I built my MPCNC last year and never really finished it. One thing I was struggling with was smooth movement of the gantry. I recently made a filament holder for my 3d printer with the same bearings I used for my MPCNC (https://www.amazon.com/gp/product/B073ST742Z) and realized that they have some kind of very sticky grease covering everything. They do not roll smoothly and the filament spool actually just spun on the rollers instead of spinning the bearings. I assumed that this was normal for the bearings but after thinking about it when I was a kid with a skateboard I never had bearing like these. I used some sewing machine oil and managed to get them spinning decently but I still had to cover the printed part of the holder with a rubber band to get enough traction on the spool for it to turn the bearings. Is there some trick to cleaning these bearings up so they spin freely? I think this would greatly help my MPCNC as I made a very large version.
I bought two different types of Chinese bearings, none of them worked well. Third timeās a charm, I ordered from Ryan and they worked flawlessly.
If you want to give the ones you have a chance, you can try soaking them in isopropanol and lubricating them with lithium grease or something else suitable.
I donāt have any isopropanol but I have plenty of acetone. Iāll give that a try and see how it does. The replacement lubricant is where Iām not sure what to go with. I did some reading and it seems everyone has their own magic lubricant for bearing but I canāt find much on CNC applications. Is there a general consensus on whatās good for CNC machines? I can imagine the bearings get extremely dirty with all the particles flying off the cutter. I almost think grease would attract too much debris.
Honestly - I have no proper clue regarding the lubricant! I once heard that lithium was good, but I just pretend to know stuff until someone more knowledgable comes along 
I think itās just as important how well they are sheilded - if thereās a gap between the shield and the metal rings. That happened to one of my aliexpress orders, they were supposed to be 2rs, but you could see through the shieldsā¦ not very debris friendlyā¦
The MPCNC doesnāt really put a lot of stress on these bearings. They arenāt subjected to any real weight, heat or RPM.
Youāre right in thinking that the biggest consideration is keeping dust and chips out of them.
If they rotate relatively easily by hand, theyāre probably good to go. If not and you want to try cleaning them, test one bearing first in the solvent to see if the acetone eats whatever they made the shield out of. Iāve never bothered with it before on small bearings, but if you look online there might be a way to remove and re-install the shields, but I doubt it.
You could try gently warming them with a hair dryer and see if some of the grease runs out.
Honestly though, unless they feel gritty and as long as you can get them to rotate by hand, I would give them a try and see how they do. My first few laps around the mpcnc were a bit gritty and rough, but it broke in and got smooth within a day. 
I used cheap bearings too. I also made my kids fidget spinners. I used a combination of brake cleaner and carb cleaner to get all the gunk out of the bearings. The fidget spinners got sewing machine oil on them. The MPCNC got 3-in-1 oil. Just a drop on the inner races to keep them from rusting.
I also tested each bearing after cleaning. If it didnāt spin freely, it got put into one pile. If it did, then I put it in another. I used all the freely spinning ones on the gantry. The less spiny ones got used on the belt guides. Iām pretty sure that doesnāt matter at allā¦ like 0 differenceā¦ ever.
If you really think about it, the amount of pull the stepper motors have, the weight of the gantry, the resistance of the bit plowing through the materialā¦ any friction from the bearings on the tubes is going to be the least of all the forces at play on the machine.
After reading the responses I think what Iāll do is make some gcode to make circles and run that for a couple hours to see if the bearings ābreak inā and spin a little more freely. I went to the garage yesterday and after looking at all the work and bolts I would have to take apart to get all the bearings out I think that made up my mind lol. I will use acetone on the bearing I have for my filament roller as an experiment to see if these bearings are even capable of spinning nicely and maybe some day down the road when the bearing do get lots of debris in them I can do it then. Thanks for all the input.
a couple of lines of code repeated a lot of times would help.
Iāve seen a lot of demos of people doing a 3-axis figure 8.
Might run the following manually to make sure I did the math right and it fits your table size, but you could do something like this: (not sure if I have the Z values good I can try this out in a little while too) This is assuming Z0 doesnāt have your spindle crashing into the board and you have enough Z height to raise 10mm.
G1 X200 Y150 Z10
G1 X150 Y200 Z5
G1 X50 Y10 Z0
G1 X10 Y50 Z5
If that works, paste it into a text file a couple of 100 times, then let her rip.
I have cleaned bearings with gasoline, a toothbrush, and a small container since I was a kid, but break parts cleaner also works well too. You gotta remove the shields first, and afterwards blow them out with some compressed air, and grease with lithium. The Cnc moves slow and sees a lot of dustā¦ so grease and shields are probably the best. After greasing it is a good idea to ābreak them inā a bit. You donāt really have to break them in, but move them enough so any grease that wants to squirt out the sides can do so before it gets all over your machine.
I usually use isopropyl alcohol as a solvent for cleaning bearings. It worked well but I never use other kinds so I donāt know how to use them.