Sherline says itt is hard to say what is the most important accessory for a milling machine. It really depends on the type of work you want to do, but certainly the boring head must rank highly in importance for any shop. What makes it so important is that it provides a means of accurately machining almost any size hole. *NOTE: Sherline Products does not sell 3/8″ collets used with the ER-16 or 3C headstock spindles. Those are available through Hardinge Inc. or other tool supply sources.