gears/rack/herringbone_rack.py → OBJECT_OT_herringbone_rack (object.herringbone_rack, "Herringbone Rack")
A straight rack sheared into a V along X: the bottom half shears one way from Z=0 to Z=width/2, the top half shears back the other way from Z=width/2 to Z=width — the gear-of-infinite-radius limit of herringbone_gear.md's V-twist, the same way helical_rack.md is the limit of helical_gear.md's twist.
Hand convention
Same rule as the helical rack: mating with a herringbone pinion needs the same hand — not the opposite hand the external-external gear-gear rule would suggest, because a rack contacts its pinion at a different relative angular position than two side-by-side external gears do. See README.md and helical_rack.md for the full derivation and empirical verification. Herringbone parts have no net axial thrust regardless of hand, but hand still determines which way the V opens and must match the mating pinion.
Properties
| Property | Type | Default | Range | Notes |
|---|---|---|---|---|
target | Object pointer | — | gears with bmech_module | Match Target; runs sync_helical_same, and if the target has bmech_tooth_count, also updates tooth_count_rack |
module | Float (mm) | 1.0 | 0.1–50.0 | Must match the mating pinion's module |
pressure_angle_deg | Float (°) | 20.0 | 10–45 | |
helix_angle_deg | Float (°) | 20.0 | 1–45 | Half-angle of the V; 15–30° typical for FDM |
hand | Enum | RIGHT | RIGHT / LEFT | Bottom half shears toward +X as Z increases when RIGHT |
length_mode | Enum | TOOTH_COUNT | TOOTH_COUNT / MATCH_GEAR | Same semantics as straight_rack.md |
tooth_count_rack | Int | 10 | 2–1000 | Used directly in TOOTH_COUNT mode; fallback in MATCH_GEAR mode if no target is set |
width_mm (labeled "Total Width") | Float (mm) | 14.0 | 2–100 (soft) | Full extent along the V axis — each half is width_mm/2 |
No bore properties, no n_slices — see Build method below for why.
Hand and Match Target freezing
Same freeze pattern as herringbone_gear.md: module/pressure_angle_deg freeze whenever any target is set; helix_angle_deg freezes only when the target stamps bmech_helix_angle_deg; hand freezes with helix_angle_deg except when gear_matching.hand_target_ambiguous(True, target) is true — i.e. when this rack (herringbone-style) is matched against a plain-helical target, since a plain helical pinion only meshes one half of this rack's V at a time, and which hand is correct depends on which half. See README.md for the full rationale.
Build method
No n_slices property, unlike herringbone_gear.py. Each half of the V is its own linear (affine) shear of Z within its own range — see helical_rack.md for the full derivation of why an affine shear needs no intermediate Z-layers to be exact, unlike a gear's rotational twist. A herringbone rack therefore needs exactly three layers total: Z=0 (shear 0), Z=width/2 (peak shear), and Z=width (shear back to 0) — not 2*n_slices-1 layers approximating a curved surface the way herringbone_gear.py needs.
peak_shear = (width/2) * tan(helix_angle)applied as x' = x + hand_sign * shear at each of the three layers, with hand_sign = +1 for RIGHT, -1 for LEFT — same sign convention as every other hand-aware generator in this family.
Reuses straight_rack.py's tooth-profile math verbatim (build_rack_tooth_profile/build_rack_profile, duplicated here per this family's per-file mesh-helper convention), tiled across X. Two straight side-quad strips connect bottom→middle and middle→top, each exact for the same affine reason described in helical_rack.md.
Swept tooth counts 2–50 × pressure angles 14.5–30° × helix angles 5–44° × both hands: 0 non-manifold edges, 0 zero-area faces on the evaluated mesh.
Hand fix — why this needed correcting after initial release
Same root cause and fix as helical_rack.md: bmech_sync_target originally called sync_helical_opposite by analogy with the gear-gear external-external rule. Verified wrong by building a 30°-helix herringbone pinion with a same-hand rack (0 mm³ mesh interpenetration, boolean EXACT INTERSECT) versus an opposite-hand rack (~388 mm³ of gross overlap) at matching module/PA/helix. Fixed by swapping to sync_helical_same — the shear math itself (each half's peak_shear formula) was already correct; only the hand-sync direction and the property/doc wording needed correcting.
Phase alignment
Same cosmetic X phase-align as every rack in this family — see helical_rack.md#phase-alignment for the full derivation. Confirmed unaffected by the V-shear: alignment is computed at the Z=0 slice, and the same-hand fix above keeps that phase in sync with the gear's own twist across the whole width once aligned there.
Output
One object, name HerringboneRack (or HerringboneRack.001, etc.), stamped:
gear_matching.stamp_gear(obj, "herringbone_rack", module, pressure_angle_deg,
helix_angle_deg=helix_angle_deg, hand=hand)Meshes with a herringbone pinion (herringbone_gear.md) of the same module, pressure angle, and helix angle, and the same hand.