Assuming you are only threading one barrel,turn off the breaker or unplug the lathe,and turn the chuck by hand,forward and reverse.use a sharp tool and light cuts. Do back out before you back up.Yeah,there are drawbacks,but it works.you have great control over where the cut stops.Each cut,try to stop just barely shy of the previous chip so you don't snap off the tip of your cutter.Use a good lube.
