Knife edge, in one meaning of the phrase, means there is little to no straight bore. Drill a countersunk hole in a sheet or plate, you get a countersink, and a straight bore hole. If the countersink is too big, so there is no straight bore in the hole, then you get a knife edge. Side
view of a hole countersunk in a thin sheet--see how sharp
the edge between the countersink and the bottom horizontal
surface? that's the knife edge:
Knife-edging is when the rivet/fastener countersink is more than 2/3rds the thickness of the sheet ... this is general practise. i'd guess feather edge means that the CSK is >=100% of the thickness