Spiral routing jig - GIF

[NortonDommi]

"did changing spool ratios generate a different helix? How was dividing managed?"

I spent some time trying to find the instructions,(with diagrams),that I have somewhere without success. From memory the drive plate had multiple divisions like a dividing plate and there were different size pulleys to alter feed rate. They had a sliding guide bar for doing tapers and the more complicated ones could trace curves as well.
Not a lot to them but turned out lovely repeatable work fast.

I only know this because I had a friend,(now passed),who made high end custom furniture and he used one to great effect.