Looking at it, it appears to be a circular saw held on two axis. I assume the saw position can be moved into or out from the central pivot, and the central pivot is a rotary axis.
Cutting outside the shape, as shown, the centre of rotation will be from the inside edge of the saw, to the central pivot, the width of the blade as it swings just cuts outside the line.
It would be difficult to calculate if you were cutting an inside edge though.