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Chasing Backlash

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jbolt

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#31
well this was before they started offering the double ball nuts they have now, but yes it's 2 chinese single nuts preloaded with belville washers.
Okay thanks. That's the way I did mine.

I used the double nuts on my router but I do not consider that a precision machine. I'm a little suspect of the Chinese double nuts. When I installed mine two of the three you could tell had some preload, the third did not fell like it had any so I put that one on the Z. I'm not sure if they can be adjusted.

The Chinese double nuts we use on the router at the high school have worked great for the last two years and that was built as a precision machine.
 

MontanaAardvark

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#32
I'm a bit late here, Tom, but did you use VXB bearings like these?

(http://www.vxb.com/7000B-Bearing-Angular-Contact-10x26x8-p/kit8872.htm)

I apologize if this is way out in left field, but the AC bearings seem like they could be part of this. I just did the Hoss DVD conversion on my G0704 and if I could upgrade it by replacing bearings, that might be a quick thing to try.


Bob
 

TomS

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#33
I'm a bit late here, Tom, but did you use VXB bearings like these?

(http://www.vxb.com/7000B-Bearing-Angular-Contact-10x26x8-p/kit8872.htm)

I apologize if this is way out in left field, but the AC bearings seem like they could be part of this. I just did the Hoss DVD conversion on my G0704 and if I could upgrade it by replacing bearings, that might be a quick thing to try.


Bob
Bob - thanks for your input. I'm using the bearings that came with the bearing blocks. They are Chinese vintage 7001 AC bearings. I am not convinced my problem is bearing related. I say this because I have preloaded the bearings as much as .015" and I still have backlash, .004' to .006" preload should be more than enough. Maybe I should just spend the money and buy new bearings. They are not that expensive. I've got a couple more tests to run when I get back in the shop. My fingers are crossed that one of them will be the smoking gun.

Tom S.
 

JimDawson

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#34
Bob, I copied this over from the Grizzly G0704 Cnc Conversion thread

I note one of the guys said "0 is acceptable backlash and with double ball nuts should be achievable". Zero? To how many decimal places? I wonder if that's realistic. If you took a production mill from someone like Tormach and measured it, would you really get 0.000? If you do get 0.000, do you have to tweak that periodically to keep the low backlash? Parts wear out, after all.
That was me. With preload on the ball nuts there is an interference fit, so therefore no axial movement is possible between the ballscrew and the ballnut. This also assumes that the ball track is not somehow floating inside of the ballnut, not sure if that is even possible. This also applies to the preloaded end support bearing, if everything is tight no axial movement is possible. Now, having said that, the overall system may have flex, stretch, or looseness in it that would manifest as backlash.
 

MontanaAardvark

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#35
Bob, I copied this over from the Grizzly G0704 Cnc Conversion thread



That was me. With preload on the ball nuts there is an interference fit, so therefore no axial movement is possible between the ballscrew and the ballnut. This also assumes that the ball track is not somehow floating inside of the ballnut, not sure if that is even possible. This also applies to the preloaded end support bearing, if everything is tight no axial movement is possible. Now, having said that, the overall system may have flex, stretch, or looseness in it that would manifest as backlash.
I answered this over on that G0704 CNC Conversion thread so I'll copy it over here, too.


If that came across as harsh or critical, I didn't mean it that way. By background, I'm an engineer (electrical - so mechanical is mostly out of my "comfort zone") and I'm just trying to understand what's going on. Obviously, nothing is zero backlash to the millionth of an inch, and probably not to a tenth (.0001"). It seems like something is always going to show up that looks like backlash. If it's always there, when do we stop chasing things? When is it "good enough"? I guess we all make that call.

As I said, I have backlash issues on my newly converted G0704, too. Quite a bit worse than Tom's (.010"). Before I take apart my mill, I'd like to have a good idea of what I'm looking for.

I had to re-ball one of the ballnuts off of one of Chai's ballscrew/nut combinations and while I know there's not supposed to be axial motion, I didn't see anything in there that would guarantee that. I didn't see a shim of any kind. I didn't take the two halves apart, though, so it's possible one was in there.

One of those things I've read is that people put larger ball bearings into the ballnuts and take out backlash that way. The way I think of it, there are something like 8 races in the linearmotion ballnuts. If you have 5 mils of backlash, maybe that means you make the balls .005/8 bigger or .000625". The ones I just bought were .1244", this would say to use 0.1250 bearings. Turns out the ebay seller I bought from sells them. Is that .005 divided over the 8 races the right way to think of this?

The thing that gets me is that ballscrews (really all screws) have imperfections from turn to turn so that the exact distance between turns wanders around the ideal. When I was looking at my screws with a dial indicator, some spots seemed to be off about .001" from one tenth (0.100") to another but averaged out to be right. It worked out that going from tenth to tenth it could be 0.1.00, 0.099 or .101, but over the few .500" intervals I measured it was perfect. Regardless of lost motion in the machine, that .001" is going to show up on some cuts. There are different "classes" of screw that have tighter or loser tolerances on this, and as you'd expect, the higher the class of screw, the more expensive. Now, having a circle out of round by .001, or a hole location off by .001 isn't always going to matter, but it's always there.

What about the way they deform under load? Under load, the balls go out of round and the thread they're in changes shape, too. It probably doesn't contribute to backlash, but it contributes to overall accuracy.

Sorry if this is rambling. As I say, I'm trying to understand all this. Is there a general tutorial approach to tracking down where backlash is coming from? A "do this, check that"? kind of thing?
 

ewkearns

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#36
I'm still trying to get my head wrapped around a .007 preload with a shaft that still turns!
 

JimDawson

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#37
Bob, not harsh or critical at all, but rather a good question.

If you bolt 2 pieces of steel together you now have 0 clearance at that joint, therefore, the static ''backlash'' is 0. If you can put enough tension on that that joint you can create clearance by exceeding the elastic limit of the joint, thus creating ''backlash''.

Having said that, in a properly designed machine tool you should never notice the elasticity of the components in normal operation. Will the components stretch a bit in use? Yes, but it should not be measurable by any tools that would normally be found in the hobby machine shop or in most industrial environments for that matter. For all practical purposes the backlash should be adjustable to 0 or at least what we would normally perceive as 0.

A recent project of mine is Alloy's Shizouka mill that is sitting in my shop. We found about 0.005 backlash in the X axis ball nut. It is a double nut arrangement and is adjusted by a shim pack. The problem was that even adding a piece of heavy duty aluminum foil to the shim pack proved to be too much preload. So what we did was drill & tap the end cap for set screws and manually adjusted for the correct preload. The ''correct'' preload in this case was what I felt was the correct amount of rotational resistance when turning the lead screw by hand with the entire assembly sitting on the bench. The table now has 0 measurable backlash in the X axis when measured with a 1um (0.000039 inch) scale.
 

MontanaAardvark

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#38
Thanks, Jim. I only have a partial mental image of what you're saying, and I'm not sure it's right. First, I assume these double nuts look like the ones that Tom and I are using. Second, when you say, "drill & tap the end cap for set screws and manually adjusted for the correct preload", I get lost. My double ballnut screw looks like this:
Both_Mounted.JPG
When you say end cap, is that the flange on this (right end of nut)? Parallel to the screw?

My only progress today is that I made a little adapter sleeve and adapted the old Grizzly handwheels to the 5/16" shaft on the ballscrew. What this does is allow me to turn the free end of the X-axis. I didn't have a reasonable way to do this until today. The motor is in place, but even now, I find I can turn the handwheel quite noticeably before the table moves. We had guests and I couldn't go off into the shop all day, but tomorrow, I'll tear down the X-axis motor end.

Chances are that whatever I did wrong to the X-axis, I probably did it wrong on the Y.
 

JimDawson

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#39
Yours is a different mounting system than on the Shizouka. The ball nut pair was housed in a steel block as I remember and had an end cap on it. So by tapping the end cap for set screws, you could preload the nuts.

Here is a picture, the new set screws are the black dots at 3,6,9,12 o'clock. You can just see the mounting block that is attached saddle.
upload_2017-2-5_21-5-1.png

It doesn't look like that would work in your case. The flange on the right is part of the nut, and is harder than He!!, not going to tap that. I'd have to think about your's for a bit to come up with a way of doing it.

Another common system is that flanged nuts enter a block from either end, but the screw holes are slotted, so by rotating one of the nuts a bit, you can preload the system.
 
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TomS

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#40
Got back in the shop today. I didn't do the test jbolt suggested because the site was down all day and I wasn't able to read his post. So I decided to strip the machine down to a point where I could remove the X axis ball nut and screw for inspection. The nut mounting bolts were tight. I'll check the ball nut mount bolts in the morning. Visually the nut and screw looked fine. I couldn't feel the .007" of backlash between the nut and screw that I measured when the mill was assembled.

The ball nut is assembled with a shim between the two nuts. My assumption is preload is adjusted by grinding the shim. Don't know for sure because I've never done it.

Not being able to identify the cause assembly will be slow and meticulous with lot's of measurements. I'm sure I will have questions along the way.

Tom S.

20170207_114005_resized.jpg
 

MontanaAardvark

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#41
How do you get a shim between the two ballnuts? In mine that dark piece in the middle wobbled around a little but I was trying my best not to let them get apart.


Bob
 

cs900

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#42
How do you get a shim between the two ballnuts? In mine that dark piece in the middle wobbled around a little but I was trying my best not to let them get apart.


Bob
well that's not right, the second ballnut should be tight up against the shim.

I also think a better option would be to put a belville washer stack in there so that you get preload that will compensate for wear on the ballscrew.
 

MontanaAardvark

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#43
well that's not right, the second ballnut should be tight up against the shim.

I also think a better option would be to put a belville washer stack in there so that you get preload that will compensate for wear on the ballscrew.
I should clarify that this is when I had the nut removed from the screw with the ballnut removal tool. Still, how do you do that? How do you take them apart to get anything in there and rebuilt it so that it's tight?
 

TomS

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#44
How do you get a shim between the two ballnuts? In mine that dark piece in the middle wobbled around a little but I was trying my best not to let them get apart.


Bob
I believe the shim is installed as part of the nut assembly procedure at the factory. Thickness is calculated prior to assembly. The shim on mine seems to be snug which is what I would expect. I tried to measure screw axial movement with a dial indicator by pushing and pulling on the screw. The readings were not conclusive but were significantly less than the .007" I measured with it assembled. What's aggravating is I haven't been able to identify the source, or sources, of the backlash. If I knew what was broke I could fix it.

Tom S.
 

TomS

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#45
Today was a good and enlightening day. First, I will say my backlash is now about .0008". Yahoo!! I'll explain how I got there. To address MontanaAardvark"s questions about the double ball nut and shim, my shim was snug but not tight. I couldn't get a .002" feeler gauge between the nut and shim. It looks like the shim is a two piece arrangement and is retained by the anti-rotation keys. I tried taking mine apart but the key started to bend so I stopped before I broke it. How the two nuts are held together I don't know.

After reinstalling the ball nut and screw and the AC bearings I checked nut backlash by mounting a DTI in the spindle with the needle against my vise. Pushing and pulling the table in both directions I got a reading of about .002" +/-.001". Not what I wanted to see but better than expected. I adjusted the X axis gib using the Bridgeport method and settled on a reading of .002". Ran a backlash test and got .007". What a disappointment after having .002" of ball nut clearance. I backed off the gib and the best backlash I could get was .0035". Better but not good enough.

I wrote a warm up gcode program several months ago and decided to run it to see if it made any difference. Ran it for about twenty minutes and tested backlash. I got .0004" repeatable readings. Things are looking up! I also found that the factory gib adjustment screws were pushing the gib to the side. I cut 1/4" off the right end of the gib making it flat and square and made a bushing that slips over a stud instead of using a bolt, with an OD that just fits into the gib screw counterbore. This works much better. I was able further tighten the gib and maintain .0004" backlash. After a bit more gib adjusting I settled on .0008" of backlash.

So after about a week of testing, measuring, cussing, retesting and remeasuring the source of my excessive backlash appears to be bad gib adjustment screws and not warming up the machine prior to taking measurements. Lesson learned. I've still got some fine tuning to do.

Thanks to everyone that contributed their experiences. Without this forum I'd still be chasing backlash.

Tom S.
 

MontanaAardvark

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#47
:clapping: What Jim said.

Before I took my mill apart, I don't recall actually measuring backlash, but I didn't feel any slop in the handwheels. I figured taking it apart would destroy that anyway. On the other hand, I think the gibs were farther in than they are now. I need to take a look at them in the morning. I think they may be loose.

I tested my mill on rapids and I can get 280 IPM on all three axes. Coming from a system where Z was 15 ipm and the fastest axis was about 50, it's amazing. If I gave up a little speed and improved backlash, that would be an acceptable trade.


Bob
 

TomS

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#48
:clapping: What Jim said.

Before I took my mill apart, I don't recall actually measuring backlash, but I didn't feel any slop in the handwheels. I figured taking it apart would destroy that anyway. On the other hand, I think the gibs were farther in than they are now. I need to take a look at them in the morning. I think they may be loose.

I tested my mill on rapids and I can get 280 IPM on all three axes. Coming from a system where Z was 15 ipm and the fastest axis was about 50, it's amazing. If I gave up a little speed and improved backlash, that would be an acceptable trade.


Bob
I don't have handwheels on my mill so wasn't able to detect backlash that way. It showed up when I did a circular pocket. The hole was out-of-round about .006". I learned a valuable lesson that proper gib adjustment and warm up routine go hand-in-hand. You can't do just one and expect to get good results, at least on my machine.

I have my rapids set at 100 IPM. Tried it at 200 IPM and it looked like the machine was beating itself to death on short moves. It's what works for me.

Tom S.
 

MontanaAardvark

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#49
I don't have handwheels on my mill so wasn't able to detect backlash that way. It showed up when I did a circular pocket. The hole was out-of-round about .006". I learned a valuable lesson that proper gib adjustment and warm up routine go hand-in-hand. You can't do just one and expect to get good results, at least on my machine.

I have my rapids set at 100 IPM. Tried it at 200 IPM and it looked like the machine was beating itself to death on short moves. It's what works for me.

Tom S.
I agree that rapids are pretty meaningless because you can never cut at those speeds. They really only matter if you have to do something with a lot of movements above the work and your time is money. I did an array of holes on the front and read panels of my motor controller box. Drill, retract bit, move to next hole, repeat, a large number of holes, followed by a "carriage return, line feed" to the next row of holes. Rapids sped that job up nicely. (Rapids on my Sherline were 15 IPM, a far cry from these speeds).

Pardon if this is like a rant, but I'm an engineer and I always see things this way. I just don't see why cutting a circular cutout should be something backlash compensation can't fix. In fact, it should be a really easy case. Backlash only shows up when direction is changed, and in a circle, each axis changes direction twice at the most. The CNC controller should know everything it needs to know. It doesn't show up when the axis stops and resumes the same motion, or slows down and resumes. Torque in a stepper is maximum as lowest speed so if the motor just slows down or stops without reversing direction, backlash doesn't apply. It's not going to slip. If it does, that's a different problem.

I guess I just don't get it.
 

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#50
Computer generated backlash compensation is only partly effective and normally only works well on operations like hole patterns. Cutting a circle is where it doesn't work very well at all. The only true backlash comp is when you have a closed loop servo system, and the encoders are on the load (table). Even that is not 100% effective, because the table can float as the comp is applied. The only real path to no backlash is to have a tight mechanical system that is heavy enough not to have any measurable elasticity in normal operation.

Take a look at this thread on the Machsupport forum.
https://www.machsupport.com/forum/index.php?topic=12143.0
 
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MontanaAardvark

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#51
Well, I was a hardware engineer (EE, not mechanical) so hardware guys thinking software guys "just don't get it" is about as big a cliche as you can imagine.

It seems like it ought to be easy. Computer (do your best impression of Disney's Goofy, the idiot dog): "Oh, I got to change direction, time to crank out those special backlash steps". If it needs to crank out something like 12-1/8 steps, that's different. That's a screw that's too steep, and a system design problem.

I could go on, but I don't want to bore everyone.
 

Metal

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#52
silly question since we are allready wandering off topic:

Where did you get your import ballscrews? I've been poking around at them for my bridgeport and prices are elusive while import screws seem to come with lots of horror stories.
 

MontanaAardvark

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#53
silly question since we are allready wandering off topic:

Where did you get your import ballscrews? I've been poking around at them for my bridgeport and prices are elusive while import screws seem to come with lots of horror stories.
I got mine from a place called Linear Motion Bearings. They are Chinese imports, so if that disqualifies them you should know. The main way to buy from them appears to be through eBay,
http://stores.ebay.com/linearmotionbearings

In my case, I chased around for them for a day or so, finally getting an email link here, in the "Grizzly G0704 CNC Conversion Thread", which I eventually seemed to have stolen from the thread starter as I documented mine.

I emailed the contact given on that page and when I told him I wanted to do a G0704 conversion, he knew exactly what to do.


Hope that helps,
Bob
 

TomS

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#54
I agree that rapids are pretty meaningless because you can never cut at those speeds. They really only matter if you have to do something with a lot of movements above the work and your time is money. I did an array of holes on the front and read panels of my motor controller box. Drill, retract bit, move to next hole, repeat, a large number of holes, followed by a "carriage return, line feed" to the next row of holes. Rapids sped that job up nicely. (Rapids on my Sherline were 15 IPM, a far cry from these speeds).

Pardon if this is like a rant, but I'm an engineer and I always see things this way. I just don't see why cutting a circular cutout should be something backlash compensation can't fix. In fact, it should be a really easy case. Backlash only shows up when direction is changed, and in a circle, each axis changes direction twice at the most. The CNC controller should know everything it needs to know. It doesn't show up when the axis stops and resumes the same motion, or slows down and resumes. Torque in a stepper is maximum as lowest speed so if the motor just slows down or stops without reversing direction, backlash doesn't apply. It's not going to slip. If it does, that's a different problem.

I guess I just don't get it.
I cut a 2-1/2" through pocket in 3/8" thick aluminum at .100" DOC, .220"stepover, 8000 rpm,
I got mine from a place called Linear Motion Bearings. They are Chinese imports, so if that disqualifies them you should know. The main way to buy from them appears to be through eBay,
http://stores.ebay.com/linearmotionbearings

In my case, I chased around for them for a day or so, finally getting an email link here, in the "Grizzly G0704 CNC Conversion Thread", which I eventually seemed to have stolen from the thread starter as I documented mine.

I emailed the contact given on that page and when I told him I wanted to do a G0704 conversion, he knew exactly what to do.


Hope that helps,
Bob
I got mine from a place called Linear Motion Bearings. They are Chinese imports, so if that disqualifies them you should know. The main way to buy from them appears to be through eBay,
http://stores.ebay.com/linearmotionbearings

In my case, I chased around for them for a day or so, finally getting an email link here, in the "Grizzly G0704 CNC Conversion Thread", which I eventually seemed to have stolen from the thread starter as I documented mine.

I emailed the contact given on that page and when I told him I wanted to do a G0704 conversion, he knew exactly what to do.


Hope that helps,
Bob
That's the same place I got mine. Price and service was excellent as was delivery. Keep in mind these are not top of the line screws and nuts. If you want to machine within a few tenths you should look elsewhere.

Tom S.
 

MontanaAardvark

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#55
I've spent a bit more time on mine and reduced the Y axis backlash to .001. I made a tool to hold the AC bearings in place and under compression while I tightened the jam nuts on the ballscrew. Just a little piece of 1/8" thick aluminum scrap I had. It looks like the motor mounts, but that's just by coincidence. I cut some long edges off it and was left with the more or less triangle shape.
Tool.JPG

I tried to do the same thing on the X-axis but had a fit problem. Somewhere along the way, I tapped the X-axis end cap for 10-32 screws instead of 8-32, so these 8-32x 3/8 screws didn't work. I'll get some 3/8" 10-32 screws and try again.
 

TomS

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#56
I've spent a bit more time on mine and reduced the Y axis backlash to .001. I made a tool to hold the AC bearings in place and under compression while I tightened the jam nuts on the ballscrew. Just a little piece of 1/8" thick aluminum scrap I had. It looks like the motor mounts, but that's just by coincidence. I cut some long edges off it and was left with the more or less triangle shape.
View attachment 226288

I tried to do the same thing on the X-axis but had a fit problem. Somewhere along the way, I tapped the X-axis end cap for 10-32 screws instead of 8-32, so these 8-32x 3/8 screws didn't work. I'll get some 3/8" 10-32 screws and try again.
I understand what you did but not sure why you did it. Your bearing housing has a cap, right? Does the cap not compress the AC bearings in the housing?

Tom S.
 

MontanaAardvark

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#57
I understand what you did but not sure why you did it. Your bearing housing has a cap, right? Does the cap not compress the AC bearings in the housing?

Tom S.
The cap is part of the motor mount, which sticks out far enough so that when it's in place, I can't get a handwheel onto the ballscrew shaft.
 

Metal

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#58
I cut a 2-1/2" through pocket in 3/8" thick aluminum at .100" DOC, .220"stepover, 8000 rpm,
That's the same place I got mine. Price and service was excellent as was delivery. Keep in mind these are not top of the line screws and nuts. If you want to machine within a few tenths you should look elsewhere.
Tom S.
Absolutely not, I'm trying to resolve on the BP:
1: my screws are worn out in the middle so if I adjust the nearly .1 backlash out I lose like half my table
2. constant adjustment due to 1: means chasing a moving target of backlash comp depending on where I'm cutting which is a huge pain in the butt.

If I could get accuracy down to a few thou reliably all across the table I would be very happy.

What size screws did you get (or the code if you are feeling nice)? from what i can tell 25mm is about as big as he gets which (to me) looks like a far cry from the size of these leadscrews
 
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jbolt

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#59
I'm running 2005 (20mm) ball screws on my PM-932 conversion on all 3 axis. The lead screws were 25mm. The size of the ball nut restricts what will fit on the X & Y axis on my machine. I would email Chai and see if he sells larger ball screws if that is what you have. He does not list all of what he sells on his ebay page. cgxfred@aliyun.com

I recently ran across this company that also does ground ball screws. I have not ordered from them but have been thinking about getting a quote. https://www.aliexpress.com/store/437307

A lot of people use the Nook ball screws in lieu of the imports. I have not priced them to see what the cost is vs import but it might be worth looking into if you are concerned about the imports.
 

TomS

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#60
Absolutely not, I'm trying to resolve on the BP:
1: my screws are worn out in the middle so if I adjust the nearly .1 backlash out I lose like half my table
2. constant adjustment due to 1: means chasing a moving target of backlash comp depending on where I'm cutting which is a huge pain in the butt.

If I could get accuracy down to a few thou reliably all across the table I would be very happy.

What size screws did you get (or the code if you are feeling nice)? from what i can tell 25mm is about as big as he gets which (to me) looks like a far cry from the size of these leadscrews
I'm using 1605 ball screws on the X and Y and a 2005 on the Z. I had Chai machine the ends for my particular application so they would not directly transfer over.

Tom S.
 
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