/* [Knob dimensions] */
outer_diameter = 30; // [20:0.5:60]
total_height = 14; // [8:0.5:30]
top_fillet_radius = 2; // [0.5:0.25:4]
/* [Nut pocket and bore] */
nut_across_flats = 10; // [8:0.1:16]
nut_pocket_depth = 5; // [2:0.1:8]
through_hole_diameter = 6.5; // [3:0.1:10]
// Optional additional clearance; zero preserves the requested dimensions.
nut_fit_clearance = 0; // [0:0.05:0.5]
hole_fit_clearance = 0; // [0:0.05:0.5]
/* [Diamond knurl] */
grooves_per_direction = 36; // [18:1:60]
knurl_depth = 1; // [0.3:0.1:1.5]
helix_angle = 45; // [25:1:60]
groove_included_angle = 90; // [60:5:110]
/* [Appearance and resolution] */
knob_color = "Silver"; // [Silver, SlateGray, SteelBlue, DarkOrange]
circumference_segments = 180; // [96:12:288]
helix_slices = 96; // [48:12:180]
fillet_segments = 32; // [12:4:64]

/* [Hidden] */
$fn = circumference_segments;
outer_radius = outer_diameter / 2;
knurled_height = total_height - top_fillet_radius;
helix_twist = knurled_height * tan(helix_angle) / outer_radius * 180 / PI;
boolean_overlap = 0.02;

// Revolved quarter-circle gives a true 2 mm outer top fillet,
// while keeping the bottom flat at Z=0 and the top at Z=14.
module knob_blank() {
    rotate_extrude(convexity=10, $fn=circumference_segments)
        polygon(points=concat(
            [[0,0], [outer_radius,0], [outer_radius,knurled_height]],
            [for (arc_step=[1:fillet_segments])
                let(arc_angle=90*arc_step/fillet_segments)
                [outer_radius-top_fillet_radius+top_fillet_radius*cos(arc_angle),
                 knurled_height+top_fillet_radius*sin(arc_angle)]],
            [[0,total_height]]
        ));
}

// The transverse V is widened by 1/cos(helix_angle) so its
// included angle normal to the helical groove is 90 degrees.
module groove_profiles() {
    cutter_extension = knurl_depth + 2;
    cutter_half_width = cutter_extension * tan(groove_included_angle/2) / cos(helix_angle);
    for (groove_index=[0:grooves_per_direction-1])
        rotate(groove_index*360/grooves_per_direction)
            polygon(points=[
                [outer_radius-knurl_depth,0],
                [outer_radius-knurl_depth+cutter_extension,-cutter_half_width],
                [outer_radius-knurl_depth+cutter_extension,cutter_half_width]
            ]);
}

module diamond_knurl_cutters() {
    // Cuts stop at the fillet tangent, leaving the entire rounded edge smooth.
    for (handedness=[-1,1])
        linear_extrude(height=knurled_height,
                       twist=handedness*helix_twist,
                       slices=helix_slices, convexity=20)
            groove_profiles();
}

module nut_pocket() {
    translate([0,0,-boolean_overlap])
        cylinder(h=nut_pocket_depth+boolean_overlap,
                 d=(nut_across_flats+nut_fit_clearance)/cos(30), $fn=6);
}

module clearance_bore() {
    translate([0,0,-boolean_overlap])
        cylinder(h=total_height+2*boolean_overlap,
                 d=through_hole_diameter+hole_fit_clearance, $fn=96);
}

color(knob_color)
difference() {
    knob_blank();
    diamond_knurl_cutters();
    nut_pocket();
    clearance_bore();
}