The MPCNC Anti Backlash solution is simple and cheap.

Backlash is a mechanical problem typically experienced by any machine that uses a threaded rod for linear motion. Basically, a moving part is connected to a lead screw (threaded rod) using a captive nut of the same thread as the screw. The problem is that there is usually a tiny space between each thread on the nut and its corresponding thread on the screw – see upper left. This tiny space causes a discontinuity or gap whenever the direction of the turning screw changes.

Backlash is described on Wikipedia as “lost motion in a mechanism caused by gaps between the parts.” When you are doing 3D printing or CNC machining, backlash can cause noticeable flaws in the work.

According to the Liutaio Mottola web site, there are three possible solutions: ignore it, compensate in software whenever you change direction, or pre-loading. Pre-loading is applied by Ryan Zellars in the MPCNC anti backlash solution. The plastic Z-axis coupler (in blue above) is designed to accommodate two nuts and a spring, in addition to the threaded rod. (An excellent example of software compensation is provided at CNC in the workshop which is where I found the good graphic demonstrating backlash above.)

MPCNC Anti Backlash Solution – How it Works

The Mostly Printed CNC Z-axis connection uses 5/16” threaded rod, as well as a 5/16” coupling nut and regular nut. Pre-loading is done with a small spring between the nuts. Pre-loading means using the spring to push each nut in the opposite direction so that there is always a proper loading of at least one nut, regardless of the direction of travel. In effect, you are creating a split nut. Hats off to Ryan for his MPCNC anti backlash approach.

I picked up the zinc-coated threaded rod and coupling nut (7/8” long as specified) at Lowes. Finding the right sized compression spring was a bigger challenge. I ended up buying a bag assortment of 101 springs for $7 at Princess Auto, and sure enough there were a few of exactly the right size. I now have a container of 100 more springs for other projects!