I have had several of the break off at the nib (the threaded piece) anyone know if that piece is replaceable and if so where to get it. Also any idea why this keeps happening?
I've never had that happen, and must have made 50-60 of them so far. Are you possibly pressing them at an angle? For replacement, try contacting the vendor you got them from.
(Tip: Get a small t-handle reamer, and slightly bevel the tube before assembly. That will help when starting to press the components together.)
I assume the part you are breaking are the threads that the bullet tip screws onto. I have broken them before also when pressing the pen together. So I have a small block of wood I have drilled different sized holes in. It allows you to put pressure on a part of the pen that won't break.