Pink tube can pivot on axle of green wheels that can roll along two brown rods.
The pink tube carries a drive of two identical gears. Motor with red spherical milling cutter is on one gear, orange stylus is on the other.
Move (by hand) the stylus on upper surface of cyan model, the cutter creates on yellow work a corresponding symmetrical surface.
Distances from the stylus and the cutter to base plan must be equal and their spherical diameters must be equal to get cut surface and model one identical (symmetrically).