Re: [CAD_CAM_EDM_DRO] ACME ( was Re: leadscrew, acme screw, threaded rod, ballscrew

David A. Frantz wrote:

>High guys;
>
>Maybe I'm missing something here but doesn't the statement below
>specifiy almost exactly the same tolerance?
>

The numbers and the way they are published can be way misleading.
According to what I have been told, you can not get any error more than
.0006 in any given inch before the screw is "Corrected"-Something about
how the screw is made and the machine making it recalculates where it
should be "rolling" to keep a good screw, a good screw. That is somehow
why they end up with +/-.003" PER FOOT. And they can give you .001" per
foot if you need it.

So, I am under the understanding that these numbers are not anywhere
near the same.

I can tell you this much, I NEVER measure anything that goes anywhere
near .002" over or under with the Kerk style screw and nut. At least not
on anything I've been able to measure up to 24" with a digital caliper
(biggest I can get my hands on) Obviously, a caliper is not THE most
accurate to use, but close enough for what I do. Still, never ever seen
anything like .009" ! Yikes !

I should not have made the "Slop" statement at all in regards the
accuracy of a screw. My gripe has always been: there is no good accurate
screw for Acme and CNC, unless you like to "fiddle". I don't. If I have
a bad Antibacklash nut, and I haven't really, All I have to do is ORDER
one ! $28-$50. Done. NO backlash.

>
>Further backlas is something that is specified over a foot. This
>sounds like lead error to me and is a function of the lead screw not the
>nut.
>

I think this too is subject to scrutiny,,,, "specified over a foot" ??

I mean every CNC controller with basic backlash really only handles: "If
at any given time I reverse direction, and I need to "take up" .xxxx" of
dead space, so, I need to ADD that to the reversed direction of the
toolpath I am running". In other words, what is the distance in inches
of the "Take-up".

Backlash settings in a CNC control are pretty much a "patch" to make
something really bad a little better. I know of some Control developers
who only added it because people thought it would fix all their
problems.. Suprise ! It won't.

Get rid of backlash, get anti-backlash nuts and build a tight system. If
you need even tighter, spend more. You can get it. Then you won't be
disapointed when climb milling circles. But you can't get it by taking
cheap stuff and fiddling with it.

CL

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