Rod vs. shaft #
In engineering parlance, a rod means a round stick and a shaft means a really round stick.
If you look at a 6 mm diameter steel rod like McMaster 8920K22, you’ll find it costs around $9 per meter, can be undersized up to 0.05 mm, and has no straightness specification.
Compare that to a 6 mm steel shaft like Mcmaster 1482K64. The shaft costs around $40 per meter, can only be undersized by 0.01 mm, and has a straightness specification of 0.03 mm per 300 mm of length.
Generally, we buy rods unless we need to make something spin. For spinning stuff, we use shafts.