Abstract: A countersinking tool comprises a disposable, flat, chip-like cutting insert mounted on a holder rotatable about its axis. The insert is readily assembled or disassembled on the holder by turning a pin which extends through holes in the insert and the holder. The insert has a flat inside surface for seating on a flat surface on the holder, and the insert has a pair of flat, peripheral side surfaces each of which engages against a respective flat surface or locating portion on the holder to locate the insert in a cutting disposition on the holder. The insert and the holder have various features which cooperate to promote the countersinking function.