select your knive, rmb click==>edges, select one of the formentioned edges and then press f3 and look in the edit polygon menu ==>selection==>convert to edge ring
when you hit this every edge around the cutting edge of your model will be selected, deselect(hold ctrl) the edges you don't want to be sharp.
press and hold shift and rmb and select collapse edges in the merge collapse menu.

hope this helped

good luck