(mouse over the "Question marks" for add'l info)
Having trolled the internet for new and interesting info I stumbled onto a post claiming the Atlas Lathes 10 and 12 could be modded to feed at 1/2 of the advertised rate so instead of .0042 you can get .0021! Also, the threading capabilities are for the most part still easily accomplished. for instance if you want 8tpi? you set it for half that so 4tpi is actually 8tip. What you lose is anything less than 8tpi why? because the lowest number on the rack is 4... and double that would be 8. If you must do some threading below that number you can always just throw the factory gears back on.
So here is the formula.
On the banjo. exists 3 gears. the rear most is a 48 and the compound gear in between is a 40.(double stacked)
What you need Gears: 1x 54tooth 1x32 tooth and 1x64 tooth .
The arrangement is:
replace the 48Tooth Stacked gears with the Single 54T and place the Bushing flange towards the banjo.
Replace the 40T gear with a compound setup. 32T on the inside and 64 on the outside. The 32T will engage the lower 54T. and the 64t will engage the smaller of the gears at the bottom of the Tumbler gears. Also, place the Bushing Flange side towards the banjo.
What you might need to do. Disclaimer: YMMV. In my case, I drilled a hole just below the hole that is used for securing/adjusting the banjo in place. The bracket has plenty of meat and was easy enough to do. the existing hole just interfered by about 1/16th of an inch. (Just my luck eh?) .
As a test. I placed a micrometer on the bed and set the gearbox at its lowest setting .0042. and then turned the spindle 10 revs. I got .021! so for each turn thats .0021. now my turns are so much smoother what a great mod. and completely reversible.
So go now and change them gears!
Many thanks to "DaveKnowsHow" (youtube channel) my source. Go watch the vid....
Added (02.20.2015) I mentioned if you wanted to thread based on the old gear setup. You could just reinstall the old set of gears. YEs, while this is and always will be an option. You could also. just flip the compound gear setup and your back to the factory gear ratio. So, no real need to put the old gears back on.