[cadguy247]

I have a question concerning the bearing setup on the cnc fusion sx2 z axis. I'm not saying anything is wrong, yet.... but something doesn't seem right to me.

I've been having issues over time with the z axis progressively get worse z depths. so i was cutting plastic last night and notice that the head was moving quite a bit. Figured it was the gibs loosening up, so took the ball screw and mount off to adjust the gibs, and the gibs checked out pretty good. So then i started looking at the bearings and ball screw and looks like the bearings are shot, as the ball screw could wiggle back and forth quite a bit. I could see the inner races move around and realized that the bearings were gone, only had this kit 7 months now and not a ton of use. Not the end of the world, bearings are cheap I can replace them.

Now my question comes to this, since I converted a PM-25MV recently and have read a ton of G0704 builds and all use either double row angular contact bearings or 2 single row angular contact bearings, It seems that the way the bearings are set up on this cnc fusion kit is incorrect, but please correct me if i'm wrong. I'd rather ask the question here and get everyones opinion first.

First of all, the finish on the diameter of the ball screw where the bearing sits has a horrible rippled finish and the diameter is .005 undersize of that of the bearing, this right here alone I feel is wrong,

Second would be that both of the bearings pockets are larger than the OD of the bearing, again, this I feel is not right either

And lastly, the bearings used in this axis are (2) 6000rs deep groove ball bearings spaced out on the outer race and preloaded on the inner races. This is what looks like caused the bearings to die out so quickly. As far as i know, these type of bearings are not supposed to be used in conjunction with each other and preloaded, i believe they are mainly for radial loads and very minimal axail loads, but i'm certainly no expert with bearings so this is why i'm asking the questions here.

My plan is to replace the bearings with a double row angular contact bearing and a thrust bearing on the ballscrew diameter being too small, I think my only option is to shim it, not ideal but the only thing I can think of other than having it laser welded and turned or ground to the correct diameter

please let me know what you think.

Thank you
Kyle

[Fastest1]

Though I would not be happy with the finish quality necessarily, the ID and OD's are not as critical IMO. If the bearings are preloaded correctly I wouldnt think the ability for the screw to move up and down would have an effect (or a very slight one at most). Now if the bearings couldnt be properly loaded, I could see an issue. In time that movement could cause some wear and lose some preload but that would be quite a ways down the road.

[cadguy247]

Though I would not be happy with the finish quality necessarily, the ID and OD's are not as critical IMO. If the bearings are preloaded correctly I wouldnt think the ability for the screw to move up and down would have an effect (or a very slight one at most). Now if the bearings couldnt be properly loaded, I could see an issue. In time that movement could cause some wear and lose some preload but that would be quite a ways down the road.

see that is the problem, i dont think they were properly preloaded as the inner races now wobble allowing the ballscrew to tilt up to 15-20° per side. And i dont really think that these type of bearings are supposed to be stacked like angular contact bearings, i believe they are supposed to be used only one ,not two stacked and preloaded. these are basically $3 skateboard wheels... Also i am pretty sure that the ID should have to have a slip fit, but the OD i could see being a little loose

I will at some contact CNC Fusion about this, but want to make sure that what i'm thinking is correct. However, i dont expect much to come of it and plan to just take care of it myself

[Fastest1]

My Sherlines use what appear to be skateboard bearings in their CNC mounts. They actually work very well though small for this intended application. I am sure yours are larger but I understand your comment.

In regards to the 15-20 degree movement, you are talking about with the screw and nut in your hand, not mounted, correct? I would be suspicious if you didnt need to re ball the nut with larger balls. What kind of backlash are you seeing? Do you have any time on this machine or is it just being built?

Also when you say stacked bearings, they are squeezing the mounting bracket from opposing sides arent they?

Yes, the ID should have a snug fit, enough to allow the bearing to slide and adjust preload. The OD should be snug enough so it doesnt spin. The inner race and balls are the only moving parts.

[arizonavideo]

Totality the wrong bearings.

Try a cheep pair of AC bearings from VBX bearing.

7200B Bearing Angular Contact 7200B Ball Bearings | eBay

[cadguy247]

Totality the wrong bearings.

Try a cheep pair of AC bearings from VBX bearing.

7200B Bearing Angular Contact 7200B Ball Bearings | eBay

this is what i figured. i wish i didnt throw away the bearings, i could show you how bad the inner races can wiggle around. as for replacing the bearings, i'm going to use a double row angular contact bearing along with a thust bearing as i am trying to reuse the mount with a little modification, part of that will be milling off their logo... and the single row ones are too large in the outer diameter to work with the mount. at some point i will make a new mount.

sucks to pay almost $900 for something that is poorly designed and built. it only lasted a few months and there hasnt been much use on the mill.