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

11) acme, as well as regular 60-degree screws generally have higher thread
count per unit of length, meaning more screw rotations are needed to advance
the screw the same distance compared to ball screws. This has several
advantages, one of which being that you can use a motor with less torque to
drive the screw. Another advantage is that you can use a motor with poorer
angular resolution to achieve the same precision as you would have with a
fancier motor and a ball screw. Of course, there are disadvantages as well.
It all depends on what you are trying to do...

Vlad

On 6/29/06, ballendo <ballendo@...> wrote:
>
> >In CCED, R Rogers <rogersmach@...> wrote:
> >/// What are the advantages of acme or 60 degree screws over
> >ballscrews in a CNC application?
>
> First, let me ask you a few questions:
> If all cnc machines would be better using ballscrews, why aren't
> they all using ballscrews???
> Why do screw manufactureres sell both types?
> Why do CNC machines come with both types?
>
> Let's list a few reasons:
> 1) Acme screws have better load capability for their size.
>
> 2) Acme sliding surfaces are less likely to fail catastrophically.
> (so where safety and maintennance are issues, you may find acme
> instead of B/S)
>
> 3) Acme sliding surfaces require less in the way of dust/swarf
> exclusion.
>
> 4) Acme screw/nuts do not rely on recirculation of the balls and
> therefore do not need/expect "clearing moves" (means moves of long
> enough travel often enough to prevent inaccuracies and wear caused
> by short movements. I already mentioned this one a couple times now.
>
> 5) Acme sliding surfaces can provide a smoother result since the
> effects of rolling balls--with rolled and other cheap B/S--do not
> show up in the work surface.
>
> 6) Acme screws can be run without lubrication which is sometimes
> necessary for cleanroom and vacuum environments.
>
> 7) Acme nuts may be cast inexpensively into shapes that save OEM's
> money and assembly time, and also reduce maintennance costs.(#1
> above is an additional factor here too!)
>
> 8) Potentially lower cost for many machine types (although as other
> replies here show; this is not the only factor. Which is what I
> already said!<G>)
>
> 9) Stub acme can have a higher column strength than a similar sized
> B/S in certain sizes. This can allow faster speeds, cheaper
> endmounts, and/or longer travels.
>
> 9A)The larger core in some sizes can also allow for larger end
> bearings with higher load capability, for a given size. (This is
> listed separately from #1 above because this load capability is not
> the screw itself, but it's end bearings.)
>
> 10) Acme screws/nuts are quieter. (Not all CNC machines are
> VMC's!)
>
> Are ten plus enough, or should I keep going?<G>
>
> Ballendo
>
>
> >>B wrote:
> >>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. "
>
> >/// So if someone building a linear CNC device has the choice
> >between the two, sometimes it's an "advantage" to use an ACME screw
> >over a ballscrew? When? Other than cost and availability, I can't
> >think of any.
> >
> > Ron
>
>
>
>
>
>
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--
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