I have no idea what you're trying to say. What is an outer cavity? Do you mean the switch housing?
In any case, the mechanical function of the switch does not affect the rotation of the pot in any way, as the shaft simply passes through a hole in the center of the pot. (The shaft/hole are not round, but notched, however, so that rotating the shaft will also rotate the pot's wiper.) If by scratching, you mean mechanical scraping noises and/or increased rotational inertia, the end of the shaft may be pushed up against something inside of the switch. Try dissasembling the pot and cleaning the part of the shaft that "hooks" onto the switch.