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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
A PERFECT CIRCLE
By now, you're quite happy with the accuracy
you can achieve with your machine, but every time you cut a circle, it
looks like it somehow got "squished" in one way or another.
More specifically, it seems to only get
"squished" at the North, South, East, and West points of the circle.
Why? You still need to address backlash.
There are some ways around it (again, this
assumes you've accepted where your "tolerance" for "perfection" lies).
If you bought the "harbor freight mini-mill"
there's really not much you can do easily to reduce backlash. You can
certainly modify the brass nuts, but this really is a short-term solution
that only works OK until the threads of the nut dissolve from
Using the example above, you bought the
standard ballscrew and ballnut from your favorite catalog. Unfortunately,
the ballnut has no "anti-backlash properties". So that needs to be figured
The typical methods to reduce backlash are the following:
single nut, "standard balls" (non-preload)
single nut, using oversized balls (single
double nut, using springs or spacers (double
Leadscrews (ACME, trapezoid, etc.)
single nut, "standard" threads of different
double nut, using springs or spacers
hybrid design using a "cam"
The purpose of the methods are to create as
much persistent contact between the "threads" of the screws with the nuts
without inducing too much friction or requiring too much force to move the
The ballnut you have has no rated backlash
number but is probably in the neighborhood of 0.005" - 0.015". (This is
just a hunch that's been shown with our test system).
Let's now say you manage to get your
backlash down to an acceptable tolerance and your circles now look like
The important point here is to make sure
that both leadscrew accuracy and backlash should be addressed in order to
bring you CNC happiness.