The smooth rods don't need to be stainless. However, getting hardened rod (not cheap drill rod) is a good idea. Cheap drill rod will end up with grooves in it from the linear bearings, and tends not to be as tight of tolerance. It will still work though if you are on a real budget. The hardened rod from McMaster is a good choice (part number 6112K42 is what you're looking at?)
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