Re: What makes a machine CNC ready?

--- In CAD_CAM_EDM_DRO@yahoogroups.com, R Rogers <rogersmach@...> wrote:

>
> The problem with ACME screws in a CNC application is backlash. CNC

machines travel faster and much further than their manual
counterparts. Adjust all of the backlash out of an acme screw and the
rotational friction increases. Then when the middle of the screw wears
slightly and that backlash is removed, the screw will lock up on the
ends of the travel.

>
> Many Manufacturers are over-zealous in their ratings. The worst

ballscrew is better than the best ACME screw any day of the week for
CNC use. If you have to move something, would you rather slide it or
roll it? Now over a long period of time and as quickly as possible..

>
> Ron

Generalities all to often overlook specifics.

On many machines, a ball screw will be rendered useless or
ineffective. ACME, even with it's potential wear problems are the
first choice for some applications.

Any time grit can work its way between the screw and the ball, the
screw or ball may suffer and the problems are numerous.

just like the potential of wear of a leadscrew in one section that is
used most often, a ballscrew can suffer the same fate, just from a
different cause.

The application needs to drive the selection. No one would recommend
using a Vee thread on a Bridgeport, but on a PCB machine where there
is almost no resistance and the forces are higher for the screw than
for moving the table, the Vee thread can obtain the same accuracies as
any other screw. Certainly better accuracies than the process requires.

For things like plasma tables where the grit is everywhere, the
ballscrews much be protected, and sliding things can actually keep the
grit out.

Ditto for wood working. Many years ago, we replaced ball slides in
favor of polymer bushings on rails. Resistance dropped, accuracy
improved and both surface finish and part precision were better than
with ball slides and ball screws.

Vee threads have a horrible resistance curve, the proverbial hockey
stick, but as long as you keep the forces away from the curve, they
work fine.

Once you get into metal cutting, I thing we all agree that Vee screws
are well past the point of use. ACME could be used, and ballscrews
are usually the absolute best choice.

So, the application should drive the choice.

Dave