Ballscrew Accuracy

[JermNZ]

I really hate it when someone sneaks in another post in between the time I have read their first post and responded to it.



Not really.



Nope. It's not. Not even close.

For my use, I believe it is. Compared to many I have seen in person and online.

[louieatienza]

Ok, I disassembled one of my Y axis screws as they are more accessible than the X axis.

It turns out that my bearing blocks look ok, except the bearings seem to be a pair of standard deep groove ball bearings (6001Z), rather than angular contact bearings. There were in fact spacers installed correctly on either side of the bearing block, perhaps they came pre-installed in the block, which is why I missed them previously.

I disassembled the whole block and i'm not sure if it needs shimming. The inner races of the bearings seem to contact each other, but only just, no pre-load.

It is my understanding that deep groove BB are not designed for axial loading. It seems to me that I should replace them if I can find a pair of angular contact bearings which would fit straight into these blocks??

What is the best way to remove these bearings? There is no access to the outer races on the side required to press them out. I guess the only way is to press on the exposed inner races and shields? But obviously this isn't ideal..

The lock nut set screw also tightens directly onto the thread, which causes slight damage to the thread, shouldn't be too much of an issue as long as I don't clamp it too many times.

Yes a very light preload is permissible, though understandably the setup is not designed for high axial loads. Which is fine, because if you exceed the load limit of even these bearings, you have other issues!! AC bearings are nice, but true matched pairs are expensive.

You may try carefully heating the housing before you press the bearings out.

As to the retaining nut, usually a small piece of brass rod is installed in the thread side which pushes against the screw, not directly with the set screw which is hardened and will damage the threads on the journal.

[NIC 77]

Don't mind me Jerm,

I can come across as a real jerk when I'm into the sauce. I need to learn not to drink and post.

My only point is that it would be nice to see definitively where the error is coming from before spending lots of $$$. I don't see the value in ground C5 screws on a machine like this.

If a significant amount of error is coming from the blocks then try shimming, and also replacing the bearings with better ones might work, and be cheaper than the SYK blocks I suggested earlier.

If there isn't any backlash from the blocks, then for a small machine like this, I wouldn't worry about it, just focus on what's causing the problem. That was my point in saying that I don't care what bearings are in them.

Carry on, and just realize that sometimes I can be a jerk, and it isn't personal.

[JermNZ]

Yes a very light preload is permissible, though understandably the setup is not designed for high axial loads. Which is fine, because if you exceed the load limit of even these bearings, you have other issues!! AC bearings are nice, but true matched pairs are expensive.

You may try carefully heating the housing before you press the bearings out.

As to the retaining nut, usually a small piece of brass rod is installed in the thread side which pushes against the screw, not directly with the set screw which is hardened and will damage the threads on the journal.

Thanks Louieatienza,

Yep I've done some reading around and you're right, I think these bearings should be sufficient for this application.

Regarding the pre-load, from what I have read, the correct approach is to insert a shim in-between the bearings. I assume the shim should only contact the inner races (and not the outers) in order to pre-load the bearings when they are squeezed together by the bearing block clamp plate?

Now that I have confirmed that my bearing blocks are indeed assembled correctly (minus a little tweaking), I'm less worried about the relatively minor screw axial movement.

My main issue is in the ballnut. It seems I have several options:
- Try a new ballnut from another seller, might be a bit hit and miss as it isn't a good match to my screw??
- Pack with oversize balls, tricky to determine correct size.
- Purchase a double-nut, probably from BST.
- Purchase new screw and nut, last resort.

J

[JermNZ]

Don't mind me Jerm,

I can come across as a real jerk when I'm into the sauce. I need to learn not to drink and post.

My only point is that it would be nice to see definitively where the error is coming from before spending lots of $$$. I don't see the value in ground C5 screws on a machine like this.

If a significant amount of error is coming from the blocks then try shimming, and also replacing the bearings with better ones might work, and be cheaper than the SYK blocks I suggested earlier.

If there isn't any backlash from the blocks, then for a small machine like this, I wouldn't worry about it, just focus on what's causing the problem. That was my point in saying that I don't care what bearings are in them.

Carry on, and just realize that sometimes I can be a jerk, and it isn't personal.

No worries,

Yep i've come to the conclusion that the ball nuts are my main issue and will try and get that sorted first.

[NIC 77]

Here's a video I found on youtube, Looks like this was done by Arizonavideo



And here, if you look at the first photo in post #4

How to pre load screws

It is my understanding that he removed the ground spacer for the double nut and replaced it with a belleville washer, which is way simpler than trying to figure out how to use two single nuts.

Personally, if I could find some cheap balls, I'd try the repack first. Interesting, in the video he mentions that you may have to try a couple different sizes of balls to get the right one, so you'd have to order a few sizes of balls. Anyhoo, he explains that in the video, it was educational for me too.

[wizard]

A couple of points.

When you have standard ball bearings used in an application where they are taking axial thrust you will have backlash unless you preload those bearings. You do that by deplacing the outer races with respect to the inner. This is where those shims in the bearing end blocks come into play. You need to be careful as to how much preload you apply because the bearings are not ground to limit preload.

Contrast this with angular contact bearings which come with a preload ground into them or possibly no preload. You wont need the shims in a normal angular contact bearing implementation.

In either case though axial backlash is a real problem in Leadscrew bearing and you are far better off minimizing it than trying to live with it.



When it comes to reballing ball screws keep in mind the need to have two sizes of balls on hand. Most ball screws do not have any sort of cage to space the balls apart to keep them from counter rotating against each other. This is addressed by the use of undersized balls every other ball in the nut assembly. If you stuff the nut with tight fitting balls and no spacing balls you will get excessive wear and binding. In general you need a good supply of different sized balls to really do a hand packing well.

Thanks Louieatienza,

Yep I've done some reading around and you're right, I think these bearings should be sufficient for this application.

If not upgrade! If you have minimized backlash in them it is better not to dwell on them and thus get the rest of your machine resolved.

Regarding the pre-load, from what I have read, the correct approach is to insert a shim in-between the bearings. I assume the shim should only contact the inner races (and not the outers) in order to pre-load the bearings when they are squeezed together by the bearing block clamp plate?

You can do this either way. It might be easier to do the outer races especially if the bearing pocket is a bit deep.

Now that I have confirmed that my bearing blocks are indeed assembled correctly (minus a little tweaking), I'm less worried about the relatively minor screw axial movement.

Worry about what you can measure. When having problems like this it is best to resolve one problem area at a time.

My main issue is in the ballnut. It seems I have several options:
- Try a new ballnut from another seller, might be a bit hit and miss as it isn't a good match to my screw??
- Pack with oversize balls, tricky to determine correct size.

Very tricky. You basically need a huge inventory of slightly different sized balls and a willingness to do a lot of trial fits. Depending upon the quality of the ball screw you may find it difficult to get a good fit that works well iver the entire length of the screw.

- Purchase a double-nut, probably from BST.
- Purchase new screw and nut, last resort.

J

A new screw with an anti backlash nut might be a good idea. Im not too certain I would want to try mixing nuts and screws from different vendors. Also don't forget that going double nut for backlash control can lead to shorter axis travels. You will always have trade offs.

[wizard]

Here's a video I found on youtube, Looks like this was done by Arizonavideo

Nice video. The only thing that i might have an issue with is packing the nut with the same sized balls. It might not be an issue on rolled screws but i was always taught that you need an idler ball between the fitted balls.

And here, if you look at the first photo in post #4


It is my understanding that he removed the ground spacer for the double nut and replaced it with a belleville washer, which is way simpler than trying to figure out how to use two single nuts.

Personally, if I could find some cheap balls, I'd try the repack first.

They are available but to get in the sizes required you end up buying a lot of balls.

Interesting, in the video he mentions that you may have to try a couple different sizes of balls to get the right one, so you'd have to order a few sizes of balls. Anyhoo, he explains that in the video, it was educational for me too.

Yes a nice video. The problem with DIY ball screw rebuilds is in fact the problem of fitting the balls. Basically you need a range of balls varying by ten thousands of an inch and ideally a precise micrometer to measure them. As noted in the video what you need size wise can vary a bit from one screw and nut to the next. Ultimately it is easier and probably cheaper for a DIY guy, working on one machine, to simply order a fitted screw and nut assembly. Otherwise you will need a bunch of balls in about a dozen sizes to get what you need for a proper fit.

I know this is a DIY forum so maybe suggesting ordering rebuilt screws does fit in. Sometimes though you need to consider it. By the way no association here with any of these suppliers.

[NIC 77]

About the end supports, I was just having a tantrum because I was curious to know if that was the problem and didn't get my answer right away. Angular contact bearings, well, you could probably order them on Aliexpress for a few bucks each, so it's probably not a big deal, but as Louie said, it won't be a matched set. You just don't want to break something trying to fix something that isn't broken. I still don't know the answer really?

As to the alternating ball sizes, well I've heard this before, but I would trust the video because he has done this a bunch of times and for customers as well. I've personally never done it, but actually, now I'm curious......I kinda wish I had the stuff in front of me to play with at the moment. Also he has done this lots for this exact rolled screw size.

Yes, you might end up buying a bunch of balls you don't need. But how expensive are they? I bet this can be found cheap cheap on aliexpress. Jerm, perhaps you should send a message to Arizonavideo, and ask him, and let him know about this thread for better info than I can give you.

Two things I wanted to mention......

1. Not all Chinese rolled screws are the same, if you are going to buy a double nut, ask about the compatibility before you order it, so you might have to use the same supplier. At the very least, you'll have to ask the question to whomever you buy from.

2. As was mentioned in the video, the size of the ball needed depends on the screw, so even if you buy a double nut, you still might have to do some repacking with some smaller / larger balls at the end of the day and a good fit isn't guaranteed, even with a double nut.

It doesn't seem too hard to me if you can get hold of a few different sizes of balls for not much $$$ to give it a try.

[JermNZ]

About the end supports, I was just having a tantrum because I was curious to know if that was the problem and didn't get my answer right away. Angular contact bearings, well, you could probably order them on Aliexpress for a few bucks each, so it's probably not a big deal, but as Louie said, it won't be a matched set. You just don't want to break something trying to fix something that isn't broken. I still don't know the answer really?

Your answer is no, it doesn't look like there's anything wrong with the assembly, a bit of pre-load probably wouldn't hurt though so I'll probbaly just try shimming it.

Yes, you might end up buying a bunch of balls you don't need. But how expensive are they? I bet this can be found cheap cheap on aliexpress. Jerm, perhaps you should send a message to Arizonavideo, and ask him, and let him know about this thread for better info than I can give you.

Yep I actually just did this before seeing your post

Two things I wanted to mention......

1. Not all Chinese rolled screws are the same, if you are going to buy a double nut, ask about the compatibility before you order it, so you might have to use the same supplier. At the very least, you'll have to ask the question to whomever you buy from.

2. As was mentioned in the video, the size of the ball needed depends on the screw, so even if you buy a double nut, you still might have to do some repacking with some smaller / larger balls at the end of the day and a good fit isn't guaranteed, even with a double nut.

Yep I had these thoughts also, will keep it in mind if I end up ordering.

It doesn't seem too hard to me if you can get hold of a few different sizes of balls for not much $$$ to give it a try.

I agree, I think I will give this a try if I can find somewhere to get some balls in a reasonable time to New Zealand. I've measured the balls in one of the spare nuts I have, they seem to be 3.19 mm, measured using a 0.01 micrometer. Hopefully Arizona can give me an idea of what increments of balls I should get to start with.

Thanks for the help everyone.

[louieatienza]

About the end supports, I was just having a tantrum because I was curious to know if that was the problem and didn't get my answer right away. Angular contact bearings, well, you could probably order them on Aliexpress for a few bucks each, so it's probably not a big deal, but as Louie said, it won't be a matched set. You just don't want to break something trying to fix something that isn't broken. I still don't know the answer really?

As to the alternating ball sizes, well I've heard this before, but I would trust the video because he has done this a bunch of times and for customers as well. I've personally never done it, but actually, now I'm curious......I kinda wish I had the stuff in front of me to play with at the moment. Also he has done this lots for this exact rolled screw size.

Yes, you might end up buying a bunch of balls you don't need. But how expensive are they? I bet this can be found cheap cheap on aliexpress. Jerm, perhaps you should send a message to Arizonavideo, and ask him, and let him know about this thread for better info than I can give you.

Two things I wanted to mention......

1. Not all Chinese rolled screws are the same, if you are going to buy a double nut, ask about the compatibility before you order it, so you might have to use the same supplier. At the very least, you'll have to ask the question to whomever you buy from.

2. As was mentioned in the video, the size of the ball needed depends on the screw, so even if you buy a double nut, you still might have to do some repacking with some smaller / larger balls at the end of the day and a good fit isn't guaranteed, even with a double nut.

It doesn't seem too hard to me if you can get hold of a few different sizes of balls for not much $$$ to give it a try.

The difference with a double nut is that one nut has the balls engage toward one side of the ball race, and the opposite side of the race with the other. Mashing the ball into the root of the bearing race on the screw I don't think is the greatest idea, because that creates variable torque needed to turn the screw, and you'll see that in the finish. Granted the bearing races are hardened by the rolling process, but the excess pressure would work harden the races eventually to the point that the surface flakes off. It can be done on a ground ballscrew effectively because the bearing races are ovoid shaped, and the balls only contact two sides of the race.

[NIC 77]

The difference with a double nut is that one nut has the balls engage toward one side of the ball race, and the opposite side of the race with the other. Mashing the ball into the root of the bearing race on the screw I don't think is the greatest idea, because that creates variable torque needed to turn the screw, and you'll see that in the finish. Granted the bearing races are hardened by the rolling process, but the excess pressure would work harden the races eventually to the point that the surface flakes off. It can be done on a ground ballscrew effectively because the bearing races are ovoid shaped, and the balls only contact two sides of the race.

My only point is that the double nut ballnuts are not guaranteed to be a direct fit and some smaller or larger balls may still be necessary to get them to work optimally.

No idea how much of a concern this is. I couldn't honestly say.

[louieatienza]

My only point is that the double nut ballnuts are not guaranteed to be a direct fit and some smaller or larger balls may still be necessary to get them to work optimally.

No idea how much of a concern this is. I couldn't honestly say.

If you preload two ballnuts against each other, it takes up any axial play, and the Bellville washer(s) allow slight variance in thread pitch while retaining tension. You'd actually want the clearance of the balls, so that to avoid spots where the screw could bind.

[JermNZ]

If you preload two ballnuts against each other, it takes up any axial play, and the Bellville washer(s) allow slight variance in thread pitch while retaining tension. You'd actually want the clearance of the balls, so that to avoid spots where the screw could bind.

I agree, this is the most economical solution as I actually have a spare ball nut sitting here. It's missing some balls but I don't think that will matter.

I just need to decide whether or not I am willing to sacrifice x axis travel.

I'm also having trouble finding balls to purchase locally.

[hanermo]

Your mounts are part of the problem.
The bearings are *not* fit for purpose as-is.

They have slop and are not preloaded, because the insides between them need to have a tiny offset between the inner and outer race.
Besides, the bearings are very poor- as usual.

Endless ways to preload the bearings.
1.
Get 2 sets of shimstock, with a 5-7 micron thickness difference +/-.
Cut 2 rings, outer thinner against outer ring of bearings (where thrust is outwards on the bearing set).

Or get one thickness, and using oil + wet dry silicon carbide 800 grit, sand the outer set thinner.
5-10 minutes.

Highly recommend also using ac bearings, as well.

Or use a very thin silk paper, and only make an inner ring. Scissors will do.
Paper will compress at first, and then stop.

[JermNZ]

Your mounts are part of the problem.
The bearings are *not* fit for purpose as-is.

They have slop and are not preloaded, because the insides between them need to have a tiny offset between the inner and outer race.
Besides, the bearings are very poor- as usual.

Endless ways to preload the bearings.
1.
Get 2 sets of shimstock, with a 5-7 micron thickness difference +/-.
Cut 2 rings, outer thinner against outer ring of bearings (where thrust is outwards on the bearing set).

Or get one thickness, and using oil + wet dry silicon carbide 800 grit, sand the outer set thinner.
5-10 minutes.

Highly recommend also using ac bearings, as well.

Or use a very thin silk paper, and only make an inner ring. Scissors will do.
Paper will compress at first, and then stop.

Hi Hanermo,

From my measurements I have determined that the deflection from my bearing block is fairly small compared to the backlash in my nut. Therefore I think I will stick with Deep Groove BB for now.

Thanks for the info on shimming. I will have a go and see if I can improve them.

[JermNZ]

Hi Everyone,

It's been a while, but I have an update.

I had trouble sourcing balls for the repack and it was going to be costly to order many sizes.

I've had heard good things about Fred from BST Linear so I sent him an email for a quote on a new ball screw. These screws were advertised as "zero backlash" but I was obviously skeptical. Over the course of several emails, Fred explained how each individual ballnut and screw is manually packed and tuned with the correct size balls to minimise the backlash and account for variation in the manufacturing of the screws. He then offered to send a ball screw to me free of charge (except shipping) for my evaluation, before making payment, I happily obliged. The screw arrived and was of significantly higher quality than my existing screws and were of remarkable value for the quoted price (USD 22.50 for an 800 mm screw). The finishing was very clean and tidy, no sharp edges or burs, they also had new style rubber end seals rather than the old plastic caps held in by tiny grub screws, these always come loose and are left behind by the nut.

I fitted the new screw to my machine and made some measurements, it was much better and probably adequate for my needs, but still not perfect. There was a small amount of play in the nut at some parts of the screw, and as mentioned before, I was worried about wearing of the contact surfaces over time and becoming worse.

I ended up going the double ballnut route with belleville washers to provide pre-tension. The results of this modification were very good, almost no determinable movement in the ballnut relative to the screw when loaded by hand. I also think this is the best solution to deal with screw pitch variation and inevitable wear in the components over time.

Some pictures below, but please visit my website for more detail and pics. https://jeremyyoungdesign.com/

[Iceturf]

I ended up tightening them as much as I could (not that much) and securing the locking grub screw.
....
I purchased the RM1605 screws from linearmotionbearings several years ago. A couple of the ball nuts seemed to have significant backlash in them so I sent them back for replacements, these were marginally better but I continued with my build.



Backlash: Screw 0.02 mm, Nut 0.07 mm
Flex (with further applied pressure): Screw 0.06 mm, Nut 0.15 mm

I've found the chinese ballscrews are pretty bad. There are two problems here,
1. How well constructed are the Bk12/BK10 bearing blocks? I found that either the bearings or the bearing block itself has slop, allowing even the best tightened blocks to enable a significant amount of backlash. I (and others) have found that spacer(s) between the 6000Z bearings and between the bearings and the housing is adequate for resisting backlash, but not a realistic solution. You would be better off buying quality bearings and pressing them into a hole.

2. I have found that sfu1204 and sfu1605's are very hit and miss. I have bought 4x sfu1204's (from one chinese vendor), 3x sfu1605's (from another chinese vendor) and 1x sfu1605 with 4 bearing tracks (from a different chinese vendor). I can guarantee you that:

A) Your ballscrew will come bent. If its bent more than 1mm, I would definitely reject it. The best solution is to pop the Bf10/12 bearing out and allow it to bounce around. Better it bounces around than it puts pressure on your linear rails. I did find my longer (~1000mm) sfu1605's flexed on their own, so I used blocks on both ends, but I could have easily gone without the Bf12 blocks.

B) Your ballscrew will have a variable amount of internal backlash, depending on randomness and the seller's general quality or reputation. I don't have a solution for you here. I did notice the last sfu1605 did seem reasonable quality, but i haven't really tested it for backlash - the extra bearing track costed me ~10$ CAD more, but it seemed O.K. otherwise.

I've read countless people saying good things about their chinese ballscrews. However, I also notice that a lot of these reviews are from ~5 years ago. I have also read a few recent reviews from people who seemed very competent and did very detailed reviews of their findings who swore off ever using chinese components. Perhaps china discovered they could cut yet another corner. I think people get good results from locally purchased ACME rods. Where I am located, this isn't practical. However, if you are in the USA - I think there are a number of sellers who provide adequate ACME rod for affordable prices.

To tighten the locking nut - I use a rag on the Bk12mm side, and I a pair of VICE-GRIP hand clamps. I don't lock into the ballscrew threads, but just past them. You will invariably mar the ballscrew metal surface. Just take a file, and file off any burs you cause when you are done. The BF10 bearing doesn't serve much purpose, except as a bit of a restriction against 'whip' so don't feel like it needs to be a perfect fit. You want an easy fit on that end. So tighten the crap out of your BK12 bearing side, and leave the BF10 loose. Heck, don't even bother with the locking clip - its up to you. Some people I saw had their BF10 bearing pop out on its own, so you may still want it.



Good luck