Parametric Adapter

// 3/4" GHT Female to 1/2" NPT Male adapter
// Dimensions are in millimeters.
// Requires the BOSL2 library in the OpenSCAD library path.

include <BOSL2/std.scad>
include <BOSL2/threading.scad>

$fn = 96;

// Thread standards
GHT_MAJOR   = 26.44;       // 3/4-11.5 NH/GHT nominal major diameter
GHT_PITCH   = 25.4 / 11.5;
GHT_LENGTH  = 18;

NPT_MAJOR   = 21.34;       // 1/2-14 NPT nominal major diameter
NPT_PITCH   = 25.4 / 14;
NPT_LENGTH  = 17;
NPT_TAPER   = 1/16;        // NPT taper, diameter change per unit length

BODY_TOP    = GHT_LENGTH + 3;
TOTAL_LENGTH = BODY_TOP + NPT_LENGTH;
BORE        = 12.7;       // nominal 1/2 inch flow passage

module outer_body() {
    // Broad hose-thread collar, with a short hex section for tightening.
    union() {
        cylinder(d=30, h=13, anchor=BOT);
        up(10)
            cylinder(d=32, h=8, $fn=6, anchor=BOT);

        // Shoulder and the smaller pipe-thread boss.
        up(GHT_LENGTH)
            cylinder(d1=30, d2=23.5, h=3, anchor=BOT);
        up(BODY_TOP)
            cylinder(d=23.5, h=NPT_LENGTH, anchor=BOT);
    }
}

module ght_female_thread_void() {
    // BOSL2 produces the helical internal thread form when internal=true.
    // The extra length at either end guarantees a clean through opening.
    up(-1)
        threaded_rod(
            d=GHT_MAJOR,
            l=GHT_LENGTH + 2,
            pitch=GHT_PITCH,
            internal=true,
            bevel=false,
            anchor=BOT
        );
}

module npt_male_thread() {
    // BOSL2's NPT primitive supplies the 1:16 tapered pipe thread.
    npt_threaded_rod(
        d=NPT_MAJOR,
        l=NPT_LENGTH,
        pitch=NPT_PITCH,
        taper=NPT_TAPER,
        bevel=false,
        anchor=BOT
    );
}

module adapter() {
    difference() {
        union() {
            outer_body();
            up(BODY_TOP)
                npt_male_thread();
        }

        // Continuous fluid passage.  It also removes the solid center of
        // the threaded primitives while leaving their thread crests intact.
        up(-1)
            cylinder(d=BORE, h=TOTAL_LENGTH + 2, anchor=BOT);

        // Female GHT thread cavity.
        ght_female_thread_void();
    }
}

adapter();