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View Full Version : Rotating ball nut, Ballscrew Pitch, RPM limits, and my particular build pickle



Bongo
12-04-2021, 12:36 PM
Hello All,
So I started building a 1.2 c 2.4m bed CNC a long time ago. It's a bit unusual in a few ways, the gantry being a frankensitne mash-up of a scrapped stairlift extrusion, steel pipes, epoxy concrete, and home cast aluminium end plates. The machine build has not proceeded the way I see most builds happening (ie a sensible CAD design that gets tweaked BEFORE construction). Thats not to say I had no design, just it's morphed quite a bit during the build. I guess you could say I am quite a thick-headed so and so, that needs hands-on experience to learn anything. Ok that's enough background for now - the dilemma, and reason for the post...

The x has two 2.4m long, 5mm pitch,32mm dia. ballscrews, which I have had for at least a year before finally getting round to fitting. They are from China (linearmotionbearings2008 on eBay) and are both a bit bent (approx 1mm off true bow). As I was going the rotating ballnut rout, I thought it wouldn't be such a problem, as the sideways force required to straighten them is not much over such a length (indeed the sag by gravity is noticeable), and I would be able to tension them a lot as the screws themselves are stationary and don't need to worry about over tight angular contact bearings.

Here's one screw rigged up to the rotating ballnut assembly I have cobbled up:


https://youtu.be/HF-OtLCGphg

Things to note: I am testing this with just one screw - I haven't yet mounted the slave ballscrew on the other side... There is very little (no detectable by eye) racking in the gantry, even though I'm driving it from one side only, which seems promising rigidity wise.

I think the 5mm pitch was a mistake! I thought that with the 400W servo (3,000 max RPM) and no gear reduction, I would get reasonable travel speeds (3000 x 5 = 9000mm/min). 9000mm/min sounded ok to me - keep in mind I'm a complete CNC newbe. What I didn't consider is the ballnut's limitations. From the word go the nut has felt a bit 'crunchy' - or at least not smooth, and also sounds a bit rattly - despite my greasing... I didn’t get them on video but, there have been a few ‘jams’ that have stalled the servo. When I turn by hand I can verify that yes, it is just the ballnut ‘catching’, and with a bit of ‘back and forthing’ I can sort of feel it past the jam. The catches have happened at relatively low speed. When going faster I guess the servo just powers through them.
So options as I see them:

1. Ditch the cheap ballscrews, bite the bullet and buy some good ones of 10 or 15mm pitch (suggestions for what would be ‘good’?).
2. Continue on, mount the other x ballscrew. My feeling is this might hide the problem for a bit... In fact, writing it out here makes this seem a very poor choice?
3. Try some ballnut fixes? push more grease in? attempt disassembly and look for swarf or other potential jamming culprits (how possible)?
4. Something that I haven’t thought of that may seem obvious to you lot??

Ok thanks a lot for reading, please share your thoughts :)

A_Camera
12-04-2021, 03:40 PM
I would start with removing and cleaning all the nuts and linear bearings. It takes time and patience but worth a try.

Bongo
12-04-2021, 03:52 PM
Hey A_Camera, thanks for this, seems a sensible start. Any idea how likely I am to be able to get the balls back in the nut or how difficult it is? Would love to hear if you (or anyone out there) has done it? Estimated time investment?
Thanks again, B.

Muzzer
12-04-2021, 05:38 PM
I had to do this on one of my ballscrews after The Stupid Fat Bloke managed to get into my workshop when I wasn't looking and rotated the ballscrew too far :highly_amused: Wasn't quite as bad as I'd feared in the end.

Best avoided if you can but there are videos out there that show how it's done. There are various methods suggested but this worked for me, as you can screw the ballscrew in as you fit the balls. You could probably do it in 20 mins or so. Or 2 minutes if you are a Chinese factory worker.


https://www.youtube.com/watch?v=e73hmOgmOw4

Others suggest holding the balls in place with grease or plastic foam and then screwing the ballscrew in place afterwards, which sounds like a recipe for frustration and disaster unless you like that sort of thing.

######

Incidentally, at 3000rpm, those balls would be flying through the ballnuts. Try to imagine them being diverted by the plastic guides and ask how long those guides will last. There are max rpm specs for ballscrews and I don't imagine those ones would be very high. Most of the manufacturers publish application notes. Looking at p33 of this one, I'd be surprised if even a top end 32mm ballscrew would be happy at 3000rpm. https://www.hiwin.com/pdf/ballscrews.pdf

A source for better quality parts might be one of the machine breakers. I've bought ballscrews from FA Parts in Korea. https://www.ebay.co.uk/str/industrialpartsshop They stock new old stock (NOS) and used parts from dismantled machines. You might find a decent quality Japanese, Korean etc ballscrew, possibly ground rather than rolled, possibly never used for the same price as a "new" Chinese rolled ballscrew. I got ground Korean 2005, 1090mm long for £109 and a 1205, 500mm long for £51 inc carriage, both delivered within a week - no comparison with the Chinesium ones I got for my Bridgeport conversion.

A_Camera
12-04-2021, 07:10 PM
Hey A_Camera, thanks for this, seems a sensible start. Any idea how likely I am to be able to get the balls back in the nut or how difficult it is? Would love to hear if you (or anyone out there) has done it? Estimated time investment?
Thanks again, B.

I don't know how long time it takes for you, it depends on many things, but no doubt that it can be done. I did clean sixteen linear bearings, and it took me a while, but the results were well worth the effort. I am sure that ball bearings are not more different, especially the larger ones.

routercnc
12-04-2021, 09:39 PM
A ballscrew I had which sounded like that had very slightly undersized balls in it. If you can determine how well they fit and can find a slightly larger diameter set of replacements that could help. However Iíve searched and not had any luck other than in large quantities.
But given that they very occasionally get stuck suggests a good clean would be a good start. I did post a YT video in my drill press series on the grease method but I do like Muzzerís quick method above !

https://m.youtube.com/watch?v=QbvH5dnUF1Q&t=3s
Starts about 26:00

Bongo
12-04-2021, 11:18 PM
Thanks so much for sharing your thoughts guys, appreciate that. @Muzzer: great link and info thanks. I will try pulling off the nut and see how I go, though it sounds like to get good rapids out of the machine I will ultimately want to change the pitch to something like 15mm and get a smaller toothed pulley for the servo...
Good call on the Place in Korea BTW, I actually got my servos, drivers, and the ballscrews for the y and z axis from that e-bay shop, and have been well pleased. I have looked a few times, and if they had anything in the right length, dia, pitch, I'd be going that route... I will keep checking and might shoot them a message. Though I don't relish the extra cost, I feel the hours I've put into the machine build would justify it...

@Routercnc Thanks for this - an interesting watch... I'll see what I get and how it goes tomorrow. I must admit, my first thought was that the balls might be a bit undersized, or possibly just unevenly sized. I can't detect any backlash (by hand).
Thanks again guys, I'll report back with the findings.

routerdriver
13-04-2021, 09:27 AM
From the little I know about ballscrews it would take a lot to persuade me to dismantle them.I think I would first pump quite a bit of grease through them to flush out anything that shouldn't be there and I might do it more than once and in slightly different positions.If it didn't improve things I think I would watch several youtube videos before even contemplating dismantling and I would take careful note of any warnings.I would only do the job on a large and clear bench with some kind of rail surrounding it for restraining escaped components.The sequence of balls may be important as I have read accounts of sizes being intentionally varied along the assembly and the variations are tiny,but necessary so that you don't get a situation akin to a line of balls trying to move but effectively having their neighbours walking on their heels and impeding free motion.

It would be much easier to clean the threads themselves very thoroughly prior to any form of dismantling and this might pick up any particles that have been attracted to the protective grease that would have been applied for initial assembly and transportation.If you are lucky it might be enough to help but as you have discovered a 5mm pitch is a bit fine for the intended use.

JAZZCNC
13-04-2021, 08:22 PM
Don't waste your time with greasing as it will make no difference and at best only mask the problem, at worst it will mix with what's grinding now and turn the grease into a very nice grinding paste.!! . . Stripping and cleaning ball-screws is a piece of cake and doesn't take long. BUT you must be set up and prepared.

It's a must that you take them apart in a container with reasonably high sides and if the container is steel place something like a kitchen roll in the bottom to stop the balls from bouncing because they have a habit of shooting off like bullets if dropped from any height. DONT put a magnet near them because you'll just magnetize them and make them attract every speck of metal dusty within a 4mile radius...:cower:

Regards the rotating nut then I've done a few now and you need at least 25mm pitch and then put a ratio on the ball-nut of 3:1 or 2:1 to get the nut rotation speed down. If using 32mm screws then I'd go with 32mm pitch as it's a common pitch size.

I wouldn't continue with those 5mm pitch screws as you will destroy the ball nut within weeks if you try to spin at anything like 3000rpm, even 2000rpm will eat the plastic tubes away in no time which then causes the balls to jam and stick turning them into 50p pieces as they slide rather than rotate. Cut your losses and sell them on because you will destroy them.

Bongo
13-04-2021, 09:23 PM
Hey JazzCNC. That sounds like very good advice. I plunged on and took the nut off!

29808

29807

I actually managed not to lose any ball bearings. Good call on the non-bouncy surface ;)
Didn't have a lot of time today but will make a small vid of the process - once I have successfully put it back together, that is!
Before Taking it off, I could feel with my hand that there was a little side to side play in the nut, though I couldn't detect any axial play that would obviously translate straight into backlash. Anyway, it made me suspect the balls are slightly undersized.

Agree, I probably am better off selling them and going to a higher pitch. What is your experience with cheap Chinese ones vs the branded,JazzCNC? 32mm pitch seems a lot, but I see that it does seem popular to match pitch to dia...

As a side note, the ball diverters are actually metal.
Thanks again for all your thoughts, keep em coming.

JAZZCNC
13-04-2021, 10:17 PM
What is your experience with cheap Chinese ones vs the branded,JazzCNC? 32mm pitch seems a lot, but I see that it does seem popular to match pitch to dia...

Depends where you buy from in China.? Chia's ball-screws where you bought those from are far inferior to those Fred at BST sell's.!

Also branded is a loose term because like most things there are good and not so good brands, Then you have to consider that you are actually trying to compare apples with oranges because it's not realistic to expect something that cost's $20 compared to something which costs $200 to be even close to equal.

My experience is that Branded screws are far superior to Chinese screws which is what I would expect given they cost 3 to 4 times the price.
However, my experience also shows me that even for relatively heavy hobby/light industrial use the better Chinese ball-screws are more than good enough for a CNC router provided you accept that they are not going to last forever and at some point, down the round, they will need replacing. But when you consider you could probably buy 4 sets for the price of one single new THK C5 ground screw this isn't such a bad thing for a hobby user or small business who can afford a little downtime.

End of the day you have to be realistic.!

Re the 32mm pitch, with a 3:1 ratio the pitch is equal to 10mm or 16mm with 2:1 however the rotation speed drops from 3000rpm to 1000rpm which is better for the ball-nut.
However, there is a price to pay in that the torque is reduced, but this is offset some by the ratio increasing torque. But for this application lowering the rotational speed is more beneficial than losing a little torque when you have more than enough spare.!

Bongo
16-04-2021, 11:59 PM
Thanks for this Jazzcnc, good advice.

The 'quick method' posted above in the youtube vid, didn't really work for me because getting a 2.5m long ballscrew vertical and then being at the top, in a convenient place see clearly into the nuts and spoon the bbs in seemed troublesome...

So yeah, after cleaning everything up with lots of brake cleaner (it was reasonably grubby in there), I used the grease dabs to 'stick' the bbs in place. I'm surprised at how well that worked! :D Did it over a
cardboard box lined with paper towel. Miraculously, number of bbs lost = 0.

29815

29816

Upon reassembly, I realized how poor the 'wiper seals' are. Just pieces of hard white plastic that don't seem to fit very well, and that get slightly deformed by the two grub screws holding them in place. They leave a thick layer of grease from inside the nut, very much ALL over the screw outside the nut, where it will attract dust & debris. Has anyone made their own seals for these ??

Anyway, it's now MUCH less gritty and rolls well. Still has noticeable lateral play mind you... As you say Jazz I think I should cut my losses.
I messaged Fred on Aliexpress. I was considering a double-nut to remove backlash, but looks like he can't do that on 3232 only 3210. Any experience or opinions on using a double nut for such a large x axis? When I asked about c5 screws Fred mentioned 'TBI' ground and TBI C5 Rolled' ballscrws. I'll admit, I have no idea what "TBI" means - anyone?

Ok thanks again guys.

Muzzer
17-04-2021, 01:05 PM
Good to see you succeeded!

Can you fit some of those telescopic covers in your machine setup? I bought some from AliExpress recently for my lathe - they are available in a wide range of diameters and lengths. My Shizuoka had them from new and nothing gets close to the ballscrews.
https://www.aliexpress.com/item/2040062237.html?spm=a2g0s.9042311.0.0.41a84c4dNrCm N6

The part numbering system is Dmin-Lmax-Lmin-Dmin where the Dmin and D max are the diameters of the ends of the telescopic springs. Lmax and Lmin are the max and min lengths. For my machine I got a couple of 20-600-40-36 (you need 2 per ballscrew usually).

Voicecoil
17-04-2021, 01:19 PM
When I asked about c5 screws Fred mentioned 'TBI' ground and TBI C5 Rolled' ballscrws. I'll admit, I have no idea what "TBI" means - anyone?

Ok thanks again guys.

TBI are a Taiwanese manufacturer (Gten are another), from what I've heard they have quite a good reputation, go here:

https://www.tbimotion.com.tw/en/category/index.html

Fred quoted me on some and they seemed to be about 60% more expensive than his normal ones, but it might vary with the type of nut you have.

Bongo
17-04-2021, 09:42 PM
@Muzzer. Thanks, there isn't a lot of room on either side for the telescopic ones, but I'll look and investigate if I can fit them in. My lathe has the same thing over its leadscrew, and I must admit I don't love the noise they make (how yours for sound levels?). Still seems like a simple solution, but I'd still feel more comfortable if the nut's seals were a bit better as well, and maybe had some kind of rubber sweeper... I don't know.

@Voicecoil. Thanks for that, I had a nose around the TBI website you liked to, I guess I'll ask Fred for a quote too. Did you go with his ballscrews in the end? I'm guessing not the TBI ones?

Voicecoil
18-04-2021, 12:38 AM
I will do - when I can find some premises large enough to accommodate the machine, but am currently being shafted by COVID, the LDNPA and urban millionaires buying up all the property round here.

JAZZCNC
18-04-2021, 11:05 AM
I messaged Fred on Aliexpress. I was considering a double-nut to remove backlash, but looks like he can't do that on 3232 only 3210. Any experience or opinions on using a double nut for such a large x axis? When I asked about c5 screws Fred mentioned 'TBI' ground and TBI C5 Rolled' ballscrws. I'll admit, I have no idea what "TBI" means - anyone?

Ok thanks again guys.

Pointless going with double nuts for a router, esp one this size, the material, and the error or flex you'll have in the machine will far outweigh any error you'll see from the nut.
Also, depending on the rotating ball nut design the longer length of the nut could cause issues. Remember you will now be spinning the nut and at high speed, so a longer nut with the flange on the end of the nut will resonate more, the shorter the better really.

This is another reason for using 32m pitch because it's designed for high speed and uses a nut design that suits high speed better by placing the flange in the middle of the nut and recirculating the balls on both sides of the head flange. For a rotating nut, this is very important as it's much better balanced.

TBI is a reasonable quality ball-screw manufacturer and if you can afford the extra they are worth investing in. Ground screws are light years better than rolled but they also come with prices that can be out of this world on the higher grades.

Bongo
25-05-2021, 03:11 PM
It's been a while, so I thought I'd update. I made a little video on the situation so far https://youtu.be/HF-OtLCGphg
And another vid of the nut cleaning saga to follow…
In other news I ordered a pair of TBI C5 rolled screws 32dia. 20mm pitch from Fred, and they are due to arrive today :D
I will need to do a bit of work on the mounts as the nut is not exactly the same. Then it’s just a question of changing the pulleys and belts…

JAZZCNC
25-05-2021, 07:16 PM
I will need to do a bit of work on the mounts as the nut is not exactly the same. Then itís just a question of changing the pulleys and beltsÖ

If it's the nut I think then they will be better for you as the flange is more central to the nut body which gives a better balance and less flex at higher speeds. Thou it might mean a re-work of the shaft and flange.?

Bongo
01-06-2021, 05:20 PM
So here's the ball nut tuning / cleaning experience in video form in case it's helpfull to anyone:


https://youtu.be/vvWdq9KyCvw

The new screws arrived! The nuts are not the ones with the flange in the middle, as that would have required new angular contact bearings, and a whole new bearing block which was more work & expense than I was ready to stomach - maybe the next machine :)

JAZZCNC
01-06-2021, 07:27 PM
So here's the ball nut tuning / cleaning experience in video form in case it's helpfull to anyone:

The new screws arrived! The nuts are not the ones with the flange in the middle, as that would have required new angular contact bearings, and a whole new bearing block which was more work & expense than I was ready to stomach - maybe the next machine :)

Watching that I think you lost more than one ball, I saw at least 3 fly off when you tipped out of that cup...:cower:

Bongo
02-06-2021, 06:40 PM
Haha, yeah, thought I'd include that in the vid in case anyone (including myself) starts thinking I am in any way proficient at this stuff... Three escaped, but only one made it to the floor. the others got held up on the extended paper towel trap. I am reasonably certain they all found their way back in - there wasn't any space for more bbs in the nut anyway...

Ok question on the new ball screw though - Started a new thread about it:
http://www.mycncuk.com/threads/14615-New-TBI-Ball-screw-How-not-bent-is-enough-How-to-test