Forming (roll) taps don't cut—they plastically displace material to form the thread, producing no chips and working in through and blind holes. The formed thread is stronger than a cut thread due to work hardening and continuous grain flow. Best for ductile materials (aluminum, copper, low-carbon steel, stainless); not brittle materials like cast iron. The tap-drill size must be larger and more precise than for cutting taps. Mainly cobalt or powder HSS with coatings; lubricate well.