The Secret to Fabricating the Ball Joints that Enable Micro Manipulation
Diffraction Limited demonstrates a new method for fabricating the ball joints necessary to build an Open Micro-Manipulator.
We covered Diffraction Limited’s very impressive Open Micro-Manipulator last year. It is an affordable, DIY option for accurate and precise manipulation at tiny scales — we’re talking sub-micron. Movement at that scale is very challenging and the Open Micro-Manipulator’s ball joints are critical to its success. In their newest video, Diffraction Limited provides a detailed overview of an improved method for fabricating those ball joints.
The ball joints are key to the Open Micro-Manipulator design. They constrain the parallel arms at each end, while still allowing for free pivoting movement around the imaginary point at the center of the balls. Elastic bands hold the rods tight against the balls, so the linkages themselves don’t have any backlash — something that would be unacceptable at the scale this works at.
But the previous ball joint design wasn’t easy to fabricate and also had some flaws. In particular, the cup on the rod didn’t match the ball, which resulted in limited contact area. The new design has an equal diameter for both, so the cup has full contact with the ball. The new design also has a better fabrication method that improves the tolerances.
That fabrication process is pretty fascinating and it’s the reason why I wanted to cover this update. The structure onto which the ball mounts is still 3D-printed, but the “pocket” the ball rests in is on a PCB. That is a very affordable way to get a complex custom part with decent tolerances.
The rods are even more interesting, because Diffraction Limited uses the balls themselves to create the cups. The rods are brass and the balls are steel, so tapping the balls into the rods will form the cups. Diffraction Limited designed a specialized 3D-printed jig that holds everything in place during the forming process, with mounted calipers to ensure the length of the rod ends up perfect.
As a bonus, Diffraction Limited even made a stepper-driven machine that automates the rod length tuning. After using a premade rod as a reference, the machine pushes the ball into the rod until it is a match. That isn’t necessary for building your own Open Micro-Manipulator, but it lets Diffraction Limited speed up the process.