Involute cylindrical gears to ISO 53 / 54 / 21771, with the true trochoidal root a hob actually cuts.
Largest fillet of the c*=0.25 profiles; best root strength. Default choice.
Shift has to stay above x min or the rack undercuts the flank, and low enough that the tip keeps 0.2 m = 0.40 mm across it. The two limits close on each other as the tooth count drops, which is what makes a small pinion hard.
A plain bore needs a grub screw, a press fit or adhesive to carry torque.
| Reference diameter d | 40.00 mm |
| Outside diameter dₐ | 44.00 mm |
| Root diameter d_f | 35.00 mm |
| Centre distance a_w | 60.00 mm |
| Ratio | 2.000 40:20 |
| Contact ratio ε_γ | 1.64 ok |
| Base diameter d_b | 37.588 mm |
| Transverse module m_t | 2.0000 mm |
| Transverse pressure angle α_t | 20.000° |
| Base helix angle β_b | 0.000° |
| Normal tooth thickness s_n | 3.142 mm |
| Tip tooth thickness s_a | 1.390 mm |
| Bottom clearance c | 0.500 mm |
| Rack fillet radius ρ_fP | 0.760 mm |
| Ratio u | 2.0000 |
| Reference centre distance a | 60.000 mm |
| Working centre distance a_w | 60.000 mm |
| Working pressure angle α_wt | 20.000° |
| Transverse contact ratio ε_α | 1.635 |
| Overlap ratio ε_β | 0.000 |
| Total contact ratio ε_γ | 1.635 |
| Tip clearance | 0.500 mm |
Nominal geometry, for machining or resin. Every surface moves in by this much, so teeth get thinner and bores get wider from the one number. It applies to STL only; STEP keeps the nominal geometry, because a CAD file that carries one printer's shrinkage is wrong everywhere else.
The DXF is the part neither STL nor STEP carries: two correctly cut gears still will not mesh if their keyways go on the shafts at the wrong relative angle. It draws both bores 1:1 at the real centre distance, at the angles that put a tooth in a space.
One external pair spans roughly 1:1 to 8:1 before the wheel gets unwieldy.