This is an anti-backlash nut for an 8mm 4-start ACME leadscrew. It's based on Tyler Montbriand's excellent TSM thread library. I had to tweak the thread measurement by 0.1mm increments to get the correct fit. You will almost certainly have to do the same. It also requires a suitable spring to keep the two halves tensioned against each other. The included STL is for demonstration purposes only.