For many years industry has used CNC (computer numerically controlled) machines to craft specialist items. CNC machines traditionally mill out metal objects with arbitrary shapes, but they are expensive, large and dangerous. In recent years a new type of CNC machine has appeared - a 3D printer - which makes 3D objects by printing layers of plastic. These can be made safe, cheap, and small enough to fit on a desktop. I will show off my 3D printer, and explain some of the maths that goes into it.