The Collaborative International Dictionary of English v.0.48:

Knife-edge \Knife"-edge`\, n. (Mech.)
   A piece of steel sharpened to an acute edge or angle, and
   resting on a smooth surface, serving as the axis of motion of
   a pendulum, scale beam, or other piece required to oscillate
   with the least possible friction.
   [1913 Webster]

   Knife-edge file. See Illust. of File.
      [1913 Webster]