If you have to change coils quite often it can be a real problem gettig some of the coiled wire through the shaft. Often the coiled wire of extra coils is larger in diameter; or in cases where it's the right diameter it tends to become wider over time , necessitating threading the whole thing through both upper and lower shafts with a piece of twine. Often too, once the whole shebang is through and secured on the detector, any mistake in extending the lower shaft too far and then trying to slide it back in place become nearly impossible. You are stuck buying extra upper and lower shafts for all your coils with wide coiled wire if you want to spend time detecting rather than threading needles. A simpler shaft design of the coil wire going around a shaft rather than through it would negate these problems, but then the tightly wound coils we have would become obsolete. Maybe that's why nothing is being done to remedy this?