threadlib emancipation

+++++++++++++++++++++++

threadlib is now its own project => remove Waterrocket Fins from this
repo.

README.rst

@@ -1,14 +1,81 @@
-PET-Bottle Waterrocket Fins
+threadlib
 +++++++++++++++++++++++++++
 
-This is a CAD-model of fins intended to be screwed onto a PET bottle to create a
-water rocket. To create the rocket, you need:
+threadlib is a library of standard threads for OpenSCAD.
 
-- bottle with PCO-1881 threading (e.g. a 1.5-l Coke bottle)
-- this part
-- Gardena tap connector G 1"
-- launcher
+In contrast to other thread libraries, it does not make you look up diameters
+and pitches and maybe even thread-profiles in tables and norms: It has these
+tables built in.
 
-(+ water + pressurized air).
+Creating a thread is as simple as
+
+.. code-block:: OpenSCAD
+
+        thread("G1/2-ext", turns=10, higbee_arc=20);
+
+to create a British Standard Pipe parallel external thread. Furthermore,
+threadlib allows for production tolerances by choosing thread dimensions well
+inside the ranges allowed by the norms.
+
+
+Usage
+===========================
+
+To create a bolt (without head) with 10 turns of G1-inch thread:
+
+.. code-block:: OpenSCAD
+
+        bolt("G1", turns=10);
+
+A nut:
+
+.. code-block:: OpenSCAD
+
+        nut("G1", turns=10, Douter=40);
+
+Note that for a nut you also have to specify an outer diameter. The inner
+diameter is implicitly given by the thread designator ("G1" in this case).
+
+If you only need the threads alone:
+
+.. code-block:: OpenSCAD
+
+        thread("G2 1/2-ext", turns=5);
+
+Then, add the support you want. In the simplest case, a cylinder (which is what
+nut(...) uses):
+
+.. code-block:: OpenSCAD
+
+        specs = thread_specs("G2 1/2-ext");
+        P = specs[0]; Rrot = specs[1]; Dsupport = specs[2];
+        section_profile = specs[3];
+        H = (5 + 1) * P;
+        translate([0, 0, -P / 2])
+            cylinder(h=H, d=Dsupport, $fn=120);
+
+Here, we have used the function thread_specs(...) to look up the threads
+specifications - including the recommended diameter of the support structure.
+
+
+Extensibility
+===========================
+
+Don't find some of the threads you need for your project? Don't worry: You can
+add your own:
+
+.. code-block:: OpenSCAD
+
+        use <threadlib/threadlib.scad>
+
+        MY_THREAD_TABLE = [
+                           ["special", [pitch, Rrot, Dsupport,
+                           [[r0, z0], [r1, z1], ..., [rn, zn]]]]
+                           ];
+
+        thread("special", turns=15, table=MY_THREAD_TABLE);
+
+Care to share? Safe others from repeating the valuable work you have already
+accomplished and get the fame you deserve: Send in your tried and tested threads
+for addition to threadlib!
 
-Use at your own risk.