The outer legs are fixed, the center one slides
The exposed section of the center dowel shows
the thickness of the bottom and leaves no hole.
The dowel has a 5/16 long brad in the end. Stick it in the center of the
plate you are throwing and it imprints the depth you need to open to.
the big rib flattens the bottom of the plate ,sets the radius of the bottom,
and sets the curve that transitions the bottom to the sidewall.
Special purpose ribs and my "chop"
This one is really handy.