Helical gears use an involute tooth that’s been swept along a helix. At any cross-section perpendicular to the gear axis, the gear profile is exactly the same as a straight spur gear – so a helical gear can be modelled as a loft between the top & bottom faces. The sketch defining the tooth profile gets offset by an additional amount, corresponding to the desired helix angle.
Both of these are spur differentials, printed about 6 months apart.
The white one was designed when I was new to both 3d printing and gears, so the involute is not quite right and the tolerances are sloppy. There is a lot of play and occasional jamming, and it was constructed using screws fixed with loctite.
The orange/blue one is more recent and uses my helical gear model. There is just enough clearance between all rotating parts to allow smooth rotation without any significant backlash. The nuts are all press-fit and sit flush with external surfaces – the shafts are designed such that the screws can be tightened in place without loctite. Overall a much better model!