How to create concave cut

 From:  zarkow 18 Aug 2017  (6 of 18) 
8554.6 
Maybe

https://1drv.ms/u/s!AgF6VFezvIYchAX5n0R5nhZNFOr-


or maybe

https://1drv.ms/u/s!AgF6VFezvIYchAbDEKlXqMC_6_d6