You can only thread PVC if it is schedule 80 or 120. Any other schedule will be too thin walled to hold a good connection. The one time I needed a piece threaded I used a hand threader for it like the following.
Manual Ratchet Threaders
You just need to make sure it is on straight and cut it slowly or you will mess the threads up.
Also as a side note some thing I found while looking into this— if you thread a piece of PVC then the working pressure limit is only half of what it would be if you used a glue-on fitting
Just as a side note because it is one of my pet peeves, When attaching a PVC pipe(schedule 40) to a metal pipe— Spend the extra to use a schedule 80 fitting (and ALWAYS go male plastic to female metal, not the other way around). It glues on exactly the same but is much much harder to crack under normal conditions. The extra dollar or two could save you digging it up again later.