Forward gears use helical cut gears, which automakers prefer to use in manual gearboxes because they’re smoother and can withstand more force. Also, rather importantly for this question ...

or helical gears, which have angled teeth, has been a complex, time-consuming process. Gear skiving, an advanced technique that uses a specialized cutting tool called a skiving cutter, has emerged ...

[Maciej Nowak] demonstrates just how to produce your own gears with a homemade cutting tool. The cutting tool for the job is an M16 machine tap, chosen for the smaller flutes compared to a hand tap.