Variable pitch hole saw, made of bi-metal. Cuts through steel, tin, aluminum, fiberglass, wood and plastic. Extra-tough bi-metal blade . we use the highest grade of tool steel to outlast competitor's blades. That beats premature saw replacement and saves you money. Steam oxide finish . no paint to gum up and slow cutting. The saw runs cooler for longer life and better performance. Extra-thick back plate minimizes vibration for smoother, easier, less tiring cutting.