Double ball nut worth it for long C7 grade rolled ball screw?

So I've been a little recently frustrated with an abrupt lack of response from these "TEN-HIGH" vendor on Amazon (before CNY- which I realize is now but this was before they had an excuse) regarding my X axis screw which I have saved for last on my machine retrofit. Still, I'm eager to get that done after my Z and Y axes turned out unexpectedly well.
I use this machine for occasional prototypes in the course of product design, not any heavy or industrial use - I frequently also use it as a manual machine.. That said, my X screw is going to be around 900mm (including end machining, and the last 4" of thread on either end won't need to engage because of the large carriage/motors/etc.

I was all settled on my purchase and upcoming challenge of installing it, when I discovered double ball nuts. In this application I have plenty of room for one without any real consequence, but it seems to be a rare offering from the manufacturers in China. I worry about the length of the screw and the additional load it will have to carry compared to the other axes and so I'm thinking if I can find a double ball it might help alleviate some of these anticipated issues. The threads I've found here seem to validate them at least in concept- I don't mind doing some extra modifications to tighten them up, as I will have to modify nearly every other part anyway in one way or the other.

Similarly, although they provide the kits with bearing blocks, I was looking into maybe spending a few more $ on a bearing upgrade as I'm sure they use the cheapest option. Has anyone tried that and do you think any of this would be worth it? In some ways I feel like its lipstick on a pig, and I'm not shooting for a world class machine- just a substantial improvement over the lead screws that were originally supplied. From another perspective though, for a few extra bucks now, if I could increase my reliability and precision I can probably convince myself its worthwhile. Just looking for a good balanced bang for the buck. Thanks for any opinions, figured I may as well use this delay as an opportunity to do more research.

Double ball nuts will be better than single nuts hands down.


Sent from my iPhone using Tapatalk Pro

In general, double ball nuts are great, but without knowing what kind of machine, what kind of materials and purpose, it is impossible to say if it is "worth it".

Trouble is its also impossible to say what it's purpose is other than prototyping- that prototyping involves cuts with nearly every material and tool type you can imagine (and some you probably can't! haha). For example, I've used this machine to plasma cut, to laser cut, to etch with a laser, to cut steel, aluminum, plastic, and to cure resin. I've cut & blown glass on this machine also. The machine started life as a Tri-power "Eldorado" bridgemill from Shopmaster but has been pretty drastically modified since then. This isn't my machine, but it is the same model (as an example). I think your general statement is probably the best answer I could hope for- thanks! I was curious more because it seems pretty uncommon to find a double ball nut. I'd expect them to be more commonly available if the advantages were more readily known.

The double nut is usually for reducing/eliminating backlash. If you're worried about too much load, go to a bigger screw. If it's the type of double nuts I'm thinking of, they don't get you anything in terms of extra load capacity.

From your description, and looking at the video of the type machine, I would say that double ball nuts would be the way I would go.

Ok, cool. I'm going to go with the double then- thanks for the input. I'll have to order from another vendor- I hope they are comparable. The "TEN-HIGH" units worked out really well. The new vendor is off ebay. 20mm is a bit larger than the lead screw diameter that came with the machine. The Z was the smallest at 16mm, but the Y is also a 20. There would be clearance issues elsewhere if I went larger, and 20mm seems conveniently standardized, so I'll go with it. I'm not super worried about loading, probably just being a little OCD. In a perfect world, each screw would be precisely proportional to its load, but I think this should work. Still could upgrade the bearing spec in that block, but we'll at least give these a try.

Just like to mention.
I bought some double 1605 size ballscrews/nuts for a PM25 type conversion.
Haven't got them fitted yet but the quality difference is CLEARLY visible:

These are good:
https://www.ebay.co.uk/itm/2-set-DFU...jRE:rk:12:pf:0
Properly machined tab slot almost perfectly spaced.

These are crap:
https://www.ebay.co.uk/itm/ballscrew...s3uu:rk:2:pf:0
Slot is pretty hacked and no spacer fitted at all!

I found on my smaller mill that thrust bearings don't seat that well on these pre-machined screws. If you go even a fraction too tight it forces a bend on the screw as they slip into the area where there is no flat spot (roll thread end).
For this new machine I'm going to use AC bearings of size 12x32x10mm on x,y and making bearing blocks using my small mill.
These bearings seem to seat really nice on the end when I checked.

Z will be using the crap screw and thrust bearings for now as I'm looking at replacing it a later date with 2005 DFU from the same seller that sent the good 1605's. Cost more but so be it.

Thanks for the feedback! I have to admit I have the same worry but there's not a whole lot I can do about it until I get my hands on the delivered assembly to assess. The original Z & Y single nut assemblies turned out to be really good. The Z in particular is maintaining .001" repeatability with almost no backlash- of course it's not carrying a heavy load or a long range but still. I wish TEN-HIGH would respond but I don't think they offer a double ball nut anyway. I'm trying a new vendor for the big X axis here, and as a bonus they can customize with some mill operations (I requested a slot on the end so that I can lock the axis to the mechanical drive and still do manual threading operations when desired). TEN high was unable to do that. They also have 100% positive feedback so that's nice. Also, they can deliver these in about 2 weeks in spite of CNY, so that's nice too.
I'll try to come back with some info after I receive them! Even if I do have to make some of my own customizations I'm hoping this offers a pretty good head start on the process.

Also I hate to add to this, but based on my research (and not experience) you might want a c6 screw for an almost 1 meter span.

In spite of that long screw, my X axis actually has around 25.5" of actual real movement, so hopefully we'll be ok. It'll be about 2 weeks until I can receive this assembly.

Just saying...
https://www.machinedesign.com/motors...king-the-myths

Quote:

---

Originally Posted by Greggan

Just saying...
https://www.machinedesign.com/motors...king-the-myths

---

Sure you can preload with oversize balls. If you have a drawer full of different sized oversize balls and want to load up a ballnut it will work. The oversize ball selection is critical and may take several tries with different size balls to get the lowest backlash without binding. The real problem is determining what size balls you need and then obtaining them. If your vendor can do this for you then it is an option, but troublesome if you are doing it yourself.

A double nut system is either adjustable or spring tensioned and therefore you install it and adjust the tension or the free play and that is all. On my double nut system on my mill I designed the system, installed it and had as near zero backlash as possible right out of the gate.

To each his own, but I'd still use double nuts.

Quote:

---

Originally Posted by 109jb

Sure you can preload with oversize balls. If you have a drawer full of different sized oversize balls and want to load up a ballnut it will work. The oversize ball selection is critical and may take several tries with different size balls to get the lowest backlash without binding. The real problem is determining what size balls you need and then obtaining them. If your vendor can do this for you then it is an option, but troublesome if you are doing it yourself.

A double nut system is either adjustable or spring tensioned and therefore you install it and adjust the tension or the free play and that is all. On my double nut system on my mill I designed the system, installed it and had as near zero backlash as possible right out of the gate.

To each his own, but I'd still use double nuts.

---

I used thin aluminium cooking foil between the slave nut and spacer to pack mine.
Used approx 0.06mm thickness on one and 0.08mm on the other.
Nut backlash is down to 0.015mm (0.00059").

When fitted to the axis the overall backlash is 0.025mm (0.00098") on my Y.
I haven't fully finished adjusting my X yet, workshop is like a fridge atm.

Quote:

---

Originally Posted by Greggan

Just saying...
https://www.machinedesign.com/motors...king-the-myths

---

From that link:
"But only a few manufacturers can hold the tight tolerances that both nut and screw require in the ball-select method."
The article was written by a Bosch Rexroth employee employee so maybe it's a partly a jab at the lower price stuff, take it for fwiw.
I have 3 budget Thomson screws refitted by ball-select (done by supplier), the backlash tested good.*

*and by good I mean better than stock, this isn't Bosch grade stuff.

Has anyone ever considered that 2 ordinary ball nuts fitted back to back or flange to flange with a rubber "O" ring between them on a rolled ballscrew is resilient and will have a constant preload even if the screw is worn in the middle over years of use?

A solid spacer between the flanges is not necessary as the "O" ring will apply quite a bit of constant pressure once fitted.
Ian.

I have a Bridgeport with 32 mm ball screws and the X axis is worn and there was some backlash, there was no room for an additional nut so I sliced the original nut into two halves giving two ball runs on each half and used an "O" ring as a kind of preload spring and the backlash is for all purposes non existent now. Not for the faint hearted I admit, I used a slitting disk to split the nut. It is just about possible to machine the rough edges to true up the edge to give a form of register. The "O" ring seemed to give the right amount of preload, I also thought of making a thick shim out of butyl rubber or polyurethane to give a bit more pressure, it is hard to say just how much preload you need, a case of trial and error I guess. Dave

Wow, now that is what I call innovative thinking......my top hat is off to you.....you're a genius.

That method would have to be the idea of the month...year, whatever......I would have attempted to fit a second ball nut on the screw to achieve the same.....I'm stunned that it could be done.

I would agree that a Butyl rubber spacer will work 100% better than an "O" ring......I have some thick red Butyl (I think) rubber that is used in tool making for springs where needed and a spring is too big to work.

You probably will get a lot more wear in the nut with the single ball track as you now have only one ball track going one way and the second track backing it up.....OK in reverse though...…. but what the heck, it works and that's what counts.

It will work better than a new single ball nut replacement due to the screw being worn in the centre area so the resilient "O" ring will keep the nut always in contact.

It would be great to see the process of cutting the nut in 2 and what it looks like afterwards......that must have taken some working out to get it right.
Ian.

I used an angle grinder mounted onto the tool post of my lathe with a 4 1/2 inch x 1mm slitting disc and carefully plunged it in, it only took a minute or so. The nuts on that size of screw appear to be case hardened so it did not take long. Most ball screws have sufficient space between the ball runs to allow you to do this. It will potentially shorten the life but in this case it was already dead, there is no way would I have tried this on a new nut. If you do try this yourself beware the sharp edges that are left they are like razor blades it took me longer to remove them than make them in the first place. I made a simple frame that screwed into the handle mountings on the angle grinder to hold it onto the tool post, it was more luck than judgement that I got it to cut square. Because of the resilience of the spacer a small amount of "out of squareness" should be tolerable. I also thought of using Belville washers they should if you need a really heavy preload. Dave

Very clever....I was wondering how you got through the case hardening.

If it works with a worn or used nut it should work well for a new one...…...the anti backlash feature would justify the expense of replacing it when it got really worn due to the reduced ball count in each direction.

That would of course be if you did not have the space for a double nut.

I think you could treble the life of a cheap nut and have a backlash free solution.

With space I would go for 2 single nuts with flanges back to back and a rubber spacer between......guaranteed no backlash for it's life.
Ian.