My MPCNC has about 100 hours on it so far. I just noticed that the finish on some projects has become bad and while debugging I found particles of dust packed onto the rollers. After removing the dust, it looks like the rollers have pitted and eroded somehow. This makes the dust pack into lumps that creates the finish problem and makes the rollers hard to keep clean.
I wonder whether there is some kind of reaction between zinc and the steel of the rollers, perhaps exacerbated by moisture in the wood waste. Note that the zinc from the conduit seems to have worn off the steel also.
The zinc doesn’t react with steel, in fact it protects it from corrosion. It’s what’s called a sacrificial anode, because the way it protects is by corroding first. Galvanization (which is likely the process that put the zinc on the steel) gives a thin layer, and it’s not hard to wear through.
However, you’re reporting issues with the bearings, right? Those don’t have any inherent protection beyond being hardened steel (which corrodes less quickly). Usually bearings live in greasy environments, and that forms an oxygen barrier so the metal cannot oxidize.
The sawdust could well be trapping moisture, and wood itself can have acetic and/or formic acids, both of which will attack metals. It certainly sounds plausible that what you’re seeing is caused by the sawdust.
If you were inclined, you could try to remove material until the bearing is smooth again. However, this won’t be straightforward since you need to remove material uniformly or else your bearing will be out of round. It might be the case that you need to replace the bearings.
I agree - it’s probably something in the sawdust that’s caused the pitting. I noticed the problem after milling a big MDF job. Accoding to Wikipedia, MDF contains the fibers bonded in a urea-formaldehyde resin with some wax. I bet the wax is making it clump under pressure on the rollers. I doubt that the resin is causing the pitting, though.
I have seen videos on testing bearing hardness and not all bearings are the same. My mpcnc squished the zinc out of the way and I had to constantly de-burr the rails for the first couple hundred hours. If your bearings are taking the abuse I would question their quality.
Is it only an issue on the bearings that ride the top of the conduit? Could you mount brushes on the front and back of the bearing carrier to keep the dust off the conduit so the bearings wouldn’t keep picking it up?
