Tom,
If Rich can drive the leadscrew and cut metal with it, then there should be more than enough power, through the very low gear ratio required, to drive the spindle.
I have cut large pitch threads using the leadscrew on my old Atlas lathe using handle power to turn the leadscrew, and the effort involved is very little. The actual cutting action of the tool is negligible when doing threading as it is only removing tiny amounts of material.
It is the only way to cut large pitch threads on these types of machines without doing damage to the gear trains. If you look at much larger lathes, they have the beefed up gearing to take the strain of driving the saddle so fast. In fact, because of the increased gearing, and taking into account the usual very slow speed of the spindle, the speed the leadscrew turns and so the saddle movement requires the reactions of a s**thouse rat to stop it before it hits the chuck. But even very large machines are limited to the large thread pitches they can cut when driving gears from the spindle.
In fact, for anyone with the smaller types of machines, say the mini lathe with their very fragile gears, I would recommend the leadscrew method when cutting the larger thread ranges that the machine manufacturers say that their machines can do. You are taking out the risk of smashing up the gears on your machine. Set up the gear ratio that the manufacturers give, but instead of turning on the spindle, turn the lathe by means of the leadscrew with a handle on the end instead, not only does it take the strain off the gear train, but you also have total control of the speed the saddle travels at.
John
