A pen mandrel is a rod of steel with a taper on one end and threads for a brass nut on the other end. There is a small recess integrated and of the mandrel. You will need a 60° center to fit that recess. The standard center that comes with a wood lathe will not work properly. Mandrels are commonly 7 mm (approx 1/4"). Berea makes a larger mandrel called a "B" mandrel that is approximately 5/16 inches in diameter. The taper portion on the right fits into the headstock of a lathe. There are several sizes of tapers depending on the size of the lathe. Check the specifications for your lathe for the correct taper needed for your lathe. Common tapers are MT1, MT2, and MT3. They are not interchangeable.