First, I have to make something clear. Several years ago, on one of the penturning forums, someone posted a tutorial on this technique. The idea seemed to die there. About two years ago, I was getting very disgusted with the ends of my turned blanks being less than perfectly round. They were very slightly oval. I credited (blamed?) this on the inherent flex in the mandrels. Most mandrels are under a 1/4" in diameter and about six inches long. Flex is a fact of life.
I recalled the 'no mandrel' technique, tried it and got the perfection I was looking for. Never looked, or went, back to using the mandrel on larger pens.
For whatever reason, when I posted my results the idea took off like a hot penny stock and I was given credit for 'inventing' the technique. Not so, not me. But, I'm pleased that others have learned to use this technique and are achieving good results with it.
The method and concept is simple. First, for pens which are larger than 7mm, put your mandrel away and forget it.
Buy yourself a machine tool dead center and a machine tool live center. Those sold as wood lathe accessories are made from soft steel and will fail quickly. The machine tool ones are hardened steel and could last indefinitely for this work.
As you can see in the first picture, put the dead center in the headstock. It is your drive.
Put the live center in the tailstock.
Take just one of the two glued up blanks for your pen and mount as shown in the second picture. Tighten the tail enough to have a secure grip and drive.
Turn your blank.
When turned to just a couple thou over the bushing size remove the bushings and remount the blank between the centers as shown in the third picture. A word of caution here. Tighten the tail only enough to get a grip and decent drive. If you overtighten the ends of the brass tube will bell out and you could split your blank. DAMHIK
At this point, I turn down the ends to bushing size. I eyeball it but maybe, after doing this a jillion times I have gotten to where I can do so correctly. But, I also cheat. With the skew first, sandpaper second, I put a tiny bevel on the end. And, I do mean tiny. This hides any sins. Seems to work well. (almost) Invariably, when assembled the parts fit flawlessly with the blanks.
With that done, you can finish sanding and do your finishing. Yes, the crud you see is from my use of rattle can lacquer.
Repeat for second blank.
I used to get some chatter when using the mandrel. That is gone with this technique. The overall quality of my pens has improved dramatically since the change. As far as I am concerned, this is the only way to go with pens over 7mm.