The invention refers to an apparatus for monitoring the operation of an electrically conductive tool (120) of a machine tool (100, 400) having at least one electrically conductive spindle (110) for holding the tool (120) and a detector (140, 545, 645) which determines an electrical resistance (190, 290, 390) between the spindle (110) and the electrically conductive work piece (130) to be processed, and wherein the spindle (110) is electrically isolated from the machine tool (100, 400) and/or the apparatus is adapted to electrically isolate the work piece (130) to be processed from the machine tool (100, 400).