Nothing worse than having a pen crack during assembly. Some things I am careful about to prevent it.
1. When gluing the tube in the blank, try to minimize the glue at each end thereby giving some expansion room for the tube inside the blank. The brass tube will expand but the wood, (especially hard woods) will not, resulting in a crack.
2. Thoroughly clean any residual glue from inside each end of the tube prior to pressing the pen together. Using sandpaper wrapped around a transfer punch will help.
3. When pressing together, be sure the parts are straight.
4. Sometimes putting the metal parts in the freezer for awhile will make them easier to press into the tubes.