This tool is a variation on the traditional thread gauge adapted to FDM printing. One face of the threads has been aligned to the print plane to allow printing of threads finer than the diameter of your nozzle. The cone allows measurement of the following sizes: M4x0.7, M5x0.8, M6x1, M8x1.25, M10x1.5, M12x1.75, M16x2, M18x2.5. The slots in the sides correspond to the shown diameters and are oversized by 0.25mm.
The center is hollow, so stacking is an option if you have multiples.

For a more traditional thread gauge please see:
http://www.thingiverse.com/thing:153724

Credit where credit is due:
Mr. Smout (http://www.thingiverse.com/Smoutech/designs) put out the challenge to come up with a thread gauge and bolt checker so I am happy to oblige.