> Maudslay also devised an ingenious method that used two imperfect screws to guide the tool cutting a third. Equal gears turned both guide screws at the same speed. A bar joined their two nuts, with the tool fixed at its midpoint, so the tool's position was the average of the two nut positions. Where one guide ran ahead and the other lagged, their errors partly cancelled. Each guide contributed only half its error to the new screw.
Actually, this invention is Leonardo Da Vinci's! Here's what his screw-copying machine looked like.
https://cdn.britannica.com/39/60539-050-BCB3019D/machine-Leo...
https://media.sciencephoto.com/image/c0661830/800wm/C0661830...
The screws can also be taken out and flipped end-over-end, averaging out the errors even further.