Slitting saw arbor. By Blondihacks. 15:02 video:
https://www.youtube.com/watch?v=-COjawgu32k
Previously:
https://www.homemadetools.net/forum/...411#post169739
https://www.homemadetools.net/forum/...083#post164894
https://www.homemadetools.net/forum/...653#post163065