Sherline3049S Boring Head with Straight Shaft (Metric) The Sherline straight-shaft boring head comes with a hex key for making adjustments; however, the cutting tools are 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 also offers a fly cutter with a straight shaft for use in ER-16 or 3C headstock spindles.