Re: [CAD_CAM_EDM_DRO] Rolled vs. Ground ballscrews

cool - thanks for the point by point answers.
i'm getting the perspective that the scale of the
machine matters a lot here on what decision to take.
on the small end, where rotational diameters approach
closer to zero, and forces are on a scale where the
bug can lift 1000 times his own weight, there i think
a screw has no serious wear issues. i know my sherline
is doing very well after much use. and i do consider a
brass nut a consumable item whenever it will wear out.
backlash is adjustable before every run. i haven't
tested how long it lasts, but i swear i can get
backlash to .001 on the x and y on a sherline! i mean
the stepper can't move a click without me seeing the
tool move. that may not sound super if you're used to
big expensive machines, but it's quite good on a cheap
little desktop.
in such a case, where i get that good a performance
and have no wear problems i guess it must be cuz the
machine itself is just plain tiny? at this level i
can't justify ballscrews. so maybe the wear issues are
just much more significant with a larger machine and
therefore do call for the ballscrews?

the native resolution thing, though, i do care.
perhaps the real best way is as suggested, a true
linear scale directly reading actual position...but
$$$$
without that, one is extrapolating. that makes me
uncomfortable. i'll use a water level rather than a
transit, for instance.

--- Les Newell <lesnewell@...> wrote:

>
> >1 - there can be no tighter bearing than flat metal
> to
> >metal
> >
> >
> True. However you cannot have a true metal-metal
> contact on both flanks
> of the screw without the whole thing binding up. You
> have to have
> clearance for the screw to turn. Acme screws also
> wear fast. You will
> soon have backlash in the nut. After a while you
> will also get wear on
> the screw itself so you will not be able to
> compensate by tightening up
> the nut.
>
> >2- as soon as you introduce additional elements,
> like
> >rollers, you are dealing with multiplied tolerances
> of
> >all the parts (read as ERRORS of all the parts)
> >
> >
> True. That is why screws are manufactured to very
> close tolerances.
> Balls can be manufactured to extremely tight
> tolerances. In fact Newall
> use balls packed into a tube as the measuring
> element for a DRO system.
>
> >3- ball screws/nuts immediately reduce native
> >resolution cuz they can not have as many tpi as a
> fine
> >thread screw.
> >
> >
> Who cares what the native resolution of the screw
> is. You can soon
> change it with suitable gearing or encoders on the
> motors.
>
> >4- they are expensive, sound hella sexy in a
> >specsmanship pissing contest.
> >
> >
> Yup they are expensive. What you get for your money
> is _consistent_ very
> low backlash.
>
> Les
>
>
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