In order to manufacture powders that are extremely fine, a ball mill is usualy used due to its low maintenance and ability to work for long periods of time without attention. It is reliable and relatively cheap to make.
A ball mill works by rotating a drum, or canister, filled with the coarse powder and metal balls. As the drum slowly rotates, the balls 'ride up' the wall of the drum and then fall back down .
When the balls fall, the impact other balls and the base of the drum at high speed, crushing any coarse powder between them into very small fragments. The only issue is that only a very small amount of powder is crushed by one collision at a time, so typical run times for a ball mill are around 6-8 hours - the longer powder is left in a ball mill, the more consistenly fine it gets.
The initial design for the ball mill was one where the drum itself had 2 holes in it to attach to a rod that was spun by the motor. The motor would spin slowly and the drum would be supported by another rod on the side opposite the motor
While this approach was considered at first, it came to light that there were several flaws with this design. The most pressing reason was the fact that it would require two holes to be drilled in the drum. This would mean sealing a potential leak of very flammable powders, inventing a system to allow the drum to 'slot' into the ball mill and the fact that the drum would contain many heavy balls and powders. The combined mass of the drum would be around half a kilogram, which could easily bend an axle or the output shaft of a motor over time.
To combat this, a new design was made. The drum would sit on two rollers. They would be vertically misaligned, so that one of the rollers would carry more weight than the other. This roller would be free spinning, its job only to take most of the load away from the motor's shaft. 
It The other roller would be the powered one. It would be wrapped in a grippy substance such as rubber so that when the roller spun, it would spin the drum. This setup has the advantage of the roller and drum acting as a gear - many rotations of the smaller roller would be needed to spin the drum once. This means no external gears are necessary.
Next, a 3d CAD model of the ball mill was made. I used Onshape, a free 3D CAD Software. Onshape's workflow is sketching a 2D shape, and then extruding it, joining it with other shapes and manipulating it until you get your desired shape. I started by sketching out a 2d circle of radius 0.47cm.
I then cut this circle and connected the cut peices of the circumference with a straight line.
This was then extruded out by 1cm to accurately model the motor's shaft. All the colors of the shapes in Onshape are randomly generated, so I decided to leave the objects as they are until the end.
Another cylinder with life-matching diameter was made that centred around the shaft, and then another smaller one for the main body of the motor. The motor we used had a wider section that housed a small gearbox.
Two small pins were then added on the back to act as the electrical connectors to the motor with +- 18V DC.
Now that the motor was complete, it neeeded a frame to hold it. Firstly,
3 blocks were made by sketching their rectangular outline and extruding
them all by 1.5cm.
Next, a drum had to be made to be placed in the frame, the cylinder that would hold all the balls and powders. It was modelled with a large circle of diamater 3.5cm, and a thin rectangle with the same length as the cylinder
The circle was extruded to make the lid and walls of the drum, and the rectangle was extruded to make a notch in the drum so it would be obvious when it span.
This process was repeated for the roller.
And for the shaft the motor connects to.
After assembling all the parts, this is the final product
The assembly was animated so that when the motor span, the shaft would spin at the same rate. The cylinder would then spin at a rate defined by (Shaft diamater/Cylinder diameter) x -1, as it would spin the opposite way. The other supporting roller would then spin the opposite way again at a speed defined by its diameter.
This is the final 3d CAD model.
