def set_speed(self, value):
        ''' set total axes movement speed in mm/second'''
        self._combined_speed = float(value)
        speed_per_min = int(self._combined_speed * SEC_PER_MIN)
        command = GCODES['SET_SPEED'] + str(speed_per_min)
        log.debug("set_speed: {}".format(command))
        self._send_command(command)
set total axes movement speed in mm/second