Brass Round

why iron weights are hexagone shape while brass weights are in round shape?

It is true that caste iron weights are usually either hexagonal or octagonal in shape. This is for better grip. For the same reason, a surface roughness is given on those weights. Another advantage is that it will not roll. But round or square iron weights are also available.
Brass, on the other hand is much softer than iron and a hexagonal or octagonal shape will invite more wear and tear on the edges in it. That is why it is round. Usually, brass weights are low weights and small. It is unlikely that rolling of brass weights will cause any damage.

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