A smooth rod is a metal rod usually used on the axis for components such as the X-carriage or print bed of a RepRap to slide on. The most commonly used diameter is 8 mm, but 10 mm and 12 mm are seen in more rigid designs as well. Stainless steel is a pretty hard steel, and doesn't rust, making them ideal to slide bushings on. (A smooth rod can be seen as a kind of cut-to-length construction material).
Minimum material requirements
Bearings/bushings have minimum requirements on rod surfaces to avoid exorbitant wear. For example, using a ball bearing on an aluminium rod wouldn't last more than a few hours.
|Slider type||Rod material|
|Ball Bearing||Hardened steel, ideally > 60 HRC|
|Brass bushing||Steel, stainless steel or hardened steel|
|Plastics bushing||Anodized aluminium or one of the above|
Not all rods are created equal, and lower quality rods might be too small or too big by a fraction of a millimeter, which will cause problems with the bearings. Unless you have adjustable bushings, ensure that your rods have precisely the correct diameter, or when buying, ensure that they are 8h6 (0.009mm) tolerance. Any bend in the rods will cause issues, so it may be useful to sight down the length of each rod to ensure it is straight.
Smoothing the smooth rod
This isn't required for rods which are made for bearings and bushings. Sometime you want to go with even cheaper rods, though, where this can reduce wear.
If the finish of your smooth rods is too rough, you can smooth it up by carefully sanding the surface with an 800 grit wet & dry abrasive paper, then a 1200 grit paper. Then put the final polish on the rod using metal polishing paste (the same stuff as what people use to polish their car wheels). In about two hours you'll have all your rods polished.
Testing wether a rod is straight
A simple procedure for checking straightness is to roll a rod over a flat surface, like a smooth table or a piece of window glass. Straight rods will roll evenly, curved ones not so.