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What is backlash?

With any screw driven mechanical positioning system, there is often some degree of slop or “backlash“, including the one used in the Mill One. To explain further, backlash occurs when there is a gap between the threads of the nut and the lead screw and the nut is allowed to move within the gap which is present. With most people, the level of precision that the Mill One provides is more than enough, but for those who want to push their machine further, or want to need their Mill One extremely high precision work, this is an important topic to discuss.

We tested the backlash on our ACME nuts by attaching a dial indicator to the Mill One’s gantry and running the gantry back and forth. We chose a point to call zero on the dial indicator, and moved the gantry past zero, moved it back, then moved back even more, and then moved it forward again. The gap between the zero and where the needle landed was our backlash. A better explanation can be found here:

We tested this way for both a brand new stock ACME nut, as well as after running for many cycles. We ran the gantry back and forth for about 8 hours, with an additional load simulated by using bungee cords as to wear the nut as far as possible.

The results of this test, are as follows:

  • Brand new stock ACME nut: 0.001″ or less in backlash
  • Stock ACME nut after 2-3 hours: 0.002″to 0.003″ in backlash
  • Stock ACME nut after 6 hours: 0.003″ in backlash
  • Stock ACME nut past 8 hours: 0.003″ in backlash

Based on these results, we can see that the backlash of the ACME nut had gone from just under 0.001″ to around 0.003″. Just to put that into perspective, 0.003″ is approximately the thickness of a sheet of paper. For most projects, this is a great level of precision. But for something like PCB milling, where the width of each trace can be less than 0.006″, 0.003″ is a big number.

Over the last few months we have been testing anti-backlash nuts on the Mill One, which “preloads” two sides of the ACME nut to eliminate the gap which causes backlash. Using the same tests, the anti backlash nuts manage decrease backlash to less than 0.001″, greatly improving its accuracy. This resulted in parts coming out with much better dimensional accuracy, with tolerances of +/-0.002″ (0.05mm) or better being easily achievable.

Here’s an example of a test cut we milled from some brass:

Because of these good results and since the upgrade is simple to do and fairly inexpensive, we have created kits to allow users to install their own anti backlash nuts. You can order a kit here: We’re really happy to make this upgrade available to let users take their projects to the next level. We will continue to do tests with the anti backlash nuts with a variety of projects, so make sure to check out the blog to find out more!