The Porter-Cable 43178PC is a Cove & Bead Router Bit. Porter-Cable Professional Edge router bit is constructed of 4140 Alloy Steel, a harder grade of steel than standard cast steel bit bodies. The bit is annealed and heat treated to Rockwell hardness of 40-45, harder than is possible with cast steel bodies. The carbide tip is made from a specially blended micro grain carbide for superior results and longer lasting sharpness. The bit 5/8 inches in diameter or large has a chip limiting design which provides more cutting control, better balance and more body mass for a cleaner cut. A black oxide coating protects against corrosion while preventing resin build-up and improving chip clearance.