can be achieved.
The polishing process
Polishing stones to a high gloss requires patience, especially as the time required can vary greatly depending on the base material. If river stones are used that already have a round shape, the polishing process is considerably shorter. Different grain sizes of silicon carbide are used for grinding and polishing. I used silicon carbide in the grits K-80, K-220, K-500 and K-800. Aluminum oxide with plastic pellets was used as the final step.
The tumbler works extremely efficiently and a thick sludge of stone particles forms after just a few hours. This sludge can thicken considerably and become solid if too little water is used. The right amount of water is therefore crucial. Over time, you develop a feel for how much silicon carbide and water are needed. I check my tumbler twice a day, morning and evening, and add water if necessary. If the mud gets too dry, the rotation of the stones stops. This should be prevented by adding enough water, otherwise the mud can become very hard. Every 2-4 days, remove the stones from the tumbler and clean them thoroughly, as the silicon carbide wears away over time. It should not be used for longer than 3-4 days as it loses its abrasive effect.
When I feel that I have finished with a certain grit, I clean the stones thoroughly with a brush to ensure that no coarse particles get into the next finer polishing stage. This is very important as the stones need to be extremely clean.
The whole process can take up to two months, but the reward is beautifully shiny stones. A fascinating hobby for stone lovers!
The polishing process
This is followed by the polishing process. Aluminum oxide and plastic granules or ceramic granules are used to slightly cushion the stones. This process usually takes around 1-2 days.
The stones are then vibrated again with washing powder for a few hours. Finally, the stones are vibrated again in pure water to give them a final clean.
The result is beautiful, shiny stones. I am extremely pleased with the machine and look forward to further trials.
For the next pass, I plan to round the stones a little first to pre-smooth the sharpest edges. I am confident that this will significantly shorten the initial grinding process with the coarse silicon carbide.
You can download the files here.
You will receive
– all required 3D files in .stl format
– a complete 3D model in .step format
– an overview drawing in .dxf and .pdf format
– a parts page with links to the purchased parts
– the circuit diagram
– the Arduino code
You need to build:
– Compression spring 1,2x12x50
– Rubber base
– Extension spring 1,2x10x50
– E-magnet
– 24V power supply
– Arduino
– L298N Motor Drive Controller Board
– some screws, melt-in threaded nuts
