Lead or Ball Screws (lathe Conversion)

[Katoh]

Friends
I am venturing into converting a lath from manual to NC, I have read prior post and peoples experience's in the conversions and there's one thing that puzzles me.
Why do they change the Z and X axis lead screws that are already part of the lathe with more expensive ball screws and have the hassle of fitting and trying to make them work. Is there a reason why the original lead screws do not make the grade? and how is it they are good enough for manual work and not for CNC?
Katoh

[Tweakie.CNC]

Katoh,

I think it all comes down to Backlash.
Under light loading ballscrews can exhibit zero backlash whereas adjusting the double nut on a trapezoid to obtain the same result produces much increased friction.

Tweakie.

[Hood]

Yes, backlash is the big one, on a manual lathe you do the majority of cutting in one direction and you tend to pull out and move back in the same direction, on a CNC often the motion is both ways when cutting. Friction is another reason as Tweakie says and lastly ballscrews tend to be more accurate but that will depend on the grade of the ballscrew and the grade of the original leadscrew, some leadsrews are very accurate.
Hood

[RICH]

Accuracy,repeatability, wear / longivity of the screw and of course backlash removal. Additionaly one may desire to have a speciific resolution or turning rate of the screw.
The original screw may have good lead accuracy but backlash exists ,and even if adjustable for backlash,you just can't maintain it without constant adjustment.

RICH

[Katoh]

Gentlemen
I agree 100% and I see my questioned has been answered, but just to through a spanner in the works. For my own curiosity.
One, backlash easily can be adjusted for in Mach! Yes ball screws can be more accurate but a lead screw that cuts threads is not to bad as well you have to admit. The other point of friction Why don't we simply use a larger motor with a reduction to the screw, solve that problem.
Please don't take me wrong, I'm not trying to be difficult, its some answers I would like to know. Its OK doing something because you have been told that's the right way, but can the wrong become the right way?
Cheers
Katoh

[Tweakie.CNC]

Good points you have raised there Katoh.

I would like to know the answers as well. I must admit I know nothing about lathes but Hood is an expert, he will know.

Tweakie.

[Hood]

Backlash comp works but is no substitution for lack of backlash in the first place, backlash comp cant take out the issue of the axis being pulled. Yes more likely in a mill but still relevant on a lathe.

I think you may find that the accuracy of your leadscrew is not that great, especially as its a relatively cheap import if I recall correctly. Threads done on a manual lathe of that size  tend to be short so accuracy is not so important as its a short distance. Using CNC requires accuracy over a distance when turning and you may find your leadscrew is not that great.

Bigger motors will overcome friction but you also have to consider friction causes heat which will also affect accuracy.

People do use the leadscrews and are happy, I wouldnt be and wouldnt consider it but you may be happy.

Hood

[Katoh]

Good one Hood answered to a "T"
One thing I would like to bring up has anyone played around with the Igus polymer lead screw nuts? They claim zero backlash, dirt and dust resistant, and a long wearing life. I use The Igus Polymer linear s on my router and they are impressive, not cheap buy any means but work under a ton of dust and work well, maybe this could take another twist.
Katoh

Sorry Should have added this for anyone interested.
http://www.igus.co.uk/_Product_Files/Download/pdf/41_GL3_UK_DryLin_TR_RZ3.pdf

[Hood]

Plastic compresses and lathes can have high loads. Granted its a smallish lathe you have but still would be my concern.
Hood

[Katoh]

Hood Again I thank you for your patience in answering my questions, I can now easily conclude accuracy and reliability can only be achieved by using ball screws, well that's the way I have to go.
Many Thanks to All.
Katoh