Problems with ball screws and nuts.

[jtriggs]

I have a CNC wood router with 24" X 36" cutting capacity. It uses Nook ball
screws with SBN7500 double recirculating ball nuts. A short time ago I had a
problem with my X axis motion. After sitting for a couple of months when I
started using the machine again the x axis travel was binding resulting in a
somewhat jerky motion causing the cutting tool to create chatter in the cut.
I have replaced the screw and the nut and the x axis worked fine but the Y
axis started doing the same thing. I called Nook and they said lube the screws
(and that would, in turn, lube the bearings in the nut. I used a non-graphite
type fine mahine oil and now both axes are having problems.
When the machine moves at anything over a slow jog speed the gantry
shakes. When I move either axis by hand it feels like the axis is bumping over
something every 3 inches or so. The closest I can describe it is it feels like
the components are going over 'speed bumps'.
Both of the screws are straight and unbent.

Any ideas what might have caused thes and or how to cure it? As I said, the
X axis worked smoothly until I put a lubricant on it as per Nook's recommndation.

Thanks for any help.
Jim

[Khalid]

what software you are using.. if u r using Mach3 then go into the config--> state--> change the exact stop to Constant velocity... Also use G64 in the top of ur g-code

[jtriggs]

I have Mach3 set at CV. This is a hardware problem and not in my software
settings. If I push either of the axes while the machine is completely shut
down I feel this bump at about every three inches of travel (on both axes).
The faster I push the axis the more sever the bump is. It also seems worse
when being moved back toward the motor (backwards).
This translates to a shaking when the router is under power.

[Khalid]

if u can post some pictures..may be some body here to help u

[ger21]

Maybe the nuts are packed with sawdust?

[LeeWay]

It does sound like trash in the ball nut. I bought one screw off ebay that worked nice and smooth UNTIL I lubed it with oil. This somehow loosend the trash in the nut and it did bind and bump like you mention. I could actually see tiny bits of the trash on the screw in the fresh oil.

Two options if this is the culprit. Lube and run by hand or machine jog over and over adding (drowning) the screw and nut with the same light oil you are using.
Carefully remove all the balls and thoroughly clean everything. Then just as carefully replace the balls.

Mine got smooth and I saw no more trash after about 15 minutes of the drowning and jogging method. No more problems since them.
Like an airtool, I like to add oil to these areas every time I think of it. You aren't going to over lube them like with an engine. Like an airtool, it will get messy if you over lube, but won't really cause any mechanical damage and will help to keep the trash washed out.

[jtriggs]

Thaks for the replies. Nook also suggested that, because of the tight
tolerances, the oil can pick up sawdust and carry it into the system causing
problems. They didn't offer any cures, however, so this is very helpful info.
My only other question is in regards to the process of removing the balls
and repacking them. How do you repack these little suckers after taking the
nut off of the screw? I pulled the nut off of the old screw (the one I had to
previously replace) and cound not see a method for replacing the balls.

Thanks again for the help.

Jim

[LeeWay]

It is probably easier to replace the balls while it's still on the screw. Depends kinda on the way your machine is built though and how much room you have.
The only ball screws I have messed with are some NSK ground ball screws and some 5/8" Thompson ball screws. Both have a steel tube on the outside. This is the ball return tube and also where you reload the nut at. Care must be taken to capture all the balls. There are balls in that return tube as well.

If you have a double nut configuration, they may not actually be preloaded with big balls and smaller spacer balls, but you should check these at least with calipers or a mic if you have one. It will be much more difficult if you have two different sized balls in them. They usually alternate one small, one big etc.

If it's easy enough for you to remove the nuts, you can use a tube or dowel inserted into the nut to keep the balls in location while you are replacing them. They come from the factory with a cardboard tube inserted in them and it fits on the end of your screw and you just slip it off the tube while threading it on.
This tube or dowel should be the same diameter as the bottom of the threads in the screw.
Really harder to explain than it is to do.

[jtriggs]

Mine are doubles. I took the nut off the old shaft before replacing the whole
assembly and, as far as I could tell, all the balls were the same side.
I'll probably try to do it without taking the nut off of the screw as you suggested.
Thanks for the help.
Jim

[jtriggs]

Lee,
I have all the balls out and clean but I'll be damned if I can get the little
suckers back in. Any tricks to getting the balls back in?
Thanks.
Jim

[LeeWay]

I have never taken doubles apart before, but they should work the same with just a little more trouble. I use some light machine oil to help lube the way. Then I just started feeding the balls from one end. You may need to twist the screw a little back and forth. Jiggle the nut some. I used a small punch with a handle to help push the balls down so the next few could be inserted. It is tedious at best. Make sure you keep the holes covered when if you roll the nut completely over. You will eventually get down to only a few balls and the amount that would fit in the tube. I fill this tube with these remaining balls and dab some grease in the ends of the tube to keep these balls in. Then I insert the tube and screw it down. Then after a couple test turns, it should work as normal.
With a double nut, just work on one circuit at a time. Split the ball count in half.

Was there a lot of trash inside? Did you find any damaged balls or metal shavings? My ground screws have seals on the ends from the factory. My Thompson screws don't, but due to the design and what I cut, this hasn't been a problem. Some guys will use hot glue to seal the ends of these nuts to the screw itself. Use a little light oil again on the screw surface and just shoot some hot glue on there and let it cool. The oil should prevent an adherence to the screw.
Aquarium silicone sealer might work pretty well for this too. These seals aren't perfect, but should really help prevent the bulk of stuff from getting inside.

[jtriggs]

Well, I pulled all the balls, cleaned and repaced them making sure I used
machine oil as I assembled them. I didn't notice anything obvious in the
parts. No shavings, no metal, no junk of any kind that I could see. All the
balls look and feel good. I reloaded the balls and now some of them are
coming out of each end when the nut is moved forward or aft. Probaly 30
or so balls came back out and I'm still feeling that bumping every 3 inches or
so.
I'm not sure what next. I guess I can see if Nook can and will reload the nuts
for me but if anyone has any suggestions please let me know.

Jim