Chester 920 - under cross slide nut

[flintandsteel]

Had the lathe striped down this week for an overhaul.
Suspected and discovered that there is a load of play in the under cross slide nut. Creating too much backlash to be comfortable.
Just emailed Chester to see if they have one. Not optimistic.
Additionally it's left hand threaded.
What chances of getting one made if Chester don't have one?

I don't know if it would work but is it realistic to consider skewing the nut slightly? Just feel it may bite on the worn threads differently and reduce the backlash?

 

[deema]

With a 920, you can use it to make yourself a new one.

 

[flintandsteel]

Love to, but my skills are limited.
Mainly used to make screws, ramrods and bits for old flintlock and percussion beasties.
Found a few very similar (but better made) on line but not sure they'll fit. waiting to hear back from the makers.
One I saw had 2 horizontal grub screws in the end then it dawned that with adjustment they would take up the backlash. Correct?

 

[Fergie 307]

If you search Harrison feed nut it similar on e bay, there is a guy who makes bespoke feed nuts. You send him your screw and nut and he will make a new one to the exact dimensions for your individual screw. Can't see why he wouldn't be able to make one of these for you. Alternatively you could cut it in two and then install two screws to enable you to vary the spacing between the two halves. This is how the adjustable feed nuts on most machines are configured. So, assuming the whole thing is threaded, just cut right through the slot then drill and tap the main body to take a screw either side, and drill suitable holes in the thinner part to be a close fit on the bolts. Now by adjusting the bolts you can make it grip the thread more or less tightly. Cap heads are good for this. I am sure if you Google adjustable feed nut or similar you will find images of this sort of arrangement. Only worth doing if the threads are still reasonably good, if they are really badly worn it would probably be better to just get a new one. Have to say I'm surprised to see that they have used that thread form. I would have expected to see a square or Acme thread on something like that.

 

[dickm]

Agree it's strange to use what looks like a basic Whitworth thread in that situation. It's quite likely that the screw is worn as well as the nut. Might the OP be best served by replacing the whole feed screw and the nut with a more robust Acme? My guess is that the screw is just a single piece, easily machined to Acme on the relevant bit. That would give the OP a really good machine.

 

[Fergie 307]

I agree, that's probably what I would do. Providing you use the same pitch thread then the micrometer dial will still be correct as well. I am afraid this is a perfect example of why Chinese machines get a bad press. Here you have a standard thread form running in a nut that appears to be a pretty crude zinc or similar die casting. The whole thing designed primarily to be cheap to make, with little regard to accuracy or longevity. What is so annoying is that making the sort of volumes they do the additional cost of using an acme thread, and phosphor bronze nut would not be that great. I would certainly be happy to pay a few quid more for the machine for the improvement that would bring. I dare say the other feed screws are probably the same.

 

[chaoticbob]

Love to, but my skills are limited.
Mainly used to make screws, ramrods and bits for old flintlock and percussion beasties.
Found a few very similar (but better made) on line but not sure they'll fit. waiting to hear back from the makers.
One I saw had 2 horizontal grub screws in the end then it dawned that with adjustment they would take up the backlash. Correct?

I think you may be overestimating the difficulty of making the part. Steve Bedair documents his mod here . It doesn't look too hard - just a matter of machining a lump of brass to size, which could be done on the lathe if you don't have a mill, then drilling and tapping a hole. He actually beefs up the screw to 7/16" - 20 which is a UNF rather than Whitworth size, so perhaps the original is 3/8" - 24 UNF if it's an imperial lathe? Whichever, Tracy Tools will have an LH tap to suit. If you need to remake the screw it could be done with a die, rather than by threading on the lathe as Steve did.

You are correct about the horizontal grubscrews - on a lot of machines, rather than the the two part nut Fergie describes, the nut is slotted vertically to maybe 90% depth. The grubscrews act to open up the slot, thus distorting the nut and tightening the fit. Sounds crude, but it seems to work!
Rob

 

[Fergie 307]

With a 920, you can use it to make yourself a new one.

That's not a bad shout, just take some careful measurements, then put the machine back together and make a replacement, should be easy if you have a four jaw chuck.

 

[Fergie 307]

If the nut is indeed a die cast part then I would think there is unlikely to be any great wear on the screw itself. As Caoticbob says Tracy tools will have a suitable tap. I would say you would be better to use phosphor bronze for the nut, rather than brass, although it will be more expensive it will last better. If you are lucky then simply making a new nut out of some decent material will solve your backlash problem. But I would make the nut extra long in case you do need to modify it to give some adjustment. Another good idea would be to introduce a way of lubricating it when it's in situ. Maybe a vertical hole through the top of the nut into the thread, lined up below an oiler in the casting it fits into.

 

[Fergie 307]

And Bob is right the split type adjusting nut would be easier for you to make, just thread it then cut a slot with a hacksaw maybe 6-8mm from the end so you have enough meat to thread for the grub screws. Or you could modify my original two part idea by threading the body so when you tighten the screws it closes the gap, whichever you find easiest.

 

[chaoticbob]

I agree, that's probably what I would do. Providing you use the same pitch thread then the micrometer dial will still be correct as well. I am afraid this is a perfect example of why Chinese machines get a bad press. Here you have a standard thread form running in a nut that appears to be a pretty crude zinc or similar die casting. The whole thing designed primarily to be cheap to make, with little regard to accuracy or longevity. What is so annoying is that making the sort of volumes they do the additional cost of using an acme thread, and phosphor bronze nut would not be that great. I would certainly be happy to pay a few quid more for the machine for the improvement that would bring. I dare say the other feed screws are probably the same.

Well, the Taiwanese/Chinese 920 lathes were cloned from the 1970's Austrian made Emco Compact 8. Which used metric 60 degree leadscrews. Tony at lathes.co.uk says "As an economy measure, instead of the proper Acme thread usually found on leadscrews, that on the Compact 8 employed an ordinary 60-degree metric type." So not just China!
I agree that it seems crazy - but maybe it makes economic sense for the manufacturers.
Rob.

 

[Fergie 307]

Yes, a curious choice for Emco. In fairness I think on that machine the feed screws for the slides are Acme though I could be wrong, relying on memory, which is not as good as it was. The leadscrew is quite large, and a fine pitch so probably not such a problem. Still a poor way of doing it though, especially with the original cast half nuts made of what we used to call monkey metal. This was always a daft idea given the amount of work the leadscrew does, if they insisted on using an ordinary thread, they could at least have put some decent nuts on it. I know you could get aftermarket bronze replacements as I helped someone out fitting a set to his. Overall I think it's fair to say the Compact wasn't a high point in Emco's lineup. I had an old Unimat and have always regretted selling it, beautifully made little machine. That of course used normal threads but really not a problem in such a tiny machine with such small loads, doesn't really scale up though.