Extension rods are one major spare part in terms of drill application area, there are two shapes of extension rods, one is round type, another one is hexagonal type.
But all extension rods are threaded and made by hollow drill steels, this means usually extension rods are hollow.This hollow hole are usually named flushing hole, used for transmitting water or air during drilling. Two ends threads can be used for connecting couplings, shanks, couplings or bits. Typically two ends of extension rods are male, while sometime one end is male and another one is female, then we call it M/F rod. We need couplings to connect extension rods, while for M/F rods, because one end is female, so couplings are not necessary.