When evaluating a PCB milling machine, prioritize spindle speed, which should ideally range between 30,000 to 60,000 RPM to ensure clean cuts on delicate copper traces. Positioning accuracy is critical; look for machines offering a precision of ±0.001 inches (0.025mm) or better. Additionally, verify the working area dimensions to ensure they accommodate your largest board designs, and check for automatic tool change (ATC) capabilities to improve efficiency in multi-stage milling and drilling processes.