Zyto cross slide screw

[ianday1972Participant @ianday1972]

I’ve got quite a bit of slop in my little Zyto’s cross slide.

I’m wondering what thread they used and if it’s possible to buy a replacement? Would an anti backlash nut help?

It’s 5/8″, 8tpi and looks like a square thread.

Thanks

Ian

[ianday1972Participant @ianday1972]

Sorry, I meant 5/16″

[SillyOldDufferModerator @sillyoldduffer]

Is it the screw that’s worn, or the nut?

The nut, hidden under the slide, is probably made of Brass.  The anti-backlash adjuster is usually crude –  even in many posh lathes – such as a sawn slot squeezed by a screw.  Or maybe the nut is rocked from the top with two fiddly adjuster screws to take up slack.   These are tweaked periodically to reduce backlash, and eventually the nut wears out.

Can you take it apart and post some photos?

Dave

[Nigel Graham 2Participant @nigelgraham2]

I am afraid you are very unlikely to find spares for a Zyto lathe.

You could try searching transmission-components stockists for lead-screw material, but 5/16″ diameter is very small.

I did buy similar material to create a short lead-screw for an early-pattern gearbox I fitted to my Myford ML7 (keeping the original intact) but that is larger, 3/4″ LH Acme.

An anti-backlash nut may reduce some of the play but really the only satisfactory solution is a new screw and nut.

It is though worth seeing how much lost motion there is in the screw’s bearing on the slide. Removing that will cut the backlash down a little bit.

Good luck anyway – it would be good to restore the lathe to somewhere near new condition.

[HopperParticipant @hopper]

I am not familiar with the Zyto but typically British lathes of that era used either an Acme thread or square thread there. Most likely Acme if it is post WW2. You can google Acme threaded rod and find suppliers.

But typically it is the brass or Mazak nut that wears most. You can make a new one. Tracy Tools sells a lot of Acme taps that may suit. Check if your thread is left or right hand first though.

The other dodge if your screw is worn is to cut the end off it, flip the threaded section around 180 degrees and rejoin it with a pin and silver solder or Lloctite etc. This moves the most worn section of screw into the least used area.

 

[Martin of WickParticipant @martinofwick]

It could also be Acme thread, it may also be 10tpi, but you say 8tpi square which although unusual for cross slides on small lathes, is not inconceivable on such a relatively crude lathe.

The only spares you will find will likely be from machines of the same vintage and similarly distressed.

If there is a separate and removable feed nut, then you are in luck. If you have some screw-cutting gears, purchase 10 tpi trapezoidal insert to make new length of feed screw from which you can make a crude tap to heat up and melt your way through a block of delrin or nylon to use as your new feed nut.

You could also use a bit of silver steel instead for the tap, harden and temper and make the new feed nut from brass.

You will need a deep pocket for commercial Acme taps and dies.

 

[JasonBModerator @jasonb]

How much baklash? a syou are generally feeding in one direction you should be able to live with a reasonable amount.

8tpi seems very coarse pitch for a 5/16 dia screw. Have you measured it or are you going by the handwheel increments. Could be a double start that gives 8tpi but does not have such a small minor diameter. I doubt the handwheel is callibrated in diameter.

[Clive Brown 1Participant @clivebrown1]

Lathes.co.uk website quotes 10tpi square thread for both cross-slide and top-slide in their description of the Zyto lathe.

No micrometer collars.

 

[JasonBModerator @jasonb]

Same section on Lathes.co also give 16tpi for some Zyto lathes.

Maybe a photo and confirmation of which model it is would help.

Also check that some of your backlash is not end play of the screw which can be taken care of more easily.

[ianday1972Participant @ianday1972]

You could be right about the 10tpi, I was typing from memory after measuring it some time ago. The homemade indicator dials are 100 thou per turn so it must be 10tpi.

There’s no nut unfortunately, the screw is threaded through the lower slide casting. The backlash tightens up at the extremes of travel so I guess it’s wear in the screw.

I’ll have a proper look at it and put some details on here.

Thanks so far

[Brian WoodParticipant @brianwood45127]

I am not surprised to hear that the thread is cut directly into the cross slide casting; early Myford ML2-4 models were made in a similar way.

You may be able to buy a new 10 tpi screw and mazak nut from Myford and drill out the old hole to accept the mazak nut, then fit the leadscrew to the cross slide; if there is sufficient metal left to contain the nut. Failing that, Hopper suggested a top and tail rework on the screw to shift the worn screw to a less used area

Good luck either way

Brian

[Martin of WickParticipant @martinofwick]

I was rather afeared that the feed thread would be cut into the casting (the halcyon days of British engineering – thank god they have been consigned to the dustbin of history).

If you can accurately measure and determine the actual feed screw pitch and type, you can at least make a replacement by making up your own tool to cut, which will be relatively easy for a square thread, bit more difficult for Acme. This may help as from your description, the wear appears to be mainly in the screw.

If it is truly a square thread, you might start by making the major diameter of the new screw a few thou over the major diameter of the unworn part of the screw to take up any wear in the ‘nut’ and ‘trim to fit’ as they say.

[BazyleParticipant @bazyle]

First check the endplay of the screw as mentioned by Jason. If this is set by a radial screw there are probably dents now in the leadscrew making it hard to set. Think of a better way. The thrust is probably taken by a too-small shoulder on the screw working and wearing against the cast iron of the slide. See if you can interpose a larger washer and clean up the cast iron face or add a brass washer too. You will need to fix washers to their respective components to ensure the sliding movement is between the right faces.

Now for the screw to saddle interface where it has worn slack. Most work is one-sided so the tool is being pushed into the work and the screw is being pushed back into that thread in the saddle casting. To reduce backlash you want an adjusting nut on the outside or handle side of that threaded hole. That nut will seldom take cutting thrust as it would only be when the tool/slide is being pulled back towards the operator.
So the nut can be imprecise, short, say two turns of thread, and a loose fit. Just have a go at making it on the lathe as it is in a bit of brass, (scrap of mazack, ali even 3D printed plastic would work)
Fit the new nut on the outside side of the saddle. Do up hand tight to lock the screw at the end or tightest part of its travel. back off a fraction and make a reference mark then move to slackest part of travel and repeat. The two marks are a measure of the backlash ie 90 degrees apart = 1/4 of the 100thou per rev. or 25 thou.
Now if you attach a lever to the nut, glue screw or soft solder, such that the lever hangs down below the slide you can just move it to take up the variable slack at different points along the length of the screw. There should be enough space too adjust it by at least 60 degrees taking out 15+ thou of backlash.

[bernard towersParticipant @bernardtowers37738]

Try Moore International for the screw and nut

[ianday1972Participant @ianday1972]

I’ve had a better look at it tonight, there’s about 25thou free movement in the cross slide along most of its travel. There’s a point where it tightens up when it’s most of the way forward (away from the operator).

I’ve attached some pictures, it may be Acme thread as it looks like there’s an angle rather than square, what do you think?

As you can see there’s not much material to play with where the thread is cut into the casting. Bazyle I think I follow you with your suggestion and I was thinking something similar myself although probably not as well worked!

Ian

[ianday1972Participant @ianday1972]

[HopperParticipant @hopper]

Can’t tell if it’s Acme from those pics. You need to photo it in focus in good light with a piece of paper behind the screw so the thread profile is visible. But if I had to guess I’d tend to think it might be Acme but not sure.

If you bore out the casting and fit your own Myford style flanged nut with retaining screws, you can use whatever thread you like as long as it matches your new screw. Acme is easier to buy threaded rod and taps for. Square thread not so much.

[Martin of WickParticipant @martinofwick]

I assume you have removed the play between the feedscrew and end plate of cross slide.

A brave attempt, but not clear enough to decide convincingly whether Acme or straight.  as Hopper suggests better photo!

You could try to take a small piece of metal say a piece of hobby knife blade or Stanley knife blade and press it so it is resting firmly against the flank of the thread with a small screwdriver. Do this check with the screw  on a horizontal flat surface.  If the blade sits vertically upright at about  90 degrees to the flat surface, it will be a straight cut thread. If the blade cants over at a definite angle of about 15 degrees, it will most likely be Acme (do this test on a relatively unworn part of the screw if possible).

If it is definitely Acme, then you may be able substitute a Myford component, however the ML7  feedscrews are 3/8in od and not 5/16in which was the dimension you provided (maybe check?), so a new feedscrew alone will not fit, so you would need to purchase both nut and screw as a complete replacement.

If you opt for a complete replacement, be careful in assuming you can bore out the existing unit to fit, as you only have a couple of mm ‘spare’ metal beneath the thread on the casting. You will need to check the OD of the Myford feed nut. If it means that boring out to fit a nut will break through the casting, this is not necessarily fatal as long as there is sufficient space between the crosslide and the saddle casting to accommodate the protruding nut (you may even be able to file a small flat in the replacement nut to assist with fitting).

 

[Phil PParticipant @philp]

You might be able to gain a bit of extra space if you lower the screw down a bit by making a new plate for the cross slide, that way you could bore out the old thread in the casting eccentrically to fit a new sleeve nut without it breaking out of the casting.

[Martin of WickParticipant @martinofwick]

You might be able to gain a bit of extra space ….

But watch out that doing that doesn’t cause the feedscrew to foul something else

 

[HopperParticipant @hopper]

<p style=”text-align: left;”>If it is a square thread, you might be able run an Acme tap through it and clean if up nicely, then use a new piece of Acme leadscrew in it.</p>

[SillyOldDufferModerator @sillyoldduffer]

Now the worst has been revealed, the screw running direct in the casting, triple yuk, is it possible to attach a nut running inside to the cross-slide?   This is how ‘better’ lathes are made, the advantages being backlash compensation and being able to replace worn nuts.

This example is of a mini-lathe conversion, where the simple nut provided by the manufacturer is replaced by a ball-screw.  It shows the principle, replacement nut ringed in blue:

Then the nut is attached to the slide.  Lathe nuts aren’t complicated: this example from a mini-lathe.

I’m not familiar with the Zyto, and can’t say how easy or difficult the mod would be.  Zyto did what they did for cheapness, not because it’s best practice.   Remarkable the lathe lasted this long before wearing out.  Is it time to replace it?

Dave

[Michael GilliganParticipant @michaelgilligan61133]

Definitely worth double-checking the details of your original screw, Ian

[ 10tpi is pretty-coarse for a 5/16” screw of any thread-form ]

This supplier is obviously ‘over the top’ for what’s appropriate … but I include the link as a useful general reference:

https://www.abssac.co.uk/go/Precision-Lead-Screws-and-Nuts/

 

MichaelG.

.

Edit: __ This page might be of particular interest:

https://www.abssac.co.uk/c/Precision+Nuts/58/

[Martin of WickParticipant @martinofwick]

In summary, options to remove 20 thou lash (if that was a bother for me) are in order of increasing difficulty and cost…

do nothing (apart from tighten up attachment to end plate)

screwcut new feedscrew to suit existing nut either with shop made or commercial cutters (assumes    OP has screwcutting and grinding stuff available)

fabricate and install a backlash compensator mechanism of some sort

quick and dirty, tap and die a 20W 5/16 feedscrew and corresponding flanged sleeve nut to fit drilled    out nut bore (requiring only press drill and lathe in question).

retrofit larger myford xslide feedscrew and nut and set up new mortgage to pay for it

Fit ballscrew or new nut on cross slide casting (assuming it is possible )and change driving           arrangement / make L/H leadscrew to suit (unless you like cackhanded feeds)

And with the possible exception of the ballscrew option, all others will still leave you with degree of residual backlash.

[Michael GilliganParticipant @michaelgilligan61133]

Picking-up on Martin’s quick and dirty suggestion:

I would be inclined to modify the the arrangement to 3/8” BSF

… a nice standard thread, of very suitable proportions.

MichaelG.

[Author]

Posts