brothers, I need some advice

[Robocop]

Hello, from the Great White North,

I don't know what you guys call cold where you are but today I had to pull an engine from a vehicle outside and I thought it was a bit nippy and decided to look at the thermometer and it read -40deg. It's little things like this that encourage me in my quest for something I can do inside in my shirtsleeves, CNC milling is just the solution. Until that day comes I have to finish building my ideal machine which I can do in a small heated shop perfect for such projects. This little preamble brings me to the real subject of this visit and as some have said, there is no such thing as a stupid question, except the unasked question. Here goes but with a slight explanation first. In my eagerness to get started on this project, I pushed ahead in the hope that I could get a great deal on Ebay and I bought a ball screw and nut with dual axial bearings at the end. The shaft is 5/8" with six threads per inch and 17-1/2" over all and I planned it as the Z axis on my machine. I was assured that the ball nut was in excellent condition and after shipping, exchange rates and customs costs, my "great deal" soured somewhat to say the least. As I ran the screw through the nut I discovered that it was very smooth, but only in one direction. Turning back the other way there was a sudden jamming that made my little heart flutter with the thought that something could be amiss........but OH! what could it be I asked myself? Have I been misled?
Unsure of what I might find, I decided to venture forth into an area which for me is uncharted territory and calmly suggested to myself LETS PULL THIS SUCKER APART, or thoughts to that affect. At first glance, all looked well but then, realizing you, my brothers of the CNC world would only be content if my investigation was exhaustive, I looked more closely. I believe I may have discovered the problem which presented itself as a slight yet noticeable secondary "groove" riding up onto the thread of the nut just at the start of said threads. Interestingly, when I turned the screw in that direction this is when the jamming effect occurred I suspect. Here are my questions.
1. if this screw and nut are old (although they look good) is it possible for the recirculating ball to wear out of spec and cause this problem?

2. assuming the balls are worn ( for some reason a dozen jokes just came to mind) would their replacement solve the problem?

3. I saved this question for last because I hoped in sympathy you would stick with me this far and not write me off as someone in desperate need of a lobotomy. Once I took the balls out of the nut, I threaded the shaft back in and as I expected all ran very smoothly in both directions. The thought then struck me and I was wondering, if properly lubed would it still work as a Z axis since it is only raising the tool and backlash shouldn't be an issue.....right? On the other hand, is the rod only threading itself on the nylon keepers at each end of the nut and with sufficient weight the Z would come crashing down onto my table? This would be a Baaaad thing.
I have inclosed a photo (I tried to put two but I think this raised me over the limit) in the event there were questions and I trust with all the expertise out there I can make a decision, or maybe I should just replace the nut and be done with it. That way I can avoid the axiom which states, "it is better to remain silent and have people believe your an idiot, then to open one's mouth and remove all doubt".
Thanks for the help, wdenis (from the Great White North)
P.S. how do these guys manage to get more than one photo on this site?

[wakeboard]

On the z-axos you are actually supposed to use an acme screw because the ball screw will have backlash....i dunno but thtat what I have been told....am i wrong in this piece of information?

[DaSigntist]

Hmmm......Cold I understand as I'm right under you in Prince Albert, SK. freezing my CNC off.

With your ball screw....You might want to call Dean at Axxus Technologies as He is in Edmonton.
Great guy and might be able to guide you in the right path.
I'm using Acme but will upgrade my machine soon with some ball screws.

[jimbo]

I dont know enough about ballscrews, but I do know you can open it up and replace the balls in it. I have taken one apart and put new balls in it. It worked for me, it rotates smoothly. The reason I replaced the balls was that the balls fell out of the nut assembly when it was shipped to me. There are professional places that can recondition ballscrews to like new but are rather expensive. If there is a physical groove that causes interference, then I am not sure what your options are. The new balls will probably be damaged by the groove.

I wonder if possibly, using a drill to drive the ballscrew back and forth with some cutting compound on the ball screw with the current balls to smooth out the groove. Then replace with new balls. This would be pretty risky though, but if it gave the possiblity of salvaging your ballscrew...

wakeboard, you can have backlash in either acme or ballscrew application. I think you may be referring to back drive on the ballscrew, where the weight of the platform on an unpowered screw causes it to rotate and thus lowering the platform. This can be overcome with the use of a brake. This usually does not happen on acme because of higher friction and often the use of more threads per inch.

[sixpence]

Hello from a sister.

We are suffering in a heat wave in South Africa.
I cannot offer any help with your problem but I do know that the SKF site offers help in the troubleshooting dept. Drop their technical dept an e-mail.
I have the fan and air cooler on at the moment am considering an air conditioner! No scratch that I need to pay for my set of lead screws for my first cnc. Prioities.

[sixpence]

OOPS! Touch of heat stroke Priorities

[bb99]

I was wondering, since you plan on using this as the z-axis (which I assume is vertical), the balls in the nut would rest only on one side of the thread. The weight of the router (or whatever) will drive the cutting head into the material as you lower the z-axis and while lifting it, the nut will ride on the lower edge of the thread. With this in mid, constant downward force on the rod, can you position it so that the smooth running direction is towards the table?

--Alan

P.S. I’m in Southern California where the temp is about 70 decrees with clear blue skies. OOPS, never mind. There’s a little fluffy white cloud way off in the distance.

[HuFlungDung]

Hi Wdenis,

I'm enjoying minus 40 degrees today, too.

Yes, I have seen ballscrew nuts with one side of the threads damaged. However, the screw was simply noisy, it still ran back and forth.

Assuming this is a preloaded ballscrew, I would wonder if there was a couple of mismatched balls. Actually, they should alternate, large ball, small ball exactly, all the way through each ball ciruit. If all balls are the same size, and they all make contact with the screw and the nut, obviously they will bind up, as anyone could tell who has ridden up behind another on a bicycle and touched front wheel to back

If you mike all the balls, there should be two groups, maybe .0005 difference in diameter. Install them alternately, and keep track. Don't pack an extra one in the ciruit if it cannot hold it. A little space will not hurt.

Inspect the ends of the ball circuit tubes, too. They must not be dented or bent, in order that they properly pick up the balls out of the screw. You have to reload the ballscrew from the holes where the return tubes fit. I've used a plastic straw from a WD40 can as a "pusher" to push the balls down into the screw. This helps eliminate gaps that will not fill.

Good luck

[Robocop]

Thanks to all the input regarding the jamming ballnut I'm dealing with. One of the responses gave me an address of a fellow, I never new existed, here right in my city. I conracted him and after further information regarding the size of the shaft he indicated that this was a standard size and a replacement nut shouldn't be too expensive. Thanks again for all the suggestions and while I wait for a replacement maybe I will try replacing the balls (which are very cheap) just to see if this makes a difference.
Sorry this is so short but I have yo go out and feed the sled dogs.
wdenis