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.
It would be very impractical to own drills and reamers for all the possible hole sizes up to 1.75" (44mm) in diameter. Another point to consider is the fact that larger diameter drills and reamers could not be adapted to fit on a miniature size machine tool. They are unnecessary too, because the boring head is infinitely adjustable so it can machine all of these holes. It is also capable of machining stepped holes for seating bearings and internal grooves for lock rings.
Boring tools cut like lathe boring tools with the difference being the tool moves rather the the work.
The Sherline boring head comes complete with a drawbolt, thrust washer, and a hex key for making adjustments; however the cutting tool is not included. The boring head is designed for use with standard 3/8" (10mm) diameter shank boring tools. For best results, the length of the tools should be kept to a minimum. You will find that a short tool gives you less chatter and it will be easier to hold tolerances.
Sherline Boring Head (Inch) Instructions
CA Prop 65 WARNING: Drilling, sawing, sanding or machining wood products can expose you to wood dust, a substance know to the State of California to cause cancer. Avoid inhaling wood dust or use dust mask or other safeguards for personal protection.
I can do all this through him who gives me strength. Philippians 4:13