how to remove materiel from round surface
I've always viewed it as cheating, but I've made a similar feature by:
- Using a split command to create new model faces as Stephen said.
- Offset/copied, or knit the surface
- Used the Move Body tool to rotate and position the surface as needed
- Created a Cut Extrude up to the newly positioned surface.
Deleting, and reforming the desired feature with surfacing tools would be less cheating, but if it works, it can't be too bad.
Another option is simply making a plug or block of the desired shape, and then using the Indent, or Combine tool to do a Boolean operation and remove the unwanted material.
Lots of ways deepening on what you are looking for
Extrude cut. You can set the end condition to be a sugar or offer from surface.
You can check out the split command and the intersect command.
If you can be more specific to the results you are looking for I'm sure people will show you a few different ways to get it done.