[lcvette]

Ok... So I was needing the ballscrew assembled to mark some holes locations and just assembled the single nut for the task.. I noticed it felt notchy and not nearly as lose á previously before disassembly. We this is odd, but on the double nut, there are different sized balls for the floated side and flanged side...

Floated side has .125500"
Flanged side has .124800"

So I guess I have a decent enough range to order larger balls.. .0007" larger made the single nut feel very notchy.. so I will try .0003" and .0005" larger and see what happens I guess. Although it may be prudent to try all the same size as I'm sure I have some of balls mixed in together that may effect movement... Will try that tomorrow and see if it smooths out.. for anyone who has repacked, how much larger of a ball we you able to get in without feeling notchy losing its smoothness?

Thanks,

Chris

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[ebrewste]

I had the same problem with it feeling notchy and had to try a few sizes. I was using a ground single nut and had < 0.001" backlash, so I though reballing would fix it. I was able to take it down a few tenths, but wasn't able to make a big improvement before getting notchy. It got pretty expensive for the resulting performance improvement. So, I don't have any useful info for you, but I feel your pain...

[arizonavideo]

Once they get tight you often find a single ball being .0002" larger can cause tight spots sometimes even less. Double nuts are a bit more forgiving because they have contact only on one side but the same problem happens.

Some screws are ruff and some nuts have ruff spots too. That last little bit is where all the work is at.

If the balls are all the same size sometimes you can have one that is not so round too and you cant really find them too easy. Replacing the whole set is the easy way.

If the nut is ruff then lapping it with 500 grit and slightly tight balls works well but then you have to clean the screw really well and junk the balls and then have the next size larger balls.

[lcvette]

Once they get tight you often find a single ball being .0002" larger can cause tight spots sometimes even less. Double nuts are a bit more forgiving because they have contact only on one side but the same problem happens.

Some screws are ruff and some nuts have ruff spots too. That last little bit is where all the work is at.

If the balls are all the same size sometimes you can have one that is not so round too and you cant really find them too easy. Replacing the whole set is the easy way.

If the nut is ruff then lapping it with 500 grit and slightly tight balls works well but then you have to clean the screw really well and junk the balls and then have the next size larger balls.

I measured all of the balls to separate the 2 different sizes and did find every once in a while I would have an outlier by a few tenths and I put those in a separate container. I ended up being able to find the following sizes to try out:

For the .1248" I found .1250"

For the 1.255" I found .1259"

I figure even if they are not as big as what might fit in there, they should help a little bit. And they are double nuts after all so hopefully it turns out pretty decent. I would be thrilled to get anything in the 0.0002" - 0.0004".

I also just purchased a brown and sharp 0.00005", yes half a tenth, not a typo test indicator so it should be able to really see what kind of backlash everything is seeing. Will probably be a week or so before I have the motors and control wired up and can begin testing.. want to make sure my lube system is finished and everything is adjusted properly and really well aligned so as not to introduce any avoidable causes for backlash.

My hope for this lathe build is for it to hold .001" tolerance repeatably without a second thought. We shall see..

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[handlewanker]

Hi....one thing to realise with a double nut is that if the balls in both nuts are slack, the spacer between them determines the backlash.

So, if you go for slightly loose balls in each nut and adjust the spacer to make the nuts go closer together,.....that takes a surface grinder to adjust the thickness of the spacer, and in your case it will be by trial and error.....one thou too little and you have a tight nut.......one thou too much and you have some backlash.

What you are doing in reality is pressing the two nuts together against a spacer of a specific thickness.....the thickness can be adjusted to get the nuts to hold the space when the backlash is just right.

if you take too much off the spacer you can then make a brass or steel shim and fit it between the nuts and the spacer.

You will notice that there is a key to prevent the two nuts from rotating once they are screwed against the spacer.

if you understand how the arrangement of the spacer and the key works you can make the nuts backlash free once again.

Simply put.......load the balls to each nut.....preferably having them slightly loose in each nut........assemble both nuts to the screw with the spacer between the nuts..........note the position of the keyway in each nut when the nuts are fairly tight against the spacer.....too tight and you wear the screw.......too loose and you will have backlash.

If the keyway in each nut overlaps then you need a thicker spacer or a shim to allow the one nut to back off and get the keyways in each nut in line......if the keyways won't get in line when the nuts are tightened you will have to grind the spacer down a few thou.

If you go for the loose balls in each nut the keyway will overlap, so you can use the same spacer and fit a shim in the gap......this is easier than grinding the spacer down if it's too tight.

BTW.......you don't need a hardened spacer so you can make one in the lathe from mild steel and face it carefully to keep it parallel.........you will need to cut a keyway clearance in the spacer too.

I also subscribe to the idea that if you fit alternate balls undersize by at least .0005" you will get less friction from the balls rubbing against one another than if they are all the same size.
Ian.

[lcvette]

Hi....one thing to realise with a double nut is that if the balls in both nuts are slack, the spacer between them determines the backlash.

So, if you go for slightly loose balls in each nut and adjust the spacer to make the nuts go closer together,.....that takes a surface grinder to adjust the thickness of the spacer, and in your case it will be by trial and error.....one thou too little and you have a tight nut.......one thou too much and you have some backlash.

What you are doing in reality is pressing the two nuts together against a spacer of a specific thickness.....the thickness can be adjusted to get the nuts to hold the space when the backlash is just right.

if you take too much off the spacer you can then make a brass or steel shim and fit it between the nuts and the spacer.

You will notice that there is a key to prevent the two nuts from rotating once they are screwed against the spacer.

if you understand how the arrangement of the spacer and the key works you can make the nuts backlash free once again.

Simply put.......load the balls to each nut.....preferably having them slightly loose in each nut........assemble both nuts to the screw with the spacer between the nuts..........note the position of the keyway in each nut when the nuts are fairly tight against the spacer.....too tight and you wear the screw.......too loose and you will have backlash.

If the keyway in each nut overlaps then you need a thicker spacer or a shim to allow the one nut to back off and get the keyways in each nut in line......if the keyways won't get in line when the nuts are tightened you will have to grind the spacer down a few thou.

If you go for the loose balls in each nut the keyway will overlap, so you can use the same spacer and fit a shim in the gap......this is easier than grinding the spacer down if it's too tight.

BTW.......you don't need a hardened spacer so you can make one in the lathe from mild steel and face it carefully to keep it parallel.........you will need to cut a keyway clearance in the spacer too.

I also subscribe to the idea that if you fit alternate balls undersize by at least .0005" you will get less friction from the balls rubbing against one another than if they are all the same size.
Ian.

In familiar with how the double but is co figured. But that is alot more involved then I want to get right now. I'm hoping they did a good job spacing it at the factory, of not I will address that after the ball resizing. The ball diameter and bit groove geometry should affect the spacer size so I will test and see what shakes out.

By reducing the correct sized balls between each other you are essentially changing the rigidity of the assembly in the thrust direction. The double nit increases this rigidity by about 10-15% according to Rexroth's studies and testing. Most likely due to the gap on the loose side of the but groove caused by the spacer. Ideally if that gap is reduced to a close enough tolerance the rigidity of the double nit should increase as each ball becomes load bearing in both directions and the bal count is doubled verse a single nut. This is obviously based on the actual thrust loads being applied and the force required to move the table/saddle.

One could go overboard quickly I can see. But in just trying to meet my own goals for the machine and once achieved I will be satisfied to leave the 0.00001" chasing for the next guy..lol!!

Chris

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[arizonavideo]

A double nut has about the same load handling as a single nut. A single nut has lets say 3 rows. In any direction three rows take the load. With a double nut only one nut takes the load but only in one direction.

You do have a lot of advantages with double nuts but load handling is about the same.

[handlewanker]

Hi....doing the nut reloading like I said means the size of the balls are not all that critical....undersize is better than oversize...... as the second ball nut is in effect a jam nut purely for taking up the backlash....no other purpose, and the fitted spacer just keeps them that way.

Fitting a new double nut to an old screw means the careful setting up of the manufacturer for pre-loading is a waste of time, but anyone can refit a spacer to enable the reloaded double nut to perform as required.

The other problem is an old screw will have more wear in the centre than the ends, so a rigid double ball nut will go slack in the middle and tight on the ends......where do you do most of your machining.........in the middle, that is why the screw has gone slack in that area, so you're still going to get backlash in the area you need it least, unless the spacer was re-made from some resilient material that "self adjusted" for the tight spots.
Ian.

[lcvette]

Does anyone have the G0704 stock table end cap bearing pocket inner diameter? Think one of my thrust bearings is bad, started making a crunching noise, need to order a couple replacements and can find the bearing size I need to order online and didn't have time to tear it down this evening.
Thanks in advance!!

Chris

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