An apparatus for detecting a planar endpoint on a semiconductor wafer during mechanical planarization of the wafer. The planar endpoint is detected by sensing a change in friction between the wafer and a polishing surface. This change of friction may be produced when, for instance, an oxide coating of the wafer is removed and a harder material is contacted by the polishing surface. In a preferred form, the change in friction is detected by rotating the wafer and polishing surface with electric motors and measuring current changes on one or both of the motors. This current change can then be used to produce a signal to operate control means for adjusting or stopping the process.