sorry to but in on old thread,
I often make variable pitch bottle screws/worms/scrolls.
I generate program in excel then save as *.txt file and load into Mach 3 and cut on 4th axis stepper motor fixed to gearbox with chuck on it.
Works well, but program is a little time consuming to generate(30 mins or so)and tweak.
Time to cut each worm if shape is awkward can be up to 6 hours 120mm dia x 600 long.