Re: anti-back lash nuts

I have watched a lot of debates over ballscrews vs leadscrews, and best I
can figure, there are thee main reasons ballscrews are better: 1) Ballscrews
can be made with much greater precision than leadscrews, and 2) ballscrews
wear much more slowly than leadscrews, because 3) the use of ball bearings
reduces friction enough to allow much tighter tolerances. But there is no
such thing as zero backlash between moving parts, and I suspect what that
really means is negligible backlash. But again, far as I can tell, all
ballscrews are not created equal, and you can buy a cheesy ballscrew with
lots of backlash. Otherwise we wouldn't see people talking about replacing
the ball bearings. Any backlash due to wear would have the same problem as
leadscrews: more wear at the center. So replacing the ball bearings would
not solve that problem. And with ballscrews there is no backlash adjustment.
You get what you get. So you have to rely on the manufacturer's reputation.
Either that or empirical measurement, which is rarely practical before you
buy. So unless you are prepared to spend the money for a new brand-name
ballscrew, you're still at risk. It's really a question of whether it's
worth the investment, how much you will use the machine, how much precision
you really need, etc. But for CNC, if you choose leadscrews, I would suggest
that backlash adjustment capability is pretty much essential.
--
Phil Mattison
http://www.ohmikron.com/
Motors::Drivers::Controllers::Software

----- Original Message -----
From: Michael Fagan <woodworker88@...>
To: <CAD_CAM_EDM_DRO@yahoogroups.com>
Sent: Wednesday, November 07, 2007 6:02 PM
Subject: Re: [CAD_CAM_EDM_DRO] anti-back lash nuts


> The main problem with modifying the nuts is that in return for reducing
> backlash, the nut must fit tightly on the screw. On a new machine, such
as
> the Taig CNC mills which have anti-backlash nuts from the factory, this is
> not as much of a problem, since the screws are in good shape. However, on
> an older, used machine, the screw may be worn, possibly significantly.
Most
> of this wear will be in the center of the travel. If you tighten the nuts
> in the center, they will bind dangerously near the ends of the travel,
where
> the wear is less. If you tighten at the ends, then it will still be loose
> in the center. IMO, the effort and expense of replacing the screws with
new
> ones and modifying the nuts is not significantly less than acquiring and
> retrofitting anti-backlash ballscrews.
>
> On Nov 7, 2007 12:58 AM, zs6bxi <bobt@...> wrote:
>
> > Apart from alot of lurking, I think that is the first time that i have
> > posted to the group. I have an elderly Boxford lathe that I am moving
> > towards using CAD-CAM on (mainly for taper turning screw cutting etc.)I
> > have pulled the cross slide feed screw and nut out and am looking to
> > see what can be done to improve the back lash. (its not bad for its age
> > and being a lathe as well)I have seen several references to slitting of
> > nuts, adding of extra nuts and spring pressure and using other sections
> > of treaded rod as a nut. But what I would like to know and get advice
> > on before I start slitting my expensive to replace nuts is, how
> > sucessful have these methods been ?
> >
> > Regards Bob
> >
>