Involute Profile · Compound Trains · 3D Print Export
Units
Motor RPM
Gear Glossary & Guide
This is a quick reference for the dimensions and concepts you'll see in the generator. The terms are standard mechanical engineering — once you know them, every gear catalogue and CAD tool uses the same language.
Gear anatomy
Teeth (N)
Number of teeth on the gear. More teeth = smoother motion but bigger gear.
Module (m)
Tooth size. Pitch diameter = N × m. A module-2 gear has teeth twice as big as a module-1 gear. Two gears must share the same module to mesh.
Diametral pitch
The imperial equivalent of module. DP = 25.4 / module.
Pitch diameter
The "imaginary circle" where two meshing gears touch in theory. Pitch Ø = N × m.
Outside diameter
The tip-to-tip diameter — the actual outside size. Outside Ø = (N + 2) × m.
Root diameter
The diameter at the bottom of the tooth valleys. Root Ø = (N − 2.5) × m.
Pressure angle
The angle of the tooth flank. 20° is standard; 14.5° is older/vintage; 25° is for high-load applications.
Backlash
Clearance between meshing teeth so they don't bind. 0.15–0.25 mm works well for FDM 3D prints.
Centre distance
Distance between two meshing gears' axles. Always = (N₁ + N₂) × m / 2.
Undercut
When teeth are too few (≤17 at 20° PA), the flanks get "bitten". Still works, but weaker — used for teaching purposes.
Gear trains
Simple train (no compounding): A → B → C. Each pair has its own ratio, but the total ratio is just N_first / N_last (the in-betweens cancel out). Direction reverses with each mesh.
Compound train: Two gears fixed to the same axle rotate together. So A drives B; B and C share an axle; C drives D. Ratios multiply: a 4:1 stage followed by another 4:1 stage = 16:1 overall. This is how gearboxes get high reductions.
Print & assemble
The exported SVGs use 1 unit = 1 mm. In Tinkercad: Import → set units to mm → drag onto workplane → set extrusion height (5 mm is a good starting thickness). The gear body imports as a solid; centre holes and cutouts import as separate shapes. Select each, set to Hole, then Group to subtract.
For compound stages, both gears on a shared axle must have the same centre hole diameter so they fit on the same axle.
Motor presets
TT Yellow
The classic kit motor. ~200 RPM at 6V, very high torque, internal 1:48 reduction. Big body (~70mm long). Shaft: 5.4mm double-D.
130 Hobby
Bare 130-size DC motor. Very fast (~9000 RPM at 6V), low torque, no internal gearbox — your gear train provides all the reduction. Small (~25mm). Shaft: 2mm round.