Tightening up lathe cross and compound with new nuts

FlyinChip · Feb 25, 2018

hi all,

my compound has some backlash, and so does the cross slide which is normal for such an old machine. I'm working on the compound gib, and noticed that the nut is brass and the compound screw is steel. So I would think all the wear is in that brass nut. They are available new for about $40, so how much improvement if I spend that $$ at some point?

The gibs take up the dove tail slot looseness. I'm not sure about the carriage slides(?).

Bottom line, does replacing these two nuts really improve things?

sorry about the newbie questions, I am new!

m-lud · Feb 25, 2018

Not if you only have lateral movement. Just crank it to snug and watch the dial. It will tell you how far you moved.
New is nice but nothing stays new.
Personal preference is yours

iwananew10K · Feb 25, 2018

It's bronze.

rbdjr59 · Feb 25, 2018

On a worn machine there is usually more wear in the place where more work is performed over time. If you make it snug there then you will get to tight at the other extremes of travel so you have to find a sweet spot that works for you. Or you can adjust it every time you switch to another area. I've run machines where I had to loosen the gib, dial in the cut, and then tighten the gib down to keep it in place for every cut. Yes, it was a worn out old Gisholt turret lathe but it would pull a 1/2 or more to the side all day. Your needs will determine what you need to do.

FlyinChip · Feb 25, 2018

ok thanks. just wasnt sure if changing those nuts was "the thing to do" to make it feel like a tighter machine. I already have to know the backlash and its no big deal at all just remember what the dial said... sounds like a low priority to spend $80

rbdjr59 · Feb 25, 2018

Often tightening the gib will help with chatter as well. Running old machines will make a machinist out of you by forcing you to learn how to get the tool to make what you want. Of course I mean in the manual world. I suspect that is true in the CNC world to some extent as well.

thomasutley · Feb 25, 2018

Look closely at the screws. Compare the Acme thread form in the center wear area to either end. If the thread crests are significantly thinner in the center than at the ends, a new nut won’t fix all the backlash that’s bothering you.

rbdjr59 · Feb 25, 2018

I was fixated on the gib and its function. The bronze half nut is the wear piece but as the previous post suggest if the screw is too far gone that wont solve all of your problems.

Rudd · Feb 25, 2018

thomasutley said:
If the thread crests are significantly thinner in the center than at the ends...

This is what I had on the crossslide, knife blade thin in the middle. I put in a new nut, and "grafted" a new piece of ACME threaded rod. I probably should have used precision threaded rod, but didn't know any better back then.
I think the bronze being softer will have abrasive bits lodge in it, and slowly lap the leadscrew.

Edit, I did this on the lathe in question - while the cross slide screw was being turned, I locked the cross slide by tightening the gib, and used the compound for feed when doing the turning.

FlyinChip · Feb 25, 2018

Mine doesnt look too bad. This is a GREAT video showing how to make a new feed screw. Lot of work, but...

EDIT: this is the correct video

Making a Compound Screw For The South Bend Lathe - YouTube

FlyinChip · Feb 25, 2018

Correct video: Making a Compound Screw For The South Bend Lathe - YouTube

tobnpr · Feb 25, 2018

Depends on the tolerances to which you're trying to work. I'm sure it's possible to get a better "feel" for it, but backlash to me has always been a bit of an inexact science. I'm
As mentioned, screws wear unevenly. Dialing 50 thou might move the slide 47, 52, etc- and it can vary depending on where you're at on the screw. Easy way to get an idea is just put an indicator on it and check actual travel to the dial.

I work to a thou on diameter (or try to!), and I couldn't hit it consistently until I installed a DRO, even with new nuts due to screws with what I would consider normal wear.

FlyinChip · Feb 26, 2018

OK you guys corrupted me. I made me a gib screw! Was amazingly easy. Flattened the gib and all seems pretty well now. The big adjuster screw doesnt look worn at all. thanks for all the advice.

SLK001 · Feb 26, 2018

Did you use a hacksaw to cut that screw slot?

FlyinChip · Feb 26, 2018

SLK001 said:
Did you use a hacksaw to cut that screw slot?

yes. not precise but it did the job

m-lud · Feb 26, 2018

FlyinChip said:
yes. not precise but it did the job

It looks good. Your crossfeed screw looks real good also
I don't think I could have got the slot that strait with a hack saw. I forgot how to hold a hacksaw. That was a good tip to grind the set off the blade. That's new to me.
I may have tried a die grinder with a cut off wheel. It's all what your use to working with and what you have on hand.

Nice repair !

jim rozen · Feb 26, 2018

From 20 feet away one would suspect the bronze nut would wear faster than the steel screw.

Actually not true. The wear is actually abrasive lapping. The bronze gets loaded with abrasive particles over
time (charged) and this is what really wears the steel screw. Typically the lash in the handle is equally distributed
between the nut and the screw.

Except of course, the ends of the screw don't get much use so they hardly wear.

Thus you can determine how much is from each source, by checking the lash at the center (screw plus nut) and
compare that to the lash at the ends of the screw (nut only).

Also be sure the dial assembly does not have excess axial play in the bushing.

FlyinChip · Feb 26, 2018

more good info.

as to the hacksaw, I didnt modify the teeth, just held it real tight and made the slot, working an angle to wident it.

I actually have a air cutoff tool, duh! should have used that instead!

MrWhoopee · Feb 28, 2018

When I received my Heavy 10, it had .140 free play on the dial (.070 lash). I started shopping for a new nut. Before ordering one, I tightened the screw that retains the cross-feed nut. The free play dropped to .080 on the dial. While I was playing with the screw, I noticed movement at the end of the screw behind the taper attachment. I took that apart to discover that there was no bearing, just the thrust washers from the bearing and the bearing cover. I ordered a replacement bearing from MSC. After I installed and adjusted the bearing, the free play was .014 on the dial (.007 lash). No point in replacing the nut.

n2zon · Feb 28, 2018

MrWhoopee said:
When I received my Heavy 10, it had .140 free play on the dial (.070 lash). I started shopping for a new nut. Before ordering one, I tightened the screw that retains the cross-feed nut. The free play dropped to .080 on the dial. While I was playing with the screw, I noticed movement at the end of the screw behind the taper attachment. I took that apart to discover that there was no bearing, just the thrust washers from the bearing and the bearing cover. I ordered a replacement bearing from MSC. After I installed and adjusted the bearing, the free play was .014 on the dial (.007 lash). No point in replacing the nut.

My SB13 was similar, with the bearing adjustment at the far end of the taper attachment, but I also replaced the nut, which was pretty worn. The screw itself was not worn very much. I don't recall the measurement now (it was a few years ago), but it was nowhere near the remaining backlash. There's .020" there now, and I suspect I could take half that out by readjusting the bearing if I were so inclined. For my purposes, though, .020" is OK for now.

Tightening up lathe cross and compound with new nuts

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