crossslide leadscrew nut

[leroy]

313973313974Hi, I'm trying to remove some slack from the leadscrew on the carriage of my AL 320 lathe and just discovered that the bronze leadscrew nut cannot be tightened against the bottom of the compound slide because the part of the nut that goes through the slide is longer by .43mm than the thickness of the slide so when you tighten the screw into the nut it just wobbles around, question is, is this so the nut can self align with the leadscrew or should it be fixed tightly to remove play?, Leroy,:?

[nearnexus]

My CQ9325 has the same setup and it pulls up hard against the base of the topslide.

All the adjustment is done by pinching the thread with the two screws in the bronze/brass block.

Try putting a shim between the block and the cross slide and see what effect it has, as you fully traverse the slide.

Rob

[Stustoys]

Which way around does that nut go?

Stuart

[leroy]

the black spacer with the Philips screw goes in the hole on top of the slide and the round section on the bronze nut fits into the hole from underneath with the two adjustment screws facing the rear of the lathe, the problem was the round section on the nut was too long so could not be tightened to the slide, Rob's suggestion of shims worked and has taken all play out of the crosslide, I was going to machine the extra metal from the nut so that it lined up as it should, not sure how critical these dimensions are but as I had to assemble it to see if the shims worked I'll wait till next time I have it apart Leroy:D

[Stustoys]

with the two adjustment screws facing the rear of the lathe,
Then I'm pretty sure you should be opening that slot, not closing it....... but I've been wrong before.

Stuart

[nearnexus]

Then I'm pretty sure you should be opening that slot, not closing it....... but I've been wrong before.

Stuart

The slit adjusters as fitted only work on compression. There's no provision to open the slot.

They actually work pretty well.

Rob

[Stustoys]

The slit adjusters as fitted only work on compression. There's no provision to open the slot.

Wouldnt that mean most of the work was being done by the thread on the thin side?

Still.... if it works it works.

Stuart

[Michael G]

The problem was the round section on the nut was too long so could not be tightened to the slide, Rob's suggestion of shims worked and has taken all play out of the crosslide, I was going to machine the extra metal from the nut so that it lined up as it should, not sure how critical these dimensions are but as I had to assemble it to see if the shims worked I'll wait till next time I have it apart Leroy:D

If the nut when it could slide up and down in the cross slide hole was loose on the screw and now that you have fixed it's position it is tight, to me that seems you have fixed the problem by pushing things out of alignment. If the fit of the nut boss in the cross slide hole was a smooth sliding fit then whether it was fixed or not did not matter as it would automatically adjust it's position to accommodate any error in the screw (eg a slight bend).
If the nut boss was loose in the hole then a better solution may be to either sleeve the hole or the boss so that it is a smooth sliding fit.

Michael

[nearnexus]

If the nut when it could slide up and down in the cross slide hole was loose on the screw and now that you have fixed it's position it is tight, to me that seems you have fixed the problem by pushing things out of alignment.

Michael

If that was the case, the thread nut would bind at one extremity of the cross slide travel (probably nearest the shaft bearing).

That's why I suggested he check full travel to see if all is parallel.

Yes, the pinch block only works on the thin section.

Rob

[lather]

Leroy,

came across a similar problem on my lathe, except the brass nut did not contain the pin, the pin was separate and basically allowed for a crappy screw alignment when manufactured due to it sliding piston style in the cross slide.
when checking the piston pin it moved about 0.3mm in the cross slide which would cause more wear over time, solved it by first adding the 2 grub screws and machined a new flanged bolt pin to fit tightly in the cross slide.
it helped but found out that the cross slide screw was not centered in the other axis.
needed to make up another flanged pin, except drilled the bolt hole about 0.4 mm off center so that the other axis could now be adjusted.
definitely improved the feel, as it didn't get any tight-spots along the slides travel.

looking at the photos of your cross slide it seems as if there's not much room to add the 2 grub screws similar to what many lathes use, even the screw oil hole is in the way for one of the screws as well as the nut not being long enough.

the other option is to use 4, M4 grub screws on the corners of the brass nut, looks as if there's room, "images attached"


fig 1, shows the 4 grub screws and flanged pin, note the optional flanged pin on the right (light and dark blue) has an off centered bolt hole which allows for adjustment if the screw is not parallel along that axis).
fig 2, bottom view, displays the brass nut pin cut off.
fig 3, no explanation needed.
fig 4, top view and shows the optional flanged pin with the off center bolt hole.

314094314095314096314097

[leroy]

Just to recap, the original problem was a lot of play in the crosslide back and forth and a tightening as it neared the end of the screw, both of these problems were fixed by adding shims between the nut and the slide, it now slides very smoothly with no binding even with the gibs tightened more than before, there is only 2.5 thou. backlash in the crosslide now, I'm thinking about drilling and tapping a hole through the nut so as to expand the gap instead of compressing it, not sure if the results would be worth the effort, or leave well enough alone, anyone's opinion is welcome, Leroy :)

[Machtool]

Can we at least call the roundy part on a square nut a journal. Thats the round male round part that floats up / or down in the female bore in the cross slide. 0.43 millimeters, you may as well call half half a mill. Its a huge amount. Randomly shoveling shims in, wheres the basis in that?

Get rid of all the shims, wind the slide back until its allmost hitting the housing at the bearing end. Thats the point where it's the most self aligned, with the nut closest to the anchor point via bearings. The other end of your screw doesnt have a housing /bearings. Then depth mic the top of your nut journal, to find out where your nut really wants to sit.
If you were in my caper, you would measure it @ 0, 90, 180. 270, and put it in the middle to average bend/ run out.

[Michael G]

Can we at least call the roundy part on a square nut a journal. Thats the round male round part that floats up / or down in the female bore in the cross slide.

Sorry. I should have used the technical term "roundy part". I thought I was close with "boss" though

Michael

[nearnexus]

Just to recap, the original problem was a lot of play in the crosslide back and forth and a tightening as it neared the end of the screw, both of these problems were fixed by adding shims between the nut and the slide, it now slides very smoothly with no binding even with the gibs tightened more than before, there is only 2.5 thou. backlash in the crosslide now, I'm thinking about drilling and tapping a hole through the nut so as to expand the gap instead of compressing it, not sure if the results would be worth the effort, or leave well enough alone, anyone's opinion is welcome, Leroy :)

This indicates that the original looseness/ poor fit was caused by too much metal being machined from the top square section of the nut, also making the journal too long.

The shims have obviously positioned the thread block on the correct alignment.

That's why I said try adding shims first, rather than simply grind a bit off the top of the journal. If shims had made it worse, then you could attack the journal length.

I'd just superglue the shim to the block so it can't dislodge and leave it as is.

Spreading or compressing the slit block would appear to have the same effect as I see it. All the thread canting will still be in the thin section, regardless.

Rob

[Ueee]

As I see it you want the large part of the thread taking the thrust away from the work. There is more likely to be more load pushing the slide out than in. So if the slit is at the back of the nut you want to compress it not jack the two halves apart. I think thats right.....

Ew

[Stustoys]

I think thats right.....
Pretty sure you're wrong........Time for a drawing....

Stuart

[Stustoys]

Ok this is how I'm seeing it.

From the top

Cross slide screw and nut un-adjusted.

Normal cutting force

Split nut adjusted "closed"

Split nut adjusted "opened"

Been wrong before though

Stuart

[Ueee]

So if the most thrust is moving from R to L, say a tool being pushed out of a cut, the force will be taken on the L flank of the screw/ R flank of the nut. That means closing the gap up right.....?

Ew

[Stustoys]

So if the most thrust is moving from R to L
Yes

the force will be taken on the L flank of the screw/ R flank of the nut. That means closing the gap up right.....?
No (well not in my drawing anyway, L and R depend on which side you're looking from)
If the thrust is moving R to L how can the L side of anything stop it?

"Normal cutting force" The screw is forcing the nut into the work (or "tool being pushed out of a cut"). The screw is trying to move the nut L to R, the nut is trying to move R to L. The thrust is on the right flank of the screw and the left flank of the nut.

Split nut adjusted "closed". If you close the gap up the small side of the nut is pull against the right side of the screw and the large side of the nut is pulled against the left side of the screw. Any thrust of the screw L to R is now carried by the small side of the nut.

As Rob(?) says this seems to work ok regardless. I wonder if there is a vertical component to the clamping? Closing the gap at the top of the nut forcing the screw down into the thread on the other side of the nut?

Stuart

[Ueee]

I was thinking compound in my head, fixed nut, moving screw....

It probably makes no difference now but how about wear down the track?

Ew

[Stustoys]

It probably makes no difference now but how about wear down the track?


Well thats my theory on why it should be opened not closed. It would last longer. But it would depend on exactly what was wearing and why.

Stuart

[nearnexus]

Actually after giving this considerable thought, I have to agree with Ewan.

All the screw thrust is taken up by the main section of the thread block.

Compression of the small section merely keeps the screw thread shaft within the confines of the profile in the block (ie. it applies tension from the other side of the thread profile to take out backlash).

As such, the small section should not wear out any time soon, as it is not taking the full load.

Rob

[leroy]

well it appears the problem was more complex than I imagined, thanks everyone for their input, I now have a much better understanding of the workings of the nut in question, I think I'll leave it alone for now and see how it goes, Leroy:)

[Stustoys]

Actually after giving this considerable thought, I have to agree with Ewan.
Didnt Ewan change his mind?


All the screw thrust is taken up by the main section of the thread block.

Compression of the small section merely keeps the screw thread shaft within the confines of the profile in the block (ie. it applies tension from the other side of the thread profile to take out backlash).

As such, the small section should not wear out any time soon, as it is not taking the full load.

If this is the case, where is in my drawing incorrect?

Stuart

p.s. maybe I'd better have another go at my drawing.

[Ueee]

Presuming the split is at the back (or away from the handwheel) then for the cross you want to spread it and for the compound you want to pinch it.....thats how i see it anyway.

When i scraped the Mars's cross slide and compound i found the re-alignment of the nuts was one of the hardest things to get right.

Ew

[nearnexus]

I'm totally confused.

Rob

[Machtool]

Isn't it all a bit academic? That nut only has provision for compressive preload, buy tweaking up the two screws. You cant jack that thin part out if you tried. Its an ordinary piece of engineering, but you have to live with what you've got, short of remaking it.

If you look at the pictures, the slit is clearly closed up at the top, too the point whare that thin section of nut must only be touching near top deep centre on the flank of its thread.

Still not sure why we are randomly tossing shims under it. Assuming that boss / journal is free to float up or down in the cross slide, if you were to wind that all the way back to the bearings and the housing were it is most ridgid. It will be somewhat self aligning, then you can measure that.

-Phil.

[Stustoys]

Isn't it all a bit academic?
Possibly :p
I was just wondering as one of these days I need to make a new nut. While I'm not to worried about what feels like the best part of a turn slop.. it does make things harder than they should be sometimes.

Might not be impossible to um "fix"(not sure thats the right word lol) cut the current bolts off through the slot. tap the current clearance holes what ever size suits. now you can jack it open........Not saying it would be a good idea but as you say, its just about toast now by the look of it.

New improved(?)drawing*.

Stuart

*no the thread isnt drawn wrong. its a two start thread ;)

[Gammaboy]

Seeings we're talking about crossfeed nuts/leadscrews...My Mars has a single SHCS and a dowel holding the cross slide nut in place, and no adjustment screws or clamping type nut adjustment - so I was thinking, if I drill and tap the cross slide above one end of the nut and fit a grub screw to kick the nut out of alignment with the leadscrew, then I can take out the lash that is in the crossfeed nut... can anyone see a serious problem with that approach?

[pipeclay]

Go for it,worst you will miss the mark,damage the nut,extra holes in the cross slide.
Then just buy another.

[Gammaboy]

I guess my question was more "is the technique of intentionally missaligning the leadscrew nut a valid technique"?There appears to have been no way to adjust the nut originally.

[Ueee]

I would not do it. What I would do is ignore pc and his usual bs.

The best thing to do is pull the nut out and split it yourself, then add some jacking screws and you should be able to eliminate a lot of the backlash. Of course parts are not available at all for the mars, but maybe an imperial hercus nut may be able to be fitted.

Ew

[Gammaboy]

I would not do it. What I would do is ignore pc and his usual bs.The best thing to do is pull the nut out and split it yourself, then add some jacking screws and you should be able to eliminate a lot of the backlash. Of course parts are not available at all for the mars, but maybe an imperial hercus nut may be able to be fitted.EwRather than pinching, I guess a spread could be achieved by drilling and running a taper tap in part way, (perpendicular to the nut axis), slitting through the middle of the holes, and then run grub screws in - once you hit the partially formed threads, it'll start spreading.Could always use the split delrin nut technique to make a new nut - I hopefully won't be leaning on it hard enough to hurt one.

[azzrock]

I would not do it. What I would do is ignore pc and his usual bs.

The best thing to do is pull the nut out and split it yourself, then add some jacking screws and you should be able to eliminate a lot of the backlash. Of course parts are not available at all for the mars, but maybe an imperial hercus nut may be able to be fitted.

Ew

that's a bit harsh. I think its good to read every ones input. admittedly it good not every one is as abrupt
as some members

[Ueee]

Rather than pinching, I guess a spread could be achieved by drilling and running a taper tap in part way, (perpendicular to the nut axis), slitting through the middle of the holes, and then run grub screws in - once you hit the partially formed threads, it'll start spreading.Could always use the split delrin nut technique to make a new nut - I hopefully won't be leaning on it hard enough to hurt one.

You can also spread by simply drilling and tapping thin section and using a grub screw to jack the split open. This is how i have done it. You have to be careful though, if your leadscrew is worn in the middle and not at the ends you have to adjust it for the ends and not the middle, leaving some backlash in the middle still.

Ew

[Gammaboy]

You can also spread by simply drilling and tapping thin section and using a grub screw to jack the split open. This is how i have done it. You have to be careful though, if your leadscrew is worn in the middle and not at the ends you have to adjust it for the ends and not the middle, leaving some backlash in the middle still.EwWell aware of allowing for the wear in the middle - the first lathe i spent any serious time with was a *seriously* flogged out government Hercus - trying to do anything remotely precision with that was a challenge! With the cross slide adjusted to bind at the ends, there was still enough play in the middle to rock the toolpost a good 2 or 3mm... had to use bodyweight to preload the post one way.Good point on the grubs.