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

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

>
> The problem I see with acme screws is wear - you simply cannot stop
> the wear. Sure, you can hide it - with anti-backlash nuts, but you
> can't stop it. Anyone that has run an older bridgeport knows about
> this. The middle area of the screw is worn more than the ends - shows
> up as backlash. So, further complicating the issue with acme screws
> and wear, is it is un-even wear.
>
> Some antibacklash nuts will automagically adjust for this, some
> won't. Some use springs to spread two nuts. This will certainly hide
> the backlash, but it won't fix it - it's sort of like back lash comp
> in the CNC control - hides it, but doesnt fix it. Back to the spring
> loaded A/B nuts, while they do hide it under light loads, if the
> cutting forces exceed the spring tension on the nut, backlash comes
> back into the picture again, only this time it is a problem
> under "special conditions" making it for a real nightmare to figure
> out.
>
> I really don't think either type of screw system is immune to chips
> and lack of lubrication like others have stated, that is just
> nonesense. Both need proper lubrication, both should be kept clean
> from debris. Just because big chips won't get "inside" the nuts,
> small particles can and will, and those are the ones that cause the
> damage.
>
> So, if you are okay with "hiding problems", Acme lead screws are fine
> as long as you know they will wear and that A/B nuts will do a pretty
> good job of hiding the wear and resulting backlash, they won't fix the
> wear.
>
> Chris (Certainly not a "High Quality" poster)

Hi Chris,

Theoretically, your auto engine wear is 98% from starting the car.
Once started, there is little if no wear because there is oil
lubricating all the surfaces. many engine parts are sliding bearings,
not roller bearings.

ACME uses sliding surfaces and not roller surfaces.

So, theoretically, you are correct.

With the above known, once can also predict that 98% of the wear of a
screw is from moving it while dry.

But, there are other considerations. First, you are also correct that
no wiper system is perfect, and also that there is a range of
particles that are horrible. Particles (chips) that are too large to
get between the surfaces are not important, they are brushed away.
particulate that is smaller than the oil film are also harmless as
they can do nothing.

but, particles that are the same size as the oil film are the ones
that do the wear. as the film is lost and thins, those suspended
particles become more agressive.

Synthetic oils are extreemly good at bonding to the metal and
preventing wear, so the wear on todays machines can be dramatically
reduced.

Ball screws on the other hand are run either dry or lubricated.
lubricated can hold the dust so the balls crush it. that crushing can
cause wear. some materials are extreemly agressive. PCB dust is
horrible. most metal chips are not agressive and will either be
pushed away or stick and be crushed.

In all cases, we try to make sure chips do not get under the surfaces.

Nothing is immune, but when you know the industry, you can design for
that, and as Ballendo highlighted, selection should be application driven.


Dave


ps: you're a high quality poster in my book !