Half nut skipping still

My thoughts are that if the engagement mechanism is not backing off when under load, then the leadscrew is flexing downwards because of the shape of the worn nut threads. The Smart & Brown model A also uses a part section nut, but any flex in the leadscrew is stopped by a bronze slipper opposite the nut. I wonder if you could fit something on your Zyto?

Where are you located?

It is possible that someone may be close enough to you to be able to come and help in your investigations, and repair / adjustment.

Howard

Thanks for the replies….

When I take it to bits again later that’s the 4th time today I’ll have taken it to bits!

I’ve watched several YouTube videos of people restoring Zyto lathes this afternoon. All of them have 3/16” pins, but all of the pins seem to fit, and the cam looks different. I think mine is either a replacement, or a previous owner has messed with it. None of them have springs.

I think Howard, your idea of turning down some thicker pins is the easiest thing to do there.

I still wonder how to fix remake half nut threads, but that might be a project for another day, presuming it works reliably once I’ve fixed the basic mechanism.

Quickest way to check if thicker pins work is to sleeve the ones you have on at the moment.

Martin C

It may be that the pins and skipping are symptoms of one or more basic faults.

I suspect that stripping the Apron and seeing the parts and how they fit together and behave may be the way to fix the problem.

It may seem to be a sledgehammer for the proverbial nutshell, but until the basic fault has been found and corrected it could be chasing a will o'the wisp fixing problems caused by the fundamental fault, which may be within the Apron

Until you understand exactly how the mechanism works, and how it goes together, in detail, the fault will remain elusive.

Howard..

Edited By Howard Lewis on 26/11/2021 17:58:29

I agree Howard. So I have had it all apart again and this time I have worked on adjusting the gibs, so that the is no slop in the movement of the half nuts. Now gravity plays no part and they basically stay where the lever/cam mechanism puts them. This removes another variable.

I’ve been looking very closely at how the half nuts disengage. The lower nut disengages properly, but occasionally, the upper nut sticks. When the apron is off, now I have adjusted the gibs, the nuts behave in a very predictable way, with no movement other than that caused by the lever.

So I think the upper half nut must be catching somehow.

The whole mechanism that supports half nuts is interesting. When the half nuts are closed, they stand a 50-50 chance of landing on a trough rather than a crest of the lead screw. Usually, one doesn’t notice this, the lead screw turns and the half nut just slips into the thread. For me, for some reason, that isn’t happening correctly.

The top half nut is in bad condition with 2 of 8 threads basically stripped. Whereas the lower half nut is in good condition.

I have also turned around the lever to match the other Zyto lathes on YouTube. This seems to make the pins look the right size.

Closer look….

Bottom half nut in good condition

Top half nut with 2 1/2 knackered threads. It also has a threaded hole bored in it to accept a screw which hold the lead screw guard. It looks worse in the pic than it actually is… and indeed has worked like this until recently. But clearly it needs renovating.

More importantly, there is a burr on either end of the top half nut which could be impeding the correct engagement and disengagement of the lead screw.

There is also some “bobbling” of the metal surface next to the dovetail, which has probably always been there, but it may not help the half nut to move smoothly when the gibs are tightened.

So, plan A is get to work with a file in the morning and fix these bits as best I can, and see what difference it makes.

Plan B is to renovate the top half nut, which clearly needs doing. I’m getting some ideas for how to do that.

I watched a really excellent vid by Joe Pie yesterday which has helped me to much better understand how half nuts and lead screws work in detail.

https://youtu.be/kESm2Kozb-4

So essentially what’s happening is the cam is moving the half nuts into a position where they will engage *next* time the lead screw is in the right position, and creating the pressure to do so. The lever is not pushing the half nuts into position directly. That explains why the pins are “too small” and there is play in the cam. There needs to be so it can accommodate the half nuts being in or sitting on the thread.

my half nuts look like they are engaging at different positions where they can, hence the “jump” that occurs. Here’s the jump by the way….

Steve

Have you got a threading dial and are you using it correctly? That multiple pass looks just like the result of engaging the half nuts in the wrong part of the leadscrew. You are getting a perfect helix but not in the same place each time.

Look at this album for information on the threading dial Understanding the threading dial

Martin C

Hi Martin, yes and yes, you kindly provided that document to me a while back. The trouble has been that when I engage the lever, the half nut hasn’t been engaging, it skips and engages 1/8 of a wheel turn after. If you see the pic of the upper half nut above, frankly it’s amazing it works at all.

Right I think I am getting there, extreme persistence paying off perhaps.

Rule 1 the half nut must be free of burrs and obstructions

Rule 2 the nuts must engage and cause no movement of the lead screw. If they do they are engaging at different times and the carriage may jump out of thread. To adjust this the left side of the apron can be shimmed.

Rule 3 the nuts must be absolutely perpendicular to the lead screw, else the half nuts will stick in the lead screw thread when disengaged. To adjust this, the right hand side of the apron can be shimmed. Too high and the top nut sticks, too low and the bottom nut sticks.

Rule 4 (which I am predicting will be the next problem) the lead screw needs to be parallel to the slideways of the lathe.

Different shims at both ends.

Edited By Steve355 on 27/11/2021 09:39:37

Edited By Steve355 on 27/11/2021 09:40:22

Edited By Steve355 on 27/11/2021 09:40:36

Edited By Steve355 on 27/11/2021 09:42:06

Edited By Steve355 on 27/11/2021 09:43:17

I still wonder how to fix remake half nut threads,

Boxford half nuts were made from a single casting that was the pair in one piece - the leadscrew thread was made by drilling & tapping the casting after machining the other features, then the casting was split along the leadscrew thread centreline. The leadscrew thread drilling & tapping was done on a fixture in a radial arm drill, with the tap being a special type with two cutting sections in series - the first section being undersize & the second section being the "finisher" - and an undersize shank. The tap was driven through the casting & dropped out onto a leather pad below the fixture so the tap didn't have to be withdrawn.

The half nuts on my Super 7 (which are a similar pattern to your Zyto) appear to have been made in a similar way.

Nigel B.

Is it possible that the half nuts have been interchanged, so that the upper is now at the bottom and vice versa, leading to damage? i e the thread in each half not coinciding with each other

This might account for the skipping since the two halves would be fighting each other to engage with the Leadscrew, reducing engagement and increasing the load on any thread that did engage..

A quick check would be offer the two halves to the Leadscrew to see if the dovetail faces align when the present "upper" is in the lower position and the "lower" is on top.

If so, does reassembling the other way up improve engagement?

Obviously no burrs to obstruct movement or positioning.

Howard

I don’t think so, as the upper half nut has the lead screw guard attached to it. It’s possible that was an aftermarket addition, but anyway if they were the wrong way around I don’t think it would even engage with the thread. I’ve had the nuts off and sat them on the lead screw and everything seems square and correct.

I got it working quite well this morning by measuring the displacement of the lead screw with no shims, then adding exactly that amount of shim to stop it happening (28 thou). I then added a 7 thou shim at the other end to stop it sticking on disengagement. That might be the solution for now. I will try cutting a thread tonight and see how I get on.

Medium term, I’m thinking of getting a 5/8”-8 acme tap and a bronze bar, taking off the ruined thread plus a bit on the mill, and then tapping, turning and cutting the bronze bar to fit. Then gluing or brazing on the new thread section. I saw a vid on YouTube of a chap doing something similar.

Steve

Edited By Steve355 on 27/11/2021 13:30:37

As the lathe has a long history behind it and has been stripped down and reassembled probably best not to assume anything. Although they're not rocket science, lathes require careful alignment and snugging down to avoid unwanted movement.

As the half-nuts worked OK before the lathe was stripped, I suspect an assembly problem. Entirely possible the worn top nut is making adjustment more difficult than it should be.

I'd double check the gibs: are they the right way round and do any dimples align with the adjusting points?

Especially if it's been off, make sure the lead-screw is horizontally and vertically parallel to the bed-ways, and that the half-nuts centre on it. Is it possible a shim or two fell off? Not unlikely an 80 year old lathe has a few.

The lathe would have been factory assembled in a particular order by a trained chap supported by a set of jigs and gauges. Home maintenance relies on intelligently tweaking the machine, which might have to be painfully learned.

Be wary of removing metal! Problems are more likely to be due to wear and tear or misalignments.

Properly adjusted half-nuts should engage a turning lead-screw without fuss. The lead-screw, apron and half-nut arms shouldn't move up-down or sideways when contact is made. Make sure this is so and investigate why not.

I think Steve is approaching the problem in broadly the right way, apart maybe from jumping in early to disassemble and repair before it's clear what the problem is. As random mending based on guesswork can lead to spiralling confusion I recommend slowing down. Frustrating to have to learn the ropes the hard way when half an hour with a working Zyto might reveal all, but we are where we are.

It's possible the problem is simply that the lead-screw and half-nut mechanism aren't quite aligned properly and that the adjustment is fiddly to get spot on even when they are. Persist!

Dave

From your post at 09:29 I would suggest stopping the lathe between passes and making sure the leadscrew is engaged at the correct point as shown by the thread dial before starting the lathe for the next pass. You will probably have to put some light pressure on the half nuts and move the carriage to the left and right to get to the point where the nuts slot into place. The other thing you can do is to turn the chuck by hand when applying light pressure to the half nuts to get them to line up with the leadscrew thread.

You may already be doing this but just in case you are not: Under normal circumstances, if you leave the lathe running you try to close the half nuts with light pressure a little before the correct number comes round on the dial (probably half way between the previous thread dial mark and your required one) then the nuts fall into place as the leadscrew thread rotates between them. Do not try to slam them into place at exactly the point where the thread dial number comes round. The chances are (from my experience) that a combination of wear, backlash and how the thread dial has been set up the fiducial mark and the numbers do not line up exactly when the half nuts are engaged anyway.

Martin C

Dave

I’ve taken a look at the lead screw mechanism as you suggested. The lead screw itself has very little wear at .625 pretty much everywhere, so that’s good. Using a m&w depth micrometer, the distance to the lead screw from the bed is .614 at the tailstock end and .592 at the change gear end, so it is slightly slanted down at the headstock end. I’m not sure if 0.022 is enough to make a difference, but I may as well sort it.

I also realised that there is about 2mm longitudinal play in the lead screw, which I’d think could make a big difference. Again, I will sort that.

I have a feeling that all this started when I reconfigured the changewheels for the first time a couple of weeks ago. The V bracket that holds them was very stiff and took quite a bit of freeing up. Looking at the location of the change wheel bracket (whatever it’s called) right next to the lead screw adjustment, it’s easy to see that I could have knocked the lead screw out of alignment.

When I escape from the kids I will sort all these things out and see what difference it makes.

Steve,

I've just had a closer look at my Zyto and taken a couple of shots of it.

When i look at your first photo there appears to be a substantial clearance between the scroll slots and your pins,( wobbly threads on another query) On my machine the pins are a reasonably close fit to the slots and my take is that your slots are badly worn or maybe modified. It is the proper fit of these pins in the slots that gives you the lock in of the half nuts. You need a, uniform width slots and b, good fitting pins in the whole length of the slots to keep the half nuts engaged.

Not to be recommended but strictly speaking a single thread on your top half nut should be sufficient to produce a good screw thread on your workpiece, at least for a limited time!

I notice that you do not show a large washer over the lever assembly. For what its worth mine has a full diameter cover washer which has a tapped hole in the centre. The pivot shaft has a screwed extension onto which the washer screws and then it is locked in position with a domed nut, which is then used to control the fit of the lever assembly . Too tight and you can't turn the lever.

Depending on your skill level I would be filing out the slots to a suitable drill shank al the way along the slots, then making two oversize pins to suit the slots and having another go.

Regards Bob D

Edited By Robert Dodds on 27/11/2021 15:52:44

Bob

it’s great to hear from someone who actually owns a Zyto lathe! It’s been a bit of an uphill battle for me, having little experience myself, and not knowing how the lathe should actually be. I’m sure they have all been modified over the years, and there would’ve been numerous revisions from the manufacturer as well.

Can I ask a question, what diameter are your pins, and what diameter is the slot?

My pins are 3/16, and several other Zytos I’ve seen look to have the same.

My slots are just under 1/4”, about 0.230.

Are there any shims between the apron on yours and the top of the carriage?

Another pic of mine with the pin in the same position as yours, looks remarkably like yours.

The problem really does seem to be the pressure that it is putting on the lead screw,

cheers and thanks for your input.

Steve

Now I am beginning to understand how the nuts (I originally thought there was only one) are moved. The pins are attached to the nuts which slide in a vertical plane. You should remove the handle with the cam and see how the pin spacing is when the nuts are manually closed, and compare that with the spacing when using the cam. The pivot should have some tension/friction adjustment or the weight of the lever could slacken off the nuts.

The picture of the threading marks is suspiciously like misreading the threading dial as each mark goes evenly throughout its length. Maybe some practice taking a just visible cut may be useful, especially to demonstrate what happens if the wrong dial marks are used, there is nothing like actually doing something to help understand what is happening.