B/S vs. acme was Re: What makes a machine CNC ready?

Dave,

Excellent post. To say that ballscrews beat Acme's in every
situation is simply untrue.

The previous poster's assertion that backlash is "the problem" with
acme is similarly over-generalised.

There are obviously MANY different means to accomodate acme backlash
that find WIDE industrial acceptance in motion control, including
CNC. Things like CNC engravers almost always use acme screws with
A/B nuts.

And while no one would compare a MaxNC to a Bridgeport, these tools
also use A/B nuts and acme screws. (There are at least several
thousand MaxNC machines out there, and at least some of them are
being used successfully in "production" commercially.)

As you've so eloquently written, the application should drive the
choice. That is a very good approach!

Not some over-generalisation like "Ballscrew good. Acme baaad."

Great post!

Ballendo

P.S. I'd like to especially single out the statement that "the worst
ballscrew is better than the best ACME screw any day of the week for
CNC use."
This is absolutely untrue. CNC is a VERY wide arena, from light duty
slow-moving machines to heavy duty fast-moving machines. The "worst
ballscrew" is one designed to be used where its EFFICIENCY is more
important than its ACCURACY. You find these in surplus venues and in
linear actuators for moving heavy loads. Perhaps also in your Car's
steering setup. Ballscrerws of this grade are also found when one
purchases cheap ballscrews from "normal" bearing houses. (I've used
these, and have commented that they CAN be a good choice; just know
what you're getting.) But that's not the point Ron has promoted
here. Let's get back to that:

The BEST acme screw will be EXTREMELY accurate, and will come
equipped with an EXCELLENT no-backlash nut. You might find this on a
wafer-fab CNC machine costing many millions of dollars.

In fact, there are MANY places in the CNC world where a mfr without
an overbearing need to conserve $$$ on screws will CHOOSE acme for
ITS advantages over ballscrews...

A statement like this "worst B/S is better than best Acme" does
little to promote actual learning and truth about the differences
and advantages of EACH type of screw.



--- In CAD_CAM_EDM_DRO@yahoogroups.com, "turbulatordude"
<dave_mucha@...> wrote:

>
> --- 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
>