When you google "DIY Flying Probe" you will notice that such a project does not seem to exist - until now. Which is a pity as we think the vast amount of desktop 3D printers and router CNC mills have laid the foundation and provided the availability of all required parts to build something like this by now. So lets do it :).
The approach is rather simple: We use the same stepper motors, sliding bearings, rails, geared belts and electronics that are commonly used in most DIY CNC mills and 3D printers today. We use two moving heads though that can move independently from each other: one for the top PCB side and one for the bottom PCB side. Spinning disks with two test pins each would use solenoids to push the test tips onto the PCBs. Another small stepper motor and threaded rod system would be used to control the distance of the probes to each other. End switches for all moving axis would make sure we don't break the machine while developing and improving the control software. The movement in general should be controlled with GCODE.
The most important right at the beginning: We do not attempt to build an industrial grade machine. We want to build a highly affordable good enough "slow" machine that can verify populated and unpopulated PCBs in a reasonable amount of time - for prototyping and low volume production runs when production speed is not that much of an issue. The goal is that anyone can replicate this machine with basic supplies (prototype with 3D printer, off-the-shelf parts, later version with CNC milled aluminum parts) and that we create a developer community around it. Each side would be equipped with a camera (an AXIOM at some point in the future maybe) for aligning of PCB positions (fiducial recognition) and doing automated optical inspection (AOI) of populated PCBs. Here the idea is that the camera scans all boards and identifies component placement errors or solder paste flow problems automatically (with the help of OpenCV).