i adjusted my chain today. i was being so anal about the slack and aligment of the wheel that i ended up tightening and loosing the axle nut about 6 times. i noticed a lot of metel chips falling from under the axle nut. is that the washer gettting worn in from the nut?? do i need to replace the washer next time i loosen the axle nut to adjust the chain or is it totally normal for that friction between the but and washer to leave metel dust.