I did this regularly for a while. Put a hand towel over each cap of an iron dumbbell. Punched it lightly about as much as it could tolerate until I couldn't anymore. Then, I folded each towel to make it thicker. Punched it a little harder until I couldn't anymore. Then folded it again. Eventually, I was able to punch the caps without a towel over them, but not that hard. These days, I don't really do it that often, but it definitely helped.