A snapped end mill rarely just “breaks.” At spindle speeds north of 10,000 RPM, a carbide cutter fails the way glass fails: it detonates, throwing micron-to-millimeter shards across the enclosure at lethal velocity. Understanding why means understanding what the tool actually is. Most production end mills are cemented tungsten carbide, hard WC grains bonded in a cobalt matrix. The composite buys extreme hardness (1500 to 2000 HV) and red-hardness that lets it cut at temperatures that would anneal HSS. The tradeoff is brittleness. Carbide fracture toughness sits near 10 to 15 MPa root-m, a fraction of tool steel, so it tolerates almost no plastic deformation before catastrophic crack growth. The flute is the weak point. A fluted cutter is a notched cantilever loaded in bending and torsion. Deflection scales with stickout cubed over diameter to the fourth (L cubed over d to the fourth), so a tool run too long or too thin concentrates force at the flute root. When bending stress exceeds...