This is yet another OpenSCAD thread library, and it was more fun to write it than to find a library that did exactly what I wanted. It's designed to be fairly fast: it generates the whole thread polyhedron as a single piece.

Thread profile is completely flexible. You can specify a counterclockwise polygon of 2D points for the profile or you can use ISO thread profiles.

There are two modules exposed:
rawThread(profile, d=diameter, h=height[, lead=lead, $fn=segmentsPerRotation]): draw thread with specified profile; if lead is omitted, it is calculated from the profile
isoThread(d=majorDiameter, h=height, pitch=pitch[, angle=angle, internal=true/false, lead=lead, starts=starts, $fn=segmentsPerRotation]): draw metric ISO thread; default angle is 30; default is external thread (male); internal thread is designed for subtraction
If you want to use UTS measurements, you can omit pitch and instead do tpi=TPI, and then use hInch, dInch and leadInch instead of the metric versions. I haven't tested the UTS stuff yet.

For left-handed-thread, just mirror things.

If for some reason you want access to the points/faces of the thread polyhedron, these are available from the threadPoints() and extrusionFaces() functions.

The extrusionFaces(pointsPerSection, numSections) function is useful for more general extrusions than threads. It generates a set of triangular faces that can be fed into the faces= argument of polyhedron() for a tube-like non-self-intersecting extrusion with two end-caps and constant number of points on each cross-section. For a demo of how extrusionFaces() can be used to do that, see twisty.scad. There is also tubeFaces() which omits the end-caps, to be used for closed knots and the like.

Updates:

• Dec 14, 2017: Fixed minor diameter calculation.
• May 10, 2017: Add starts option for multi-start thread.
• May 16, 2017: Fix tpi bug. Thank you to AlexeyBobkov.
• May 31, 2017: Refactor functions, making for user-usable extrusionFaces() and tubeFaces()