I think the best way to cut external threads using a tap is to grind away all but two or three teeth, or ideally, even all but one. A single tooth, well relieved on either side of the tooth, would give you a nice cutting profile and then it is all about the feed rate. Using a carefully aligned tap as an external threading tool, you can scratch pass nicely, but once the tap starts to cut, you are trying to cut along quite a long linear front and that would demand extreme rigidity and good power. The reason that single point threading works so well is that it is a single point. A small amount of pressure can be presented to a very tiny spot and if the tool is properly ground, properly aligned to the part, and held with adequate rigidity, it's gonna cut even on a mini lathe. A bigger lathe doesn't even know it's there. It doesn't take a lot of pressure or rotational power to cut, when that pressure is applied over just a very tiny spot. Multiply that by say 20 tiny spots, and that is a lot more difficult. Add the intervening cutting edge of each tooth and as you cut deeper you end up with a half inch of threading representing over an inch of cut width. Clearly this is going to be difficult and for the typical small lathe, maybe not even possible or practical.
Reducing the threaded length to the bare minimum would make the 3% pitch error less of an issue. A nominal metric pitch of .5 with an actual pitch of 3% finer would be instead of 20 threads over a centimeter, be 20.6 threads. Over a half thread off. In brass it will probably wear in. But if you double the threaded length to two centimeters then the final error is more than an entire thread. Half it to a half centimeter threaded length and the final engaged thread is only a hair over a quarter tooth out of place. With only say 5 threads engaged, 3% pitch error in brass is almost not even an issue. talking 3/20 part of a thread off at one end, with the other end perfectly engaged, or with the center perfectly engaged, either end will be off, one in one direction and one the other, of 75/1000 of the pitch distance. To say that is no biggie is a gross under (over?) statement.
Taps and dies at this pitch are available from China and also from Australia and UK. The Chinese are all over fleabay and amazon, and even the Chinese ones will do when cutting soft stuff, if you don't mind the wait for delivery.
I, too, find it a chore to get all the stars in alignment for single point threading at such a fine pitch. My threads just don't make the cut when I get over 32tpi and yeah I know I could spend the time to address every single issue and make it work on much finer threads, but I just don't roll that way and neither does my Vevor 7-incher. If I ever HAVE to do it, using a single tooth of a tap of the correct pitch at least gives me a good tool profile and I just have to tighten stuff up good and check alignment and stuff.
