Instead of widening a hole using a lathe, which turns the workpiece while the cutting tool stays still, use these holders to turn your boring bar in a milling machine and keep the workpiece stationary. They’re good for creating holes that are too large to drill or a diameter you don’t have a drill bit for. To help you bore holes to precise diameters, these holders have an adjustment dial with fine increments. Mount them in your milling machine using their shank. They’re also known as boring heads.