When building the D-Bot CoreXY 3Dprinter by spauda01 (http://www.thingiverse.com/thing:1001065), you need to tap M5 theads into several V-slots.
To help me keep the tapping tool centered and vertical, I designed this little tool guide that slides on top of the 2040 Vslot ends where needed.
Designed for an M5 thread tapping tool with less than 6 mm diameter, will work with similar tools of similar diameters.
Very high quality, but wanted to make sure it can hold up to use. I used PLA on a cold build platform (Polycarbonate with thin layer white glue).
You can print it standing up, with the prongs facing up, then you need no support. But then the prongs break off easier. Or at least did so in my prototype prints.
You can print it on the side, rotate it 90 degrees in your slicer. Then you will need supports, but the prongs will not break off as easy. On the other hand it may split along the barrel if the tool is thicker than 6 mm in diameter.