Whilst I understand both sides of the program vs gearbox discussion above, and don't wish to take either side, other than to say that a machine with a gearbox will only have two options to change ratios to correctly align metric and imperial. I will add that my own Triumph will cut both, with the proviso that the half nuts are not dis engaged - been there and found it to be true, unsurprisingly. The problem is you cannot turn a big lathe, fully engaged backwards, and if your single phase motor does not have a reversing switch it cannot run backwards...
So what's the fix? Well I recently had to cut a metric thread, and my answer was to turn a smaller diameter on the end of the shaft, cut a thread into that and use it to restart each new cut - obviously setting the correct depth for every advancement. It worked really well I have to say.
