Thread: crossslide leadscrew nut

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

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

Originally Posted by Ueee

So if the most thrust is moving from R to L

Yes

Originally Posted by Ueee

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

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

Originally Posted by Ueee

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

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

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

Originally Posted by nearnexus

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

Didnt Ewan change his mind?

Originally Posted by nearnexus

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.

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

I'm totally confused.

Rob

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.

Originally Posted by Machtool

Isn't it all a bit academic?

Possibly
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

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?

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