Linear bearing shaft quality requirements

I have a linear motion setup that I am building for a 3d printer I've designed.

I'm using misumi recirculating bearings of the 12mm variety.
(MISUMI USA: Industrial Configurable Components Supply)

I'm running them at very low speed and low load on 12mm hardened, chromed precision linear guide rail.
I am having issues with the bearings feeling crunchy and gritty, the bearings feel pretty bad, but best I can tell, the components are all new (granted from eBay)

When I got the rails, I micd them and checked for roundness, hardness, and surface finish.
They're straight and round within a tenth, and 12mm within a tenth as well. They're around 65hrc, and The surface finish is around 14micro inches.. I have a hunch this is my problem. The surface finish feels like it's from a centerless grinder, and to a fingernail it's just slightly "catchy"

I did end up disassembling and repacking the grease in one of the bearings to see if that may be the problem. I couldn't quite fit my profilometer in the bore to check that, but they don't appear to be used or worn at all, and the surface finish feels really nice.

What do you guys think might be the issue? Do recirculating bearings on round shaft just normally feel pretty crunchy? Or did I just get a poor quality set of rails from eBay.

You bought them from eBay, noted they feel rough, and they're probably not even chrome to begin with. There's your problem.

Buy good Thompson shaft, or whatever quality stuff you can buy. Many machines where I work run on linear bearings/tracks for millions and millions of cycles and are still working just fine. I do not know, but I doubt they're chrome.

Does take a bit to wear in and it takes very little grit to get in them to make em feel rough as guts tooo!

adama said:

Does take a bit to wear in and it takes very little grit to get in them to make em feel rough as guts tooo!

Click to expand...

Same with flared cup needle bearings. One piece of dust and it's a paddlin!