OVERVIEW: The Toolholder is the essential connection between the machining center and the cutting tool. The toolholder fits into and is secured by the machining center’s spindle, and in turn secures the cutting tool such as a drill or end mill by clamping onto its shank. The taper of the toolholder matches the toolholder interface of the particular spindle. Toolholder tapers are often conical, including CAT and BT taper specifications. A different kind of taper, HSK, is not tapered at all, but instead includes a variety of flanges for securely locking the toolholder in place. Toolholders use different mechanisms for clamping the tool. A set screw can clamp onto a corresponding flat in the tool’s shank. Or, a collet can be compressed around the tool by tightening a nut that also surrounds the tool. More unusual clamping mechanisms include hydraulic toolholders, which compress a bladder of hydraulic fluid, as well as shrink fit toolholders, which clamp and release the tool by heating and cooling the toolholder’s metal to take advantage of thermal expansion and contraction. Other toolholder types use still other methods of clamping. Various clamping mechanisms aim to provide not just secure holding of the tool, but also tool concentricity to the centerline of the spindle that is accurate to a level of runout error appropriate to the tool and the machining process.