A simple set of guide holes for thread tapping. I've added space for magnets on top so it snaps into place underneath my tap holder, allowing me to easily grab the tap holder and guide block together whenever I want to tap something.

I sized the holes to be a tight clearance fit for the tap cutting threads according to this chart: [https://www.datametal.com/wp-content/uploads/2018/03/Metric-Tap-and-Clearance-Drill-Sizes.pdf](https://www.datametal.com/wp-content/uploads/2018/03/Metric-Tap-and-Clearance-Drill-Sizes.pdf)

Due to the nature of FDM 3D printing, the holes will come out slightly undersized. You can correct for this in Cura by setting a negative horizontal hole expansion (you'll need to tune this for your printer, for me it's about -0.2 mm). However, I left mine slightly undersized so the threads just barely cut into the guide hole. This ensures the tap is held firmly and doesn't wander as you tap into your work piece. Just make sure you're tapping straight the first time you use the guide hole.

OnShape document available here.