Also remember there are ballscrews and there are ballscrews. Ground better than rolled and even then there are different classes of them.
Look at the linear accuracy of the ones you are intersted in and see if the accuracy is as good as you want. Again with a smallish lathe  it is not so much of a concern but still worth noting.

Hood

Yes your dead right there. I remember when I built the router the 1.8m screw on the y 3/4"  and the 300mm on z same size were only available from limited sources, I got mine out of the states and again weren't cheap, but I have noticed the market is flooded with all sorts and at no were near the cost of the old stuff.  I cant say how good or bad they are, a lot of people bag the Chinese stuff but some of it is pretty darn good. Its hard to tell any more, personally I think it all comes out of the same shop just re-badged.
Katoh

IMHO, you are much better off with a really good quality lead screw than a cheap ball screw. In the hobby CNC world 'ball screws' have this magical aura about them and many think that you can't build a good machine without them, which is non-sense. It all depends on how you are using your machine and what you want out of it.

If you are using a machine in an industrial situation than the money spent on a good quality ball screw is money well spent. You are using your machine all day long and time spent tweaking things is money down the drain. As your machine size goes up then the additional efficiency offered by ball screws is also a big benefit. A more efficient drive train means small motors, smaller drivers, smaller power supplies, a smaller foot print, and less power used. The cost of energy to run machinery is huge factor on an industrial scale and should not be ignored. (Since Hood depends on his machines for a living then ball-screws are a great investment.)

For hobby use or small scale commercial use a ball-screw may not be as good an investment. Most hobbyist spend more time tweaking their machines than running them (as it is fun), so a few minutes spent every now and then to adjust the backlash nuts is not a big deal. Also as I mentioned before a good quality lead screw is much better choice than a cheap ball screw. A cheap, inaccurate ball screw is a waste of money.

Open loop backlash compensation, like Mach does, has limitations and it cannot make a sloppy machine work like a precise machine. Think of it like driving an old truck that has a lot of play in the steering wheel. Your brain is really good at trying to compensate for the slop in the steering but you'll still be darting around in your lane on the highway more. What happens when you hit a pot-hole though? The truck is pushed around by external forces that you have a much harder tiem trying to compensate for and you cna loose control. The more slop in the steering the worse the situation can be.

Jeff
Problem is  we are talking about re-using the leadscrews on a small import lathe so they are not likely going to be of high precision.

Katoh

The Chinese can make as good as anyone else, just depends what you are paying that normally determines the quality. I have Chinese  ER collets that are as good for runout as any UK or USA made ones and heat treatment seems right as well as they dont seem to be losing their spring and have not cracked yet.

I got ballscrews from a place in the UK, might be worth contacting them. Also worth contacting  Dan13 to see what he has to advise as I know he also got some from them but had some issues but think he got them sorted. Mine were 32mm Dia class 5 ground and I am happy with them. This is the place
http://www.motionandbearing.co.uk/Home.html

Hood

Hood, yes the stock screws are likely junk. Good quality leadscrews will likely be a less expensive alternitive to ballscrews and may well fit the application of the OP.

Jeff Cant really understand your saying that lead screws are better than ball screws in some circumstances but to use ball screws? What about my lead screws are they really that bad?
Hood you get what you pay for 90% of the time. The other 9% I buy good name products and they end up being absolute CRAP! and then 1% I buy cheap stuff and its better than anything you ever bought? Go Figure.
I should make something clearer, this is no industrial lathe nor is it going to be used in an industrial situation, it was only bought to make parts for a boat project, still none the less if your going to do something then might as well do it right.
A friend  of mine laughs at my 11x30 lathe, and only places it on turning pieces 10mm dia or less and its far to small and weak for an NC work, you really need to use something like his 15hp 5ton beast for anything substantial! I would dearly love to prove him wrong!
Katoh

Obviously if you want to take 20+mm off in one pass then you need something a bit bigger but even the wee Sherline lathes can make good parts.

Hood

Jeff Cant really understand your saying that lead screws are better than ball screws in some circumstances but to use ball screws?

No that is not what I am saying. If cost is a concern you will likely be better off buying the good quality lead screw than a cheap ball screw. Think of it like buying a new flat panel TV. Your friends might be telling you that you 'have' to buy a 50" model. When you look at the various TVs in the price range you want to spend you find that the picture on the 42" model is better. So you can buy a cheap 50" TV with a horrible picture or a slightly smaller 42" TV that has a fabulous picture.  The cheap 50" TV, or a cheap lead screw, does not really get you much.

What about my lead screws are they really that bad?

No idea. You will have to measure them to know for sure. Chances are if it is a cheap import model than they are not really useful for CNC purposes.

The question should be asked the other way around.  Why do manual lathes not use ball screws? That is because lead screws are self locking ball screws are not.  But in the case of CNC where steppers or servos (instead of your hands) do the work they do the self locking for you.  In a manual lathe the operator usually takes a measurement before the last cut, so he compensates for most of the lead screw problems.

If you want to consider a ´half way´ solution then you could go for a ball screw only on the X axis.  In lathe work the diameters usually need to be very accurate, lengths not that much.

Jorge

Interesting to get a variety of opinions on the matter.

Katoh,
You can get a lot out of a "system" and get good prefomance and accuracy for some dollar amount.
As the desired accuracy and preformance increases you pay for the increased system. So only you can decide and define the cost, effort, and end
system quality for what you want or may ever want  to do. You can't compare systems of different quality ie; hobby to  industral. Your's is what you have or will have should you desire to improve it.That my friend is just a fact of life!

As my music teacher said....YOU CAN HAVE ANYTHING ON THE SHELF BUT ONCE YOU DECIDE YOU WANT IT THEN YOU MUST PAY THE PRICE
Then he said .....NOTHING EVER STANDS STILL IT  PROGRESS OR REGRESSES AND THAT IS JUST NATURE

What you see is what you get , but, maybe what you see is not necessarily what you think it is. Ask someone how much  it cost to replace their bearings in a say a Hardinge lathe, or it's lead screw. Can the lathe cut a class one thread, a class two thread, or does the nut just go on the thread?

Practical info on screws:
They are not all created equal. The ones removed from both my Atlas lathes and mill were ground.
You are not going to get ground lead screws for cheap and certainly not on the imports for their machine pricing.
There are lead screws in my engraving machine and the nuts have backlash removal and provide for 20 micron accuracy over their travel 
( yes that's .000020", but i could only verify to .000040", so i will believe the certification papers).
 I know of friends that have cnc lathes using all thread  / crappy cheap threaded rods along with .080" backlash and they have done some nice work using backlash comp. I wonder why they changed them out to ball screws?

I have a Sherline lathe with the anti backlash attachement, the lead screw is quite accurate, but, even with the backlash adjustment, as compared to ball screw it sucks in accuracy. Not bashing Sherline but it depends on what your trying to do with it.

My Rockwell manual lathe will produce threads that rival my cnc converted Atlas lathe.
But the Atlas is very reliable on producing the part over and over even if this dumbo defines it incorrectly.

The "system" is only as good as the sum of it's components and installation. A high end ball screw with poor  / improper bearing installation
 could be just as bad or no better than a higher end screw.

So should you replace the current lead screws with ball screws?
Only you can decide as stated above.

BTW, A non-backlash accurate cnc machine is just pure joy to use. 

RICH