I've done a couple of closed-end fountain pens.
Easiest way (in my opinion) is to make a closed end mandrel from a piece of 1/4x20 allthread rod. Get a bit of plastic - I used a small plastic spacer that was originally intended for undercabinet mounting of a Black & Decker coffee maker (don't throw away ANYTHING - it could be useful later). Threaded the spacer onto the end of the rod and mounted it in a Jacobs chuck in the headstock. The, I used a skew to turn down the spacer until it just barely fit through the brass tube of the pen kit that I was basing the closed-end design on. Then, I used an overlength blank and drilled a hole that was deep enough to accommodate the inner workings of the pen (the hole ended up deeper than the length of the brass tube. Finally, I glued the tube into the blank, flush with the open end of the hole (which meant that the hole actually extended further into the blank than the length of the tube).
Using the Jacobs chuck with a single pen bushing, I mounted the blank with the plastic spacer inserted as far into the hole in the blank as it would go. I used the tail stock to provide support for the closed end while turning, sanding and finishing the barrel of the pen. Finally, I parted off the end of the blank, and with the tailstock backed away, carefully turned down, sanded and finished the closed end.
Obviously, the cap end of the pen has to be done separately.
