Interesting result. Is the original bit of machining as rough as it looks? I know that photographs are often misleading when it comes to surface finish. And can you use the existing thread, or are CBN threading inserts available? I've never looked past carbide in the catalogues - prices go up as you turn the pages...

And for completeness, what lathe were you using? Possibly not using the kinds of speeds/feeds/DOC that a "hobby-class" machine could achieve?