Securing the center of the lensboard isn't sufficient. You need to secure the outside edges too, so that the board doesn't move as the cutter goes through it. Put a piece of waste underneath and tighten everything down.
That is what the geared rotary table is for, the board is clamped to it, then the table rotates slowly, as the mill cuts the whole. Probably easier to understand for those who have a milling machine.