This is the most sturdy and accurate means for cutting small glass circles; simply apply a small amount of oil to the area that you'll be scoring, then apply gentle vertical pressure onto the handle and turn one full circle. Do not go over the score line more than once as this will cause the head to become blunt.
This lens cutter cuts a variety of different sized circles ranging from 12mm (1/2") diameter up to 127mm (5") diameter.