Like others, I find that polyurethane glue is the best choice for gluing pen tubes. There can be bonding problems with CA, and while is is an excellent glue, epoxy is messy to work with. It is true that polyurethane foams and expands, so the joint must be clamped until the glue cures (or in the case of pen blanks, clamping is required to keep the foaming from forcing the tube out of the blank while it cure), and some cleanup is required to get rid of the foam. But its always necessary to true up the ends of blanks after gluing, and that addresses the polyurethane foam problem.
The penetration property of CA decreases with increasing viscosity. In other words, the thin stuff will soak into wood fibers to some degree, but medium and thick tend to stay on the surface. However, the void-filling properties of CA improve with increasing viscosity. That means that the thick stuff works very well at filling gaps, medium is ok, and thin doesn't do well at all.
The result is that the medium and thick CA then to stick only to the surface fibers, and while they may fill a gap between the tube and the outer body material, the bond between the CA and that body material is weak because only the surface fibers inside the hole are engaged in the bond.
However, CA does adhere to itself very well, so if I'm doing a wood glue up that calls for CA (basically, anything where I don't want to have to deal with the tendency of polyurethane to foam and expand out of the joint), then I first apply a coating of thin to the bare wood and allow it to soak in and cure. This leaves a layer of acrylic that extends into the wood, bonding very well with the wood. Then, I complete the joint using either medium or thick CA (depending on how large the gap is). My experience is that this results in a far stronger bond.
