Z24R - Zirconium. Tough zirconium grain. High performance grinding for the toughest applications. Ferrous Metals: iron, steel, welds. Stainless steel, high tensile alloys. Specifications: Max RPM - 6600, 5/8-11 Super-Lock hub. Tech Info: The highest quality zirconium grain. All sizes made with uniform grain throughout, no filler grain used. Super-Lock Hub on all 5/8-11 threaded arbors is the safest, vibration-free design.