Summary

Allows creation of standard nuts and bolts or allow threads to be included in projects.
THIS IS OLD VERSION, please see http://www.thingiverse.com/thing:311031

Instructions

top of file includes calls to make bolts: M10x16, M8x16, M6x12 nuts: M10, M8, M6. simply uncomment a line and build then print, or create your own sizes or include into your own project so you can have threads.
Photos should show black and white nuts and bolts in M6, M8 and M10 sizes, black versions are standard bought steel, white versions are ones I printed in white ABS.
I have a Reprap Metric Mendel Prusa with LM8UU bearings, fishbone wade extruder, own design hot-end, MKII hot bed. Only got it going yesterday, not yet calibrated awaiting micrometer from ebay.
This is my first design in OpenSCAD, maths in not my strong point, but I think these are as close to accurate ISO Metric as I can get.

Hot-end has 0.5mm extrusion hole, used 0.2mm layers, I had to go down to 5mm/s speed to get these printed, I have not yet tried smaller sizes.

OK, so that was a shock, knocked up the original file quickly for self-use, uploaded as an after thought, did not expect that much interest!
There have been some concerns with CGAL rendering times with bolts.
So, see new version IsoThread_20120823.scad.
I have moved positional calculations into variables to they are now calculated only one per thread part and I have now included quality tables.

Results:
CGAL Rendering times
DELL Vostro Desktop - Intel i5-2400 @ 3.10 GHz - 16GB RAM

Original 15/07/2012
Windows 7 Home Premium - OpenSCAD 2012.04.01 (git c869c6e):
M10 nut 7 secs
M8 nut 9 secs
M6 nut 4 secs
M10x16 5 mins 35 secs
M8x16 8 mins 43 secs
M6x12 7 mins 27 secs
Ubuntu 10.04 LTS - OpenSCAD 2012.02.28:
M10 nut 4 secs
M8 nut 4 secs
M6 nut 2 secs
M10x16 2 mins 52 secs
M8x16 4 mins 36 secs
M6x12 3 mins 45 secs

New 23/08/2012
Windows 7 Home Premium - OpenSCAD 2012.04.01 (git c869c6e):
M10 nut 9 secs
M8 nut 10 secs
M6 nut 3 secs
M10x16 2 mins 16 secs
M8x16 2 mins 7 secs
M6x12 1 mins 4 secs

I may re-visit again later, I have an idea that 'may' work that could make it faster.

OpenSCAD ISO metric thread library / functions