Ballscrew ball size calculation

I'm thinking about reballing the screws in my mill in the near future and I wanted to go about it sensibly, only needing to order 2 or so sizes of balls.

I've read that ballscrews have a gothic arch profile ground into them, which makes sense to avoid skidding. However, this places the majority of the load on a small surface area, which is likely the major component of the wear.

Getting new, or having the screws professionally serviced is out of the question. The X screw is about 53 inches long and 1-3/16 in diameter, with a 4 or 5 pitch.

I seem to recall reading that you can calculate the precise ball size needed based on the pitch diameter and the measured backlash. I could model this simply in a CAD program, however the gothic arch ogive data is the gotcha I don't have.

Does anyone have a formula or spreadsheet for calculating the ball size? I got lucky on my last screw and guessed right, but it had much less backlash.

At 18 cents per ball, I'd like to try and contain my costs as much as possible. As it is, I'm probably going to spend over $2k in parts to repair and upgrade this mill (a Shizuoka AN-S).

I learned a few things here when looking for parts recently:

http://www.nookindustries.com/ball/BallGlossary.cfm

I looked over the site you refered to. Their is a ton of good information but you will need time to digest the info. I just printed it off, so I can file it and refer to it when I need it.
Thanx for the info.
Regards Walt...

Why not measure them?

The balls are of two different sizes, with a small one between each pair of large ones.

That being the case, measure the small ones, which should not exhibit any significant wear.

Then measure some of the large ones to determine their actual diameter, which should be .001" to .002" larger.

- Leigh

The real Leigh said:

The balls are of two different sizes, with a small one between each pair of large ones.

That being the case, measure the small ones, which should not exhibit any significant wear.

Then measure some of the large ones to determine their actual diameter, which should be .001" to .002" larger.

- Leigh

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Based on my research, screw rebuilders don't do that anymore. They put all on-size balls in the screw and this difference has a very significant improvement on longevity, on the order of several times.

The wear isn't in the balls, it's the ballnut or the screw. That's what I found before. The last screw, which is the same age as the others, had balls that were exactly .1250. I ended up going up to .1258 with a new screw and original ballnut.

In this situation, the backlash is the same over the length of the screw, which suggests the ballnut is the culprit. The ballnuts are usually the softest portion of the equation based on research.

My point is, I know ball size is D, backlash is B, and the Pitch is Pd, with these variables I should be able to determine the new D value to make B equal zero, then add a tenth or 2 for preload.

My adv. is just don´t do it

Buy a new one. 100% ok. and is propably cheaper if you compear it with working life/time repairing old one and mystakes, perheps.
Off cause Im not sure what kind of Ball screw you got but if it is anything like this http://steinmeyer.com/english/technik/kgt/gesuml/gesuml_f.htm
You will know what I meaning
(sorry about my spelling, if any errors)

I've "rebuilt" my ballscrews myself. Not really rebuilt but I did just as you are planning, to go to larger balls.

Surprisingly a couple of tenths can take up a couple thou slop. This is because there is little or no wear at the root in the nut and screw. So now there is perhaps more pressure at 12 and 6 o'clock instead of where the ball was contacting the nut 10 and 2 o'clock and for the screw 4 and 8 oclock. For this reason all screws would be different so there probably is no formula to get the correct size. On one screw with .005 slop I went up .00040 to .00050 and one with .002 slop I went up .00020 to .00035. Give them a range so that they can match a size that they have in stock because a special grind would be very costly.

With more pressure at 12 and 6 o'clock it is sure to wear in faster but it only cost me about $50.

As for measuring the balls I would suggest to send the supplier a couple of samples and specify how much larger you want them. Don't bother trying to measure with a micrometer. They have much better measuring equipment.

The toughest part might be finding a supplier. Searching will find millions of matches but not what you need because the terms are so common: ball screw repair, bearing-balls, etc. Ball screw rebuilders will not help you. They will tell you that you shouldn't do it, refuse to sell just the balls, and will not refer you to a supplier.

Dave

This screw would cost over $5000 to have remanufactured, and a new one would probably cost similar, since the amount of work is similar. The machine is only worth $5k on a good day, but a machine in the hand with a known history has a lot more worth than just book value. This machine is a 5000lb knee mill and although it's an open type machine, it is a very good machine.

I already have a supplier for precision balls, my cost is around 18 cents per ball in special sizes. At $58 per batch of each size, I don't want to be playing hit or miss with balls, I'd ideally like to narrow the size down to 3 different options. I did this last time and loaded the nut with each size and checked how much the nut would cock on the screw, then I arrived at using the largest ball.

Right now the X screw has .00272 backlash and the Y screw is around .0005. These are ground screws and should be very accurate once reloaded. I was just hoping someone actually had some concerete formulas or data.

I'm also planning on replacing the variable pitch drive with an inverter drive motor and doubling the spindle speed in the process. The spindle bearings are rated for 7krpm, and it's only capable of 3800 right now, so a 7.5hp inverter drive motor will replace the existing motor and retain the power ratio.

Perry Harrington said:

Right now the X screw has .00272 backlash and the Y screw is around .0005. These are ground screws and should be very accurate once reloaded. I was just hoping someone actually had some concerete formulas or data.

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Was wondering how you went about confirming these backlashes... did you have them removed to check ??

MJM

Perry-
The cost for replacement you quote is suprising, given you can get a set of x and y ballscrews for a Bridgeport for less than $1k. Why is this screw so expensive?

sudtechcnc said:

Was wondering how you went about confirming these backlashes... did you have them removed to check ??

MJM

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I carefully measured and adjusted the backlash until I got repeatable results. I used a 50 millionths indicator and plenty of patience. I also have a probe on this machine and was able to verify by measuring a known size gage block.

Bruce Griffing said:

Perry-
The cost for replacement you quote is suprising, given you can get a set of x and y ballscrews for a Bridgeport for less than $1k. Why is this screw so expensive?

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Because bridgeports use small screws, are widely used, and usually they are rolled screws.

The quote to refurbish the .75-5 screw that drives the quill was $1600. That was to clean, inspect, and reload the existing screw. I purchased some rolled screw stock, machined a new screw, and reused the old nut and loaded the screw myself.

Keep in mind, the screws in the machine are ground screws that are 30 years old, from a reputable Japanese manufacturer. To get the same quality of screw, I would have to pay good money. A ballscrew rebuilder would take a piece of material and grind it to get the ends of the screw and machine a ball nut from scratch, then harden it, then grind it.

The biggest screw is about 53 inches long and 1-3/16 inches in diameter. Similar to a 40x20 VMC in size, but I don't have the benefit of calling up the manufacturer and ordering a replacement unit.

Perry:
I do not doubt that you have done some homework re your ball screw repair/re-ball.
Just went through this with my Deckel FP3NC....Did the Z axis screw myself after talking at some length with a very nice man at "Express Ball Screw Repair" He gave me some very good advice on doing my screw..and since it was on the "Z" and has the advantage of gravity to help with backlash i went ahead and did the job myself. I was not as fortunate as you in getting to the correct size balls. Took me i believe 7 sets of balls to get it where i thought it was good. I bought the balls from "BallTeck" in So Cal. and i used grade 25 balls at about $.25 ea.
Trouble is that you most likely can't measure the real size of the existing balls close enough to get the correction by a formula...you need to measure and order within at least 25-50 millionths....That is the size range the balls should be coming in to get close enough in fitting to eliminate backlash and not have it too tight (can kill the nut in almost no time if you load balls that are too big)
The man at "Express" told me that they do all the fitting by actual loading the balls and feel of the nut and screw, and then test for backlash...left the impression that they at least did not depend on any calculations to gt the final ball size.
He also told me that they used grade 2-4 precision balls which were much tighter in tolerance than anything i was likely buy.

Long story short, when it came time to re-do the screw on the "X" axis, i sent it to them and had it done professionaly. Now my screw is shorter than your unit at 36", but it is ground (not rolled) and it was fully serviced with precision lapping of the ball tracks, and screw, and new balls. I think the cost was about $1200.00. If you consider the cost of trying several different sizes of balls , this rebuild cost becomes very attractive....
Might want to call for an estimate of repair cost....might be cheaper than you think.

Express Ball Screw Repair: 800-711-2153
Believe the gentleman i spoke with is John ....spoke with a very pleasant "Scottish" accent.

The usual disclaimer..not affiliated in any way ..have had two screws done at Express and both have been excellent and the service is very good. Can save some cash as well by not getting the work done in a rush. Think it costs less to run the job if you allow them a week or so turn around...and that was fine for my case.

Cheers Ross

+1 for Express Ball screw Repair. Very nice to deal with, and he will spend time on the phone with you whether you are ready to buy from him or not, he'll give good advice and help you decide what your screw needs. I've used others before, I'll definitely use them again.

i called barnes last time, maybe ill give express a try.

Perry

i called barnes last time, maybe ill give express a try.

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Barnes are probably about the most expensive in the industry. Their about the only ones that can properly regrind the flight of the screw. And the have the equipment to manufacture new nuts. Its horribly expensive equipment.

They do good work, but they charge for it. Most every one else, myself included just re-ball and adjust. I think R.T Gillman now SKF used to do it. I think they have even dropped regrinding now, just preferring to sell new SKF screws.

Keep in mind, the screws in the machine are ground screws that are 30 years old, from a reputable Japanese manufacturer.

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What are we talking here. N.S.K, Koyo, or Tsubaki? Any chance that’s a double nut with a preload control spacer. If so the ball size wont be so critical, if you can replace that spacer with a larger one.

Regards Phil.

Perry Harrington said:

Right now the X screw has .00272 backlash and the Y screw is around .0005.

Click to expand...

Holy cats you must do fussy work. Where I work .008" slop is not enough to get a machine serviced But then this is at a company that once locked up the way oil and ran a machine dry instead of fixing the oil leak because "We are going through too much oil."

Dave

Perry,
Did you go through with the re-balling? Anyone ever take the table apart on an Shizuoka AN-S that could tell me if there are double ball nuts in there that can just have their gap adjusted? My backlashes are 0.0005 and 0.0015 for y and x respectively.

I helped my buddy pull his X ball screw out of his new to him ANS just yesterday. His screw was really bad with .011 backlash in it. Yes ANS has double ball nuts on the screw. He was able to get the lash down to .001 by using a .008 shim as a temp fix. Not sure your lash is worth fixing if this is a CNC can you comp it out.

Eliminating the X backlash isn't on the top of my list of things to go fix; however, it would be nice to just set-and-forget when programming and not have to think about the reaction forces and how they will either shrink or enlarge my part or if they will hose up the finish. I am rebraining the bandit with Linuxcnc; but I am not sure that i am a firm believer in backlash compensation. I have a very sloppy manual machine that give me periodic reminders about why it is important to lock the table! With no ability to lock the table on the AN-S and me using feed forward control only with steppers, I feel from time to time I would get bit. I would like to share what I learn and compare notes with your friend as he pulls his machine together. Could you please ask him if I could call him some time to compare notes? I'm reverse engineering all of the wiring right now. ANY kind of drawings that may be common would be a gift! Perhaps I should start a build thread?