The rotary cutters I have had did wear out eventually. The stem becomes worn and is not round. When that happens, the whole thing is tossed. Cheaper to buy new and have it work properly .
I have a mis-shappen left index finger thanks to not paying attention. I sliced a pretty good chunk off, but after racing to the dr. and being treated by EMT, the dr. threw the piece in the trash. Too small to sew back on. I still have the scar.