An apparatus for detecting machining conditions in a machine tool characterised by electro-magnetic bearing means (10) for magnetically supporting at least one spindle (9,15) of the machine tool; a detector (20) for detecting a variable which depends upon the electro-magnetic state of the bearing means (10) and for producing detection signals in relation thereto; a plurality of different frequency band pass filters (32-35) coupled to the detector (20) so as to receive the detection signals from the latter; and comparator means (36a-36d) for comparing output signals of said different frequency band pass filters (32-35) with respective set values.