/* [Gear Mesh Parameters] */
// Module of the gear pair (mm)
gear_module = 2.0;          // [1.0:0.1:5.0]

// Tooth count of pinion (Gear 1)
teeth_gear1 = 20;           // [12:1:35]

// Tooth count of wheel (Gear 2)
teeth_gear2 = 40;           // [20:1:80]

// Pressure angle (degrees)
pressure_angle = 20;        // [14.5, 20, 25]

// Face width (gear thickness, mm)
face_width = 8.0;           // [4:1:30]

// Bore diameter for both gears (mm)
bore_diameter = 6.0;        // [3:0.5:20]

// Keyway width (mm)
keyway_width = 2.0;         // [1:0.5:6]

// Keyway depth into hub (mm)
keyway_depth = 1.0;         // [0.5:0.1:4]

// Backlash allowance (mm)
backlash = 0.15;            // [0.0:0.05:0.6]

// Rotation angle to demonstrate meshing (degrees)
mesh_rotation = 0;          // [0:1:360]

/* [Visual & Details] */
// Gear 1 color
gear1_color = "RoyalBlue";

// Gear 2 color
gear2_color = "DarkOrange";

// Add lightening holes to the larger gear
lightening_holes = true;

// Number of lightening holes in Gear 2
hole_count = 5;             // [3:1:8]

// Diameter of lightening holes (mm)
hole_diameter = 11.0;       // [6:1:20]

// Show mounting shafts
show_shafts = false;

// Show pitch circle lines (wireframe indicators)
show_pitch_circles = false;

/* [Calculated Geometry] */
pitch_radius_1 = gear_module * teeth_gear1 / 2; // 20 mm
pitch_radius_2 = gear_module * teeth_gear2 / 2; // 40 mm
center_distance = pitch_radius_1 + pitch_radius_2; // 60 mm

// Position gears so the pitch point of contact is at [0, 0, 0]
gear1_pos = [-pitch_radius_1, 0, 0];
gear2_pos = [ pitch_radius_2, 0, 0];

// Angular alignment:
// Gear 1 has tooth centered along +X (pointing to Gear 2)
// Gear 2 has tooth space centered along -X (pointing to Gear 1) -> half-pitch offset = 180 / teeth_gear2
gear1_angle = mesh_rotation;
gear2_angle = (180 / teeth_gear2) - mesh_rotation * (teeth_gear1 / teeth_gear2);

// Math helper functions
function deg2rad(a) = a * PI / 180;
function rad2deg(a) = a * 180 / PI;

// Involute function: returns involute angle in degrees
function inv_deg(alpha) = (tan(alpha) - deg2rad(alpha)) * 180 / PI;

// Pressure angle at radius r given base radius rb
function alpha_at(r, rb) = acos(max(min(rb / r, 1.0), -1.0));

// Half tooth angular thickness at radius r in degrees
function tooth_half_angle(r, rb, rp, N, inv_alpha0, bl, m_val) =
    let(
        s_p = (PI * m_val / 2) - (bl / 2),
        psi_p = (s_p / (2 * rp)) * 180 / PI,
        inv_r = (r > rb) ? inv_deg(alpha_at(r, rb)) : 0
    )
    psi_p + inv_alpha0 - inv_r;

// 2D rotation helper
function rot2d(p, deg) = [
    p[0] * cos(deg) - p[1] * sin(deg),
    p[0] * sin(deg) + p[1] * cos(deg)
];

// Generate 2D boundary polygon for an involute spur gear
function spur_gear_2d_points(N, m_val, pa, bl, steps=6) =
    let(
        rp = m_val * N / 2,
        rb = rp * cos(pa),
        ra = rp + m_val,
        rf = rp - 1.25 * m_val,
        r_start = max(rb, rf),
        inv_a0 = inv_deg(pa),
        psi_start = tooth_half_angle(r_start, rb, rp, N, inv_a0, bl, m_val),
        pitch_deg = 360 / N,

        // Left involute flank: radius goes from r_start up to tip ra
        left_flank = [
            for (i = [0 : steps])
                let(
                    r = r_start + i * (ra - r_start) / steps,
                    psi = tooth_half_angle(r, rb, rp, N, inv_a0, bl, m_val)
                )
                [r * cos(-psi), r * sin(-psi)]
        ],

        // Tip arc point along centerline
        tip_arc = [ [ra, 0] ],

        // Right involute flank: radius goes from tip ra down to r_start
        right_flank = [
            for (i = [steps : -1 : 0])
                let(
                    r = r_start + i * (ra - r_start) / steps,
                    psi = tooth_half_angle(r, rb, rp, N, inv_a0, bl, m_val)
                )
                [r * cos(psi), r * sin(psi)]
        ],

        // Straight radial root flanks if root circle is below base circle
        left_root_pt = (rf < rb) ? [ [rf * cos(-psi_start), rf * sin(-psi_start)] ] : [],
        right_root_pt = (rf < rb) ? [ [rf * cos(psi_start), rf * sin(psi_start)] ] : [],

        // Root space arc between this tooth and adjacent tooth
        root_arc = [
            for (j = [1 : 3])
                let(ang = psi_start + j * (pitch_deg - 2 * psi_start) / 4)
                [rf * cos(ang), rf * sin(ang)]
        ],

        // Full single-pitch contour
        tooth_contour = concat(
            left_root_pt,
            left_flank,
            tip_arc,
            right_flank,
            right_root_pt,
            root_arc
        )
    )
    [
        for (k = [0 : N - 1])
            for (pt = tooth_contour)
                rot2d(pt, k * pitch_deg)
    ];

// Individual spur gear module
module spur_gear(
    teeth,
    mod_val,
    pa,
    width,
    bore_d,
    key_w,
    key_d,
    bl,
    has_holes = false,
    n_holes = 4,
    hole_d = 10
) {
    rp = mod_val * teeth / 2;
    rf = rp - 1.25 * mod_val;
    hub_radius = bore_d / 2 + key_d + 3.0;
    rim_inner_radius = rf - 3.5;
    hole_placement_radius = (hub_radius + rim_inner_radius) / 2;

    difference() {
        // Main gear tooth body
        linear_extrude(height = width, center = true, convexity = 12)
            polygon(points = spur_gear_2d_points(teeth, mod_val, pa, bl));

        // Center shaft bore
        cylinder(d = bore_d, h = width + 2, center = true, $fn = 48);

        // Standard rectangular keyway slot
        translate([-key_w / 2, 0, -(width + 2) / 2])
            cube([key_w, bore_d / 2 + key_d, width + 2]);

        // Lightening holes (for larger gear)
        if (has_holes && rim_inner_radius > hub_radius + hole_d) {
            for (h = [0 : n_holes - 1]) {
                rotate([0, 0, h * (360 / n_holes)])
                    translate([hole_placement_radius, 0, 0])
                        cylinder(d = hole_d, h = width + 2, center = true, $fn = 36);
            }
        }
    }
}

// Pitch circle reference ring for verification
module pitch_circle_ring(r, width) {
    color("DimGray", 0.4)
    rotate_extrude($fn = 96)
    translate([r, 0, 0])
    circle(r = 0.3, $fn = 16);
}

// Assemble the meshing gear pair
module gear_assembly() {
    // Gear 1 (Pinion - 20T)
    translate(gear1_pos)
    rotate([0, 0, gear1_angle]) {
        color(gear1_color)
            spur_gear(
                teeth = teeth_gear1,
                mod_val = gear_module,
                pa = pressure_angle,
                width = face_width,
                bore_d = bore_diameter,
                key_w = keyway_width,
                key_d = keyway_depth,
                bl = backlash,
                has_holes = false
            );
        if (show_pitch_circles) {
            pitch_circle_ring(pitch_radius_1, face_width);
        }
    }

    // Gear 2 (Wheel - 40T)
    translate(gear2_pos)
    rotate([0, 0, gear2_angle]) {
        color(gear2_color)
            spur_gear(
                teeth = teeth_gear2,
                mod_val = gear_module,
                pa = pressure_angle,
                width = face_width,
                bore_d = bore_diameter,
                key_w = keyway_width,
                key_d = keyway_depth,
                bl = backlash,
                has_holes = lightening_holes,
                n_holes = hole_count,
                hole_d = hole_diameter
            );
        if (show_pitch_circles) {
            pitch_circle_ring(pitch_radius_2, face_width);
        }
    }

    // Optional mounting shafts and test base
    if (show_shafts) {
        shaft_length = face_width + 12;
        base_thickness = 4;
        base_width = center_distance + 35;
        base_height = max(pitch_radius_1, pitch_radius_2) * 2 + 15;

        // Baseplate
        color("SlateGray")
        translate([gear1_pos[0] + center_distance / 2, 0, -face_width / 2 - base_thickness / 2])
            cube([base_width, base_height, base_thickness], center = true);

        // Shaft 1
        color("Silver")
        translate(gear1_pos)
            cylinder(d = bore_diameter, h = shaft_length, center = true, $fn = 36);

        // Shaft 2
        color("Silver")
        translate(gear2_pos)
            cylinder(d = bore_diameter, h = shaft_length, center = true, $fn = 36);
    }
}

// Render the assembly
gear_assembly();