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 Charts

1. PREFACE
2. INTRODUCTION
3. BACKLASH IN A MANUAL WORLD
4. CNC TO THE RESCUE?
5. EXPECTATION
6. A PERFECT CIRCLE
7. OTHER DESIGN CONSIDERATIONS AND THE PROS AND CONS
8. CONCLUSION
9. RESOURCES

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 wear.

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 out.

The typical methods to reduce backlash are the following:

Ballscrews

• single nut, "standard balls" (non-preload)

• single nut, using oversized balls (single nut preload)

• double nut, using springs or spacers (double nut preload)

Leadscrews (ACME, trapezoid, etc.)

• single nut, "standard" threads of different classes

• 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 assembly.

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 this:

Much better!

The important point here is to make sure that both leadscrew accuracy and backlash should be addressed in order to bring you CNC happiness.