British Standard Pipe Thread

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

Added module to create british standard pipe thread. For now,
only the parallel version, not the tapered one.

BSPthread.scad

@@ -0,0 +1,86 @@
+/*
+Britisch Standard Pipe Thread
+++++++++++++++++++++++++++++++
+
+British Standard Pipe Thread is based on a triangular profile. The angle between flanks
+is 55 deg (symmetrical). We use the following nomenclature:
+
+- crest: The "summit" of a thread
+- groove: The lowest point in the valley of a thread
+- rcrest, rgroove: Radii of crest and groove, respectively
+- t = abs(rcrest - rgroove)
+- rpitch = mean of rmajor and rminor
+
+The crest and groove are both truncated by t/6. The radii at these flats are called
+rmajor and rminor.
+
+Note 1:
+
+The specification requires that the flats are smoothed by circles that tangentially
+match the flanks. We use a simpler version here: We create a triangular profile
+and truncate the crests but not the grooves.
+
+Note 2:
+
+The specification requires allowances. This module creates threads with the ideal
+rpitch for both external and internal threads (i.e., they just touch each other).
+This is exactly at the edges of the given allowances and will not reliably work
+in practice!
+*/
+
+use <thread_profile.scad>
+
+phi = 55;
+
+module BSPP_external_thread(pitch=2.309, turns=3, dminor=30.291, higbee_arc=45, fn=120)
+{
+    t = pitch / (2 * tan(phi/2));
+    rgroove = dminor / 2 - t / 6;
+    rcrest = rgroove + t;
+    section_profile = [[0, -pitch / 2], [0, pitch / 2], [+t, 0]];
+    rmajor = rcrest - t / 6;
+    intersection() {
+        straight_thread(
+            section_profile=section_profile,
+            higbee_arc=higbee_arc,
+            r=rgroove,
+            turns=turns,
+            pitch=pitch,
+            fn=fn);
+        cylinder(h=turns * pitch, r=rmajor, $fn=fn);
+    };
+}
+
+module BSPP_internal_thread(pitch=2.309, turns=3, dmajor=33.249, higbee_arc=45, fn=120)
+{
+    t = pitch / (2 * tan(phi/2));
+    rgroove = dmajor / 2 + t / 6;
+    rcrest = rgroove - t;
+    section_profile = [[0, +pitch / 2], [0, -pitch / 2], [-t, 0]];
+    rminor = rcrest + t / 6;
+    difference() {
+        rotate(180) // rotate by half a turn to fit external thread
+            straight_thread(
+                section_profile=section_profile,
+                higbee_arc=higbee_arc,
+                r=rgroove,
+                turns=turns,
+                pitch=pitch,
+                fn=fn);
+        cylinder(h=turns * pitch, r=rminor, $fn=fn);
+    };
+}
+
+// testing:
+
+intersection() {
+    color("Green")
+        translate([-50, 0, -50])
+            cube(100, 100, 100);
+                union() {
+                    BSPP_external_thread(pitch=2.309, turns=3, dminor=30.291,
+                                          higbee_arc=45, fn=120);
+                    BSPP_internal_thread(pitch=2.309, turns=3, dmajor=33.249,
+                                         higbee_arc=45, fn=120);
+                };
+};