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.

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.

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.

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.

Try Moore International for the screw and nut

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).

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

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

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.