This ought to be simple so I'm probably missing something. I'm trying to cut a 1.5mm thread on an ML7 with 8TPI lead screw. I have a 127 tooth gear but no 21 tooth. Duncan Munro's Ml7 Gear calculator suggests 60 on the spindle and 127 on the lead screw with various idlers between. The is not enough space between spindle and lead screw to make that work. Via a Google search I found a suggestion taken from Martin Cleeve's book for a compound train using the 38 tooth and 65 tooth gears. Once again I can't make the train fit the space available.
My memory says that I have cut metric threads in the past but I probably shouldn't trust that.
What am I missing?