When a Tap Breaks: Getting It Out, and Not Doing It Again

One: the tapping drill was too small, so the tap is being asked to cut far more metal than it was designed to. Two: the chip was never broken — no quarter-turn back — so it packed solid. Three: a blind hole bottomed out. Four: the tap was started crooked and then forced. Five: sideways pressure, usually from a spanner on the square instead of a tap wrench. Every one of those is a decision made before the break, and four of them are made before the tap even enters the hole. The small sizes break most. An M3 tap will snap under a force you would not notice applying, and below M4 the sensible approach is a tapping stand or a pillar drill used as a guide by hand, never a wrench swung freely.

A tap broken off above the work can often be gripped and wound out. Lock a pair of grips or a small pin vice onto the stub and turn it gently backwards — gently, because what is left in the hole is even easier to break again. Flood the hole with penetrating oil first and leave it. The tap is jammed by packed chips more often than by the thread itself, and oil plus time does real work here. If it will not turn backwards, try rocking it: a few degrees back, a few forward, gradually increasing. This breaks the chips loose the way the quarter-turn should have done, and it recovers far more taps than brute force does.

A TAP EXTRACTOR has fingers that slide down the tap's flutes and grip from inside. It works well on a clean break in a larger tap and poorly on a shattered small one — and its own fingers break. SHATTERING works on hardened steel because it is brittle: a sharp centre punch and light taps can break the tap into pieces that pick out. It is genuinely effective and it can also crack the workpiece, so it is a decision, not a default. ALUM SOLUTION dissolves hardened carbon steel and leaves aluminium and most workpiece metals alone. Hot saturated alum, several hours to a day, and the tap is gone. It is slow and completely non-destructive, which makes it the right answer for a valuable part — and the wrong answer for an HSS tap, which it barely touches. EDM will remove any tap from any part, and it means paying a specialist.

When the tap will not come out, stop trying to save that hole and start thinking about the part. Drill the hole out to a larger size on the same centre — if you can get past the tap — and fit a threaded insert or a larger fastener. Often the better answer is to abandon that hole entirely and put a new one a short distance away, or to fill it and re-drill. A part with an extra hole in it usually still works; a part destroyed by three hours of increasingly violent extraction does not. Decide early. The damage in these situations is nearly always done in the attempt to fix the first mistake, not by the first mistake — and the same is true of most things in a workshop.