When sourcing tapered end mills, you must prioritize Substrate Material, typically Micro-grain Tungsten Carbide for high hardness and wear resistance. Pay close attention to the Taper Angle (Side Angle) accuracy, as even a 0.1-degree deviation can ruin precision molds. Additionally, verify the Flute Geometry; a Variable Helix design is preferred to reduce chatter and vibration during high-speed machining. For longevity, ensure the tools have advanced coatings like AlTiN (Aluminum Titanium Nitride) or nACo for heat resistance in dry cutting conditions.