Here is a photo of one that cracked. It isn't happening on every pen.
That's a great pity -- it was a nice looking pen, and all the hard work has been done at this point. If it makes you feel any better, I can tell you that it's the kind of thing that still happens to me if I don't pay attention to what I'm doing.
The usual reasons why it happens to me:
1) There's some glue from when the tubes were glued in that got inside the brass tube.
It only takes a very small amount -- roll up a small piece of 220 grit sandpaper and polish the inside of the tube end. Any left over glue will show up under a bright light -- I usually scrape it out with an X-acto knife, then sand again to check that I got it all.
2) There's a burr on the inside edge of the brass tube.
You can remove the sharp inside corner using an X-acto knife, a metal-worker's de-burring tool, a countersink drill bit ... putting a chamfer on the corner makes it easier to get the components lined up and started when you are pressing them in.
3) The component goes in crooked -- or -- the pressing tool twists while the component is being pressed in.
I started out using wooden screw clamps, but found the two parts would easily get out of alignment. Then I tried the squeeze-grip clamps, and I found they'd over-squeeze the components.
I used my drill press for a while -- that worked very well, but now I use my lathe. The great thing is I can let go of the crank handle and it doesn't spring open, it just stops exactly where I got to.
4) It went in too far -- applied too much pressure and it forced the component into the tube beyond where it was supposed to stop.
Again, I find the crank handle on the lathe gives much better fine control and lets me stop at exactly the point where the parts are "in" but not "in too far".
HTH
