Hi Mike, 12 mm tenon should work, I wouldn’t go to much less than that or you will end up with flat tops on your threads.As a relative newcomer to kitless pens, I asked this question about how to calculate the size of the tenon for using a die in a recent forum, and I had an excellent response from Monophoto. My die for the internal cap thread and the corresponding male thread is 12mm- .75. The 12mm tenon worked well on acrylic blanks, although being a novice at kitless pens, I am still waiting for a finished pen I can be proud of!!
However, after recently seeing an article on the Web about making an aluminium pen with a slimline kit on a woodturner's lathe, written by Walter Hall in Woodturners magazine a few years ago, I decided to attempt a kitless pen in aluminium after making a decent attempt using a kit.
However, I turned the tenon down to 12mm, but the die would not even attempt to make a thread, and in the end, I had to gradually turn the tenon down significantly to enable the thread to start. The pen was not successful but I am still trying. I bought a 1 metre length of 15mm rod for £10 (English) so I still have a lot to play with without it costing a lot, and I am learning a lot in the process.
However, the question is, what was wrong with the 12mm tenon size in aluminium for the die I was using?.
I have also now ordered a brass rod-so I can give that a try!
Keep safe and Cheers
Mike, I believe not having a chamfer at the end of the tenon is the main factor causing trouble. Some materials that are soft (acrylic acetate,, ebonite, celluloid) don't put up as much of a fight, but for the most part it's necessary to have some kind of ramp that the leading teeth of the die can slip over before they start to bite into the material.... I also need to experiment more with the chamfer ... I have been using WD40 as a lubricant ...