Pink double crank carrying orange stylus and red spherical milling cutter can pivot on axle of green double slider that can move along two brown rods.
Move (by hand) the stylus along the cyan model and turn (by hand) blue gear crank, the cutter create 3D surface on yellow work corresponding to the model 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.