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- Jul 28, 2017
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For some time I've had a noise problem when I turn the crank for the X axis on my desktop mini mill. It often emits a loud screeching sound so I figure it's more than just a minor problem. The noise is clearly coming from the bearing bracket on the right side, but when I took the thing apart I didn't really find anything obviously wrong. I could mitigate the problem by slathering grease over everything I could find, but it would always return in short order. The fact that it sounded like metal-on-metal was an indication that I needed to fix it or wind up with more perhaps serious problems down the road.
For just a little background, the X axis on these machines is designed to use a pair of thrust bearings inside the bearing bracket. Each bearing uses a ball bearing race between two grooved, hardened (I hope) washers. The bearing bracket has a pocket for each bearing, one on the inside and one on the outside that faces the hand wheel. There are some clearance requirements there because the handwheel must be snugged up against the outside thrust bearing, but not rub on the bracket. I determined that the clearances were OK -- while the bearing was slightly below the plane of the bracket, the handwheel's hub protruded sufficiently (and was small enough in diameter) so no mechanical interference was present. Clearance isn't huge, but enough, on the order of .020" or so for my machine.
So what was the problem? Well, based on some recent learnings about thrust bearings I did some measurements. I'd purchased some for modifying the Y axis on my mill and found that the ID's of the bearing washers were different, and for a reason (their OD's are the same BTW). One of the washers has a slightly larger ID, and that is to ensure that (if it's the fixed one), it doesn't rub against the shaft as it rotates. Since the race will rotate more slowly than the shaft, you really don't want it to contact the shaft, either. Ones I bought from VXB were marked to make it easy to distinguish which washer was which, because that would make a huge difference in the longevity of a machine using them. Examination of the OEM thrust bearings didn't reveal any way to tell them apart, although my measurements did show similar differences between the two washers. Lesson one: as they come, they're not marked, but that doesn't mean their orientation doesn't matter! The owner's manual is silent on this point.
BTW, for those of you who have disassembled that part of your mini-mill, you may want to check to see if you put the thrust bearings back in there correctly.
Lesson two: My measurements found that the OD of the thrust washers was about the same as the diameter of the pocket they go into; and that the pocket diameters are not stepped to provide sufficient clearance between the rotating part of the thrust bearing and the pocket. I proved this by greasing just the edges of the rotating thrust washers, reassembling and assessing the noise problem. Gone.
So, to ensure a long-term solution to my problem I found I needed to do two things.
1. Mate thrust washers with the smaller ID's to the rotating components. This will ensure that the shaft is constrained properly. Grooved-type thrust bearings can do a good job of forcing the shaft to be concentric with the bearing so it's necessary to take advantage of that.
2. Modify the bearing pockets so the rotating part of the thrust bearing can't rub against the walls of the pockets. This can be done by boring the pockets out a bit, but only deep enough to accommodate the rotating thrust washer and race. You still want the fixed washer to be constrained so it doesn't drop and permit the rotating component to rub against the bearing bracket.
For just a little background, the X axis on these machines is designed to use a pair of thrust bearings inside the bearing bracket. Each bearing uses a ball bearing race between two grooved, hardened (I hope) washers. The bearing bracket has a pocket for each bearing, one on the inside and one on the outside that faces the hand wheel. There are some clearance requirements there because the handwheel must be snugged up against the outside thrust bearing, but not rub on the bracket. I determined that the clearances were OK -- while the bearing was slightly below the plane of the bracket, the handwheel's hub protruded sufficiently (and was small enough in diameter) so no mechanical interference was present. Clearance isn't huge, but enough, on the order of .020" or so for my machine.
So what was the problem? Well, based on some recent learnings about thrust bearings I did some measurements. I'd purchased some for modifying the Y axis on my mill and found that the ID's of the bearing washers were different, and for a reason (their OD's are the same BTW). One of the washers has a slightly larger ID, and that is to ensure that (if it's the fixed one), it doesn't rub against the shaft as it rotates. Since the race will rotate more slowly than the shaft, you really don't want it to contact the shaft, either. Ones I bought from VXB were marked to make it easy to distinguish which washer was which, because that would make a huge difference in the longevity of a machine using them. Examination of the OEM thrust bearings didn't reveal any way to tell them apart, although my measurements did show similar differences between the two washers. Lesson one: as they come, they're not marked, but that doesn't mean their orientation doesn't matter! The owner's manual is silent on this point.
BTW, for those of you who have disassembled that part of your mini-mill, you may want to check to see if you put the thrust bearings back in there correctly.
Lesson two: My measurements found that the OD of the thrust washers was about the same as the diameter of the pocket they go into; and that the pocket diameters are not stepped to provide sufficient clearance between the rotating part of the thrust bearing and the pocket. I proved this by greasing just the edges of the rotating thrust washers, reassembling and assessing the noise problem. Gone.
So, to ensure a long-term solution to my problem I found I needed to do two things.
1. Mate thrust washers with the smaller ID's to the rotating components. This will ensure that the shaft is constrained properly. Grooved-type thrust bearings can do a good job of forcing the shaft to be concentric with the bearing so it's necessary to take advantage of that.
2. Modify the bearing pockets so the rotating part of the thrust bearing can't rub against the walls of the pockets. This can be done by boring the pockets out a bit, but only deep enough to accommodate the rotating thrust washer and race. You still want the fixed washer to be constrained so it doesn't drop and permit the rotating component to rub against the bearing bracket.
