RE: Easing Into CNC -- Ball Screws?

Comments mixed in below.

> From: "Paul R. Hvidston" <p.hvidston@...>

> I decided it was time to join the group since I'm considering
> moving towards making my RF-31 dual-use.

Welcome to the group. You came to the right place. There are a lot of very
knowledgeable individuals on this list and all are willing to share. This is
surely the best group that I have ever joined.

>I'm still wondering if I can "get away" with retaining the stock
> lead-screws and
> nuts for the near term. I understand some of the issues of backlash
> (climb-milling and accuracy), but given that I can minimize
> backlash with
> bearing pre-load and a well-adjusted nut (:-o), what else is
> there to worry
> about if using acme screws instead of ball screws? Friction?
> Would I need
> much bigger steppers to offset friction (eating up any savings in lead
> screws)? Your advice cheerfully accepted.

Most people who have tried this (I am not one of them), are using one of the
following solutions:

1) A cast nut from a material called "moglice". This nut is casted around
the screw, eliminating backlash. See the archives for more detail about
this.
2) A split nut (most of the time the existing nut). The two split halves are
either "pulled apart" or "pushed toghether" with some sort of screw
arrangement, taking up the backlash. Some mill / drills are sold with such
an arrangement already in place. I might be wrong, but I think the RF 31 is
supplied with it.
3) A split-nut similar to above, but with a spring between the two halves.
It is thus not fixed. The pressure of the spring pushes the halves apart,
taking up backlash. The limitation is of course that the maximun force that
can be transmitted before backlash sets in is equal to the spring force. The
higher this force, the higher the friction and the higher the wear.

The bigest problem with methods 1 and 2 above is that the leadscrew wears
more at certain positions than others. The maximun wear usually takes place
in the centre area, because more machining is done in this area. This
results in backlash in this area. Because of the uneven wear this backlash
cannot be eliminated, as it will result in a nut which is OK in some areas
but very stiff in others.

Ballscrews eliminates all of these problems, but at a much higher price.
Take your pick. I have a mill / drill and have decided to go for ballscrews
when I come to that stage. Not the super accurate, very expensive ones, but
ballscrews nevertheless. There are people on this list who used
Mcmaster-Carr ballscrews and made their own anti-backlash nuts by combining
two regular nuts is a similar fasion than points 2 and 3 above. Maybe it is
time for them to report back on their success.

Just my 2 cents worth.

Deon Styger