This 3 piece set includes: 3.0", 3.5", 4.0" - Often called "dimple dies", flared hole dies are used in sheet-metal fabrications to provide rigidity and strength to flat, unsupported panels. Due to the resulting high strength -to-weight ratio applications promoted by the use of this technique, flared holes are found in many aircraft and race vehicles designs. Beginning with comprehensive finite element analysis supported by extensive prototype testing, our Flared Hole Dies are engineered to provide the optimum combination of performance attributes: Designed to eliminate edge tearing even when used on high strength 4130 chromoly sheet (normalized condition), with laser cut holes (hardened edges) Optimized form (flare angle, bend radius and length of run) provides maximum stiffness and strength Manufactured from premium hardened high carbon steel for maximum tool life Unique shape for ease of handling Up to max sheet thickness of .090 for chromoly and .125 for mild steel.