These are precision parts that, in time, are subject to wear. This wear is dependent on your shaving method and beard type. The foils should be inspected visually for nicks, dents and wear patterns before each use. The cutter blocks are not self sharpening and will wear out over time. To maintain optimal shaving performance you should replace these parts when worn. Generally, with good cleaning habits and an average shaving frequency, Braun recommends changing foil and cutter block every 18 months.