A mandrel is nothing more than a steel rod threaded and center drilled at one end. You need to size it to the type of pen/pencil kit you are making. The ones I make (and I believe the most common size) are 7mm. That is, the inner brass tube/sleeve fits into a 7mm hole in the wood blank. Since you are going to have to get a kit anyway, get one and measure the inside diameter of the tube. I'm guessing it's roughly .25 inch, but you need to check (I'm at work and can't right now). Thread one end (centerdrill as well) for whatever size nut you want to put on it. Can be a standard hex nut, or a knurled thumbwheel. You'll need to purchase the bushings, or be able to harden anything you make. Bushings are particular to each type of kit. Although the tubes are all 7mm kits, different pen styles use different bushings. I have three sets. They are cheap, so I just bought them.