The construction of a mechanical neutron monochromator for the cold neutron region with 80% transmission is described. The apparatus is specially designed to be used in connection with a neutron crystal spectrometer for filtering out higher order reflections by placing it between the crystal and the counter. An experimental test of the machine has been performed by measuring the gold cross section and the neutron intensity as a function of the angular velocity of the rotor for a certain wavelength. The result shows that no higher order neutrons are transmitted.