mandolin-neck-fender.py

"""
Parametric Fender-style bolt-on mandolin neck in CadQuery.

Units: millimeters.
Default design intent:
- 13.875 in / 352.425 mm mandolin scale
- 22 frets
- ~10.25 in / 260.35 mm nut-to-pocket-end neck length
- Fender-style flat bolt-on heel
- Radiused fretboard, default 12 in / 304.8 mm radius
- Simple non-angled solid headstock with 8 tuner holes

Run:
    python mandolin_fender_style_neck.py

Exports:
    mandolin_bolt_on_neck.step
    mandolin_bolt_on_neck.stl
    mandolin_neck_pocket_cutter.step

CadQuery install example:
    conda install -c conda-forge cadquery
"""

from __future__ import annotations

import math
from dataclasses import dataclass

import cadquery as cq
from cadquery import Vector, Edge, Wire, Solid

MM_PER_INCH = 25.4


@dataclass(frozen=True)
class Params:
    # Core mandolin layout
    scale: float = 13.875 * MM_PER_INCH          # 352.425 mm
    frets: int = 22
    nut_to_heel_end: float = 10.25 * MM_PER_INCH # 260.35 mm

    # Fretboard
    fretboard_radius: float = 12.0 * MM_PER_INCH # 12 in radius
    fretboard_thickness_center: float = 5.5
    fretboard_overhang_after_last_fret: float = 6.0
    nut_width: float = 29.0
    fretboard_end_width: float = 38.0
    fret_slot_width: float = 0.55
    fret_slot_depth: float = 1.25

    # Neck shaft and heel
    neck_top_z: float = 16.0                    # underside to board bottom at nut/shaft
    nut_neck_width: float = 26.0
    shaft_end_width: float = 35.0
    heel_width: float = 39.0                    # pocket should be slightly bigger than this
    heel_length: float = 55.0
    heel_thickness: float = 19.0                # flat heel thickness before fretboard
    heel_pilot_dia: float = 2.4
    heel_pilot_depth: float = 14.0

    # Headstock, deliberately simple so you can reshape later
    headstock_length: float = 88.0
    headstock_width: float = 48.0
    headstock_thickness: float = 12.0
    tuner_hole_dia: float = 6.2

    # Nut
    nut_thickness_x: float = 3.0
    nut_height: float = 6.5
    nut_depth_y: float = 31.0

    # Export quality
    stl_tolerance: float = 0.05
    stl_angular_tolerance: float = 0.15


P = Params()


def fret_position(scale: float, fret_no: int) -> float:
    """Distance from nut to fret, equal temperament."""
    return scale - (scale / (2 ** (fret_no / 12.0)))


def lerp(a: float, b: float, t: float) -> float:
    return a + (b - a) * t


def width_at_x(x: float, x0: float, x1: float, w0: float, w1: float) -> float:
    t = max(0.0, min(1.0, (x - x0) / (x1 - x0)))
    return lerp(w0, w1, t)


def fretboard_section_wire(x: float, width: float, thickness_center: float, radius: float) -> Wire:
    """Closed Y/Z wire at length position X. Flat bottom, radiused top."""
    half = width / 2.0
    sag = radius - math.sqrt(radius * radius - half * half)
    edge_z = thickness_center - sag

    p1 = Vector(x, -half, 0.0)
    p2 = Vector(x, half, 0.0)
    p3 = Vector(x, half, edge_z)
    p4 = Vector(x, -half, edge_z)
    crown = Vector(x, 0.0, thickness_center)

    edges = [
        Edge.makeLine(p1, p2),
        Edge.makeLine(p2, p3),
        Edge.makeThreePointArc(p3, crown, p4),
        Edge.makeLine(p4, p1),
    ]
    return Wire.assembleEdges(edges)


def neck_back_section_wire(x: float, width: float, top_z: float, center_back_z: float = 0.0) -> Wire:
    """Closed Y/Z wire for a simple C-ish neck back with flat top."""
    half = width / 2.0
    shoulder_z = top_z * 0.35
    shoulder_y = half * 0.92

    p1 = Vector(x, -half, top_z)
    p2 = Vector(x, half, top_z)
    p3 = Vector(x, shoulder_y, shoulder_z)
    p4 = Vector(x, -shoulder_y, shoulder_z)
    back = Vector(x, 0.0, center_back_z)

    edges = [
        Edge.makeLine(p1, p2),
        Edge.makeLine(p2, p3),
        Edge.makeThreePointArc(p3, back, p4),
        Edge.makeLine(p4, p1),
    ]
    return Wire.assembleEdges(edges)


def make_fretboard(p: Params) -> cq.Workplane:
    last_fret_x = fret_position(p.scale, p.frets)
    fb_len = min(p.nut_to_heel_end, last_fret_x + p.fretboard_overhang_after_last_fret)

    sections = []
    for x in [0, fb_len * 0.25, fb_len * 0.5, fb_len * 0.75, fb_len]:
        width = width_at_x(x, 0, fb_len, p.nut_width, p.fretboard_end_width)
        sections.append(fretboard_section_wire(x, width, p.fretboard_thickness_center, p.fretboard_radius))

    solid = Solid.makeLoft(sections)
    board = cq.Workplane("XY").add(solid).translate((0, 0, p.neck_top_z))

    # Cut fret slots down through the radius. These are shallow visual/locator grooves,
    # not prescriptive luthier fret tang dimensions.
    for n in range(1, p.frets + 1):
        x = fret_position(p.scale, n)
        if x > fb_len:
            continue
        w = width_at_x(x, 0, fb_len, p.nut_width, p.fretboard_end_width) + 4.0
        cutter = (
            cq.Workplane("XY")
            .box(p.fret_slot_width, w, p.fret_slot_depth, centered=(True, True, True))
            .translate((x, 0, p.neck_top_z + p.fretboard_thickness_center - p.fret_slot_depth / 2.0 + 0.2))
        )
        board = board.cut(cutter)

    # Dot markers as small shallow inlay discs. These intersect the curved top slightly.
    marker_frets = [5, 7, 10, 12, 15, 17, 19]
    for n in marker_frets:
        if n >= p.frets:
            continue
        x1 = fret_position(p.scale, n)
        x2 = fret_position(p.scale, n + 1)
        xm = (x1 + x2) / 2.0
        dots_y = [-5.0, 5.0] if n == 12 else [0.0]
        for y in dots_y:
            dot = (
                cq.Workplane("XY")
                .center(xm, y)
                .circle(1.6)
                .extrude(0.7)
                .translate((0, 0, p.neck_top_z + p.fretboard_thickness_center - 0.25))
            )
            board = board.union(dot)

    return board


def make_neck_core(p: Params) -> cq.Workplane:
    shaft_end_x = p.nut_to_heel_end - p.heel_length

    sections = []
    for x in [0, shaft_end_x * 0.33, shaft_end_x * 0.66, shaft_end_x]:
        width = width_at_x(x, 0, shaft_end_x, p.nut_neck_width, p.shaft_end_width)
        sections.append(neck_back_section_wire(x, width, p.neck_top_z))

    shaft = cq.Workplane("XY").add(Solid.makeLoft(sections))

    heel = (
        cq.Workplane("XY")
        .box(p.heel_length, p.heel_width, p.heel_thickness, centered=(False, True, False))
        .translate((shaft_end_x, 0, 0))
    )

    neck = shaft.union(heel)

    # Fender-like 4 pilot holes in the heel bottom. These are pilot holes in the neck,
    # not body clearance holes.
    screw_xs = [shaft_end_x + p.heel_length * 0.34, shaft_end_x + p.heel_length * 0.74]
    screw_ys = [-p.heel_width * 0.27, p.heel_width * 0.27]
    for x in screw_xs:
        for y in screw_ys:
            pilot = (
                cq.Workplane("XY")
                .center(x, y)
                .circle(p.heel_pilot_dia / 2.0)
                .extrude(p.heel_pilot_depth)
            )
            neck = neck.cut(pilot)

    return neck


def make_headstock(p: Params) -> cq.Workplane:
    L = p.headstock_length
    W = p.headstock_width
    t = p.headstock_thickness

    # Simple Fender-ish paddle outline, non-angled. Nut is at x=0, headstock extends negative.
    outline = [
        (-L, -W * 0.30),
        (-L * 0.82, -W * 0.48),
        (-L * 0.25, -W * 0.44),
        (0, -15.0),
        (0, 15.0),
        (-L * 0.18, W * 0.32),
        (-L * 0.78, W * 0.45),
        (-L, W * 0.30),
    ]
    hs = cq.Workplane("XY").polyline(outline).close().extrude(t)

    # Blend transition into nut end with a small rectangular tongue.
    tongue = cq.Workplane("XY").box(10.0, 28.0, t, centered=(False, True, False)).translate((-5.0, 0, 0))
    hs = hs.union(tongue)

    # Eight inline tuner holes. Change to 4+4 if you prefer a Gibson/mandolin look.
    start_x = -L + 14.0
    spacing = 8.8
    y = W * 0.20
    for i in range(8):
        x = start_x + i * spacing
        hole = cq.Workplane("XY").center(x, y).circle(p.tuner_hole_dia / 2.0).extrude(t + 1.0)
        hs = hs.cut(hole)

    return hs


def make_nut(p: Params) -> cq.Workplane:
    return (
        cq.Workplane("XY")
        .box(p.nut_thickness_x, p.nut_depth_y, p.nut_height, centered=(False, True, False))
        .translate((-p.nut_thickness_x, 0, p.neck_top_z))
    )


def make_pocket_cutter(p: Params) -> cq.Workplane:
    """Body-side helper: neck pocket cutter/block, with a small fit clearance."""
    clearance = 0.25
    depth = p.heel_thickness
    shaft_end_x = p.nut_to_heel_end - p.heel_length
    return (
        cq.Workplane("XY")
        .box(p.heel_length + 0.5, p.heel_width + clearance * 2.0, depth, centered=(False, True, False))
        .translate((shaft_end_x - 0.25, 0, 0))
    )


def build_model(p: Params = P) -> cq.Workplane:
    neck = make_neck_core(p)
    fretboard = make_fretboard(p)
    headstock = make_headstock(p)
    nut = make_nut(p)
    return neck.union(fretboard).union(headstock).union(nut)


if __name__ == "__main__":
    model = build_model(P)
    pocket = make_pocket_cutter(P)

    cq.exporters.export(model, "mandolin_bolt_on_neck.step")
    cq.exporters.export(model, "mandolin_bolt_on_neck.stl", tolerance=P.stl_tolerance, angularTolerance=P.stl_angular_tolerance)
    cq.exporters.export(pocket, "mandolin_neck_pocket_cutter.step")

    print("Exported mandolin_bolt_on_neck.step")
    print("Exported mandolin_bolt_on_neck.stl")
    print("Exported mandolin_neck_pocket_cutter.step")
    print(f"Scale: {P.scale:.3f} mm")
    print(f"Nut to heel/pocket end: {P.nut_to_heel_end:.3f} mm")
    print(f"Pocket end to nominal bridge: {P.scale - P.nut_to_heel_end:.3f} mm")
    print(f"Fretboard radius: {P.fretboard_radius:.3f} mm")