The inner face/bottom of the cut is curved. In soft wood and with glue ìt doesn't show but in hardwood of 8mm & over it shows as a faint elipse. This application I want for visible box joints for visible decorative work, not for long housings where the 'groove' is filled with a panel edge.But you'd never see it unless squinting down the open end of the slot?
I would be cutting a maximum of 12mm in a pass.