I just skimmed through your responses. I'm too lazy to read them all at the moment. I will offer an observation about lathes.
Older lathes get the bed worn more near the headstock. On inverted V ways this is readily detected by running your fingernail up the side of the V ("Up" is Vertical, towards the sun and moon). If you feel a ridge near the top of the V, the bed is worn. You can inspect the bed at the tailstock end of the lathe for comparison. This kind of wear causes the lathe to turn a taper. Typically bigger near the chuck and smaller as you go away from the chuck, and truly parallel when you are a long way from the chuck. You can correct some... sometimes, by using the tailstock, but really the only thing to to is to fix the bed. The cause of the taper can be readily realized if you draw a circle on a piece of paper, and put line from the center to due west on the circle. The circle is the work you are turning. The Westward line is the line that the cutting tool is supposed to placed on when it touches the circumference of the work. Put an arrow head on the line at the point of contact to the circle to represent the cutter. Drop the arrow due south, and watch the space between the arrow head and the center grow. There is your taper. It doesn't matter if the cause is a worn bed, or some other innacuracy in the lathe which changes the tool height. Changing the tool height while cutting is a mistake.
With a worn bed, a dial indicator on the carriage, measuring to the surface used to support the tailstock will show you the drop. Errors from a worn bed usually show up as greater innacuracies on small diameter turnings than on large turnings. I.e.,you can't correct the taper with the tailstock or cross slide once and have it always be corrected. The compensations are only good for one diameter.
If this is your problem, you either have to fix it or have eternal hassles when trying to make accurate parts. My first 9 inch southbend had the worn bed problem. I re-ground the bed carriage support inverted V's with a Unimat used as a post grinder in a very hacked-up arrangement. Not perfect, but MUCH better than a worn out, not straight bed.
David Rutledge