A parallelogram mechanism of violet conrod and two blue cranks can slide on two brown rods. The pink tube can pivot around the axle that connects green sliders.
Yellow stylus and red spherical milling cutters are on the violet conrod.
Move (by hand) the stylus on upper surface of cyan model, the cutters create corresponding surfaces on yellow works.
The stylus and the cutter diameters must be equal to get cut surfaces and model one identical.