"The bit broke again" is one of the most frequent complaints at a tap-hole crew. A broken bit not only wastes money but, worse, leaves a piece in the hole; if it cannot be retrieved, the whole tap hole is affected. When this happens, do not just fit a new bit—first work through the following five causes.
Cause one: bent rod or poor alignment. After long service, a rod bends permanently; as it drills, the bit sees alternating bending stress and the wing root fatigues and cracks. Fix: check rod straightness every shift and retire rods that exceed the tolerance.
Cause two: excessive feed. Operators in a hurry push too hard, the buttons bite too deep, and the bending moment at the wing root spikes until it fractures. The right approach is light feed with fast rotation, letting the bit cut naturally rather than forcing it.
Cause three: insufficient cooling air or water. A clogged port or low pump pressure lets bit temperature soar, softening the carbide and body, and the next shock snaps it. Always start water and air first and confirm return flow at the hole before drilling.
Cause four: improper thread make-up. Dirty, under-torqued or over-torqued joints all cause the bit to wobble in the hole and see extra bending moment. Torque to specification; do not rely on feel.
Cause five: hitting metal accretions or residual iron. Over long campaigns, accretions and residual iron form on the lining; when a bit hits such a hard lump, it is metal on metal, and the wing chips off. In that case, pull back, drill a pilot hole with a smaller bit, then ream out to full size.
Run through these five points and most "mysterious breakages" will have a cause. What remains is a genuine bit quality problem.