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

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)