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11-22-2010, 11:32 AM #1
How to get balls back in ballscrew nut
I have a CNC Bridgeport clone with ball screws. I was having an issue with some lost motion in the "y" axis so I took the table off to make sure the casting that houses the "x"/"y" nuts was tight and to see if I could see any mechanical issues. So I didn't see any problems but what I did manage to do was to dislodge some balls from the nut when I pulled the screw out. How do I reload the balls into the nut and how do I know how many balls I need? If I remember correctly, you load balls in until you can't load any more?
11-22-2010, 12:22 PM #2
If you have external return tubes you can easilly enough put them back in. As many as you can fit is prolly good enough.
I had one in here last spring that needed re-done and it has internal returns. I'm not sure how that's done - so I sent it up to Robbert.
Think Snow Eh!
11-22-2010, 12:33 PM #3
The internal returns are easy to fill too.
I think that the internal return nut might be easier to re-load than the external returns.
Set the screw up on end, and place the nut over it. You can drop the balls into the space between the nut and screw, one at a time, and push them into the groove.
Turn the nut a tiny bit for each ball as it is added. Be sure and DO NOT let any balls get in the parts in between the tracks if there are multiple circuits.
Also, check you balls and make sure they are all the same size. Some ball screws have an alternating pattern of larger and smaller balls. Just a few tenths difference between them. The idea was that the larger balls carried the load, and the smaller balls between them kept them from rubbing together as they turned.
11-22-2010, 01:45 PM #4
Been there done that got the T shirt! Use some nice sticky grease helps a lot and a few shapped pointy lolly pop sticks. Keep things clean and flush out with solvent - oil genourusly post assembly. Don't over load the nut with balls either they need a bit of space to roll otherwise they rub on one anouther. If its a multi track nut get the same numbers in each. Mine was off a anilliam retrofit and the return tube jumped about 2-3 turns. Reading online kinda convinced me that i had found all the balls but i could probaly have squezed anouther 2-3 in but that would have probaly made it too tight. I found it easyest to have the return tubes on the nut and gradualy feed the balls in from the end whilst screwing the leadscrew in. Its fiddaly and a pain.
Above all look at the screw and make sure you load it the right way around. Due to how mines worked out my power feed - cnc motor drive jumped ends! No way am i spending a afternoon doing that again! Was easyer to just swap the table end brackets ;-)
11-23-2010, 03:26 AM #5
Ok ... thanks guys. I think I got all the balls loaded (I dropped some on the floor but I think I found all of them). It was a real pain. I loaded them through the return tubes and jiggled and fussed to get the balls in and then I would install the return tubes and jiggle some more and then take the return tubes back off and load more balls and then jiggle some more and then reinstall the return tubes and jiggle some more and ... It probably took me 2 1/2 hours to find the balls I dropped, to clean everything, reload the balls etc. I do have the screw in place in the right direction. Today I will put the table back on and continue my search for the lost motion in the "y" axis. Thanks for all the help.
11-23-2010, 04:38 AM #6
Joe, whenever I had to reload a ballscrew, I'd use a plastic straw off a WD40 can as a 'tamper rod' to push the balls against one another. This is really only necessary for the first full turn, after which the ballscrew can be slowly rotated, always keeping a fresh ball on top of the last one just before it actually enters tangency with the screw.
11-23-2010, 11:29 AM #7
HU - a straw from WD-40 - nice idea. OK...I got all the balls back in and everything is back together. I tried it out and it seems smoother than ever but I still have a lost motion issue in the "y" which is what this whole thing is about.
You guys that are running CNC knee mills, what kind of lost motion in the "y" are you living with?
11-23-2010, 11:32 AM #8
HU - another thing, were you loading the balls from the end with the screw held vertically? Or were you loading through the return tubes? I had my screw in its normal horizontal position and loaded the balls through the return tubes. Was that the bass ackwards way to do it?
11-23-2010, 11:55 AM #9
With the external return tube style, I reload through the tube ports. Make sure that not a single ball gets in the 'no ball zone' between the two return tubes, as when that errant ball comes up against the outside of either return tube, it will act like a sprag clutch and want to lock up the screw. This effect is pretty noticeable, and right away, too, so I'm sure you'd notice it. I think this is why it is impractical to try to load this type of ballnut from the end, because you would not be able to keep the balls out of the 'no ball zone'.
If the ballnut has a bit of wiggle to it, you may benefit from buying new balls that are a wee bit larger. If the ballnut freely spins its way down the ballscrew when oriented vertically, that is too loose. How loose? Hard to know, but preloaded ballnuts have a very smooth feel when rotated, but are not freewheeling when brand new.
Another possible source of backlash is the thrust bearing on the screw. Good quality thrust blocks use angular contact bearings, or even plain axial thrust ball or needle bearings so that backlash can be adjusted to a practical minimum. A lesser quality thrust block might be using ordinary deep groove ball bearings, which do not have a strictly defined path in the raceway, because of clearances. They can wedge and seem tight, but still not really locate the screw firmly enough for high accuracy applications.
Edit: any slop in your X or Y slideway gibs can also add mysterious lash to either axis. The slop in the X gib will show up as Y lash, and vice versa. It might not be backlash exactly, more like random position errors, with apparent overshoot or undershoot of true intended position.
11-23-2010, 12:02 PM #10
X and y on my bridgeport are under a thou of lost motion. But my ball screws both have twin nuts with beviell springs bettwen them maintaining a genourus preload. The bearings either end are just the standard bridgeport anugular contacts nicely cleaned out and grease. Theres realy only a few posible causes. If its a belt drive it could be worn toothed pulleys - belts, i think its mostly belt stretch causing my lost motion. But im cheating and running the belts a bit tight to help counteract it :-)
11-23-2010, 01:17 PM #11
HU - I've been careful here to not use the term "backlash" as I am not seeing a backlash issue but a "lost motion" issue. If I put an indicator on the screw pitch I am seeing about what adama is seeing - about one or two thousandths "backlash" so I can live with that. However, somewhere I am losing motion and I am just not seeing it.
I was hoping that when I took the table off and checked the ballnut bracket that I would find that the bolts weren't snugged enough or something but sadly that was not the case. I put a wrench on everything and everything is nice and tight and I am not seeing anything out of the ordinary.
I am going to go over the gibs again and see if I can't get another couple of thousandths out of the "y". Right now I am seeing about .006" in the "y" and about .001"-.002" in the "x". If I can get the "y" down to about .003-.004" I have told myself I will live with that and just program the error out (a bit of a pain the a@# but it is a knee mill after all).
If business picks up I will look for a real CNC machine but in the mean time I have no choice but to make this work.
Adama - your setup sounds intriguing to me. Where did you find such fancy ballnuts? Also, are you telling me that you can mill a round circle within .001 or .002" on your Bridgeport? That's pretty darn good!
Thanks to all.