Thread Reaming Tool

These tool is designed for "cleaning up" M3 to M6 holes. When 3d printing these, the threads are often too tight, or the bolt does not fit.  You could also use this on a regular, threadless, printed hole, but I would not recommend at as the cutting tip gets clogged quickly.

The only hardware required is a hex head bolt and a bolt of a matching size. To make this function as intended, use a dremel or similar to cut away a notch from the bolt. This creates a cutting tool 

If your bolt head is slightly too big for the handle, scale it up by a few percent until it fits your hardware. Scaling may be needed especailly if your hardware is cheap.