Shrink fit toolholders rely on the thermal expansion characteristics of metal to provide extremely accurate toolholding, with very high grip force. The bore of the shrink fit toolholder is engineered to be slightly smaller than the diameter of the cutting tool. When the toolholder is heated, the bore expands, allowing the cutting tool to drop in. When the toolholder cools, the bore contracts, firmly clamping the cutting tool in place.
A special shrink fit machine is used to properly heat and cool the toolholders quickly, reducing the time needed to change cutting tools, and ensuring that toolholders are not overheated.