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7x10, 7x12, 7x14 Mini-Lathe Information
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BACKLASH AND LEAD ACCURACY
What is backlash
and why does it matter? What about lead accuracy? How does it affect me
and my machine? Or in other words... "In search of 'perfection'"
Also: Ballscrew and ACME Leadscrew Accuracy
- BACKLASH IN A MANUAL WORLD
- CNC TO
- OTHER DESIGN CONSIDERATIONS AND THE
PROS AND CONS
OTHER DESIGN CONSIDERATIONS AND THE PROS
Assuming performance ratings being equal,
with ballscrews, your design and installation choices are limited
primarily the manufacturer's ballnut dimensional specs.
The problem is that there's not a lot of
room in your mini-mill with which to play around. Thus, material removal
of your machine may be required to ensure no clearance issues exist and
table travel is not reduced.
With ACME leadscrews (or another other type
of leadscrew), you can often times make your own nuts since there are
commercially available taps (and they're expensive!) or ways of making
taps from extra leadscrew stock. The advantage here is that you can choose
from materials such as brass, bronze, and acetal as well as make the nuts
in almost any shape. You can also buy the nuts either standard or
anti-backlash but again you would be limited by the manufacturer's
A quick comparison between the two are the
following (this is neither an exhaustive nor detailed list):
efficiency: ballscrews are ~90% efficient
(but are prone to backdriving - sometimes a good thing!) whereas
leadscrews are ~30% efficient (but have more thread selection which means
easier handwheel handling by a human operator)
cost: ballscrews are generally more
expensive than leadscrews
accuracy: while leadscrews can have
severe lead errors, they generally are on par with ballscrews
anti-backlash nuts: both offer solutions to
combat and reduce backlash. there are more options available with
"smoothness": ballscrews with ballnuts offer
a different kind of "smoothness" than that of a leadscrew with acetal
nuts. If a human operator is involved, leadscrews will offer more
"feedback" than the ballscrew setup.
Having addressed the pros and cons of just
the screw/nut as one sub-assembly of the entire table assembly, it's
important to also talk about gibs and table alignment. We'll do so briefly
since they can both be written about extensively.
The points to remember are the following:
Even with a ballscrew setup, if the
components are misaligned, you will experience binding or looseness during
certain areas of travel. This not only can damage your components, it can
also lead to positional errors, "missed steps", and a slew of other
problems (mostly noticeable during CNC).
If the gibs are too tight and you have a
more compliant material like acetal being used for nuts, there could be
some induced flex and "stiction" before enough force is applied to move
the table. This will generally lead to the problems mentioned above as
The lesson here is to maintain your table
adjusted "just right" in order to reap the benefits of any ball/leadscrew
setup whether you still maintain manual operation, go CNC only, or both.
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