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Re: Dismounting the headstock bearings

 

Gotcha. If it was me, I'd start with two things. First would be pulling the headstock apart and addressing the bearing adjustment problem. The inner race on the outer bearing needs to be able to slide on the spindle to set the preload. I did mine the old fashioned way like you'd polish engine crankshaft journals. If you haven't done that it done by bolting the spindle down firmly, wrapping sand paper around the spindle, wrapping a long shoelace or the like around the paper a few times and pulling the ends of the laces to turn the paper. There are probably YouTube videos that show what I'm describing. It works better than just trying to hand sand as it keeps pretty even pressure all the way around the part. If you decide to do it use fine grit paper, like no courser than 800 to start, as yours is probably pretty close. I may even start with 1,000 as yours is probably within fractions of a thousandth if the bearing already goes on but doesn't slide. Go slow and check fit often. As we know It's easier to remove material than put it back. :-) ?You just want it slide, just. If you go too far then the spindle can move in the inner race and we don't want that. It's a time consuming process, lots of cleaning and testing for maybe only two or three rotations of the paper before cleaning and testing again but it's worth the time. You'll be able to set it perfectly.?
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My second thing to check would be the lead screw and half nuts to see if there's a burr or anything causing irregular movement and adjust of the base, compound and cross slides. That may cause the ghost thread. You can check if it's from the compound or cross slides by locking them down tight with the gib adjustment screws and taking a test cut. I'm sure you know it's really a process of eliminating variables until you find which part is causing the problem and then addressing that part to eliminate the issue.?
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Ryan
On Feb 9, 2025 at 10:23?AM -0500, Pierre-Raymond Rondelle via groups.io <pierreraymondrondelle@...>, wrote:

Ryan,

I'm not speaking of alignment accuracy that would mainly lead to a machine a taper instead of a cylinder but of runout of the chuck that is currently resulting in an irregular machined surface. And you're right in your rationale, telling that, depending on the location the machining is preformed, the effect is greater when the distance to the chuck is increased. I admit not having checked the conjunction of the bad surface with the distance from the chuck. I take note to do that during the final verification after fixing the trouble.

I don't also remember if I said this : the roughness is drastically improved as well by decoupling the gear train from the 40-teeth drive gear at the end of the spindle. This phenomenon lets me thinking that the two bearings aren't pre-constrained enough. I don't understand yet why the ghost thread it's close to the lead screw pitch (2mm). I gonna change the ratio of the gear train to check it.


On 08.02.25 21:38, Ryan H via groups.io wrote:
1/100mm is about .0003". Over what distance are you trying to maintain that? Just curious as it shouldn't be a problem on shorter lengths with everything properly adjusted and aligned especially the saddle and headstock. I may have missed where you said but if you're saddle travel isn't perfect you're not gonna get .0003" unless you work only in the "good" area of travel. Then the headstock has to be done but anyway...
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The inner race of the inner bearing can be an interference fit on the spindle albeit very slight. Like tap it with a wooden hammer handle only to fully seat it.?
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?The inner race of the outer bearing should be a very snug slip fit on the spindle. That race has to be able to slide just like on a vehicle. ?It's fairly time consuming process. ?Sounds like your spindle wasn't properly fit for the bearing. If it was the race would move so you can set it. I think you're first step is going to be to rectify that and from there you're going to be set on that part.
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