Helical Annulus Gear

gears/ring/helical_annulus_gear.py → OBJECT_OT_helical_annulus_gear (object.helical_annulus_gear, "Helical Annulus Gear")

Helical internal gear — twisted involute teeth cut into the bore of a solid ring. See README.md for family-wide conventions and annulus_gear.md for the swapped addendum/dedendum geometry shared by all three annulus generators.

This is where the hand convention flips

Read this even if you've already read the helical/herringbone external gear docs. External-external helical pairs need opposite hands; a helical annulus and its mating helical pinion need the SAME hand. Right annulus meshes with a right pinion, not a left one. This is the single most common mistake this library's Match Target system exists to prevent — which is why picking a target here runs sync_helical_same, not sync_helical_opposite.

Properties

Build method

No boolean, no Solidify modifier — same direct-bmesh rewrite as the straight annulus gear (annulus_gear.md), extended across twisted Z-slices. This used to be a solid outer cylinder minus a boolean-DIFFERENCE helical cutter (EXACT solver); measured ~10-40x slower in testing (330ms-5.0s for tooth counts 8-100 vs 9-133ms direct).

_build_helical_annulus_solid builds:

• The inner (toothed) wall: n_slices twisted Z-layers of the tooth profile, twist computed relative to z=0 (hand_sign * z * tan(helix_angle) / pitch_radius) — simpler than the old boolean cutter's twist formula, which had to be relative to the cutter's own padded bottom face to avoid a boolean seam artifact that no longer exists here. hand_sign is +1 for RIGHT, -1 for LEFT, same convention as every other helical primitive.
• The outer (cylindrical) wall: a PLAIN, UNTWISTED, independently- spaced circle (outer_segs points, only 2 Z-layers). The outer surface of a helical annulus doesn't twist — only the inner teeth do. As with the straight annulus gear, this is deliberately NOT built with one point per tooth-profile point at a matching angle (that reintroduces zero-area triangles at the tooth profile's collinear dedendum-circle point insertion).
• The two end caps: bmesh.ops.triangle_fill fed the boundary edges of both the (twisted) inner ring and the (untwisted) outer ring together at each end — see annulus_gear.md for the fuller rationale.

Pressure angle clamp has extra margin here too: _derived() clamps to gear_matching.max_pressure_angle_deg(...) - PA_TRIANGLE_FILL_MARGIN_DEG (0.2°), not the theoretical limit itself — triangle_fill is more fragile right at that boundary than the old EXACT-solver boolean was. Swept tooth counts 8-100 × pressure angles 10-45° × both hands: 96/96 clean.

Total twist (width_mm * tan(helix_angle) / pitch_radius) and normal module (module * cos(helix_angle)) are shown as read-only info lines.

Panel warnings

Only tip_r <= 0 → "Module too large — tip radius ≤ 0" (ERROR, blocks). No rim-thickness warning, matching the straight annulus gear.

No print-in-place clearance logic in this generator — it produces a rigid single-part ring meant to be printed separately from its mating pinion.

Output

One object, stamped:

gear_matching.stamp_gear(body, "helical_annulus", module, pressure_angle_deg,
                          tooth_count=tooth_count,
                          helix_angle_deg=helix_angle_deg, hand=hand)

Meshes with a helical pinion (helical_gear.md) of the same module, pressure angle, and helix angle, and the same hand.