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big job
04-17-2010, 06:51 AM
I have been searching for a thread dial for a SB 9A X 41/2 (long bed). I
know there are plenty on Ebay and new repros. The problem is most
stupid and thats retiring. Im kind of looking for one in someones junk box.
Noted for some crazy ideas I may be looking for input on this one. After
all (to me) this dial is simply a timer correct? Imagine this - A small pulley
at the end of my lead screw (far right this area not used) to a calulated
disk to a micro switch to a micro beep speaker. A slight delay on this disk
(a little trial/error) I hope as soon as it beeps > the half nut is engauged.
Actually there will be two disks adjusted to each other for lines and
numbers, a small round head screw to close the micro switch on the
rotating disk. 2nd thought is to drill and tap the end of the lead screw &
just mount this disk there eliminate belts. Lathe off engauge 1/2 nut then
mark this disk to trigger micro sw. Nice for some input. Broke in Taxachussets
sam

philbur
04-17-2010, 07:45 AM
It's not really a timer it's more a device to allow you to record at which particular thread in a series of adjacent threads on the lead-screw that you engaged the half-nut.

Phil:)



After all (to me) this dial is simply a timer correct?

big job
04-17-2010, 07:55 AM
Yes you are correct, I should have reworded that however I am still going
to experiment with this theory.

oldtiffie
04-17-2010, 07:56 AM
Sam,

no need to over-think it, go to any real expense (if any) or go without.

All that a threading dial for you machine is really is a shaft in a block that is fastened to and can tilt on your lathe carriage and with a simple dial marked off with 8 equally-spaced radial lines at the top of the shaft and a 24-toothed gear on the bottom of the shaft that engages the lead-screw.

Assuming that your lead-screw is 8 threads per inch (most are), the "gear" can be as thin as 1/16" (say 1/8") if needs be and the tooth shape, size etc. doesn't need to be too flash either. As long as the lead-screw turns the gear and the shaft and dial it will work. Back-lash is not an issue either as it is soon "taken up" as soon as the half-nuts are engaged. There's lottsa "slop" in the fit of the half-nuts as well as end-play in the lead-screw too - yet with all this it works well.

As long as the gear will roll along the lead-screw it will be OK. There is no need to use or make a "tap" or a "hob". You can make it all by hand but if you have a mill and a rotary table it will be much easier. All that will needed is a bit of patience and time. A file will soon "ease" any rough spots well enough.

Those are the bare bones basics. Anything more is just for looks.

Go and take a look at and a few pics of the threading dial as well as a few measurements on someone else's lathe and you will be well on the way.

There are some very nice "home-shop made" and "shop-made" threading dials around, but irrespective of how well they look or are made, they all operate on the same principals.

A pure "farm" or "hill billy" item can work just as well as one made to "tenths" or "microns".

I can sketch the gear for you if you wish.

big job
04-17-2010, 08:19 AM
Yes thanks my heavy Greaves didnt have the gear either so I made one
from a large brass washer and it works perfectly however ill have to make
some kind of housing for this SB. Today ill play with my disk idea thanks

GadgetBuilder
04-17-2010, 11:01 AM
Not sure I understand what you intend to build with the disks on the end of the leadscrew but... the threading dial is not a timer. It's more of an analog computer that accepts both carriage position and leadscrew rotation and uses the combination to indicate when the tool will sync to the thread being cut.

Unless I missed something, your disk idea seems like it ignores carriage position and that is a necessary input - that's why the threading dial is mounted on the carriage. Not trying to rain on your parade, just want to ensure you're considering both inputs so you don't go down the garden path.

John

Scishopguy
04-17-2010, 11:19 AM
Sam...Think of the threading dial kind of like the timing wheel that mechanics use on the camshaft when they are building an engine. It gives the relative position of the shaft. ;)

big job
04-17-2010, 03:07 PM
Yes yes you are all correct, however I did manage to work my disk idea.
Just to much counting. Old man brain dead ! Carriage v/s lead screw
make it go around with 8 lines dumb & dumb, ashamed head down , cant
believe being that stupid idea
thanks again try again tomorrow sam

xeddog
04-17-2010, 05:41 PM
If you are still looking for a thread dial, you might send email to Ted at latheman2 at aol dot com, or Ronnie at ipenelopepitstop at aol dot com. Both of these men deal in used SB parts. I have dealt with both of them and I highly recommend them. They probably have the full range of thread dials from prime condition to something that someone dumped out of the bottom of a toolbox.

Wayne

Ken_Shea
04-17-2010, 09:31 PM
oldtiffie,
At one time you had explained this in a post and I took mental note of it, but...................

My Clausing has a 8tpi but uses a 32 tooth gear on the thread dial, what effect does this have?

Thanks
Ken

oldtiffie
04-17-2010, 09:44 PM
Sorry Ken.

Thanks for the "please explain".

You are dead right and I am dead wrong - it was a silly mistake for me to have made. I inadvertently related the carriage travel per revolution of the thread dial to 3" instead of the correct 4".

4" of travel over an 8tpi lead-screw = 4 x 8 = 32 lead-screw pitches = 32 teeth on the gear on/of thread chasing dial.

Self-administered severe admonishment and kick in the ar$e applied.

Its harder on my metric lathe which has a 3mm lead screw and a 24 toothed gear which works surprisingly well as it covers most common threads

Ken_Shea
04-17-2010, 10:05 PM
Sorry Ken.

Thanks for the "please explain".

You are dead right and I am dead wrong - it was a silly mistake for me to have made. I inadvertently related the carriage travel per revolution of the thread dial to 3" instead of the correct 4".

4" of travel over an 8tpi lead-screw = 4 x 8 = 32 lead-screw pitches = 32 teeth on the gear on/of thread chasing dial.

Self-administered severe admonishment and kick in the ar$e applied.

Its harder on my metric lathe which has a 3mm lead screw and a 24 toothed gear which works surprisingly well as it covers most common threads

Well..OK, since you properly admonished yourself, it won't be necessary to do more :D

Actually, I never caught the error, was just a question, any reason a 24tooth could not be used?
The dial would only move faster, correct?

J Tiers
04-17-2010, 10:46 PM
the idea is for the dial to relate to the distance in which the threads repeat.

Consequently , the usable marks on teh dial differ for different gears....

With a 2" travel dial, which for 8 tpi would be 16 tooth gear, a thread with a half turn per inch, like a 4.5 tpi, etc, will repeat in one dial turn.

A 24 tooth would be a 3" travel dial, and so you could do 1/3 threads, ones with an extra 1/3 turn per inch, but there are not so many of them.

However, OTHER threads might not work out well.... a thread with an extra half turn per inch would have 1.5 half turns per 3", and would be out of phase again at 3", same as at the first inch.

So the 3" travel 24 tooth gear dial would work, but would be rather impractical for the typical threads.

However, if your machine has 6 tpi leadscrew, you would have a 4" dial, and all would be gas and gaiters.......

oldtiffie
04-18-2010, 12:02 AM
Well..OK, since you properly admonished yourself, it won't be necessary to do more :D

Actually, I never caught the error, was just a question, any reason a 24tooth could not be used?
The dial would only move faster, correct?

The lead-screw to gear is a form of worm and wheel with the same velocity ratio principles. With the 32 tooth gear you have a 32:1 reduction at the screwing dial. With the 24 tooth gear it is a 24:1 reduction.

The 32 tooth = 4" travel works better than a 24 as threads that have a lead that is an odd number of tpi of say 5.25tpi or 5.75tpi have respectively 5.25 x 4 = 11 threads at 4" and 5.75 x 4 = 23 threads at 4". In these cases the threading half-nuts can be engaged at the same point on the threading dial no matter where the carriage is on the lathe bed - ie every or any 4" point on the threading dial - with a 32 tooth gear.

It works just as well for threads with a 1/2 in the TPI - ie a 5 1/2 tpi would make 11 turns in 2" so the half nuts could be engaged at any two points on the threading dial that were 2" = 1/2 turn of the threading dial apart.

My 24 tooth gear on my 3mm lead-screw = 24 x 3 = 72mm per turn of the threading dial. Most common metric pitches are covered but its easy to get it wrong. So I and a suspect quite a few others with metric lathes or lead-screw leave the half nuts engaged most times - just as we have to when cutting inch threads on a metric lathe.

"Inch" is far better than metric when it comes to screw-cutting with a thread-chasing dial on a lathe.

Spin Doctor
04-18-2010, 07:34 AM
This is the perfect reason for building a Single Tooth Reversing Dog Clutch. One of those and you can throw the thread dial away

philbur
04-18-2010, 07:50 AM
I would be interested to better understand the workings of such a clutch, do you have a link or could you give some clues.

Phil:)


This is the perfect reason for building a Single Tooth Reversing Dog Clutch. One of those and you can throw the thread dial away

Spin Doctor
04-18-2010, 09:11 AM
Basically the gear train is turning all the time the spindle is running. At the out put side of the gear train there is a stub shaft that carries a gear and half of a dog clutch. The gear drives another gear/dog clutch that is mounted on the leadscrew with a bearing. The first gear drives the second gear through two intermediate gears so their rotation is opposite. On the leadscrew is the sliding element of the dog clutch. when slid toward the gear train it drives the leadscrew for forward/RH threading. When slid the other way it drives the leadscrew for reverse/LH threading. Because the stub shaft is always in time with the spindle and the dog clutches can only engage one way the leadscrew is always in time with the spindle. Even when it is not turning. The thing is the half nut is always engaged through the threading process. Inch, metric, DP or module it does not matter.

http://img.photobucket.com/albums/v19/markandannie/Leadscrew%20Reversing%20Drive/LeadScrewReversingSystem5.jpg

Look at the Hardinge page on Tony's web site http://www.lathes.co.uk/hardinge/index.html for good explanation or the Hardinge yahoo group http://groups.yahoo.com/group/Hardinge-Lathe/

There was some discussion on the site a while back as to how the Hardinge system works as the gears on the Clutch shaft are 50 and 48 teeth with 50 and 48 on the reverse shaft. The reason for this is there has to be an idler gear between the two 48 teeth gears to have the LH clutch run in reverse rotation

GadgetBuilder
04-18-2010, 11:20 AM
I would be interested to better understand the workings of such a clutch, do you have a link or could you give some clues.

Phil:)
Martin Cleeve describes the dog clutch concept and has pictures of one he fitted to his Myford in his book, "Screwcutting in the lathe".

I copied Cleeve's notion to add a dog clutch with auto-stop to my 7x12:
http://www.gadgetbuilder.com/Dog_Clutch.html
My scheme is purely mechanical but solenoids and micro switches might make implementation easier on some lathes.

I don't see how the design in Spin Doctor's drawing works. My understanding is that a dog clutch fitted on the leadscrew would not work for metric threads if the leadscrew is imperial (as well as needing care for many imperial threads) so there's something I'm missing here.

Read the instructions for use of the Artisan dog clutch at post #16 in the thread below for why I think there may be some limitations to the leadscrew dog clutch approach:
http://bbs.homeshopmachinist.net/showthread.php?t=32150

John

Mike Burdick
04-18-2010, 11:50 AM
As I look back on my projects it seems to me that 99% of my threading on the lathe is somewhere between 1/2 and 3/4 of an inch in length... and very seldom more than 24 TPI. So, for me, it's faster to just put the lathe in reverse then to engage/ dis-engage the half nuts.

My point - If you don't have a thread dial it's really not that big of deal - especially for the average HMS'er!

If you want to make one here's a drawing that you can use as a go-by. It wasn't designed for the SB but with a few measured modifications it'll work. You can make the gear or you can buy one - your choice. You can make it as fancy or as plain as you want! The one pictured below was made from 4140 because I had that in the scrap bin but you can use anything you want - even aluminum. The main modification for the SB will be the "Holding Flange" which will only need to be a solid piece that will be held in place by a setscrew located on the apron. This will make the project even easier!

http://img.photobucket.com/albums/v157/MikeBurdick/THREADDIALASSEMBLY.jpg

http://img.photobucket.com/albums/v157/MikeBurdick/combined_dial_pic-1.jpg

Spin Doctor
04-18-2010, 09:45 PM
I don't see how the design in Spin Doctor's drawing works. My understanding is that a dog clutch fitted on the leadscrew would not work for metric threads if the leadscrew is imperial (as well as needing care for many imperial threads) so there's something I'm missing here.

The Hardinge HLV-H has a 1"-8 acme lead screw. It doesn't matter what the pitch is the leadscrew is always in time or sync with the thread. When you reach the end of the thread and either kick the dog clutch out of engagement manually or use the stop rod the leadscrew stops turning. Back the tool out either on the cross slide or with the quick acting lever and shift the dog clutch to the left. The leadscrew will now be running in the opposite rotation and return the carraige to the start point. Slap the dog clutch back into neutral feed the tool in and shift the clutch into forward. because the dog clutch can only engage one way it cannot get out of phase to the leadscrew and the tool will pick up the thread. Now the dog clutch that shift back and forth slides on the leadscrew shaft. But it is keyed to it to transmit power. For the idea I came up with the forward gear dog clutch drives a lay shaft made out of pinion stock. This layshaft drives another layshaft which drives the reverse gear/dog clutch. The reverse gear/clutch spins free on the leadscrew shaft. Because there are two idler gears the two dog clutches run in opposite direction. And as long as the spindle is turning they are too. Now I realize I was most likely re-inventing the wheel but I had other people look at it and so far I haven't had anybody poke holes in the concept. Now I just have build the darn thing. Part of the reason I bought the lathe I have is because there is no QCGB so I have more than enough room to install something like this. And you don't need to use layshafts. Bevel gear would work just as well

Ken_Shea
04-19-2010, 09:40 AM
Thanks JT and OT.

Had to look up the "gas and gaiters" :D

GadgetBuilder
04-19-2010, 09:58 AM
Sorry Spin Doctor, I'm still struggling with your concept.

If you were cutting a 4tpi thread the 8tpi leadscrew would be turning twice for each turn of the spindle. Wouldn't your dog clutch allow connecting on either rotation of the leadscrew?

Or, when cutting 9tpi (per the Artisan example) there would be 8 leadscrew turns between favorable pick up points. It seems like your design could connect on any leadscrew rotation, including the 7 unfavorable ones?

John

Spin Doctor
04-19-2010, 11:06 AM
Think of the leadscrew being in two pieces (which it would be) when you re-engage the RH it is picking up right where it left off. The dog clutches can only be engaged one way. I just ran this idea past one of the mechanical engineers at work and he agrees. As long none of the gearing or the half nut is dis-engaged every thing will be in time for the next pass

Mike Burdick
04-19-2010, 12:13 PM
...As long none of the gearing or the half nut is dis-engaged every thing will be in time for the next pass

So... how is the carriage going to be returned to the start with the half nuts engaged?

GadgetBuilder
04-19-2010, 12:47 PM
In cutting a 9 tpi thread with your design, my thought is that the LH section of the 8tpi leadscrew is turning continuously and favorable pick up could occur only on every 8th turn; crossed threads would occur if pick up occurred on any of the other 7 turns.

Your dog clutch connects the two halves of the leadscrew together in one way but my question is how does it select the one out of 8 leadscrew rotations that is favorable.

You seem serious about building this and my notion is it may not work as you expect.

John

S_J_H
04-19-2010, 05:46 PM
I'm not really following this thread but since my Artisan was mentioned, I just want to make sure everybody understands how it works.

The Artisan does not have half/nuts nor the usual rack and pinion to move the carriage when the halfnuts are disengaged.

The old Artisan has a coarse 2tpi leadscrew and the carriage has a long solid nut mounted to it.
The stationary handwheel near the headstock moves the carriage via a set of bevel gears that turn the leadscrew. Below that handwheel is a lever for the feed. This lever engages/disengages a 8 tooth dog clutch for the leadscrew.

Since we know the leadscrew is 2tpi and always drives the carriage and it has a 8tooth dogclutch, then it's pretty simple math to understand that moving the disengaged dogclutch 1 tooth, moves the carriage 1/16" of an inch. So any thread divisible by 16 can be cut very fast without ever stopping the spindle or counting revs of the carriage handwheel.
This lathe has a spindle brake and it plays a role in threading. At the end of a thread for any TPI not divisible by 16, you brake the spindle, disengage the 8tooth dogclutch and run the carriage back the correct number of turns to position the carriage, engage the dogclutch and engage the spindle for another pass.
It's all very fast to do and pretty slick if you ask me.
Steve

http://i109.photobucket.com/albums/n48/S_J_H/vintage%20Artisan%20lathe/THREADINGTEXT.jpg
http://i109.photobucket.com/albums/n48/S_J_H/vintage%20Artisan%20lathe/Artisanlathe023.jpg
http://i109.photobucket.com/albums/n48/S_J_H/vintage%20Artisan%20lathe/artisan2ndway004.jpg

Spin Doctor
04-19-2010, 09:11 PM
So... how is the carriage going to be returned to the start with the half nuts engaged?

When you shift the sliding element of the dog clutch to the left to engage reverse or the LH thread cutting side of the dog clutch it causes the lead screw to turn in the opposite rotation as it was when you were taking the cutting pass. Because the lead screw is now running "backwards" it drives the saddle with the half nut attached to it towards the tail stock. Remember the LH side of the clutch is using the lead screw shaft as an axle on which its bushing runs. On really long threads it can actually be a pain because the return takes just as long as the cut. On short threads it is much faster. Plus if such a system has a stop rod linked to the shifting handle then you can thread up to a shoulder with ease. Of course one could just go with an electronic lead screw but some of us have trouble getting the right color electricicles going down the right color wires. :rolleyes:

Don Young
04-19-2010, 09:30 PM
There seems to be more to this system than I (and apparently some others) understand. Re-engaging the leadscrew on the wrong revolution seems exactly like re-engaging half-nuts in the wrong thread. It seems to me that it would be necessary to return the carriage to the exact same starting point and have the dog clutch on the spindle side of any gearing other than 1-1. Then re-engagement could be accomplished with both the carriage position and the spindle orientation unchanged.

You sure can't disengage the leadscrew, rotate it a full 180 degrees, and re=engage it without losing the correct sync unless the work pitch is a multiple of the leadscrew pitch. In that case you don't need anything as any place the half-nuts close is okay.

I will have to follow this thread and learn more, as I am sure the system works, I just don't quite understand why.

Don Young

The Artful Bodger
04-19-2010, 10:12 PM
There seems to be more to this system than I (and apparently some others) understand. Re-engaging the leadscrew on the wrong revolution seems exactly like re-engaging half-nuts in the wrong thread. It seems to me that it would be necessary to return the carriage to the exact same starting point and have the dog clutch on the spindle side of any gearing other than 1-1. Then re-engagement could be accomplished with both the carriage position and the spindle orientation unchanged.

You sure can't disengage the leadscrew, rotate it a full 180 degrees, and re=engage it without losing the correct sync unless the work pitch is a multiple of the leadscrew pitch. In that case you don't need anything as any place the half-nuts close is okay.

I will have to follow this thread and learn more, as I am sure the system works, I just don't quite understand why.

Don Young

Maybe that real coarse lead screw is the clue.

Don Young
04-20-2010, 09:15 PM
Maybe that real coarse lead screw is the clue.
I seems to me that, regardless of the leadscrew pitch, randomly re-engaging the leadscrew in the same rotary orientation without any regard as to which revolution it is on is no different from randomly engaging half-nuts in any thread pitch.

What I have found so far about the Hardinge system still leads me to believe that it returns the carriage to a fixed location each time and has the dog clutch between the spindle and any thread change gearing. I can understand how starting the carriage from the same position with the spindle in the same orientation will correctly track the thread.

The spindle chaser system can do something similar also. If you have the carriage against a fixed stop and press the follower toward the master thread, it will engage that thread and start moving the carriage when the spindle is correctly oriented. That happens once every revolution of the spindle.

It seems to make sense to me, but I am not really sure I understand it right.

Don Young
04-20-2010, 09:31 PM
Aha! I think it doesn't matter where the carriage is! If you are threading using a dog, you can remove the work, rotate it as many times as you want, and re-install it back in the lathe without losing sync on the thread, as long as you install it in the same driver slot. If you put it in a slot 180 degress away you will get a two-start thread. Any revolution of the spindle is just like any other revolution and that is true anywhere in the gear train as long as it is 1-1 with the spindle. Once there are other ratios, it is not true, as one revolution of the gear can be equal to fractional revolutions of the work being threaded.

I now believe the only requirement is that the dog clutch have a 1-1 ratio to the spindle, not the leadscrew. I think the spindle is the only place in the train that you can slip the drive a full revolution and it does nothing to change the spindle to "carriage" relationship. I am still open to learning different.

Don Young
04-21-2010, 09:18 PM
I have received confirmation that the Hardinge HLV-H threading dog clutch does run at a 1-1 ratio with the spindle, not the leadscrew.

Spin Doctor
04-23-2010, 09:06 AM
For those worried about being contaminated avert your eyes

http://www.practicalmachinist.com/vb/bridgeport-hardinge-mills-lathes/hardinge-hlv-h-dog-clutch-ratio-202859/#post1341499

An Amimation of the HLV-H gear train through the dog clutches

GadgetBuilder
04-23-2010, 10:28 AM
Spin Doctor, the animation shows the dog clutch operating in sync with the spindle per the description in Cleeve's "Screwcutting in the Lathe".

Do you now agree that a dog clutch won't produce the desired result if it operates in sync with the leadscrew? I can't tell from your comment...

John

Spin Doctor
04-23-2010, 02:14 PM
Gadgetbuilder;

Well, maybe I do have to rethink the position of the device but its overall design is sound IMO. One of the things I get from reading through the HLV Dog Clutch ratio thread over at PM is the the position of the Dog Clutch in the of no consequence. It merely effects the number of turns the spindle must make before the clutch will align.
With apologies to Don at PM and Clive

"Don

Book No 3, "Screw-cutting in the Lathe" by Martin Cleeve, in the Argus/Nexus/Special Interest publications (publisher name depends on vintage) Workshop Practice Series includes what appears to be an authoritative treatment of the behaviour of the single tooth dog clutch. So far as I recall the relative speed ratios between lathe spindle, dog clutch and lead screw is irrelevant so long as neither the screw or clutch runs faster than the spindle. Obviously the slower the clutch runs the more turns the spindle must make between engagements which will slow down the rate of working. In practice this slowing won't be a problem as a dog clutch equipped lathe can screw-cut at spindle speeds approaching those used for normal cutting os the waiting time will be small.

Martins book is good but, in attempting to make things clear to the novice, the maths are more than a little long winded and the various cases for different pitch lead screws covered individually. This makes for hard reading without pen and paper to hand and a ruthless willingness to skip parts as needed. The dog clutch analysis is based on the concept of Synchronous Distance defined as the distance the saddle needs to move to pick up the next thread. Probably not the easiest concept to get your head round but its the only way to get correct answers. Any rotation based analysis is doomed to fail.

Clive"

So I guess I will be heading over to local library to have them get me the book so I can do some more research. It will be a while before I start building the darn thing anyway. Lets this dog :rolleyes: sleep for awhile.

Spin Doctor
05-01-2010, 11:08 PM
Going through the copy of Cleeve's book I got on an inter-library loan. While it makes for some dull reading it also is useful for answering some questions I have been having determining gear ratios for an idea I'm kicking around

Page 91, Screwcutting in the Lathe

"Dog Clutch Ratios

Although Figs. 14 and 15 show a 1:1 ratio between the lathe spindle and dog clutch , pick-up can be held with 1:2, 1:3 etc reduction gearingbetween the lathe spindle and the clutch., correct pick-up moments then occur at every second, third etc revolution of the lathe spindle, and of course , at the lower clutch speeds the shock at the moment of engagement is correspondingly reduced."

If I read this right a Single Tooth Reversing Dog Clutch positioned at the lead screw should work while having the problem of potentially longer lead times in terms of spindle revs per pick-up. But if you are threading mild steel at 350 or 400 RPMs and brass at 500 or more so what.

Don Young
05-02-2010, 10:40 PM
SpinDoctor, I think you are correct "IF" the spindle to leadscrew ratio is exactly 2/1, 3/1, 4/1, etc.. In those cases you would be cutting threads exactly 2, 3, or 4 times the leadscrew pitch. In other words, 16, 24, or 32 TPI. Those are the same cases you do not need a threading dial for, as the half nuts can be closed on any leadscrew thread. Consider carefully what would happen if you were cutting 13 TPI. One leadscrew revolution is then equal to 1 5/8 spindle revolutions, two leadscrew revolutions would be 3 1/8 spindle revolutions, etc.. Note that one disconnected leadscrew revolution would move the carriage 1/8" which would not put the tool point into another thread groove. If instead you rotate the disconnected spindle one revolution, the tool still fits into the same work thread groove.

Just try it on your lathe without it running. If, after making a rotation, the tool aligns with the thread when stopped, it will continue to do so when running. If, after rotation, the tool does not align with the thread, starting the lathe will not make it align, except for backlash.

You can disconnect the spindle from the gear train for testing by releasing the tumbler gears. Releasing the leadscrew depends on your lathe but the same effect as rotating it a full turn can be accomplished by slipping the half-nuts by one thread.

I am sure you will observe that sync is not lost by full spindle turns. I am also sure you will observe lost sync by full turns of the leadscrew unless your work is an integral multiple of the leadscrew pitch.

Hope I am correct and this helps. I am still open to other possibilities.