Nick,
I think you are getting confused because of the different threads that are required to be cut.
I will try to explain the reasons behind what we are on about.
Imperial Lathe.
These are almost always fitted with an imperial leadscrew, and because the imperial system uses TPI (threads per inch), then by using the drop in dial that is matched to that leadscrew, and with the correct change gears fitted, imperial threads can be cut directly by dropping in the correct numbers of the screwcutting dial.
When metric threads are being cut on an imperial machine, a transposing gear is usually fitted into the gear train to allow the leadscrew to turn at the correct rate for metric threads. Unfortunately, because metric threads use a pitch system instead of a TPI one, you cannot use the drop in dial, but instead you have to leave the half nuts engaged, and reverse the machine backwards to wind the carriage back to the beginning, to carry out the next cut.
Usually, metric threads are not true pitch when cut on an imperial machine, they are an approximation of the pitch, but usually close enough for a normally cut metric nut or bolt to fit. The problems arise when you cut a long metric thread that has to be used with a long normally cut metric hole or bolt thread. They can very easily bind up solid because of the missmatch.
Metric Lathe.
These are normally fitted with a metric pitch system of leadscrew. But unlike the imperial system, you also have to compensate on the drop in dial for the different pitches being cut. Usually this is done by having change gears on the bottom of the dial that pick up the readings from the leadscrew. There are 3 on my machine, and these little posts I did might explain what they look like.
http://madmodder.net/index.php?topic=870.0http://madmodder.net/index.php?topic=973.0Unfortunately, even with the compensating gears, some metric threads still have to be cut by leaving the half nuts engaged, and setting up the gear train for the correct pitch.
For cutting imperial threads on a metric machine, it is basically the same as cutting metric threads on an imperial machine. A transposing gear is fitted, the gear ratios are changed to give what TPI you require and the thread is cut with the half nuts permanently engaged. The same problem with the fit also rears it's ugly head, they are only very close approximations of the thread required.
Some very expensive lathes have all these facilities built in, just select on the gearbox. But on the sorts of lathes we normally use, we have to compromise.
I hope this has made it a little clearer.
Bogs