Why screws strip and break
A stripped screw is one whose recess has been chewed out, so the bit spins without turning the screw. The usual causes are a bit that does not fit, too little downward pressure, a worn bit, and a driver turning too fast. A recess shape that cams out easily makes things worse, and the comparison of screw drive types explains why some designs strip more readily than others.
A broken screw is one whose head has snapped off or whose shank has sheared. This happens most often when a screw is driven into hardwood without a pilot hole, when it is overtightened, or when a brittle screw meets resistance. Rusted screws can also snap when twisted.
It helps to start gently. Each method below is escalated in aggressiveness, and the early ones are least likely to damage the workpiece.
Methods for a stripped head that is still above the surface
- Use the right bit. Check that you are using the correct size and shape, and a fresh bit if the old one is rounded. A slightly larger or smaller size may bite better.
- Press firmly and turn slowly. Most of the work is pushing the bit into the recess. Slow speed and heavy pressure often beat a fast, light touch.
- Add grip. Place a wide rubber band, or a piece of steel wool, between the bit and the screw head. Some people use a bit of abrasive grip compound made for this purpose.
- Try an impact driver. An impact driver delivers short rotational blows along with the downward force, which can break loose a stuck screw without as much cam-out. Hold it square and use a good bit rated for impact use.
- Grip the head directly. If the head stands proud, locking pliers or a small pipe wrench can clamp it and turn it out.
- Cut a new slot. Use a rotary tool with a cutoff wheel, or a hacksaw blade, to cut a straight slot across the head, then use a flat screwdriver. Wear eye protection, since cutoff wheels can shatter.
- Use a screw extractor. A screw extractor set typically has a drill-and-extract design. The tool bites into the damaged head and turns it counter-clockwise. Left-handed drill bits sometimes catch the screw as they cut, and back it out on their own.
Methods for a screw sunk flush or below the surface
- Extractor bit in a drill. Drill a small pilot hole in the center of the screw, then insert the extractor and turn it slowly in reverse. Hardened extractors are brittle and can snap off inside the screw, which is a far worse problem than the original, so apply steady pressure and do not force them.
- Drill out the head. Use a drill bit slightly larger than the screw’s shank diameter to remove the head. The head comes away and the piece you were fastening may lift off, leaving the shank sticking out. Once the parts are separated, grip the exposed shank with pliers and spin it out. Drilling metal needs sharp bits, light oil and a slow speed to avoid ruining them.
- Use a hole saw or a plug cutter around the screw in wood, to cut away the surrounding material so that the shank is exposed. This is aggressive and leaves a hole to repair.
If the shank is broken off in the material
A shank snapped off below the surface is a common headache.
- If a piece is protruding, grip it with locking pliers and turn it counter-clockwise.
- If it is flush with the surface, drill a pilot hole and use an extractor, as above, or file a small flat or notch in the end so a small flat screwdriver or punch can turn it. A small center punch tapped at an angle against the edge can sometimes rotate it counter-clockwise.
- If it won’t budge, the practical option may be to leave it, move the new fastener a short distance away, and fill or plug the old hole. In thick wood, drilling out the screw and gluing in a dowel can restore the hole.
Rusted or seized screws
Corrosion makes everything more difficult. Applying penetrating oil and letting it soak, and tapping the screw head lightly to help the oil creep in, can free many stuck fasteners. Heat is sometimes used, but heat near finishes, fuel, wiring or flammable materials is a safety risk, so be careful. Rust is also covered in the article on fastener corrosion.
Preventing the problem
- Match the bit to the screw and replace worn bits.
- Drill pilot holes in hardwood and near edges. The article on pilot holes explains how.
- Set the clutch on your drill so it slips before the head strips. Clutch settings are covered in a separate article.
- Hold the driver straight and push firmly.
- Choose better drive types such as Torx or square recesses, which resist cam-out better than basic Phillips.
- Use quality screws. Soft, cheap screws strip faster.
- Back off at the end. Slow down as the head seats, so you do not overtighten.
Safety
Wear safety glasses at all times with these methods, because broken extractor and bit fragments can fly. Clamp or secure the workpiece. Unplug the tool or remove the battery when changing bits. Use gloves with care around spinning tools, and avoid loose sleeves. If the screw is part of a safety-critical component, like a brake part, steering component or structural connection, consult a qualified professional rather than improvising.