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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 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

OTHER DESIGN CONSIDERATIONS AND THE PROS AND CONS

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 dimensions.

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 leadscrew nuts

• "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 well.

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.