README
Tr
Entity
T
EntityId
- ::newRawEntity::new
- ::to_stepRawEntity::to_step
T
Ref
T
UnknownEntity
V
fmt_num
V
step_str
V
unknown_entity_witness

#38 — Planetary Gearbox

This folder contains a working example that generates a STEP model for a print-in-place herringbone planetary gearbox: a sun gear, three orbiting planet gears, and an enclosing inward-facing ring gear.

It is a port of go-bjk/cmd/make-planetary-gearbox and Blackjack's HerringboneGear node (herringbone_gear.lua).

The intent is that you can run the code here to emit a STEP file, open it in a CAD viewer, and/or import it into your slicer to 3D print and iterate.

#What this example demonstrates

porting a mesh generator to a B-Rep model
defining each gear's outline once and sweeping it through numElbows + 1 twisted extrusions
internal (ring) gear envelopes generated from a mating planet gear
exporting a clean multi-body STEP solid you can reuse in other tools

#Why it is small

Blackjack evaluates a herringbone gear as a triangle mesh in (numElbows + 1) * 2 * resolution horizontal slices. This example builds the 2D outline a single time and only instantiates the numElbows + 1 elbow cross-sections, so the STEP output is smaller than the equivalent STL (about 36 MB vs 64 MB for the default parameters).

The number of cross-sections adapts to the twist: each elbow is subdivided so that adjacent cross-sections differ by at most 24° of profile rotation. Bands whose four corners are not coplanar are split into two planar triangles, so every face in the STEP file is planar and the mesh tessellates watertight. The default parameters need only the elbow boundaries; small modules with a long gear are subdivided automatically (--module 0.5 --gearLength 30 twists 142° per elbow and is subdivided 6x).

#Orientation

The STEP is written Z-up, with the gear axes vertical, so it sits flat on the build plate: 30 mm print height and a 136 mm footprint. The gears are built with their axes along +Y and the whole design is rotated +90° about X at export time. Pass --yUp to keep the native Y-up orientation.

#Scaling the design

Every length parameter is in millimetres. To get a uniformly scaled copy of a model, scale gearLength, outerThickness, tolerance and holeRadius by the same factor as module; angles and counts stay unchanged. For example, a 6x smaller copy of the default gearbox:

./run-example.sh 38 --module 0.5 --gearLength 5 --outerThickness 1.6667 \
  --tolerance 0.041667 --holeRadius 1.6667

Note that -gl is not a flag here (the short form is -g); use the long options above.

#Parameters

The parameters mirror the Go program:

Go flag	This program	Meaning
-mod	--module	Gear module (tooth size)
-gl	--gearLength	Gear length
-ha	--helixAngle	Helix angle (0–45°)
-hr	--holeRadius	Hole inradius
-ht	--holeType	None, Squared, Hexagonal, Octagonal, Circular
-ne	--numElbows	Number of herringbone elbows
-nt	--numTeeth	Sun gear teeth (default 12; must make sun + ring divisible by the planet count)
-planets	--numPlanets	Number of planet gears
-planetTeeth	--planetTeeth	Planet teeth (0 = same as the sun)
-pa	--pressureAngle	Pressure angle
-ringTeeth	--ringTeeth	Ring teeth (0 = sun + 2·planet)
-tol	--tolerance	Target flank clearance
-outerThickness	--outerThickness	Ring wall thickness
-res	--resolution	Points per curved section of the outline
-o	--out	Output file