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Your calculations seem to depend on the spindle turning an integral number of times the rate of the leadscrew. Since the lathe has a gearbox, that ratio may well be altered enabling cutting of threads not related to a 1.5mm pitch.
I vaguely recall seeing a photo of a SB threading dial attachment with multiple gears for just this situation although it was for metric threads on inch leadscrew.Here is one solution: http://www.practicalmachinist.com/vb/south-bend-lathes/cutting-metric-threads-156039/Gerrit
Hi Joco,I have a Hafco AL320G and IIRC the information in the User Guide is misleading.Some time back I decided to calculate what could and could not be done with the as provided change gears.Attached is a spreadsheet which I hope you find useful.RegardsBob