4″ Knot Wire Cup Brush – Carbon Steel. Milwaukee offers the finest quality Knot Wire Cup Brushes to give you long service on the toughest cleaning jobs. These cup brushes are ideally suited for cleaning castings, structural steel and sheet metal. Milwaukee offers a full range of Knotted and Crimped Cup Brushes as well as a full line of Stringer Bead and Full Cable Twist Wire Wheels. This Knot Wire Cup Brush is designed to be used on 7″ Milwaukee and competitive Grinders with 5/8-11″ threaded spindles. Made of carbon steel.