I see the exact same behavior on the USB SS and the latest few versions of Mach (.062 and now .066) and SS plugin ('d' and now I think 'f'). I have it tuned close but have some variance on the extreme ends. Threading works with the threading pulse, but for general lathe turning I'd like it get the extreme RPM ranges a bit more accurate. It stops the spindle (I had it running to start) and acts like it is going through the procedure, but the spindle never comes on..